LiFePO4 Battery in Series

As a novice in battery making I see that so many people are connecting 4 LIFPO batteries in series to make 48v, and for my understanding this seams to be a very bad practice as there is no way to keep the batteries balanced and the failure of one battery or BMS can create huges problems and thermal runaway as the voltage is way to high for the 12v batteries, it would seam like you would at least need a external type of BMS to monitor the 4 string voltage of individual battery that would trip a breaker to shut off the charging. If one of the mosfet for charging fails as the 3 other ones are closed and the 3 batteries get down to resting voltage the battery with the failed mosfet will see som frighteningly high volts.
Am I being stupid or is this a real problem?

LiFePO4 Battery Series​

Just don't do it.
These aren't the previous generations lead acid batteries.

As Tim said, poor idea.

Hold on now. There are balancers that will keep the 12 volters in series alignment. Some 12 volters are rated to go 4 series without issue.
Must know the equipment is rated for the project. For the unusual failure there should also be a fuse in place. LFP is actually quite difficult to induce thermal runaway in normal operation failure.

Otherwise yes I recommend a 48 volt battery at the planning stage.

The only reason for putting BMS's in series is if you are trying to work with what you already have.
Otherwise, you should buy the correct battery to begin with.

If they made similarly sized 48v batteries with say 25AH and bluetooth, I'd be all over them for about $300 each.

It’s no problem, you just need to take steps to ensure the individual batteries are looked after. I prefer to use individual charge controllers for each battery when I do pre-made 12V in series.

For RV, this works well as you can then also use 12V loads from the batteries as well as 24V. (eg using a separate 24V inverter for large loads like aircon, and 2 x 12V inverters for other loads.

I’ve found that multiple smaller units run cooler, are easier to package in smaller spaces, and are often cheaper.

toms said:

It’s no problem, you just need to take steps to ensure the individual batteries are looked after. I prefer to use individual charge controllers for each battery when I do pre-made 12V in series.

For RV, this works well as you can then also use 12V loads from the batteries as well as 24V. (eg using a separate 24V inverter for large loads like aircon, and 2 x 12V inverters for other loads.

I’ve found that multiple smaller units run cooler, are easier to package in smaller spaces, and are often cheaper.

Click to expand...

I can see that in a RV it can be a OK solution and 2 batteries in series is not as bad as 4 in series and if you are charging them individually as 12v banks the problem of them being exposed to high charging volts is non existing

toms said:

It’s no problem, you just need to take steps to ensure the individual batteries are looked after.

Click to expand...

Except when they don't and you get a blowup like the Battleborn situation in the other thread.

Generating your own power is not a trivial task and I see so many people come on here asking questions that scare me for them.
But they have watched so many glowing smiling happy reviews on the tube with the giggle " oh look it works" and here are my affiliate links.

Quattrohead said:

Except when they don't and you get a blowup like the Battleborn situation in the other thread.

Click to expand...

Multiple BMS batteries works well in parallel since they are not interdependent. The system can still run if one is taken off line for maintenance.

Asking them to work in a cooperative series environment is a roll of the dice since the Bms of each operates independently. If one fails things go wrong.

I'm confused or reading it wrong. Surely all LifePo 48v batteries are made up usually of much smaller cells in series with a BMS to manage balancing etc and so it would seem absolutely fine to do this with 4 X premade 12v batteries. Each of those 12v will have their own BMS and then you just add a 4S 48V bms to the 4 batteries or have I read this wrong? If I'm wrong then you could always just open the boxes up and connect a 16S (if each battery is 4S) BMS etc. Again I'm very new to this stuff so might have got it completely wrong.

A single Bms would be better. That way it can watch all the cells. One of the benefits of DIY and server rack batteries.

There is also a big difference between possible and feasible.

Interconnected batteries as proposed would need to ensure that adequate bus bar connections are performed for all cells. Not something I would personally perform since this might compromise the original battery builds.

I defer to those with more experience.

Each individual 12v LFP four cell BMS will balance itself. You do need to get charging absorb voltage high enough to trigger balancing.

For a cell overvoltage shutdown due to imbalance, each battery's BMS will manage its four cells. Any of the series connected BMS's can shut down the whole stack.

When it comes to resistor dump balancer based BMS's, which all self-contained LFP 12v batteries use, it will be no different than if all cells were on a single 16s resistor dump balancer based BMS.

You do have to be careful that the battery's BMS MOSFET's in the 12v self contained LFP battery is rated for a 48v system. Battery spec should state what the maximum number of series stacking is allowed.

Some cheap 12v LFP batteries use lower voltage MOSFET's in their BMS and should not be used in series stack. When a single 12v LFP battery's BMS MOSFET series pass switch opens for any fault reason, the open MOSFET will get subjected to full system voltage so MOSFET breakdown voltage rating must be high enough to handle the higher system voltage.

Most common issue folks have with 12v LFP batteries is they don't fully charge them for months which does not allow BMS to do balancing. You need to periodically (at least every couple of months) charge them to about 14.2v and hold the 14.2v for a couple of hours to allow some balancing time.

It is probably better to get a 12v LFP battery with internal BT RF link so you can monitor individual cell voltage.

Steve Fractals said:

I'm confused or reading it wrong. Surely all LifePo 48v batteries are made up usually of much smaller cells in series with a BMS to manage balancing etc and so it would seem absolutely fine to do this with 4 X premade 12v batteries. Each of those 12v will have their own BMS and then you just add a 4S 48V bms to the 4 batteries or have I read this wrong? If I'm wrong then you could always just open the boxes up and connect a 16S (if each battery is 4S) BMS etc. Again I'm very new to this stuff so might have got it completely wrong.

Click to expand...

Each pre-built battery has its own BMS, yes.
There's no BMS for multiple BMS controlled batteries.
You could use a battery balancer, at extra cost. But as stated, it depends on if Each individual batteries BMS is capable of the total series voltage.
In short, just buy the right battery for your system. And avoid the extra headaches and effort.

I wouldn't be concerned at all about it as long the battery manufacturer says it's fine. Victron says it's fine to connect up to 4 of their 12v LFP batteries in series for 48v. Obviously it depends on each manufacturer but this is what their tech document says:

1) To reduce required balancing time, we recommend to use a little different batteries in series as possible for the application. 24
V systems are best built using 24 V batteries. And 48 V systems are best built using two 24 V batteries in series. While the
alternative, four 12 V batteries in series, will work, it will require more periodic balancing time

Now on the other hand, I've seen people say on here that mixing battery types and ages in parallel packs is ok... No, it's not... Don't ever do that.

timselectric said:

There's no BMS for multiple BMS controlled batteries.

Click to expand...

If that is a question, I think there is a model of Batrium that can do that.

Ampster said:

If that is a question, I think there is a model of Batrium that can do that.

Click to expand...

Batrium has something like that , yes.
But I'm pretty sure that all of the pieces must be Batrium products.

Quattrohead said:

Except when they don't and you get a blowup like the Battleborn situation in the other thread.

Generating your own power is not a trivial task and I see so many people come on here asking questions that scare me for them.
But they have watched so many glowing smiling happy reviews on the tube with the giggle " oh look it works" and here are my affiliate links.

Click to expand...

If you use individual charge controllers on your prebuilts when you put them in series there is zero extra risk.

I agree that if you are too stupid to see how that works you probably shouldn’t be setting up your own systems.

The biggest issue is people setting up a system for how it behaves now, not realising that it should last for 10+ years, and the balancing requirements of the system will be different as it ages.

Skagit said:

Now on the other hand, I've seen people say on here that mixing battery types and ages in parallel packs is ok... No, it's not... Don't ever do that.

Click to expand...

What failure mode have you experienced with mismatched prebuilt 12V batteries in parallel?