Lifepo4 Supercap Issue

[Seawalker]

Hi folks,

I've got a thorny issue that I hoping better understand with your help.

I have built a hybrid energy storage system (hess) using the follow components.

4 - Shunbin 12v 400ah lifepo4 batteries.
4 - Quantum Magnetics 16v 2000 farad super capacitors

I have wire the batteries and super caps together in parrallel creating four sets of lfp-supercap pairs. I then wired these paired sets together in series.

This configuration performed normally being charged with a 110 charger and a solar charger. The load on the battery bank was fairly light. - Refer compressor, lights, water pump, fans - and everything function perfectly.

I then completed the installation of a 20kw PMAC motor to replace the diesel engine and which the batteries were designed for. When I took the vessel out for sea trials I ran it up to 4.5 knots which was a load of 286 amps. The result was the BMS in the first lfp on the positive end of the series failed allowing only 3-5 volts. This failed BMS then prevented any energy out of the system except for the supwercap that was attached to that lfp. The electric motor then simply drained that one supercap and stopped.

So.
The BMS IMHO should have been able to handle 286 amps but it apparently didn't.

Did the supecaps overpower the BMS since they have less internal resistence?

Would it make sense to remove all of the BMS and replace with one BMS for the entire series?

Any insight of any kind greatly appreciated!

 

[gnubie]

The supercaps would only overload the BMS if they were directly dropped across a charged battery while they were discharged (either completely or significantly). The capacitor should be precharged to avoid this. Once they charged and set in parallel with their partner battery the voltages on both will remain in step so there won't be a big load on the BMS.

If the BMS on the battery(s) in parallel with a capacitor disconnect so the supercap is the only thing holding up that bank the capacitor will discharge completely and when the BMS reconnects it will effectively see a short as massive current flows into the supercap. That will trip the BMS again, and perhaps might well damage it too if the control circuit can't respond fast enough.

In the sequence of events you are describing I'm guessing the BMS just couldn't handle the constant 286 amps you were drawing.

 

[Seawalker]

The supercaps would only overload the BMS if they were directly dropped across a charged battery while they were discharged (either completely or significantly). The capacitor should be precharged to avoid this. Once they charged and set in parallel with their partner battery the voltages on both will remain in step so there won't be a big load on the BMS.

If the BMS on the battery(s) in parallel with a capacitor disconnect so the supercap is the only thing holding up that bank the capacitor will discharge completely and when the BMS reconnects it will effectively see a short as massive current flows into the supercap. That will trip the BMS again, and perhaps might well damage it too if the control circuit can't respond fast enough.

In the sequence of events you are describing I'm guessing the BMS just couldn't handle the constant 286 amps you were drawing.

Yeah, we're on the same page. I've checked the other BMS and they show varying degrees of damage. Less damage move away from the positive end. This seems to confirm that the additive amperage was overwhelming each BMS more severely than the last.

 

[Seawalker]

It now appears that the BMS used in the Shunbin Lifepo4 batteries are unable to handle the amperage that these batteries are capable of delivering. I now need to figure out if I can upgrade these BMS systems with ones that are more robust AND that make sense given my battery bank.

Each 12v battery contains 8 cells in 4s2p connections with the commensurate 5 wire BMS. I would like to remove the existing BMS from each battery and build a new one that addresses all of the cells with one BMS board. Can I do that with one 4-cell BMS wired only to the main terminals on each of the four batteries or must it be wired to all 16 cells?

If either method is viable what are the pros and cons of each?

 

[electric]

This BMS not only can't handle so many Amps, it also can't handle the voltage of 4 series connections, so it blew up quite easily.
If you need a 48V battery for your vessel, you should build a single 16S battery with a single 16S BMS, which controls a heavy duty contactor, because MOSFET BMSs can't handle this much load. This BMS could work https://www.energusps.com/shop/product/tiny-bms-s516-150a-750a-36?category=4

Also, your supercaps are just waste of money, they don't do anything in this application. Could have spent that on larger battery capacity with 1000x better return on investment.