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mixing batteriy sizes

swimmer_spe
Explorer
Explorer
I have 3 deep cell batteries. One of them is bigger (and newer) than the other 2. What happens if I try to use a big and small one together? What about putting all 3 together?
12 REPLIES 12

JaxDad
Explorer II
Explorer II
Veebyes wrote:
JaxDad wrote:
Another option for you to consider is to create two ‘banks’ of battery power by running the newer battery by itself and the other 2 (together) as a second battery.

This can be done via a cheap marine battery switch (bat. 1, bat. 2, both, off) or by just swapping cables.


With one, or two, battery switches you can combine or isolate batteries any way you wish.

Did that with a boat. I had a total of five group 31 gel cell batteries arranged in three banks that were switchable. Two banks of two batteries in parallel and a single, all switchable for a single or combo that I pleased.


That was *exactly* what I was thinking.

swimmer_spe
Explorer
Explorer
wa8yxm wrote:
swimmer_spe wrote:
I have 3 deep cell batteries. One of them is bigger (and newer) than the other 2. What happens if I try to use a big and small one together? What about putting all 3 together?


I hear a lot of "Stuff" like "The weaker battery will draw them down">> Bull droppings.

What will happen is the batteries will work it out.. The Ideal socialist society will be formed (Each according to it's ability, each according to it's needs)

Imagine you have 3 batteries one is 100 AH capacity one 200 one 300
You are drawing 60 amps and you meter each battery
You will find the 100 battery is putting out 10 amps. the middle one 20 and the 300 30.. They work it out. Same when charging.

NOW when batteries age one of two or three things happen and this will affect you ONE of them is "of concern"

First. The internal resistance may go up.. in that case at low current you won't notice it but at higher discharge rates this one will simply not provide as much juice.. when the discharge rate goes down it will likely provide a greater share however for a time till they balance out So this is not a problem.

The second is loss of capacity. also not a problem save the battery basically "Disappears" electrically.

Finally the only concern. A Shorted cell.

OF course this can happen with a battery that's a week old (And did to me once. what's more that battery had a lifetime warranty so the replacement was free). (Pro installed too).

And I have behind me as I type 3 batteries. all different sizes all different ages and that's precisely how they act.


So, will the older, smaller batteries prematurely kill the newest, biggest battery?

wa8yxm
Explorer II
Explorer II
swimmer_spe wrote:
I have 3 deep cell batteries. One of them is bigger (and newer) than the other 2. What happens if I try to use a big and small one together? What about putting all 3 together?


I hear a lot of "Stuff" like "The weaker battery will draw them down">> Bull droppings.

What will happen is the batteries will work it out.. The Ideal socialist society will be formed (Each according to it's ability, each according to it's needs)

Imagine you have 3 batteries one is 100 AH capacity one 200 one 300
You are drawing 60 amps and you meter each battery
You will find the 100 battery is putting out 10 amps. the middle one 20 and the 300 30.. They work it out. Same when charging.

NOW when batteries age one of two or three things happen and this will affect you ONE of them is "of concern"

First. The internal resistance may go up.. in that case at low current you won't notice it but at higher discharge rates this one will simply not provide as much juice.. when the discharge rate goes down it will likely provide a greater share however for a time till they balance out So this is not a problem.

The second is loss of capacity. also not a problem save the battery basically "Disappears" electrically.

Finally the only concern. A Shorted cell.

OF course this can happen with a battery that's a week old (And did to me once. what's more that battery had a lifetime warranty so the replacement was free). (Pro installed too).

And I have behind me as I type 3 batteries. all different sizes all different ages and that's precisely how they act.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times

CA_Traveler
Explorer III
Explorer III
Go for it! Use one at a time or connect all three together. What's to lose as they are older and used? Yes good idea to use new batteries differently but older used, try it and see.
I'd probably use the one larger one and then the 2 smaller ones.
2009 Holiday Rambler 42' Scepter with ISL 400 Cummins
750 Watts Solar Morningstar MPPT 60 Controller
2014 Grand Cherokee Overland

Bob

swimmer_spe
Explorer
Explorer
OP here.
One thing is, this is only going to be a once a year thing. Basically, I go hunting for a week and may not have access to a generator.

time2roll
Explorer II
Explorer II
No issues as long as they are wired with balanced connections.

The three together will yield more energy due to the Peukert effect.

valhalla360
Explorer III
Explorer III
In a boat multiple battery banks makes sense. If one is ran dead, you can start the motor with the second bank. Of course one is typically starting and one is house.

On a trailer it makes less sense. The faster you discharge a bank, the fewer Amp hours it will produce. A single large bank provides more than 2 small banks of the same total Amp hour rating. The exception would be trying to milk the last bit out of an end of life battery where it will kill the new battery but then you probably aren't getting much out of the old battery anyway.
Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV

Veebyes
Explorer
Explorer
JaxDad wrote:
Another option for you to consider is to create two ‘banks’ of battery power by running the newer battery by itself and the other 2 (together) as a second battery.

This can be done via a cheap marine battery switch (bat. 1, bat. 2, both, off) or by just swapping cables.


With one, or two, battery switches you can combine or isolate batteries any way you wish.

Did that with a boat. I had a total of five group 31 gel cell batteries arranged in three banks that were switchable. Two banks of two batteries in parallel and a single, all switchable for a single or combo that I pleased.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter

2007 Alpenlite 34RLR
2006 Chevy 3500 LT, CC,LB 6.6L Diesel

Ham Radio: VP9KL, IRLP node 7995

pianotuna
Traveler
Traveler
Wire them in a balanced manner. I had two unequal banks for quite a while with switches. I could use one while charging the other, or if there was going to be a heavy load (microwave) I could have both supplying power.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

way2roll
Traveler
Traveler
Connecting batteries will equalize them and eventually all will only be as strong as the weakest one. When you connect brand new batteries to used batteries of the same model (never mix and match batteries that aren’t identical in the same bank), the new ones inherit the capacity of the used ones since they must all charge and discharge in unison. So unless your current batteries are only lightly used and are providing nearly 100% of their rated capacity, you will miss out on some of the capacity of the new batteries you buy for expansion.
2023 FR Sunseeker 2400B MBS

JaxDad
Explorer II
Explorer II
Another option for you to consider is to create two ‘banks’ of battery power by running the newer battery by itself and the other 2 (together) as a second battery.

This can be done via a cheap marine battery switch (bat. 1, bat. 2, both, off) or by just swapping cables.

CA_Traveler
Explorer III
Explorer III
The weaker batteries will cause a shorter life for the new battery. Given they are used per your other thread it's a good idea to load test all 3 to make sure they are in reasonable condition. Given that and they are available go ahead and use all 3.
2009 Holiday Rambler 42' Scepter with ISL 400 Cummins
750 Watts Solar Morningstar MPPT 60 Controller
2014 Grand Cherokee Overland

Bob