BMS- LVC

[JamieWlcox]

Today I was riding my ebike and wanted to test the mileage I was around 20 miles or so on one charge. Normal use for my 48 volt battery is between 51.4- 57.4 volts

Suddenly all power shut off and I assume the battery had reached its low voltage shut off. I was surprised because I did not receive any Warning by low speeds decreased acceleration.

When I got home I check the voltage it was 22 volts 10 amps from the discharge plug

It indicated 36 volts 15 amps from the charge plug.
I then plug the charger in, and the discharge plug was 48 volts 15 amps.

Can anyone explain to me how the battery can get below 40 volts if the BMS is set for lvc 40 volts and my controller is set for lvc 41.5 volts

Was my battery over discharged.
Or does the BMS switch off power to discharge not allowing good reading

Thank you for your support

 

[eee291]

First of all, how the hell are you measuring amps?

I had similar issues when my battery connections failed because the welds were too weak but still kinda making contact.

If you can open you pack, then I would check all cell connections and voltages, maybe even pry the tab up with a piece of plastic or something.

 

[JamieWlcox]

This is not an inconsistent or sporadic connection problem. The voltage differences were measured from two separate ends of the BMS. The discharge and charge plug measure different volts and amps which is normally not the case when the BMS is functioning normally I presume the BMS stop current flow to the output side. I am using a standard volt amp meter

 

[JamieWlcox]

Not to sound rude but I certainly won't be prying any tabs up to sacrifice the Integrity of a spot weld. Most people including myself don't have the equipment or skill and experience in order to get a good spot weld without overheating the battery

Just an update for anybody else that is reading this post the battery is doing fine I brought it back up to 57.4 volts it was indeed shut off by low voltage either the BMS or the controller did its job correctly

The question is still out there when the BMS stops discharge / lvc does this affect accurate reading due to the interference the BMS places on the battery at the time of lvc shut off. The battery clearly read 22 volts 10 amps which is way too low for this 48 volt 20 amp battery

 

[eee291]

 

[Alan B]

When the BMS turns off the FETs for charge and/or discharge, they will still conduct slightly (microamps). This will allow a sensitive meter to get a reading, but it will not be accurate. Essentially the voltage is divided across the FET and the meter. So it could read quite low, but this is not an accurate reading.

If a single cell group goes low before the others the overall pack voltage may not sag much so you may not have the warnings, the BMS will just shut off the output suddenly to protect the low cell group. Sudden BMS shutdown may be an indication of a weak cell group in the pack.

There should be no current reading. When measuring current properly with a meter it presents a near-short across the probes and must be placed in series with the circuit. Don't measure current across the BMS ports, this could lead to damage or at least trigger the BMS short protection.

Only measure voltage. Be very careful to never short anything. Wear eye protection, and leather gloves are advisable for shock and burn protection. This is serious stuff, a mistake could easily ruin the BMS or even the battery, or cause a fire.

Consider working with your battery vendor to resolve this. They are the experts on your battery and should support you.

 

[JamieWlcox]

Thanks for your help Alan that explains it