Using 9 LIFPO4 cells for a 24 volt system

[Boonita]

I’m thinking that I might be able to add another cell to my 24 Lithium battery pack to make a 27 volt battery. Since Victron says I can go up to a 33 volt input, it seems plausible. My concern is that Victron also has the charging voltage at 28.8 and I don’t see where it could be changed in my BMV.

Ideally I would want something like 32 volts to charge, but 28.8 would seem like it’s still high enough for a 27 volt battery - equating to about 25.6 on a 24 volt battery. Not ideal, but workable?

Is this something that would work or am I missing something?

Thanks.

 

[timselectric]

I’m thinking that I might be able to add another cell to my 24 Lithium battery pack to make a 27 volt battery. Since Victron says I can go up to a 33 volt input, it seems plausible. My concern is that Victron also has the charging voltage at 28.8 and I don’t see where it could be changed in my BMV.

Ideally I would want something like 32 volts to charge, but 28.8 would seem like it’s still high enough for a 27 volt battery - equating to about 25.6 on a 24 volt battery. Not ideal, but workable?

Is this something that would work or am I missing something?

Thanks.

You would need to get above 31.05v to be able to get any balancing.
I would recommend that you stick to the standard 8 cells.

 

[sunshine_eggo]

It's viable.

33V/9 = 3.67V/cell

3.45 * 9 = 31.05V - minimum practical absorption voltage to achieve 100% charge (or close enough to not matter).

Assuming Quattro/Multiplus and "II" models, charge voltages are 100% programmable. Download victronconnect and use the demo library to explore the ranges. IIRC, my 48V Quattro can be set for 64V absorption with a maximum operating of 66V. Would assume you could get to 32V for certain.

9S will reduce the number of available BMS.

Options needed to program VE.Bus inverters:

VRM + VEConfig and remote upload/download
PC/Mac + VictronConnect + MK3-USB adapter
PC+VEconfig+ MK3-USB adapter

I considered 15S (normally 14S) for my 48V NMC bank for similar reason, but that complicated too many things.

 

[chrisski]

That could work, but would it run your equip?

24 and 48 volt is easy to find equipment. 36 volts is harder, but available.

27 volts may climb above certain tolerance for equipment. Would the inverter handle that extra voltage and be in limits and would being higher voltage push it out of peak efficiency?

I can see the temptation if it was within tolerance. A different way to increase voltage and drop amps, and get an easy 12% increase in Wh.

 

[sunshine_eggo]

That could work, but would it run your equip?

24 and 48 volt is easy to find equipment. 36 volts is harder, but available.

27 volts may climb above certain tolerance for equipment. Would the inverter handle that extra voltage and be in limits and would being higher voltage push it out of peak efficiency?

Temperature compensated lead-acid charging can easily push above 30V, so most "24V" equipment designed to attach to a lead-acid battery is going to handle it.

My 4S2P Trojan T-1275 lead acid bank would routinely hit >62V at freezing and posed no issue with my Quattro's 66V max limit.

I can see the temptation if it was within tolerance. A different way to increase voltage and drop amps, and get an easy 12% increase in Wh.

and 12% decrease in current for a given power.

It's really a mixed bag. There are a lot of good reasons to do it, but only if it's worth whatever you have to pay for it. In my case, 15S REALLY complicated the cell arrangement, and simply expanding the battery by 40% drastically simplified config with 14S, so in the end, it wasn't worth it.

 

[Boonita]

It's viable.

33V/9 = 3.67V/cell

3.45 * 9 = 31.05V - minimum practical absorption voltage to achieve 100% charge (or close enough to not matter).

Assuming Quattro/Multiplus and "II" models, charge voltages are 100% programmable. Download victronconnect and use the demo library to explore the ranges. IIRC, my 48V Quattro can be set for 64V absorption with a maximum operating of 66V. Would assume you could get to 32V for certain.

9S will reduce the number of available BMS.

Options needed to program VE.Bus inverters:

VRM + VEConfig and remote upload/download
PC/Mac + VictronConnect + MK3-USB adapter
PC+VEconfig+ MK3-USB adapter

I considered 15S (normally 14S) for my 48V NMC bank for similar reason, but that complicated too many things.

Thanks! I was expecting it to be a simple adjustment in settings, but didn’t see the option when viewing the bmv in Victronconnect. I’ll pull up some manuals and see what I need to do.

 

[Boonita]

Looks like I need to upgrade to the Cerbo GX - which I was going to do anyway - and then I can adjust the Multiplus charge voltages appropriately. I’ll let you guys know how it works out.

 

[sunshine_eggo]

If you're not ready to commit to a Cerbo, the MK3-USB adapter enables you to reprogram the inverter