However, there is a general method that works for most ebike batteries, and it involves the following steps:
Recently (as in a few months ago) I had a problem with my e-Bike where my battery wouldn’t hold a charge, and wouldn’t accept a charge from the charger.
Normally, when I plug my eBike’s battery in, the charger’s cooling fan whirls to life and one of the lights changes colour, indicating it’s charging. This stopped happening!
I did a suite of tests, tried a few techniques recommended on forums and by the lead tech at Luna Cycle, and eventually confirmed my BMS — Battery Management System — was broken and needed to be replaced.
So in this guide I want to outline
My electric bike is semi-custom. It is built like a custom, but by Luna Cycle in the USA. It has a Bafang (, “Eight Sides Electrical Appliances”) motor with a 50A controller that Luna Cycle calls their “Ludicrous” controller, though I have it set to operate in 25A mode.
Because 2020 was a crazy year, it ended up being stored fully charged, and allowed to fully deplete, while in storage. This isn’t good for an eBike battery, it turned out, and so a few things needed to be done to get it back on the road.
The BMS (again, Battery Management System) of a battery pack is the part that regulates both input (charging) and output (discharge) voltage and current from the cells.
Even though this might appear to be a guide for one kind of electric bike battery,it’s generally applicable to all e-Bike batteries, and even batteries on other devices like scooters and mobility devices.
When the charging system of an electric bike fails, it means one of a few things:
The first (and easiest) thing to test is the charger.You measure the output voltage. For a 52V battery like mine, it should be supplying about 58V. For a 48V battery it should supply around 54V.
After you measure the output voltage, you do what’s called the “light bulb test” — where you use an incandescent bulb hooked up to the outlet. This is easier in America (or Japan I guess) where the voltage supply is 110V, but it still works with 220V bulbs.
Second, test for mechanical failures.Probe around with a multimeter and make sure you read operating voltage in the places where you should.
Also, open up your e-bike battery and check all the wires are intact, and that none of the solder joints have broken. Bikes get beaten up and it’s possible — likely— that a joint will fail at some point, especially if your battery has gone flying across the road because you forget to lock it (guilty! Actually I lost the key for a while…)
Finally, you have totest the internals of the battery.
One trick for testing voltage across the pins of the BMS is that they’re often coated with silicon. You should scrape it away gently before checking the voltage.
A BMS is a delicate issue. There are actually people who believe in charging batteries without a BMS, like this guy on :
He makes a lot of good points. Essentially, if you have a charger that can balance all the cells, you don’t need to use a BMS for that purpose.
Firstlywhat does a BMS do?
The main role of the BMS in an e-bike’s battery is to make sure all the cells of the battery are in balance during charging and discharging.
“Cells” of a battery pack are actually cell blocks, made up of multiple 18650 cells in parallel (to give additional current capacity). Each cell is thus the voltage of one battery, which is around 3.6V.
If a cell starts becoming over-depleted for some reason (an internal fault), the cell can get damaged further as current flows from another battery cell into that cell.
Many BMS also do temperature monitoring. Batteries deliver a lot of current while maintaining a constant voltage. There’s a risk they can catch fire if they get too hot or if they get physically damaged. Temperature monitoring functions shut off batteries when they get too hot.
A battery BMS generally fails because
A common way in which a BMS can fail is one of its over-current protections trips.
Most BMS have either a solid-state circuit breaker and/or a fuse. If too much current is drawn it’ll trip.
The most common way in which a BMS over-current protector can trip is when they’recharged from being completely flat. This can produce enough current to trip the breaker.
Another way is if acell inside the battery fails.This can happen, but it’s not common unless a) the cells are poor quality, or b) you’ve just had the battery for ages/many charge cycles.
If you’re lucky, you can “reset” the circuit breaker on your BMS. See the next section. But if a component or a fuse has open-circuited permanently, you’re S-O-L.
As mentioned in the last section, if your battery has failed because of over-current protection, you might be able to rescue the BMS.
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