Mixed Ah LiFePO4 strings in parallel

[keimor85]

I want to setup a 48V system for a sailboat the has four 330Ah batteries in series 330Ah at 48V and parallel a second string of 200Ah LiFePO4 for a total of 550 Ah at 48V. I'm getting mixed responses from suppliers but saw a video from Will that said you can do this.

I'm looking for find this video from Will but can seem to find it again.

Does anyone know what it was called?

Does anyone have experience doing this.

 

[TomC4306]

 

[keimor85]

thanks tom

 

[Vigo]

I have different capacity Lifepo4 in parallel.

I can't say that i've tracked or verified how well it works because i tend to operate by just having a large enough margin for error that 'good enough is good enough' and don't bother seeking perfection, but...

My understanding is that given the charge efficiency and low internal resistance of lifepo4 batteries, when charging and discharging different lifepo4s in parallel, the 'inequality' that actually affects anything is more likely to be the resistance of the cabling/circuits between those unequal cells/batts, than any IR difference of the cells/batts themselves. So whichever bank has the lowest TOTAL resistance to the charger/load will both drain and charge 'first' and thus will cycle more than the other bank. I can easily imagine a case in which much smaller capacity cells end up being the ones doing all the work because of cabling/resistance issues between the larger and smaller banks, in spite of the fact that the larger 'raw' cells probably lower IR.

The question i think is how much it matters. If your daily usage is <80% of the wh of your smallest paralleled batts/cells, i would say it's guaranteed not to matter much. But that would mean you have a fairly overbuilt battery bank, so it's not likely. In all the remaining scenarios that are actually likely, it's safe to say that whichever batts have the lowest total resistance to charger/load are going to 'cycle out' first, and while that's not ideal, it's also probably years and years away and you are better served worrying about the storage temp of your unequal paralleled banks than you are about which cells are cycling more over time. Just my .02.

EDIT: There may be more issues with cells going out of balance in whichever batt/bank/array is getting cycled more often, if it is being cycled dramatically more than the rest.

 

[Steve_S]

I run 174AH & 280AH in a split bank.. Split meaning that 174's are on one (1) side of a switch & 280's on another (2). This is a 105AH difference and IMO the practical limit for difference because when Charging the 174's EndAmps (8.7A) is reached and allows the 280's to continue to saturate a bit deeper to 3.425Vpc. The time that this takes is minimal and while the 174's are FULL @ 4.425, they sit idle taking no charge, the 280's continue to take the extra till 100% and then goto idle as well. At this point they may exchange a few volts between packs as settling is ongoing. (Normal & Expected)

On Discharge, there is NOTHING Different while within the Working Voltage Range of 3.000-3.400, all packs discharge "Proportianiately" according to their capacity. Simplistic #s, Say load is 24A draw, the 2x 280's will deliver 10A ea while the 2x174's will deliver 7A each. Again this is normal & appropriate.

Once you drop BELOW Working Voltage Range that's where things get fun. The packs will try t stay propertional BUT once they cross 2.850 and lower then you will get the divergence when the 280's over compensate and voltgaes drop fast after 2.850, Remember this is the Bottom Cliff Fall of the "Allowable Voltage Range".

Having 200AH next to 300AH should not present major issues till at edges. BUT that is the limit because if it was 200AH & 400AH the 400's would try to back fill losses as there is not enough in the lower AH packs to keep proportion. In ALL CASES, the Larger Batter Pack will transfer some power to the lower packs to maintain overall "Bank Balance" even if it is just "surface charge".

In THEORY, if you remain within the Working Voltage Range (3.000-3.400) they will discharge proportionately but once at the edges charge & discharge will diverge. The greater the AH Difference the more significant it will be.

Hope it helps, Good Luck... I know clear as stirred mud.

 

[jpimlott]

I agree with Steve-s . I have a similar setup and i see the same behavior with my 180 ah and 2 280 ah banks.