Buffalobwana
Observer
What are the drawbacks of mixing different size AH batteries in your setup? Assuming condition of batteries are good, are there serious negative consequences?
Mixing different AH batteries in your setup.
- Thread starter Buffalobwana
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Buffalobwana
Observer
All 12v LiFePO4. It would be a battery bank fed by solar running an inverter.
dreadlocks
Well-known member
Drop in LFP? One bad cell kills em both, I would not do it unless they shared a BMS and cells were all matching spec..
Two different size or age batteries in the same bank tends to lead to ping-ponging, where at same SOC they do not have the same voltage.. so one battery is providing a charge to the other and then vice versa.. with different capacities the smaller one will crash sooner then steal power from the other trying to find equilibrium which is alot of wear and tear and heavy losses to storage inefficiencies at best.
Two different size or age batteries in the same bank tends to lead to ping-ponging, where at same SOC they do not have the same voltage.. so one battery is providing a charge to the other and then vice versa.. with different capacities the smaller one will crash sooner then steal power from the other trying to find equilibrium which is alot of wear and tear and heavy losses to storage inefficiencies at best.
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Buffalobwana
Observer
Kind of what I figured. But needed to hear it from others. Thanks.
luthj
Engineer In Residence
Lithium batteries (assuming they use the same core cells and BMS) are less sensitive than lead acid. Mostly because their voltage is so stable for most of the discharge curve. For low rate discharge applications (say 5% of capacity or less), you would be likely be okay. This assume the batteries are basically the same age, and use the same cell type internally.
Generally I would avoid mixing, but if you already have the hardware, it may be worth considering.
Generally I would avoid mixing, but if you already have the hardware, it may be worth considering.
jonyjoe101
Adventurer
I got 2 x 110 ah lifepo4 connected together in parallel for the past year, I feed them with a 240 watt solar panel/20amp mmpt controller. I wouldn't even think twice about adding another one. I never tested full capacity of each battery but I doubt that each is 110 ah. Once the voltage of both batteries is the same, nothing happens. I have amp meters on both batteries and I can see if amps are going from one battery to the other.
A while back I had 2x li-ion 11.1 volt batteries in parallel, a 50ah and a 44ah, total 94ah, again they performed well together. The voltage between both batteries equalize once connected together.
If each battery has its on bms, the lower ah one will go offline when it gets depleted but the other will keep running.
Just make sure the voltage is the same on both batteries when connected together.
A while back I had 2x li-ion 11.1 volt batteries in parallel, a 50ah and a 44ah, total 94ah, again they performed well together. The voltage between both batteries equalize once connected together.
If each battery has its on bms, the lower ah one will go offline when it gets depleted but the other will keep running.
Just make sure the voltage is the same on both batteries when connected together.
Martinjmpr
Wiffleball Batter
I've been running a 2 dissimilar battery setup in my Suburban since 2016 with no ill effects.
Main or starter battery is a 55ah AGM sourced from a national auto parts chain, house battery is a 68ah X2-brand AGM battery that mostly runs the fridge (or occasionally a 2.1a USB port for phone charging.) This is in an "isolated" system though so the only time the 2 batteries are "connected" is when they are being charged.
In the trailer we are running 2 identical FLA 6v golf cart batteries hooked up in series.
Main or starter battery is a 55ah AGM sourced from a national auto parts chain, house battery is a 68ah X2-brand AGM battery that mostly runs the fridge (or occasionally a 2.1a USB port for phone charging.) This is in an "isolated" system though so the only time the 2 batteries are "connected" is when they are being charged.
In the trailer we are running 2 identical FLA 6v golf cart batteries hooked up in series.
dreadlocks
Well-known member
if they are only connected during charging it does not matter at all what size or shape they are in, thats the point of isolating em when under load.
why do battery have to be discharged separately? For exemple. i have 2 new FLA 6v in series and i want to add a 12v FLA 1-5ah in paralle to those 2x 6v. any issue with this as a battery bank?
dreadlocks
Well-known member
See Post #4, your battery bank is only as strong as its weakest cell..
Batteries are a combination of cells to reach the desired voltage, when those cells dont play well together the battery dies.. a battery bank is a combination of little batteries to make a big battery bank, just like all the cells inside each battery.. but now all the batteries and all the cells need to have the same performance characteristics or the whole bank dies.
There is a reason why when you crack one big battery up all the cells inside are identical, and there is a reason why if you look at big battery banks all the batteries inside are identical.. of course its your money and your free to burn it however you wish.
Batteries are a combination of cells to reach the desired voltage, when those cells dont play well together the battery dies.. a battery bank is a combination of little batteries to make a big battery bank, just like all the cells inside each battery.. but now all the batteries and all the cells need to have the same performance characteristics or the whole bank dies.
There is a reason why when you crack one big battery up all the cells inside are identical, and there is a reason why if you look at big battery banks all the batteries inside are identical.. of course its your money and your free to burn it however you wish.
luthj
Engineer In Residence
If the batteries have the same charge voltage specs, are similar age, and are wired in a semi-balanced manner. AND your discharge currents are not over about 0.1C, then you can get by with mis-sized batteries in parallel. In series, its an absolute no, as you will end up driving the smallest cells/battery to zero before the whole pack does. In parallel, their internal resistance helps to balance out the discharge currents.
You can expect a capacity reduction compared to ideal scenarios, but it may only be on the order of 10% in some cases. Eventually one of the batteries will start downhill fast. If you don't notice this, it can drag the other with it.
Once you get over 0.1C, the batteries internal resistance difference means that one battery will end up providing more than its share, and due to peukert's law, it will end up lower than the other on every charge cycle, thus power exchange occurs between the batteries. This can double the effective cycle rate, and dramatically accelerate battery aging.
You can expect a capacity reduction compared to ideal scenarios, but it may only be on the order of 10% in some cases. Eventually one of the batteries will start downhill fast. If you don't notice this, it can drag the other with it.
Once you get over 0.1C, the batteries internal resistance difference means that one battery will end up providing more than its share, and due to peukert's law, it will end up lower than the other on every charge cycle, thus power exchange occurs between the batteries. This can double the effective cycle rate, and dramatically accelerate battery aging.
thank you for the details: So last month, i got 2x Rolls 6v FLA 235ah and now, i only have space to add 1 battery. i could get a group 27 Rolls 12v 105Ah of the same line of battery as the 6v. within 1 month of each other in age and same suppose cell characteristic.... is this close enough or simply doesn't cut it?
same charge spec,
usage is van conversion so, fridge, tv, computer charging, lights, diesel heater, and a 1500w pure sine wave invertor with biggest load been 1000w espresso machine.
and following this conversation, i sent the question directly to Rolls battery to see what they think of mixing there battery!
thanks you all for the help, greatly appreciate as i m navigating a subject that i do not understand very well
usage is van conversion so, fridge, tv, computer charging, lights, diesel heater, and a 1500w pure sine wave invertor with biggest load been 1000w espresso machine.
and following this conversation, i sent the question directly to Rolls battery to see what they think of mixing there battery!
thanks you all for the help, greatly appreciate as i m navigating a subject that i do not understand very well