Any one know of 48V batteries with BMS that can be connected in series, at least 3?

[MrStimpy]

I was thinking about setting up a EG4 AC/ DC 12K heatpump with at least 3 each 48V batteries connected in SERIES.. Since it needs at least 90 volts DC to run, more than 2 each 48V batteries would be needed. Thanks.

 

[timselectric]

Why would you want to do that?
And how do you plan to charge them?

 

[MrStimpy]

To run it on battery only, without the efficiency loss from a inverter.
As for charging, a 200v pv array direct to battery. Perhaps with a high voltage MPPT...

 

[timselectric]

Instead of putting BMS's in series. (Not recommended)
Why not just diy your own high voltage battery, BMS, and SCC?
Bare in mind that if done correctly, this is going to be expensive. No matter how you do it.
And it's assuming that the mini split will be happy with a battery feeding its MPPT.

 

[MrStimpy]

You are correct in feeding a battery to a MPPT, since the current is huge, it may take more than it can handle and smoke it...lol.
As for BMS in series, I have a bank of 4 each 12v lifepo4's in series, and the manufacturer states 4 in series is OK. I charge it with 60 volts just fine too. While from a matched battery stand point series electronics are not a optimal situation. I suspect in the end, it would be safer just to charge a battery bank and invert to 120 or 240 vac to run the EG4...thanks

 

[LydMekk]

60V sounds too high for 4x12V LIFEPO4s.
56.8V are 3.55V per cell if 4 cells in a 12V box.

 

[MrStimpy]

You are correct but there is a different voltage from charge to rest. Plus tiny resistance of connecting wires and the voltage drop across the BMS. Most 48V chargers put out at least 58.4. I have found the extra 1.6 volts ( 0.4V per 12V battery) helps account for minor losses...IMO....see attached chart.

 

[timselectric]

Voltage drop only happens when current is flowing.
Once the batteries are full, and no current is flowing. The voltage will be over the limit.

 

[MrStimpy]

Yes, it is 1.6 Volts over limit. The batteries i have have a charging voltage of 14.6V +-0.2V.
14.6 X 4 batteries = 58.4 Volts
After the Battery Management board in the battery detects the peak of charge, it will cut off the charge and the cells will fall back to a resting state voltage. The overvoltage lockout on most BMS boards is 15 to 16 volts.
I think the confusion we are talking about is LiFePo4's connected in a battery with NO BMS board or Battery's WITH a BMS board. Bare cell battery's are rare in a series connected state without a BMS. You certainly could get the cells and make one, and then the voltages applied and voltages at rest would be closer to the ideal, but almost all batteries sold come with a BMS of some type.

 

[timselectric]

They're your batteries.
Abuse them however you choose.