Voltage Drop 67 v down to 19 volts

kovert · Nov 30, 2023

I have a electric bike that i hadn't used in a while . Its a 60v battery when i came to use it it did not work.
Plugged it into charge as i thought it was a flat battery and worked Ie wheel turns round. When I unplug it doesn't turn around I tested the battery and it shows its charged 72 v When I turn on the bike the voltage drops to 13 - 19 volts Turn the bike off again and it shoots back up . I have a spare battery and when that is connected bike works fine .

The bike is a old street legal moped 250 watts 48v That i was overvolting to make go faster .

If anyone has an idea for me id be very grateful

amberwolf · Nov 30, 2023

That means the battery's BMS has detected some problem**** that when a load is placed on it it shuts off the output to protect the cells from damage. The very low voltage is a "ghost" voltage that is just leakage thru teh BMS FETs when they're turned off.

**** could be any of the following, or others:
-- a problem in the controller that causes it to draw too much current (blown FETs, LVPS, etc), not likely because the bike works with a different battery.
-- a problem in the battery, such as:
--- unbalanced cells so one or more groups is already so low it drops to LVC
--- aged cells so that when any load is placed on them they drop to LVC
--- physical damage or humidity/water exposure that corroded something, causing the same effect
etc.

If it's the battery, and it's just unbalanced, you can leave it on the charger for at least overnight, possibly several days or even much longer, and it may rebalance the cells to the point the pack can be used, but a pack that has reached this state will never be like it was when new (balance just means the cells are all the same voltage at some point in the discharge or charge curve, usually at full, and doesn't mean they are all matched (equal in capacity and capability)--the latter only occurs when all the cells are brand new (and only if the manufacturer did this process, whcih they usually don't), and they all only grow more different with time and usage.

If you like you can open the battery up and test the cell (group) voltages, starting with teh ones at the most negative end (at the fat black wire from the block of cells), and post those voltages here. If it's a 60v battery, then assumign it's not LiFePO4 that's usually 18s, or 18 cells (groups) in series, so there should be 18 separate voltages when you're done. If it's balanced, they'll all be exactly identical, at full charge that's 4.2v each, or 75.6v total pack voltage. .

Chalo · Nov 30, 2023

Let's back up a couple steps. What kind of battery are you talking about? Lead acid, lithium ion, or something else? 72V isn't a fully charged voltage for any common pack.

Can you furnish photos that would help us understand what you're working with?

If it's a normal-ish lithium ion e-bike battery, everything Amberwolf said applies. If it's something different, maybe not.

kovert · Dec 3, 2023

amberwolf said:
That means the battery's BMS has detected some problem**** that when a load is placed on it it shuts off the output to protect the cells from damage. The very low voltage is a "ghost" voltage that is just leakage thru teh BMS FETs when they're turned off.

I Think this is the issue . When I try to plug the battery in the charger stays green so no power is going in to the battery. Ill add some photos . its a 60v battery ... I may think water ,may have got into the battery area .