What BMS is good for 2-24v batteries in series?

[John Frum]

Jehu has a video on the subject

 

[Ampster]

I assume we could use separate BMSs for each 12v pack and then another BMS to connect them together for 24v or 48v.

That is the issue that has been raised. The best solution would be one BMS for a 48 volt pack. Disassembling it conveniently into 12 or 24volt subgroups is the challenge. It gets further complicated if one wants to use those sub groups. I do not think that was the OPs intent. I have not seen examples of multiple BMSs in a pack but will watch Jehu's video with interest.

 

[Speedr56]

Good video, I think in my situation it would be best to go 16s BMS for 48v rather than series 2 -24v packs.

 

[John Frum]

Good video, I think in my situation it would be best to go 16s BMS for 48v rather than series 2 -24v packs.

Absolutely!

 

[Thingol]

I am still learning but this seemed legit.

Can I connect Lithium batteries in series?

A common question we get asked about in relation to Lithium batteries is how to best connect multiple batteries together to achieve correct voltage or capacity. At Valen, we don’t recommend series connecting Lithium batteries. However, Lithium batteries can be placed in parallel if required.

blog.valen.com.au

So why can’t Lithium batteries be connected in series?

It’s not recommended to install Lithium batteries in series because when a Lithium battery is charging, the BMS is actively controlling the charge to the cells within the battery.
The BMS prevents the cells from over-charging, balances the cells and performs other tasks to ensure that the batteries are maintained correctly. Once the cells are fully charged, the BMS will go open circuit to prevent further charge going into the battery. This stops the battery from becoming over-charged. When Lithium batteries are connected in series and this occurs, the charging circuit is broken and the charger will stop charging the batteries.
The battery that first gets to this state will be fully charged while the other batteries in the series string will not be. The remaining batteries will be unable to continue charging due to the single battery having gone open circuit.
If you are going to series connect Lithium batteries, we recommend putting one BMS over all the Lithium batteries in a series which will control the charge to each battery and ensure all the BMS’s go open circuit at the same time, preventing any single BMS from completely stopping the charging circuit.
However, we strongly recommend to avoid series connecting Lithium batteries. We instead recommend using Lithium batteries that are designed and manufactured to the required system voltage.
For example, a 24V 100Ah caravan battery system could be built by utilising 2 x 24V 50Ah batteries in parallel or a single 24V 100Ah battery. However, we do not recommend using 2 x 12V 100Ah batteries connected in series. Once these cells have been built, they can be safely connected in parallel to add up to the required capacity. This way, you’ll always have full control over the voltage.

For those of us already with 4x 12V LiFePo batteries in series, what BMS can we use to manage the whole bank? I suppose it would be a 4S, 48V one but I cannot find such a product anywhere, any leads?

 

[TorC]

For those of us already with 4x 12V LiFePo batteries in series, what BMS can we use to manage the whole bank? I suppose it would be a 4S, 48V one but I cannot find such a product anywhere, any leads?

You'd probably have to go custom electronics for a 4S, 48V, because 12V is a battery voltage, not a cell voltage. Not impossible. If you have some coding experience, you could probably manage the balancing with a few relays with power resistors controlled by an Arduino.

Regarding the opening question, I think it's fairly common for the balance leads on 16S BMSs to come out of two ports. The connectors aren't really designed for regular disconnection, but for occasional moving you can certainly make up 8S units and plug the leads from the second one into the primary. Just don't mix up which connection goes where. Much safer than series connecting something not properly designed for such, IMHO.

 

[ikedag]

Will something like this work? I have 4 12V Lifepo4 in series, I have a 48V charge controller across all 4, and I believe it is charging the batteries unevenly...

Battery Balancer on Amazon

 

[Ampster]

I have a 48V charge controller across all 4, and I believe it is charging the batteries unevenly...

It is not the fault of the charge controller. It is the nature of LFP batteries that may have different internal resistance. In your case each of your 12 volt batteries should each have their own BMS so the four cells within each 12 volt battery are probably getting balanced but between each 12 volt battery there could be differencces. Did they all start out at the same SOC? It would be hard to tell unless you put them in parallel and charged them to 14.4 or 14.6 volts. I have no experience with that device but it may be what you need and probably a lot easier than disassembling your pack and charging the 12 volt batteries in parallel.