How is overcharging possible?

[Benjahmin]

My new Insat battery comes with a generic instruction sheet that warns of the detrimental effect of over charging. It says that the charger should be turned off as soon as the led goes green.
I thought that the led went green when the battery voltage equated with the charger voltage, if so how would an overcharge occur?
Also I thought this was when cell balancing happened, or is that the latter part of normal charging?
I have, on occasion, left my Ezee battery on overnight, say after a night ride and I need it next morning. Is this not a good idea? I'm aware that there is some fire risk but the Ezee stuff seems good quality and has never had a glitch in 3500 miles.

 

[flecc]

You won't harm your Ezee battery or charger by charging overnight, the battery's BMS turns off the charge when the battery reaches the designed full charge level.

That will be true of any e-bike battery having a BMS, but I don't know the Insat batteries or whether they have them. I'd be very surprised if they didn't, so perhaps they are just being cautious in issuing the warning to avoid possible liabilities.

Lithium batteries can develop defects that can result in an internal fire, not necessarily due to charging at the time.
.

 

[Benjahmin]

Thanks Flecc. Jimmy says he uses a 25A constant discharge BMS, (Theres a picture of the battery with it's BMS on my Guest bike thread). He's supplied a 2A charger so no great stress there.

 

[Deleted member 4366]

I don't know the characteristics of your paticular cells, but most common ebike batteries have 18650 cells. For common 18650s, there can be minimal degredadation when they're above 4.2v, according to some life cycle tests I've seen.

Chargers normally charge to 42v (4.2v per cell), and they hold the voltage there. The BMS has bleed resistors on each cell group that bleed the voltage down to about 4.17v per cell, where there is less chance of degredation. So, all the time you leave your charger connected after the light has gone green, the longer your cells are close to or in the danger zone. As soon as you disconnect it, the cells are bled down to a safer zone.

Not all chargers behave the same. Some switch off all power to the jack when the light goes green.

Any degredation would be minimal, so nothing to panic about, but it can have a significant affect on the life of the battery if you leave it on overnight every time.

Cell technology is changing all the time, so anything you read on the internet probably doesn't apply anymore, including this post. All the battery makers are continuously inventing secret ingredient X, which changes the characteristics of the cell.

Whatever you do, don't start disconnecting your charge before you get the green light. You need to charge to 42v for the balncing to work. Again, it's northing to panic about. You can do it occasionally without doing any harm at all, but you mustn't do it all the time.

 

[flecc]

According to their sales details below, Insat are unusual these days in using pouch cells:

"Our batteries use pouch cells which can pack more energy density and weigh a lot less than cylindrical cells."

Here's Benjahmin's battery photo:

 

[Benjahmin]

That's the one. They are Boston Power Swing Cells. Here's a link that Jimmy sent:
http://www.boston-power.com/products
There's info on a pdf there that is beyond me.

The BMS has bleed resistors on each cell group that bleed the voltage down to about 4.17v per cell,
So that's why my Ezee battery measured 41.4v on Sunday, after a charge on Saturday. So that would be 0.03v/cell lower than when new. That battery is nearly 2 years old.

 

[Deleted member 4366]

The 4.17v is only an estimate. Different BMSs are set to different voltages. Also, your cells might be a bit out of balance.