Battery fuse keeps blowing

I have a 48v 9ah battery that is about 2 years old and taken care of.

I have a BBS02 on it which has a 25A contoller.

The fuse inside the battery got tripped twice in the past 2 weeks. My multimeter showed no conitinuity both times so I replaced the fuse with new ones.

The fuse being a 20A 250V.

It seems odd to me that the battery is using a 20A fuse though when the controller puts out up to 25A. Could that be the problem?

Also, I have other fuses that are F5AL 250V. What does the F5AL mean?

I wasn't riding the bike either times the fuse blew... my friend was. He said it blew going downhill after climbing a hill. He was using PAS 3 out of 6 which should be 16% max voltage I believe, which at 1200 peak would only be 192 so I don't know why the fuse would blow then.

I did find out that he was using gear 5 out of 7 which was too high a gear when climbing which I later lectured him about, but at level 3 PAS I don't think that was the cause.. also since he said the power went out when going downhill.

Any ideas? Thanks!

Yeah, don’t let that friend ride your bike

F5AL is a fast blow fuse for electronic circuits. A slow blow fuse would tolerate brief periods of excess amps without popping.

If your battery is rated for 30A discharge or if your controller will never draw more than max rated amps, using a 30A fuse shouldn't be a problem. Typically these fuses are meant to protect the pack from external shorts and reversed polarity and a 30A fuse would pop in these cases.

Wow just blew a third fuse!

I ordered 25a... Will have to try those...

Turns out that was a F5A I had used so no wonder it blew so quickly. I'll have to wait for the 25A ones to arrive...

What's strange I'd that my original was only 10A which lasted a long time and my original 20A replacement lasted about a year too...

I would use a 40A fuse. The controller has current limiting, which is the primary safety. The BMS will trip if the controller draws too much. The fuse is for when the BMS fails and something shorts.

Go to the fuse manufacturers website, there should be a time-current chart there, showing you how long in x seconds it can handle y current. It may be contained in a datasheet.

Here is an example, using the fuse I am buying. > http://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/bolt-down-fuses/littelfuse_mega70v_datasheet.pdf

The yellow line is a 100A fuse, you can see that if you pull 200A, it will blow after 10 seconds. If you pull 300A, it blows after 1s, and so on.

Is that how fuses work then? I have slow and fast fuses and I was under the impression that the speed referred to how fast the fuse would blow... as in, it would blow either way, just either slowly or quickly.

But from what you say, it sounds like the speed actually is another qualifier for if the fuse will blow at all?