Two banks of two batteries - different size - no-no?

[MaestroPNW]

Adding more capacity to the camper.
In-camper I've got to size 24's. Thinking of putting two 110AH in front of the wheel well in the truck bed, run the cable through an Anderson connector.

Searched this board and the rest of the web high and low, and see a lot of comments on not putting different capacity batteries in parallel, but also see a lot of write-ups on people doing it with seemingly no issues.

My options, so far, are:
1) Just run everything in parallel
2) Manual battery switch to switch from one bank to another.
3) anything else?

Thanks!

[MEXICOWANDERER]

When joining old batteries of exactly the same construction with new batteries, some tests MUST be done before hand to see if the old batteries have aged their way out of the same league of chemistry.

A) Charge and equalize 6 cells of the old batteries until specific gravity reaches absolutely full, brand new 1.280 for instance. If gravity varies by more than .05 this means that marriage is a no-no. The bigger the variance the bigger the complaint.

B)If batteries pass the specific gravity match test, perform a 50% CCA discharge using a carbon pile load tester on 6 cells of the new battery and 6 cells of the old battery. If voltage varies by more than three tenths of a volt at the end of a 15 second discharge, the batteries are not compatible.

You will have to hunt up a genuine battery shop to perform the test. If they own an 800-amp AUTO METER adjustable carbon pile load tester, or SUN VAT 33 or 40 or even a Harbor Fright load tester they can do this test.

[BFL13]

rjsurfer wrote:
I'm having a hard time understanding the correct way to wire up two banks of batteries. In my case I have 2 four year old T105's and I'm adding two more.

It looks like from what I have read I can avoid using a selector switch and join them all together for better battery life.

Is there a diagram that someone could point me to so I can see exactly
hows its done. At my age a picture is worth 2 million words 🙂

Thanks

Ron W.

With that age difference IMO keep them separate on their own floats except when actually camping when it won't hurt the new ones to be paralleled with the old ones. (when camping they don't get fully charged anyway)

[pianotuna]

Hi,

If the batteries were all new then only method #3 on this page would allow for perfect balance of the system.

How to connect multiple 12 volt batteries

Since that is not true, I would use two simple Napa switch on each bank so that you can select either bank, or both. That allows the flexibility to charge one bank while using the other.

rjsurfer wrote:
I'm having a hard time understanding the correct way to wire up two banks of batteries. In my case I have 2 four year old T105's and I'm adding two more.

It looks like from what I have read I can avoid using a selector switch and join them all together for better battery life.

Is there a diagram that someone could point me to so I can see exactly
hows its done. At my age a picture is worth 2 million words 🙂

Thanks

Ron W.

[rjsurfer]

I'm having a hard time understanding the correct way to wire up two banks of batteries. In my case I have 2 four year old T105's and I'm adding two more.

It looks like from what I have read I can avoid using a selector switch and join them all together for better battery life.

Is there a diagram that someone could point me to so I can see exactly
hows its done. At my age a picture is worth 2 million words 🙂

Thanks

Ron W.

[RJsfishin]

Quote:
Two banks of two batteries - different size - no-no?
------------------------------------------------------
Yes yes, to the different size/capacity. Voltage will stay equal on charge and discharge, no matter the size/capacity. Nit pick'n the cables etc, mite change the 3rd digit after the decimal point.

[wa8yxm]

Many myths have a root in fact..

The myth: In a multi-battery set up ALL batteries MUST be identical.
The rooting fact: When you put batteries in SERIES such as a pair of GC-2 to make a big 12 volt (Basically a 4D) or a pair of 12's to make 24 volts for a Semi truck or construction gear.. THOSE batteries MUST be identical.

But when you parallel them.. This is not the case. They need to be the same type and class. (IE: all flooded wet cell or all maintenance free). though even there some folks have mixed AGM and Flooded with little issue.

GO FOR IT

Suggestion: use very high current Andersons.. Like tow truck jumper cable connectors.

[BFL13]

Some people like to use one battery and then switch to the second. They are losing out.

Due to Peukert Effect, you can get way more than twice one battery's worth by drawing from both at the same time.

[tony_lee]

MEXICOWANDERER wrote:
I have a document from Exide that gives acceptable chemistries that will work "OK in parallel. eg AGM in parallel with standard flooded cell lead acid work well enough, but GEL in parallel with just about anything else is a no-no."


Can you pretty-please send me a link to the document? Thank you.

It is a printed instruction sheet that came with the GEL batteries and it is in the motorhome parked up in Germany. Due to get it out of mothballs in about three months for a trip to Turkey so you will have to be patient.

[JimBollman]

When I added a second battery I put in a switch that would allow separate and in parallel but planned only to use them individually to get around all the problems disgust so far. I also like the knowledge that when I switch I have used roughly half my capacity and can plan around that. I tend to use the older battery first and only switch to the newer one when we are off the grid long enough to need the extra capacity.

[BFL13]

When adding the extra batteries to the existing bank while camping, it works ok to have them as the "downstream battery." Although it is not ideal it still works.

In that set-up, you will find that the downstream set does not take as much of the load-- as seen on its loaded voltage, but after the load is removed, the voltage slowly recovers as they equalize.

So you do get the benefit of more AH available while camping, but not quite the same benefit as a balanced set-up if using the microwave/inverter for how far down the voltage gets with the load on. (if getting close to inverter alarm at 11v this matters)

Better than not having the extra batts though. It just takes time for the extra batts to share with the main set. You might not even notice the difference if you are not "instrumented" for all that, and are just a normal person going RVing 🙂

Once home and on Float, it helps to disconnect the extras from the main set and let the converter Float the main set of batts and use a separate floating charger on the extras. That way each set will float properly and one will not lose out on whatever sharing might happen with just the one converter on the upstream batteries.

[4X4Dodger]

Rather than go through all of the cabling/switching etc why not just install one 4D Marine battery which has more available power over a longer period and weighs about the same as the two separate batt's?

I would put this up front (in a TT) in a marine battery box. And use Heavy duty cables and if possible put the battery in a storage compartment and vent it to the outside.

I would look into this as an option to the two battery idea.

[Bigfoot85]

We chose option 2 ... it gives us extra safety in case of a single battery/bank failure.



We have two banks in the bed of the truck, and one bank up front (3-way switch).

[MEXICOWANDERER]

It should be less about electrical performance than matters of chemistry. Batteries cannot survive very long in years, with chronic charging to 97% then discharging them.

[wintersun]

naturist wrote:
If you plan to charge them off the same charger or to use them together at the same time, they also will need to connect to the charger/house system with the same length/same size cable. If the cable to the new batteries is a different length, the batteries on the longer cable will never charge fully, and will discharge to a different depth than the originals, which will cause problems down the road. From the sounds of it, you will have to do the switchover thing.

That might be true if the second set of batteries was 100 feet further away from the charger and 12ga wire was used. At 20 feet with 8ga wire the voltage drop is going to be only 2.15% or 0.31 volts. Instead of 14.5 volts the voltage will be 14.19 volts and this is not going to make a noticeable difference when charging 12 volt batteries to 12.5 volts.

It is not difficult to be anal about this subject. If a 80 Ah battery is at "only 97%" then you can draw 38.8 amps instead of 40 amps from it with a 50% DOD. Does anyone really think this is of critical importance with an RV application?