Can I parallel different ah lifepo4

[thomm225]

Originally Posted by mbartosch

As Newhaul wrote this is not correct. Two paralleled batteries form an electric mesh with two nodes (common plus and common minus). These two nodes are connected to the charger. Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

Now in the real world there are no ideal conductors, hence the potential at the battery poles will be different, and the potential will also vary with the current running through the individual batteries. Now, if the battery capacity values are not too different and the wires are of similar length and identical cross section the difference is usually negligible.

This means that properly wired, paralleled batteries with identical chemistry will result in the same SoC for each battery.

So are you saying the larger and smaller batteries will then charge and discharge at the same rate?

The point is, it's not good to connect batteries of different sizes in parallel

I got away with it for a few years but due to the different sizes, the battery(s) always charged up much faster than they did after I put two of the larger ones in parallel. The larger battery failed a couple years early than the smaller one did, and the larger battery was a year younger

How did it work the last time you tried it?

[mbartosch]

Originally Posted by newhaul

the best way to wire multiple 12v batteries would be pos to pos neg to neg with the pos and negative leads coming from opposite ends of the bank. ( this is for a 12v bank)

ie 2 batteries both positive wired together and both negatives together with the positive for the bus coming off of battery A and the negative comes off of battery B ( when there are two batteries in parallel. )

That's right.

An interesting side note from our special case: in our setup (two small mobile LiFePO4 banks for dinghy propulsion in a charging station, see above) the mobile banks are electrically "more remote" from the loads than the house bank.
The (properly dimensioned and fused but physically longer) wiring and the 1-2-both-off switch actually noticably adds to the internal resistance of the mobile banks and effectively contributes to distributing the current between the big house bank and the smaller mobile bank, favoring the main house bank.

This is actually quite beneficial in our case. The mobile banks contribute to serving the house loads but to a lesser extent than the big house bank. The effect is that when running big loads, such as the windlass or power winch the bulk of the load is handled by the bigger house bank which is electrically "nearer" to the load (less wire resistance). I see proportionally smaller currents served by the smaller banks than from the house bank.
As the smaller banks are fused and wired for only 50 A each this is very desirable with bigger loads.

[newhaul]

Originally Posted by thomm225

So are you saying the larger and smaller batteries will then charge and discharge at the same rate?

The point is, it's not good to connect batteries of different sizes in parallel

I got away with it for a few years but due to the different sizes, the battery(s) always charged up much faster than they did after I put two of the larger ones in parallel. The larger battery failed a couple years early than the smaller one did, and the larger battery was a year younger

How did it work the last time you tried it?

Think about it for a second of course it charged faster.
When you have as an example a 220 ah battery and a 100ah battery in parallel your total capacity is 320ah .
With 2 high capacity 220 ah batteries in parallel you now have 440 ah capacity of an additional 120 ah so of course it will take longer to charge the bank with the same 30 amp charger ( the capacities and charger are for illustration purposes )

[mbartosch]

Originally Posted by thomm225

So are you saying the larger and smaller batteries will then charge and discharge at the same rate?

No, I am saying they will stay at the same SoC. This implicitly means that individually they charge and discharge at different rates.

I got away with it for a few years but due to the different sizes, the battery(s) always charged up much faster than they did after I put two of the larger ones in parallel. The larger battery failed a couple years early than the smaller one did, and the larger battery was a year younger

How did it work the last time you tried it?

Lead Acid batteries significantly "suffer" when not being charged to 100 % SoC - which might have caused your particular problem IMO. If you have not connected them "diagonally" as mentioned by Newhaul this will have led to incomplete charge for the battery "farther away" from the charger.

With LiFePO4 this is not a problem, they do not care if they are not charged to 100 % - on the contrary they like NOT being at 100 %.

[newhaul]

Originally Posted by mbartosch

No, I am saying they will stay at the same SoC. This implicitly means that individually they charge and discharge at different rates.



Lead Acid batteries significantly "suffer" when not being charged to 100 % SoC - which might have caused your particular problem IMO. If you have not connected them "diagonally" as mentioned by Newhaul this will have led to incomplete charge for the battery "farther away" from the charger.

With LiFePO4 this is not a problem, they do not care if they are not charged to 100 % - on the contrary they like NOT being at 100 %.

I work with all the various battery types and if properly wired up and maintained they are quite long lived. I would bet that thomms issue was a PSOC issue due to how the bank was wired. I just switched from Fla to Lfp and I don't expect to ever need to purchase batteries again for my defender. Unless it is to increase the ah capacity beyond the current 150ah

[thomm225]

Originally Posted by newhaul

Think about it for a second of course it charged faster.
When you have as an example a 220 ah battery and a 100ah battery in parallel your total capacity is 320ah .
With 2 high capacity 220 ah batteries in parallel you now have 440 ah capacity of an additional 120 ah so of course it will take longer to charge the bank with the same 30 amp charger ( the capacities and charger are for illustration purposes )

The point is that it creates a mismatch and one battery will suffer. Best to connect like batteries (capacity, voltage, etc) when connecting in parallel

[thomm225]

Originally Posted by newhaul

I work with all the various battery types and if properly wired up and maintained they are quite long lived. I would bet that thomms issue was a PSOC issue due to how the bank was wired. I just switched from Fla to Lfp and I don't expect to ever need to purchase batteries again for my defender. Unless it is to increase the ah capacity beyond the current 150ah

My "bank" was two 12 volt batteries connected in parallel.

The larger one died first after five years. The smaller one lasted seven years.

I bought two of the same after that, and next time I will get larger "real" deep cycle batteries. I'm still experimenting

[newhaul]

Originally Posted by thomm225

My "bank" was two 12 volt batteries connected in parallel.

The larger one died first after five years. The smaller one lasted seven years.

I bought two of the same after that, and next time I will get larger "real" deep cycle batteries. I'm still experimenting

get yourself 4 gc2 golf cart batteries they are reasonable price wise and will give you 400 plus ah and many years of service when properly wired up and maintained for not a lot of money.

[thomm225]

Originally Posted by newhaul

get yourself 4 gc2 golf cart batteries they are reasonable price wise and will give you 400 plus ah and many years of service when properly wired up and maintained for not a lot of money.

Sounds good.

I've heard lots of good things about those batteries and may go that route after these fail

[john61ct]

So is there now a consensus?

A "It's NP, but a bit less than ideal" ?

B in the sense one pack will wear faster than the other

?

[newhaul]

Originally Posted by john61ct

So is there now a consensus?

A "It's NP, but a bit less than ideal" ?

B in the sense one pack will wear faster than the other

?

well depends on how its wired but with Lfp as long as you have a separate bms on each battery in the bank ( which IMO you should anyway) there will be no issue that I can see.

[Cpt Pat]

I concur with newhall. Also make sure when you first connect the batteries in parallel that both sets have very close to the same terminal voltages.


I install fuses (in my case 50 amps) at each set of positive terminals, just in case they ever get seriously out of sync.

[newhaul]

Originally Posted by Cpt Pat

I concur with newhall. Also make sure when you first connect the batteries in parallel that both sets have very close to the same terminal voltages.


.

Why same terminal voltage?
You just need to have the same relative voltage.
How else are you going to charge your electric trolling motor battery when its low.
They will equalize and charge just fine.

[john61ct]

Originally Posted by newhaul

as long as you have a separate bms on each battery in the bank

Well that's a can of worms qualification.

The term BMS is **functionality** to me, not a single object you can just buy OTS.

And per-cell monitoring and control, completely different level vs per bank, or intermediate per-pack design.

Here we are talking about managing multiple top-voltage packs

OP was 8s 24V, but could be 4s 12V just as easily

combining them in parallel

perhaps for charging only, as with swapping out portable powerpaks for EP

or as with the OP permanently for both charging and discharging.

Which specific aspects of "BMS" functionality, and

at what level, cell vs pack?

do you think is required to make such a setup work well?

Let's **please** not drift into "BMS vs No BMS" in the more general context, but keep the focus specifically on the needs driven by these specific use cases.

List of functions I can think of in order of relevance IMO, not necessarily traditional BMS related:

Circuit Protection, wiring vs protecting packs /cells

Current Limiting, without stopping flow

OVD

LVD

temperature protection, charging vs discharging

cell balancing

others?

______
The extreme case scenario is a smaller-AH pack at low SoC, combining to the larger bank at a high SoC.

With lead, the internal resistance keeps the amps current rate much lower than with LFP. For simplicity, let's keep the "LA/LFP hybrid" variation out of the discussion.

What sort of kit is required to do this safely, and without undue wear on LFP cycle lifetime?

The case when a charge source (of big enough amps) is active, is likely quite different from when there is no charging going on.

[CatNewBee]

you can parallel them cell wise and let the bms do his magic. Charge all cells first to the same voltage to 100% (3.65V) before paralleling.

it will be similar to a configuration of 3p4s, the larger cell behaves towards the small like 2 smaller parallel cells, the smaller is the 3rd in the parallel setting.

The resulring current corresponds to the cell size, the larger the cell, the smaller the resistance and larger the current. The SOC is even distributed between the parallel cells and the voltage is the same.

The setup will work as long as the cells are in balance. If one of the smaller cells fails by a shortcut, it will blown and explode, because all current of the larger cell will discharge on this cell.

This is also the main problem of any configuration with small parallel cells. Therefore the lipo cells have built-in fuses in large batteries like Tesla etc..