Hi all,

I've been looking at battery pages for quite some time now and thought I'd share what I've found so far. Some it is heavy duty reading, and there are a *lot* of pages.

http://evbatterymonitoring.com/

http://www.batteryuniversity.com/index.htm

http://www.smartgauge.co.uk/technical1.html

http://www.smartgauge.co.uk/technical2.html

http://www.powerstream.com/1922-FLA.htm

The last item is somewhat dated--but a fascinating read all the same--with lots of good information and some super pictures.

Can you recommend if any of these are of the "Batteries for Dummies" level? I need something that is no higher than unskilled layman level.

If you wanted to buy some group 27 size AGM batteries with as much amp hours as you could get, what brand would youuse? As usual price and value are considerations.

Hi abbygolden,

I guess it depends on how carefully you wish to read. The battery university is very comprehensive so I might start there.

The smartgauge site is a "fun read" (because he has an interesting sense of humor) and has a wealth of information.

Hi Lindsay,

There are only two major manufacturers of lead acid batteries left. Johnson Controls, and Exide.

I'm not really a fan of AGM batteries as they cost a lot more per amp-hour than standard flooded units. If I were going to buy AGM format, I'd look for six two volt cells and wire them in series.

AGM batteries have no way to replace lost electrolyte. Therefore they may *not* be equalized. Even the most conservative information suggests that equalization is a necessary feature and must be done at least twice per year. (some sources suggest once a week!)

If they *must* be group 27, and *must* be AGM, I'd simply phone/email until I found the best price per amp-hour, so long as total amp-hours are the same. I would favor 12 volt units because of their ability to produce higher amperage for use with an inverter.

It is far cheaper to equip regular flooded batteries with a "filler" system than to buy AGM batteries.

I am going with the AGM batteries for the ease of use. I have 27's now and wanted to use the biggest that would fit. I have 3 gel batteries now that are about 3 1/2 years old and are not holding a charge as well as they should. We are going on a long trip in April and I plan on replacing them before then. Thanks for your help.

Lindsay,

If it will fit the AGM GPL-8DL 12 volt will do the job. Weight is 162 lbs each. Discharge rate is 475 minutes at 25 amps. We use two for the house batteries and find they work great plus no water or care needed. Only downside is high cost. We can run everything in motorhome two days before we need to recharge. We had 8D Gel cells before the AGM set. AGM's are hard to damage.

I would love to have them, but I have an Itasca with the battery compartment under the stairs and they would not fit. I already measured before. I could put them in a nearby compartment, but I don't think I could stand to lose the space. (My wife's rock collection you know.).I am going to have to stick to group 27's. My line to my wife is that these will last twice as long, so we can pay twice as much and she seems to have bought into it. They are not in the Wally world online info, but I didn't think they would be. Have to go to the store. Has anybody ever had batteries shipped to your home?

Hi Lindsay,

I'll bet one reason for your early failure on the current battery bank is improper balancing of the wiring. For three 12 volt batteries *only* method three works correctly.

correctly interconnecting multiple batteries

How do I know? I wrote the owner of Smartgauge and he was kind enough to reply!

I typed 3 batteries, but I only have 2 gel type house batteries and 1 engine battery. all under the stairs as typical Winny style. The engine battery of course is seperate and hooked up only to the engine. The 2 gel batteries appear to have equal length wires. Sorry for the confusing typo. Do you consider 3 1/2 years, 26,000 miles, and about 300 nights (about half boondocking), as a premature battery failure? This is my first experience with gel type. If you think it is premature failure, please don't tell my wife.

Hi Lindsay,

Gel or AGM?

If Gel I think they have done quite well. If AGM I think it is a little premature.

My previous 12 volt walmart wonders have lasted seven years, 360 cycles (as low as 10.5 on more than one occasion) and are still serving the new owner of the RV. They were flooded cells maintained by a 30 watt solar system--and watered once per year. They were wired in method #2, not by knowledge but by accident.

Method #2 would be correct for a 12 volt pair, or Method #3.

My existing two coach batteries are gel. When we bought our first coach (97 Rexhall). It came with one dead 12 volt. I replaced with the two gel batteries and then put them into my current coach (2004 Itasca). I have read so many good things about AGM and we tend to boondock so much, they seemed like a good choice. If you figure how much usage you get, it works out to only a dollar or two per night on the gel and I think I can get a lots more on the AGM. Save a lot of campground money also. The gels are not doing as well as before and we have a 3 month trip coming up in April, so I plan on replacing them before then. Only shorter trips between now and then.

Lindsay,

I spent 40 plus years working with big batteries in telephone offices around the USA. Our 48 volt systems had batteries that each one had a weight of 2200 lbs and put out 2.20 volts at 2000 amps and we used 23 batteries to get 50.6 volts. Over the years we change flood cells over to Gel that had a weight of 350 lbs each with number of banks to make up the number of amps we needed.

You may have killed your Gel battery by now due to wrong voltage level. You need a three stage charger that can keep voltage at 14.0 to 14.2 and no less. Most charges only are single stage set at 13.1 to 13.9 tops. Also charger needs a setting for warm or cold weather. The AGM battery will use the same charge level as a flood cell 13.1 to 13.9. You will never dry one out, it will die from sulfate damage first. No need to equalize an AGM battery. Life on Gel batteries we have use in MH is around five years. Only problem is high cost and high current that will pull the acid off the plates and this will damage the cells. AGM's do not have this problem and will take more damage to kill it. The AGM's in MH now are couple years old and you do not know they are in place, no problems.

I was called on by GTE in AL to come up and see what was wrong with telephone office power plant. Found maintenance people had reset the voltage level to wrong levels. We had the correct level at time of service. It was off by .2 of a volt per cell and this killed 46 batteries (two racks) in two years. The cost to replace was in the $100,000 range, GTE was sick. All the cells went to the junk yard.

It pays to get it right.

Pianotuna: I went to our Super Wal-Mart and could find nothing about AGM batteries. The staff was no help. It did no have an automotive center. What should I be looking for?

Hi Lindsay,

Maintenance Free Batteries. (i.e. no filler caps)

So if they have no battery fillers, they are AGM? I am down at my daughters and they have a automotive center at their Wal-Mart. Thanks a lot.

Hi Lindsay,

Probably. I'd definitely ask that question.

Don and Lindsay,

Some mix up here on AGM and filler caps. If battery has a filler cap it is a flood cell. You will not have a filler cap on AGM or Gel battey, no need for it. Only a valve vent. You will not find an AGM or Gel at Wal-Mart. Only place to find them is a battery supply house or on line. Due to the weight shipping is not a good plan.

Hi John,

Eight years ago I did purchase maintenance free starter batteries at Walmart Miles City Montana. I do not know the battery technology for sure--and my old RV is a bit of a hike from here so I'll not be able to check that out for some time.

Walmart keeps changing things too! So it may be that they do not sell deep cycle maintenance free batteries. Or AGM either.

I went to Wal-Mart and looked. They sell deepcycle maintance free, but they are wide plate and not AGM. They are made by Johnson Controls and are about $85 for a group 29. They say for marine and RV use. They have sealed tops. I talked to the folks there, but they knew much less than I did and I suspect that will be just about everywhere. I did a search on Johnson' Control, but could not trace any AGM to Wal-Mart. I did see where several online ompanies will ship them Fed-X ground for about $10. I will be doing something before my 3 month trip starting in April.

Hi Lindsay,

Thanks for letting us know what is available. I hope I didn't waste too much of your time.

I'm not sure what you mean by "wide plate".

Don,

Engine start up batteries have thin plates (high amps). The wide plates are the deep cycle type batteries. The T-105 have the heavy wide plates. Wal-Mart will never have an AGM due to high prices. All battery shops carry the AGM's. You can find them everyplace you go. Johnson Controls is here in Tampa and builds batteries under 20 or more names, Wal-Mart, Sears, K-mart, NAPA, Auto Parts, and you name it. They also recycle all the old batteries for hundreds of miles around as well. So buy the one you think it best but most are the same in different in battery labels.

Hi all,

Here is another page to read:

http://www.mpoweruk.com/leadacid.htm

Don,

Batteries have been a pain in the butt for the hold 140 years. Look at GM and the new electric car. GM has worked on the battery not the car for years now. May be ready late next year, short range of 100 miles or less in hills and you are out of gas. GM had to add a gas engine back in, first pass was to drop the the engine to save weight and cost. First person to build a long life battery that will work for lots of years will have more money that they could ever spend.

The write up covers all the correct information.

AGM batteries ranging in size from 50 Ah to 150 Ah are commonly used in UPS (Uninterruptable Power Supplies) in big IT departments like banks and big business operations. Due to the need for 100% reliability of millions of dollars worth of data they often replace their AGM's every 2-3 years even though they can give excellent service for up to 12 years if properly cared for. I found an "inside connection" through a friend who worked at such a IT firm and was able to purchase twelve three-year old 127 Ah AGM batteries for $7.00 each (recycle/scrap price) that had an original retail of $384.00 each. All were near perfect as load tested with a 500 amp carbon pile tester. The size of these batteries was closer to a group 29 - each weighs 102 lbs.
Two have been in my fiver for over a year. Later I picked up twenty-four 50 amp hour AGM batteries that were three years young and had never been installed (they are now part of a solar power bank) They are easily parallel connected for whatever reserve power rating you want. I paid $4 each for these batteries.

My reason for sharing this is not to brag about my find but to call attention to a source of perfectly good, cheap AGM batteries that is often overlooked. If you have or can make such connections for re-cycled AGM batteries it will pay you high rewards in the pocketbook. DO NOT call an IT firm and ask a stranger if they sell their used UPS batteries. You will get a flat NO due to disposal rules (environmental stuff). You really need to know someone on the inside who knows you are not going to dump them in your back yard.

Most all AGM's are multi charge deep cycle type batteries. One real positive for RV use is their vibration damage resistance is virtually 100%. Regular flooded deep cycle batteries often fail in a RV from the 4.3 magnitude earthquake jolts they experience - not age. Second big plus is no gassing out -- so you can put them in an un-vented compartment or even a living space. Number 3 is position -- sideways, upside down, whichever way they fit best.

The major downside to AGM's is that overcharging will ruin them very quickly. Therefore, ALWAYS use a minimum of a 3 stage regulated charger specifically made for AGM batteries. A conventional RV converter charger is NOT the best charger for AGM's and battery life will be shorter using a charger designed for flooded cells.

I purchased a new Charge Wizard from Progressive Dynamics for my PD converter for AGM charging. It was something like $35.00.

As for equalizing AGM's like conventional vented lead acid batteries -- don't worry about it (4-stage chargers are not needed or good for an AGM). They do not accumulate sulfate on the plates like conventional wet cells. In fact, equalization charging is potentially harmful to AGM batteries.

BTW - I'm an EE and one of the courses I teach covers batteries extensively.

Hi all,

The Battery as a Dog

Enjoy.

Professor,

Very good write up on AGM's. Most people miss the earthquake jolts part we all have in RV travel plus no water in cells. The two items will kill flood cells off at a high rate. We use two AGM 8D type and find they work great with our three stage charger system. Only down side is high cost but no acid damage or water problems off set that item as well.

Don,

Yes, batteries are a little like a old dog. Good write up.

Two 8D's? My gosh, their combined weight must be close to 500 pounds! I carried 4 of the AGM group 29's in the front compartment of my fifth wheel as auxiliary batteries for my 150 watt solar array and 3,000 watt Vector MAXX inverter. This was in addition to the two identical AGM 127 Ah batteries for the house side and the 4,000 watt generator.

At 102 pounds each I was adding about 700 pounds to the pin weight of my fifth wheel with my additional equipment. The cheap Lippert electric landing gear drive assembly crapped out with the additional load. So, I had to move the 4 AGM's and inverter on the aux side to the TV bed, which had excess load capacity.

Anybody adding batteries needs to consider the added weight and the impact on the axle and tire loading.

BTW - in response to a PM from pianotuna, I did not mean to imply in my last post that AGM's could not develop sulfation - only that the issue of a regular equalization charge is not part of the AGM maintenance routine. As with any lead-acid battery, which AGM's belong to, sulfation is inevitable if the battery is not properly maintained.


This is a shot before I took the 4 AGM's out of the front compartment to reduce weight load on the landing gear drive. The protective cover was removed for this photo. And yes, the batteries in the photo were my $7 each AGM's cycled out from a IT company's UPS battery bank.

Hi professor95,

It is interesting to see that the original installation uses aluminum buss bars. Do they obviate the need for balancing the wiring harness?

correctly interconnecting multiple batteries

Method #3 or #4 works for four twelve volt batteries.

pianotuna - My MH has three batteries, two for the coach and one for the engine. The coach batteries have a disconnect switch which I use when in storage. I assume that there is no disconnect for the engine battery. Would there be anything wrong with removing the battery while in storage and keeping it charged at home? Would I be able to exercise all other systems run by my two other batteries if the engine battery is removed (seems like a stupid question, but...)? My MH manual is not at home so I can't look up to see if the generator is connected to the house batteries or to the engine battery. Is it the same for all MHs? I have an Itasca Suncruiser gasser. Thanks.

Don,

If you look again I believe you will see that they are balanced. The Vector Maxx has four 12 volt inputs - two positive and two negative. These four are distributed along the aluminum buss bars so as to pull current at the center point of the two pairs.

The balancing method shown in the link you provided above is for those 1% who want every last available amp and millivolt from a battery. It may be beneficial in extremely high amperage demand situations like one would experience with an electric vehicle or 5,000 watt inverter trying to start a roof-top A/C. But, the majority of RV'ers adding batteries want to extend time, not demand current.

On an additional note, the writer's statement that balancing is needed when recharging does not apply to what we experience when recharging our RV batteries -- unless we are fast charging the bank at 200+ amps, which I would NEVER do. My on-board 120 volt powered charger for the auxiliary bank is (was) 75 amps. This gives 75/4 amps to each battery, or a little under 20 amps initial boost to a 10.5 volt battery. It, of course, tapers off as voltage in the battery rises. This charger is considered BIG for an RV battery bank.

My preference for buss bars would have been copper. But, alas, solid copper buss bars of sufficient size are not too easy to obtain (or affordable!). While aluminum does have a higher resistance than copper, it is available and sufficient for my application as the total length of the buss bars is relatively short.

I can't say that doing all of the balancing would never be needed in an RV, but I can say they are overkill for the average RV install and the peak current demand that will be experienced. IMHO there are too many other issues to worry about than the fractional added resistance of a "standard" parallel battery interconnection.

Couple of more photos for show and tell:

The one below is a vented wooden case with three group 29 Wal-Mart deep cycle batteries interconnected with a 3/4" copper pipe made into a buss bar. The lid on the case can be closed and sealed. Hydrogen is vented to the outside through a 1-1/2" flexible plastic tube in the top. Fresh air is drawn into the bottom in a similar manner. Proper venting of open lead-acid batteries kept in a closed environment is extremely important.




The three photos that follow show the solar panels on the roof of our fiver. This was taken yesterday from a upstairs bedroom window in our home after we returned from a trip. On a clear sunny day I generate something like 10 amps at the output of the controller for the batteries. Nice, clean, free power. There is some loss due to the flat position of the panels, the inability to track the sun and shadows from the front A/C. But, we live with it and are happy for the power we get.








The last photo is the control/monitor panel I built for my batteries/solar/generator/inverter operation and monitoring. It sits under the OEM switch panel (the meters are in the black rectangular shapes to the left). I can monitor voltage of both house and auxiliary battery banks, charge current from any source, discharge current from either bank, A/C voltage and frequency from the inverter or generator on the large display LED meters. I can auto-start the generator and select either genny or inverter power to the RV's ATS. The generator is wired so it will start automatically and run for two hours to charge either battery bank if the bank voltage drops to or below 11.8 VDC. I can, of course, over ride this function if generators are not allowed.

This is the way we use batteries to augment our hunger for electrical power when dry camping. I have actually run my 6,000 BTU roof-top bedroom A/C for as long as six hours off of batteries/1200 watt inverter alone. Nice on a hot night when generators are not allowed.

Hi abbygolden,

There would be nothing wrong with it--but if the starter battery is fully charged there may be no need to remove it. Just start your RV whenever you go to exercise the generator--or run a battery charger from the generator.

I can't say with any certainty that all RV's are wired the same way--but I think most generators use the "house" battery bank for starting.

Professor,

Weight on the 8D's are 162 lbs. each so you have 324 lbs. of lead in a box. Batteries are on a roll out rack and not hard to work on. Some Foretravel models use three 8D's as house batteries. We also have an 8000 watt diesel generator in front end. Weight is not a problem with our unit.

Abbygolden,

Need to keep power on the batteries or they will die. See Don's information. If you can power up the AC in MH that will help or as you said take them home and hook up a small charger to all. Batteries will lose around 3 to 5% charge per day. Before long you have scrap metal. The generator is connected to the house batteries and that starts the engine.

A fully (100%) charged battery will loose 3 to 5% per day. Once the battery reaches 50% (12.25 VDC) the discharge lessens to a negligible amount. A small 500 milliamp to 1 amp float charger will maintain the 50% level, a point where minimal sulfation and gas out will occur for long time storage.

Disconnect your batteries from the MH engine side if you are storing over the winter without at least a 1 amp float or trickle charge. Better yet if you cannot plug in during storage - use at least a 15 watt solar panel as a battery maintainer. The MH ECM or PCM, possibly even the radio memory, will draw enough current to discharge the battery. Of course, your ECM will need to "relearn" certain functions upon reconnection and you may have to reprogram your radio presets and clock.

You guys that own huge MH's on bus or JD type chassis don't have the same added weight problems those of us who have towables must live with. Not too many (if any) fifth wheels or TT's come with 3-8D batteries on a pull-out tray. We were lucky to get one group 24 battery with the new coach, which is not enough reserve power to get through a "normal" 24 hour dry camping day.

We have a small solar charger that came with our Itasca to keep the batteries topped off.

Hi Prof,

I'm glad your install is balanced, there is no way to see that from the photograph as the cables are not identified. I was hoping buss bars were a "way around" balancing--but no joy there!

I disagree with some of what you have suggested. Most RV folks are going for larger and larger inverters as the cost drops. Mine is probably in the middle at 2500 watts (5000 watts surge), so far more than 1% of us need correctly balanced wiring.

Can you explain why balanced wiring is not important for charging? Resistance is still identical at even very low amperage, so the last battery in the chain may never fully charge, or if it does then the first one may over charge? It can't be both ways?

If the balanced load points are already in place on the load side--there is no reason on earth to not use them for charging. It costs very little extra to wire in a balanced manner--sometimes even zero cost--if for example there are only two 12 volt batteries.

I am fortunate enough have two banks of batteries--four with a copper buss bar system--and three with method #3 balanced load. Total capacity is 875 amp-hours.

For a forum where I've documented what I've done you may wish to surf here:

Technology

and click on solar boondocking

If I were to charge my engine battery when running the generator for about an hour, would that make any difference? I normally also run the engine at about 2000 rpm for about 15 minutes or so at least once, normally twice, a month. I have no access to power where I store my MH.

Hi abbygolden,

Yes it will make a nice difference. Depending on the battery chemistry and the charger.

If the charger is one of the new "smart" chargers and the battery is down to 80% state of charge an hour may bring it back to say 90% (again depending on chemistry).

If I were you, I'd:

-start the generator, so the converter can work its magic on the "house" battery bank

-start the engine for that 15 minute warm up

-get the charger "ready to roll" on the chassis battery bank

-turn off the engine

-turn on the charger powering it from the generator.

But then, if I were you, I'd already have a solar system in place to take good care of the "house" battery bank.

Don,

Try this formula:

Voltage drop = k x p x wire length in feet x current in amperes divided by the wire area in circular mills

For Copper, K=11
For Aluminum, K=18

P is Phase Constant. In this case P=2

Let’s use #2 AWG copper as our interconnect cable. The area in circular mills will = 66,400

Our interconnect cables will be 1 foot long each

Again, let’s make an assumption the amperage draw from the batteries is 120, typical for a 1,000 watt inverter load.

Run the numbers:

11 x 2 x 1 x 120 = 2,640 divided by 66,400 = .039759

So the voltage drop on that 12” piece of #2 copper is less than 4/100th of a volt.

Since my example assumes two batteries, the interconnect cable will only carry 50% of the current. Thus the voltage drop on the parallel cables will only be 2/100 of a volt – or better said 20 millivolts.

A 20 millivolt loss is no big deal in a battery bank set-up for an RV – even if we have more than 1 one foot interconnect cable.

Manufacturers of high power inverters often recomend two size “0” cables in parallel. (106,000 CM each, total 212,000 CM).

Run the above numbers again for a lowered charging current. The voltage differential amounts to nothing significant!

More specifically, if you have four batteries in a parallel string with no “balancing” and connect a charger, the maximum current will go to the lowest value battery first. Once it is equal to the next lowest they will “appear” as a single battery and combine with the remainder as they are charge equalized. Even 4 brand new batteries in parallel will not exhibit the same state of discharge. Manufacturing is far from being that perfect.

I’ll say what I did previously again – for RV use all this “balancing” stuff is unnecessary unless you are totally battery anal. There are other uncontrollable variables in the system that will have a greater impact.

As for intentionally wiring so that a bank is "balanced"; yes – sometimes it is just as easy and will not require an additional length of cable to configure it as such. But the question always remains when setting up batteries in an RV as to where they will be mounted. Often they will be some distance apart. Not every RV’er that wants to expand battery capacity has a big motor home with a huge pull-out battery tray (but it would be nice if we did ).

As for the 1% - We’ll just have to agree to disagree. My thirty years of association with RV’ers gives me different numbers than you project. Besides, what difference does it make if one of us is off a few percent? The fact remains that the majority of Rv’ers are NOT high power inverter users (I am in the minority – I have a 3,000/6,000 watt inverter.).

BTW - your solar set-up is extremely nice.

Hi Prof,

You have not allowed for the internal resistance for the batteries, nor the resistance for each connection. These amount to more than the resistance in the wire.

With only two batteries balanced wiring need not cost a dime extra. In my case it cost nothing extra for the buss bar system--and about $20.00 for the wiring the triple battery bank.

I agree that no two cells are likely to have identical capacity--perhaps the same chance as two snowflakes being identical--which happens but is extremely rare.

I do not have your expertize, nor do I have a spreadsheet to "crunch the numbers" so I can only quote from the smart gauge web site.

"Now in all fairness, many people say 'but the difference is negligible, the resistances are so small, so the effect will also be small'.

The problem is that in very low resistance circuits (as we have here) huge differences in current can be produced by tiny variations in battery voltage."

I'm always interested in learning more. May I ask you to elaborate on "other uncontrollable variables in the system that will have a greater impact." Are there solutions to these variables?

How can the charging current "go to the lowest charged battery first"? This seems counter intuitive to me. In an unbalanced wiring scheme the resistance will be highest to the last battery in the chain. It seems logical to me that this battery would receive the least amount of charge--and get it last as well.

Folks here hate polls--but I think I'll do one now on inverter size, just to "put a finger in the wind" to see which way it is blowing.

Thanks for the complement on my solar system. I got my first "solar cell" when I was 7 years old--and have wanted a working system since then. 56 years is a long time to wait!

I do also but it apparently isn't enough or never has worked. If I don't exercise my MH at least once a month, my engine battery will be so low I have to use the auxilliary boost to start it. I've never had a problem with the two house batteries other than having to replace them after five years.

Thanks for the info, but I'm still confused (despite having read the links you sent to me, thanks incidentally). If I read your suggestion correctly, are you recommending that after I shut off the engine that I charge the engine battery while the generator is running or the house batteries (I'm not sure if "chassis" batteries are the same as engine batteries, but I don't think so)? It's the engine battery that I want to charge. Also, can I charge the engine battery while it is running or do I need to wait until I turn off the engine?

Also, as I wrote to Lindsay, I have a 5w solar panel that came with my Itasca that I thought was supposed to keep the engine battery topped off. If that is the purpose, it hasn't worked. I also have a 5w plug in to the cigarette lighter panel but if I use the battery disconnect switch, it won't work.

Sorry I can't get this stuff into my pea brain so I can comprehend it. I guess my brfain has been clogged with golf thoughts for so many years that there is little room for anything else!

Don,

All batteries have internal resistance and the low cells (resistance) will past more current than the higher ones. So they will charge up faster. This is call impedance if I remember right. In the telephone offices we used 23 large flood cells to make up 50.6 volts. Each cell should read out at 2.20 volts but some would read at 2.10 or 2.05. Then others would go high like 2.25 or 2.27. Bottom line, each month the reading would change around. High cells will drop and low cells will go up. You can never get it on the money. We changed over to Gel cells later and found close to the same problem. Each Gel cell had a weight of 250 lbs. and a 20 year life same as flood cells.

Hi abbygolden,

Yes chassis battery = engine battery.

15 minutes of run time is not enough for the alternator to do much charging. That's why I suggested using the generator and a battery charger on the engine battery for another 45 minutes.

5 watts won't keep up with the parasitic loads on a modern engine. 15 watts might.

Hi John,

I was aware it is impossible to get all cells equally charged--and that "equalization" charging was an attempt to help "weak" cells catch up.

Are you saying (trying to learn) that a battery that is at 50% state of charge has lower internal resistance than one that is at 80% state of charge? And therefore it will charge faster? I thought that the acid turned to water--which is a poorer conductor and increases resistance?

And what about the connection resistance? How does that "play out"?

Hi Prof,

Hello Don,

You have not allowed for the internal resistance for the batteries, nor the resistance for each connection. These amount to more than the resistance in the wire. The internal resistance of the batteries has nothing to do with the resistance in the interconnect cables. Clean, properly torqued connections with a surface contact area comparable to the circumference of the interconnect cable should introduce negligible resistance.

With only two batteries balanced wiring need not cost a dime extra. In my case it cost nothing extra for the buss bar system--and about $20.00 for the wiring the triple battery bank.

The method your referenced material shows for balancing four or more batteries requires extra lengths of cable. This is counter intuitive to what you are trying to accomplish by reducing cable resistance. All of this needs to be evaluated and the added resistances computed before making a decision to implement wiring in this manner.


I do not have your expertize, nor do I have a spreadsheet to "crunch the numbers" so I can only quote from the smart gauge web site. A spreadsheet is not required. All that is necessary are basic math skills and Ohms Law. A reference text or some checking on-line will give you the resistance for particular metals, lengths, AWG sizes and CMC. Without an understand of these mathematical concepts or the ability to perform the computations one will not have a clue as to what is going on in a circuit.

"Now in all fairness, many people say 'but the difference is negligible, the resistances are so small, so the effect will also be small'.

The problem is that in very low resistance circuits (as we have here) huge differences in current can be produced by tiny variations in battery voltage." I am not sure where you are getting this information. Comparisons using the terms huge, current, tiny and battery voltage do not exist in the typical parallel battery connection found in a RV. But, if you are wiring a battery bank for a Tesla Motors vehicle we may indeed need to factor in variations in current and voltage from individual batteries because the sum of variations in hundreds of batteries can become significant.

I'm always interested in learning more. May I ask you to elaborate on "other uncontrollable variables in the system that will have a greater impact." Are there solutions to these variables? Temperature is, of course, the major "uncontrollable" we encounter.

How can the charging current "go to the lowest charged battery first"? This seems counter intuitive to me. In an unbalanced wiring scheme the resistance will be highest to the last battery in the chain. It seems logical to me that this battery would receive the least amount of charge--and get it last as well. A completely dead or "flat" battery will have an extremely high resistance. In fact, it may never recharge. What we are looking at are good batteries with a normal state of discharge (up to about 75%). The battery in a parallel bank that has the lowest state of discharge will also have the lowest internal resistance. Thus, it will take the largest share of current available. Again, this is a principle of basic electrical circuit understanding. Current in series circuits will be equal at all points. In parallel circuits it will divide between the resistances. Since you stated that you were unable to complete the calculations without a spreadsheet, you may find the following link beneficial. All you need to do is enter the values in the appropriate boxes. http://www.csgnetwork.com/parallelresistcalc.html

Folks here hate polls--but I think I'll do one now on inverter size, just to "put a finger in the wind" to see which way it is blowing. Any poll conducted on this thread would be statistically insignificant. Since this is a battery thread and is of interest to those with a desire to expand their battery capacity it would give extremely one sided results. If you want to run a poll that provides statistically significant results I would suggest a broader sampling of all RV owners. Maybe you could knock on camper doors the next time you visit a large campground and see what type of responses you receive?

Gentlemen, I stopped here as a result of our host pointing me in this direction after I posted a campground review. My sole intent was to share a source for perfectly usable AGM batteries at an extremely low price or even free.

My old fingers are badly swollen by arthritis and extensive typing causes me great pain and discomfort. As much as I would like to hang around and debate the validity of information posted on other web sites, I simply cannot.

I wish you all well... and I am impressed with the work and accomplishments you have apparently achieved in expanding battery capacity in your personal RVs. Your work should be shared as it serves as an example of what can be accomplished when one wants to expand beyond the largely inadequate OEM battery set-up.

Randy

Hi all,

As Prof has indicated he doesn't wish to continue the discussion I'll not reply to his comments. I do appreciate his efforts.

Thanks Prof for the suggestions about cheap source of very good batteries.

It has been food for thought and one of those thoughts is that unbalanced 12 volt batteries, if there are only two 12 volt batteries, cause the first battery in the chain to work approximately 15% harder.

I've also learned that lead acid batteries *do* increase in resistant as they discharge.

changing resistance with state of charge

Scroll right to the bottom for a graph for lead acid cells.

I've also learned that the resistance rises as temperatures become lower.

internal resistance vs temperature

Scroll down five screens.

Hi all,

I found another good battery information page.

Battery review

I found these guys particularly useful on sealed lead acid battery advice

Sealed Lead Acid Battery Advice

Hi,

I found a nice page about batteries and freezing here:

http://www.cdtechno.com/custserv/pdf/7953.pdf