Hey guys!

I recently fell into a deal for a 16Cel LiFePo4 battery and an inverter.

However the battery is a bit small for the inverter (2.5kW for a 5kW Inverter) and I'd like to upgrade at some point.

The manufacturer of this battery doesn't make the 16 cel version anymore and only make a 17 cell version. And those in parralel is not an option.

However other people do sell 16 cell LiFePo4 batteries. And I'm thinking of getting one of those (In the future)

Now electrically it won't matter, but the BMS standards I'm fuzzy on. How do the communication protocol differ between battery manufacturers?

Has anyone here had experience with a multi-vendor battery pack?

Manufacturers use different BMS, many not being compatible with each others. Manufacturers may even use different BMS in the same range of batteries, only depending on supplies or costs at the time of assembly.

it's easier to get communication between all batteries & the inverter but highly unlikely if you don't use the same brand of batteries & often not the exact same type.

It's easier but not crucial : as long as each battery is protected by its own BMS, you can have the inverter managing the batteries by voltage or simulated state of charge instead of communication with the BMS ...

I would recommend the same as zivva. Let the small battery BMS take care of itself and put emphasis on your future larger batteries. You need at least 5kWh (not kW) more, better 10 kWh. Standard packs come with 100Ah, with 16 cells that's 5kWh.

Yeah. This particular pack came in 15 cell I believe (No 16 cell, sorry about that.) And it's a 2.56kWh pack at 1C of discharge(50A). I know it's too small but given my load rarely exceeds 1kW I think I'll be fine for now.

 

Thanks for your replies guys!

I really appreciate your help!

 

I know I should upgrade in the future and I fully intend to.

Edited January 20, 20232 yr by Adagio

On 2023/01/19 at 6:11 PM, zivva said:

Manufacturers use different BMS, many not being compatible with each others. Manufacturers may even use different BMS in the same range of batteries, only depending on supplies or costs at the time of assembly.

it's easier to get communication between all batteries & the inverter but highly unlikely if you don't use the same brand of batteries & often not the exact same type.

It's easier but not crucial : as long as each battery is protected by its own BMS, you can have the inverter managing the batteries by voltage or simulated state of charge instead of communication with the BMS ...

This is very true, also why Pylontech is so far ahead of anything else with battery compatibility.

I have 3x 5.1kWh batteries from the same supplier, one was bought 6 months after the first two. 

In that time the batteries received a new BMS and some changes were made to the cells that are used.

End result is the newer battery charges full at 55.2v, the two that are 6 months older only charges full at around 56.4v.

5 hours ago, I84RiS said:

This is very true, also why Pylontech is so far ahead of anything else with battery compatibility.

I have 3x 5.1kWh batteries from the same supplier, one was bought 6 months after the first two. 

In that time the batteries received a new BMS and some changes were made to the cells that are used.

End result is the newer battery charges full at 55.2v, the two that are 6 months older only charges full at around 56.4v.

So... The lower voltage one just stops charging and then comes back when the voltage drops enough?

21 hours ago, Adagio said:

So... The lower voltage one just stops charging and then comes back when the voltage drops enough?

Correct, the BMS switches off the charging as soon as it is full, the other two (that require higher voltages to charge full) then continues to charge until they are full. 

Same on the discharge, the newer one (with the lower voltage) always starts to discharge a few minutes after the older two also due to the voltage differences. 

Edited January 22, 20232 yr by I84RiS

I did NOT see that coming.

I always thought that even though there is a BMS the charge voltage still needs to be correct.

This changes everything.

🤔

I wanted to ask about it being bad for the batteries to be in a constant state of OVP but knowing what I know about mosfets I know that's not relevant.

On 2023/01/22 at 9:44 PM, Adagio said:

I did NOT see that coming.

I always thought that even though there is a BMS the charge voltage still needs to be correct.

This changes everything.

🤔

I wanted to ask about it being bad for the batteries to be in a constant state of OVP but knowing what I know about mosfets I know that's not relevant.

So what happens is that the battery that only needs 55.2v to reach full charge will given an overvoltage alarm (as soon as any given cell gets to 3.6v) and then an overvoltage protect (as soon as any given cell reached 3.65v per cell). At this point the BMS of that specific battery cuts off the charge current and will momentarily discharge (maybe 1 to 2 seconds). This discharge is enough to drop the voltage on that one the cell that is at 3.65v to below 3.5v (there is really very little additional capacity in LFP cells above 3.4v per cell). It will then balance the cells and will rest as about 3.35v per cell and will not accept any further current even though the other two batteries are still being charged at a voltage that exceeds 55.2v. 

The above will happen irrespective of whether I use the BMS/inverter communication to take care of the charging parameters or whether I set these manually.

The BMS charge voltage for these batteries are actually set at 57.6v. So if the BMS takes care of the charge paramaters it will charge at this voltage during the constant voltage charge phase. For these specific batteries the constant voltage charge phase is from 98% SOC to 100% SOC. Interestingly the batteries will obviously never reach 57.6v as the BMS stops the charge current as soon as any given cell reaches 3.65v which is approximately 55.2v for the new battery and 56.4v for the two older ones.

Another interesting observation  is that as soon as any cell gets to 3.65v per cell the SOC will immediately go to 100%. My understanding is that this is done as the BMS will only balance the cells if the SOC reaches 100% AND no current is flowing into the battery (so if there is still current flowing into the battery it will balance the cells if there is a difference of more than 30mv, but as soon  as the current stops it will stop the cell balancing unless it is at 100% SOC).

Sorry for the long post, but another interesting observation is that during discharge the master battery will report the SOC to the inverter of the battery with the lowest voltage (and therefore the lowest SOC). During charging it will report the SOC to the inverter of the battery with the highest voltage (and therefore the battery with the highest SOC). This can cause sudden jumps in the SOC % as reported by the inverter.