Not sure what size BMS for newly purchased 280ah batteries

[happy]

I joined in on a group buy for the 280ah eve cells but have been putting off buying a BMS as I'm quite new and keep getting overwhelmed when trying to figure it out. I will have a 4s 12v system and am planning on having a 1500w inverter. Outside of the inverter, the loads would be a 12v fridge, some lights, and a phone or two charging, so the extra dc loads would be minimal. Based on that, I should find a BMS that can handle loads of 125A, correct? That would mean the overkill solar BMS would be out of the picture since they only have 100A or 120A models. I realize I could put them in parallel, but that doesn't seem as cost effective as just buying one that can support 150A.

Is it also possible to wire up a system so that the BMS is not subject to the loads? I'm not too concerned on the discharge rate, since a 280ah lifepo4 battery can handle much faster discharge than I'll be doing, but I'm not sure if it's possible to wire it up that way?

Does anyone have any ideas on a good BMS for this system? I saw some references to chargery bms8t, and of course the daly and overkill that are mentioned on will's site. Ideally I'd find one in the US since the batteries are arriving next week already and I'd like to do some balancing and capacity test to make sure my batteries are ok.

Thanks in advance for the help!

 

[snoobler]

It's not the size of the battery, it's the current. Simply based on 1500W, you'll need:

1500/12.8 = 117A

It's possible to wire it up so the BMS is not subject to the loads, but then you have disable the BMS's ability to protect the cells - like fastening your seat belt behind you...

I'd go with the Overkill.

 

[SoundinSpirit]

It's not the size of the battery, it's the current. Simply based on 1500W, you'll need:

1500/12.8 = 117A

It's possible to wire it up so the BMS is not subject to the loads, but then you have disable the BMS's ability to protect the cells - like fastening your seat belt behind you...

I'd go with the Overkill.

Correct me if I'm wrong... but that formula doesn't account for the inverters efficiency loss, correct? Dang me, but I can't figure out the math based on approx. 88 percent efficiency .... Counting that, the overkill would be maxed out or slightly over current, correct? This is my issue as well, I need about 180 amp output (2000 watt inverter) from similar cells. Only for short duration though, microwave. However... I am building up our RV Trailer as a solar generator too for grid down situations so I can plug it into the house for less generator use. 4 more cells are on the way for me...

 

[happy]

Correct me if I'm wrong... but that formula doesn't account for the inverters efficiency loss, correct? Dang me, but I can't figure out the math based on approx. 88 percent efficiency .... Counting that, the overkill would be maxed out or slightly over current, correct? This is my issue as well, I need about 180 amp output (2000 watt inverter) from similar cells. Only for short duration though, microwave. However... I am building up our RV Trailer as a solar generator too for grid down situations so I can plug it into the house for less generator use. 4 more cells are on the way for me...

Fwiw I did go with the 120a overkill solar bms. Steve said it should be able to handle short loads at full capacity no problem, and surges above it for a small amount of time as well.

For the efficiency you're right. The inefficiency means that for 2000w output, you'd need 2000/0.88 = 2300w coming from the battery (88% efficiency). So at 12v that'd be about 190amps.

I feel like 1500w is about the max I'd feel comfortable pushing with 12v without going the relay route. I also wouldn't feel too terrible having the load side bypass the bms for all inverter loads, since I'm more worried about overcharging than undercharging for my own system at leasy

 

[SoundinSpirit]

Fwiw I did go with the 120a overkill solar bms. Steve said it should be able to handle short loads at full capacity no problem, and surges above it for a small amount of time as well.

For the efficiency you're right. The inefficiency means that for 2000w output, you'd need 2000/0.88 = 2300w coming from the battery (88% efficiency). So at 12v that'd be about 190amps.

I feel like 1500w is about the max I'd feel comfortable pushing with 12v without going the relay route. I also wouldn't feel too terrible having the load side bypass the bms for all inverter loads, since I'm more worried about overcharging than undercharging for my own system at leasy

That's a REALLY good point about the inverter load. It would cut off before the BMS would anyway....not to mention I have 700 watts of solar on the roof (for now). Steve did reply to me as well, saying that he does not recommend the Overkill be used in parallel.... I've tried it myself now and I can't get more than 135 amps and it only lasts 5-10 seconds before shutting down. 12 volt loads shouldn't ever add up to be an issue as we have a propane fridge, even on our new 2021 trailer.
Thanks for the formula! I really felt like a dope admitting that one! Lol...
It's weird though as Will has tested the 60 amp Daly in parallel... worked great I thought...

 

[Dzl]

It's not the size of the battery, it's the current. Simply based on 1500W, you'll need:

1500/12.8 = 117A

Correct me if I'm wrong... but that formula doesn't account for the inverters efficiency loss, correct? Dang me, but I can't figure out the math based on approx. 88 percent efficiency .... Counting that, the overkill would be maxed out or slightly over current, correct?

Correct the above does not consider inefficiency, the math I use for guestimating inverter current, is more conservative than the math above. Dunno if its more or less right, when it doubt I try to err on the conservative side.

[Inverter Output Power] ÷ [Inverter Low Voltage Disconnect] ÷ [Inverter Efficiency] = [Inverter input current]

E.g.
1500W / 12V / 0.88 = 142A
2000W / 12V / 0.88 = 190A

Then I would multiply by some factor of safety to arrive at fuse size, wire size, BMS size, etc.

This math is based on somewhat conservative assumptions of maximum power at minimum voltage, where it isn't conservative is efficiency, we are just taking the manufacturers word for it, and as I'm sure we are all well aware, cheap Ali*/Amazon/Ebay brands often inflate or overrate the specs.


edit: the above intentionally leaves out any consideration of continuous vs peak power (the formula would look the same, but the numbers would differ), since this is a bit of a grey area, and depends on the type of inverter and the usage. A robust low frequency inverter for instance might be able to sustain a decent surge for minutes, or a modest surge for as much as a half hour, whereas a high frequency inverter can probably not sustain a surge for more than a few milliseconds. In the case of the latter I think surge can mostly be ignored, in the case of the former, its a little less clear. With a low frequency inverter the conservative approach would be to include surge rating in your calculations. I'm not informed or experienced enough to know if this is strictly necessary, or at what point it becomes necessary.

 

[SoundinSpirit]

I joined in on a group buy for the 280ah eve cells but have been putting off buying a BMS as I'm quite new and keep getting overwhelmed when trying to figure it out. I will have a 4s 12v system and am planning on having a 1500w inverter. Outside of the inverter, the loads would be a 12v fridge, some lights, and a phone or two charging, so the extra dc loads would be minimal. Based on that, I should find a BMS that can handle loads of 125A, correct? That would mean the overkill solar BMS would be out of the picture since they only have 100A or 120A models. I realize I could put them in parallel, but that doesn't seem as cost effective as just buying one that can support 150A.

Is it also possible to wire up a system so that the BMS is not subject to the loads? I'm not too concerned on the discharge rate, since a 280ah lifepo4 battery can handle much faster discharge than I'll be doing, but I'm not sure if it's possible to wire it up that way?

Does anyone have any ideas on a good BMS for this system? I saw some references to chargery bms8t, and of course the daly and overkill that are mentioned on will's site. Ideally I'd find one in the US since the batteries are arriving next week already and I'd like to do some balancing and capacity test to make sure my batteries are ok.

Thanks in advance for the help!

Hey @happy , what was the price and shipping for those cells? I need to order another set of four... Trying to get a reference point. I think mine were $440 with shipping before.