Prebuilt LifePO4 - series or parallel ?

[sendero]

I am a newb although I do presently have a system working (2KW with AGM batteries). I want to add some prebuilt LifePO4 to my 24V system. I am having a hard time deciding between two 12v 200AH batteries or two 24V 100 AH. The 12V batteries would provide more "flexibility" in that it could charge them or use them on a 12V inverter separately in an emergency. OTOH, after a lot of confusing study I am given to wonder if putting these batteries in series poses some kind of challenge to keeping cells balanced or perhaps other issues.

Thoughts?

 

[chrisski]

The only that would stop me from putting them in series would be if they are not rated for it. There was one 12 volt battery I looked at that could only go up tp 48 volts. I think I remember others that could not go that high. The MOSFETS in the BMS need to be able to handle the voltage or the internal BMS could be ruined.

 

[Bob142]

My current preference is to not put batteries in series to achieve higher voltage. I'd rather buy or build a battery that is at the desired system voltage and put some in parallel to achieve the required capacity.

This preference has everything to do with my recent experience with changing my AGM system from 12V to 24V. I had 4 12V batteries so I arranged them as a 2S2P configuration. Some months later, one of the batteries (they are really old) finally died. To keep the system at 24V I had to take one of the working batteries out of service. So a failure of one quarter of my batteries lost me half my capacity. I ended up putting them back into a 12V configuration to take advantage of all three batteries.

Since then my plan has been to build or buy batteries at the system voltage and have at least two in parallel for better fault tolerance. This strategy also allows me to more easily add capacity to the battery bank as I only need to buy/build and wire in single batteries at a time rather than multiple.

 

[chrisski]

I misread what you said, so I am changing to parallel.

If the batteries are physically the same size, putting them in parallel is a better idea if wired correctly. In my signature block, there’s a document on paralleling batteries with math that says why some ways are better than others. ParalEllington the wrong way can cause one battery to die quicker than the others and dies quicker and gets worst if done incorrectly the more batteries that are paralleled to the system with incorrect wiring.

 

[Steve_S]

I am a newb although I do presently have a system working (2KW with AGM batteries). I want to add some prebuilt LifePO4 to my 24V system. I am having a hard time deciding between two 12v 200AH batteries or two 24V 100 AH. The 12V batteries would provide more "flexibility" in that it could charge them or use them on a 12V inverter separately in an emergency. OTOH, after a lot of confusing study I am given to wonder if putting these batteries in series poses some kind of challenge to keeping cells balanced or perhaps other issues.

Thoughts?

Some prebuilt batteries can be put in Series and some cannot, it all depends on the internal BMS and if it can handle such, most cannot by default. IF you have an existing 24V System then it is best to use 24V Battery Packs in Parallel. Simply Put: Don't put tires from a Chevette on a Corvette !

With Battery Packs in proper Parallel, they will divide Load/Charge Evenvly throughout their cycle, should any cutoff for any reason, the others should carry on and if properly configured, any single pack can act as "Last Man Standing" in critical situations.. That Faul Tolerance is worth its weight in gold. Battery Packs in Series CANNOT do this.

The "Authoritative Guide" we generally aim people at was produced by Victron, starting on page 17 it addresses using LFP and properly Paralleling Battery Banks. https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

The most common, simplest & effective method is to wire all packs to a Common DC-BUS with equal length, same gauge wires, with each pack fused.

 

[rmaddy]

The decision of using 2 12V 200Ah batteries in 2S or using 2 24V 100Ah batteries in 2P is best made based on the max current discharge that you need and the rated continuous discharge current of the batteries. Let's say both individual battery sizes are rated for 100A max continuous discharge current. Then with the 2 12V in 2S you get a max discharge current of 100A. But with the 2 24V in 2P you get a max discharge current of 200A (due to the 2P).

24V @ 100A means a max inverter size* of 2400W.
24V @ 200A means a max inverter size *of 4800W.

* It's really the max load on the batteries but typically that's mostly from the inverter.

In short, if you are planning on an inverter over 2400W then you must choose the 2 24V batteries in parallel.

If you only plan a 2000W inverter or smaller then either will work. But I'd still lean toward the two 24V in parallel. Two in parallel are easier to balance than two in series. Two in parallel are more fault tolerant. If one battery goes bad you can keep going on the one remaining good battery (at reduced system capacity of course). When in series, one bad battery and your whole system is down until the one battery is replaced.

The only advantage of using 2 12V in series vs 2 24V in parallel is it might be easier to find a 12V charger in a pinch if the need were to ever arise.

 

[sendero]

Thanks everyone for your thoughts. They pretty much aligned with what I already thought. but I was leaning toward the series 12V before, now I'm leaning towards the 24V