Run 4 LFP 12V batteries in series while pulling 12v loads?

[justinm001]

In my RV chassis system I have 4x lead acid batteries running series/parallel to power the 24V engine and aux loads as well as each 12V side runs all the electrical components. The system uses a 100a Vanner equalizer like below to keep the batteries in balance.

https://marinepartssource.com/vanner-100-amp-equalizer?gad_source=1&gclid=CjwKCAiArfauBhApEiwAeoB7qNHS7ClKQRzs6EI_zYq0S5ERTho3RmtQvcUwry3D6VXGwPteO8A5ShoC9PoQAvD_BwE

Why can't I run something similar for a 48v lfp system? Isn't it the BMS's role to accept voltages and then it equalizes each cell? Say I want to have 2x 48V batteries in parallel then have 4x 12V batteries in series connected to those 48v batteries all using a large charger. As long as the charger is running at 57.6V it should be recharging the 12v batteries in series as they're at 14.4v.

When not charging the 12V batteries are draining separately but the other 12V batteries in series should pickup the voltage and level it out, while the 48V batteries also level it out? Or would something else happen since LFP batteries don't drop much voltage when discharging, hence the need for an equalizer like that Vanner?

What exactly happens if you connect a 48v battery at 10% to another 48v battery at 100%.

I'm also curious what'll happen if one of the 12V batteries is down to 20% while rest are all at 90% then we get a trickle solar charge. Would it charge each battery evenly or would the 12V take most of it.

 

[Vigo]

In a series string, all batteries receive the same current because current is the same everywhere in a series circuit. However, batteries in series will not balance with each other. Even if no unequal loads are hooked to them, their slight differences in internal resistance will cause their actual state of charge to drift apart from each other due to some of them making slightly more heat than others while charging and discharging. Basically, some of them will charge and discharge more 'efficiently' than others, so they will drift apart in SOC even though they all receive the exact same 'charge'.

So, you need to run a battery equalizer (which is like an external balancer that balances between whole batteries instead of individual cells) to compensate for this, otherwise you'd periodically need to break down the series string and charge them in parallel to rebalance the pack before putting back into series.

These things are about $50.
Battery Equalizers

Say I want to have 2x 48V batteries in parallel then have 4x 12V batteries in series connected to those 48v batteries all using a large charger.

I have basically that at my house, 3x 48v 50ah batteries in parallel with one series string of 4x 12v 280ah batteries which has one of those equalizers on it. It's been working fine for months.

The only difference is mine are ALL lifepo4 whereas you are talking about paralleling lead acid and lifepo4. In general it is fine to do so but you may find that the absorption voltages actually recommended by the battery manufacturer are higher than what the lithium wants, so you'll be limited on 'properly' charging the lead pack IF that were the case. Like, my giant Rolls pack would actually want 60-61v whereas the lifepo4 wants more like 57 and will go into high voltage disconnect if allowed to go to 60v.

 

[justinm001]

In a series string, all batteries receive the same current because current is the same everywhere in a series circuit. However, batteries in series will not balance with each other. Even if no unequal loads are hooked to them, their slight differences in internal resistance will cause their actual state of charge to drift apart from each other due to some of them making slightly more heat than others while charging and discharging. Basically, some of them will charge and discharge more 'efficiently' than others, so they will drift apart in SOC even though they all receive the exact same 'charge'.

So, you need to run a battery equalizer (which is like an external balancer that balances between whole batteries instead of individual cells) to compensate for this, otherwise you'd periodically need to break down the series string and charge them in parallel to rebalance the pack before putting back into series.

These things are about $50.
Battery Equalizers


I have basically that at my house, 3x 48v 50ah batteries in parallel with one series string of 4x 12v 280ah batteries which has one of those equalizers on it. It's been working fine for months.

The only difference is mine are ALL lifepo4 whereas you are talking about paralleling lead acid and lifepo4. In general it is fine to do so but you may find that the absorption voltages actually recommended by the battery manufacturer are higher than what the lithium wants, so you'll be limited on 'properly' charging the lead pack IF that were the case. Like, my giant Rolls pack would actually want 60-61v whereas the lifepo4 wants more like 57 and will go into high voltage disconnect if allowed to go to 60v.

No I'm wanting all LFP batteries, just using the lead acid chassis batteries as an example as it works so well.

Those battery equalizers are exactly what I'm thinking!!

Right now I have 2x100ah 48V batteries and 2x400ah 12v batteries on completely isolated systems with separate inverters and everything. The 12V runs all my 12V loads and the 5kw inverter is just a backup but used to charge those batteries when on shore. I'm wanting to increase total capacity and adding 2 additional 400ah 12v batteries makes the most sense as I can then have 30kw total 48v batteries. Separate the 12V loads on 2 batteries which are like 100w idle up to 400w when in use. I could even separate them into using all 4 12V batteries so its more of an equal draw.

I'd be fine if the 12V loads draw down the 12V batteries equally then the 48V batteries draw from all equally as its the main load. I'd then keep the 12V inverter off incase issue with 48v inverter to save another 40w or so idle consumption.


I'm still confused on if a 48v battery with 10% is connected together with another 48v battery at 90% and its connected to a charger at 57.6v, how it won't charge both batteries to 100%? I totally see the batteries internal resistance causing the batteries to drift apart if they never get a full 100% charge that lasts for x hours/days and the BMS has time to equalize every cell and make it perfect. This is for an RV setup where it'll be on shore at least once a week, with solar all the time helping boost the charge during the day.

 

[Vigo]

Separate the 12V loads on 2 batteries which are like 100w idle up to 400w when in use. I could even separate them into using all 4 12V batteries so its more of an equal draw.

You COULD keep your existing huge 24 > 12 step down converter and simply hook it to 2 of the 4 12v batteries and then just allow the 'equalizer' to gradually make up for the slight imbalance you will cause by drawing from only 2 out of 4 batteries to power your 12v loads. It does allow you to buy the minimum of extra parts vs buying a 48v to ~13.8v step down converter or using a small MPPT solar controller with the 48v system as the solar input, to keep a separate 12v battery maintained.

I'm still confused on if a 48v battery with 10% is connected together with another 48v battery at 90%

IF both are lifepo4 they will try to flow a ton of current from the higher battery into the lower battery and overheat (MELT) your conductors. It's not safe to parallel lifepo4 that are so far apart in SOC.

 

[justinm001]

IF both are lifepo4 they will try to flow a ton of current from the higher battery into the lower battery and overheat (MELT) your conductors. It's not safe to parallel lifepo4 that are so far apart in SOC.

They have BMS so worst case it'll trip both BMS for overload although I don't think that's true because the voltage has to be so much different for it to charge. I'd think it'll just drain the 90% battery first. For instance with DC/DC converters if I set it to 53v like I did before it'll just use the DC for the idle loads and batteries will slowly charge to 40% or so and just level out.

 

[Vigo]

Its true that the bms's SHOULD trip for any serious overcurrent but if you plan to rely on that just make sure that your wiring can take that current for longer than the BMS's can. I've seen an experiment where a guy hooked one cell to another cell at different SOCs and flowed well over 200a with just a couple tenths of a volt difference between the cells.