No BMS - Do you use a fuse?

If you aren't using a BMS do you use a fuse or circuit breaker? If so how do you size it?

There's no reason you can't use a fuse with a BMS. For the fuse rating, 1.5x your maximum expected load should be about right.

Any cheap DC 50-100V ~50A fuse exists?

https://www.youtube.com/watch?v=sqcaRamjr4M

You can use an automotive fuse. if one 50 amp is hard to find, you can use two wires, and a 30 amps on each positive wire.

My naked packs I don't fuse,, this is because they are pouch type RC cells, and the tabs vaporize any time there is a short that bad. So the battery cell is sort of self fusing.

But a pack with round cells and spot welds, won't do that. I have a bms as a fuse on all my round cell packs.

Teklektik used a breaker. He tripped it a few times and it has always worked perfectly. Its the 2WD Yuba Mundo (with Headway cells)

dogman dan said:

You can use an automotive fuse. if one 50 amp is hard to find, you can use two wires, and a 30 amps on each positive wire.

Click to expand...

Cannot find any over 32V DC

I used to use these kind below. I believe ice cube used them also. He might still sell them. They have ones that go up more also. Note: Ice cube mentioned something about the higher up you go, the fuse can be more explosive or loud pop. Look at his videos. I'm using breaker now. Much easier.

Install Bay MANL50 - 50 Amp Mini ANL Fuses (2 Pack)
https://www.amazon.com/dp/B004WK4TDS/ref=cm_sw_r_sms_api_O.q5xb0QFZ4DP

Have a look at this thread: https://endless-sphere.com/forums/viewtopic.php?f=14&t=83091

If so how do you size it?

Click to expand...

One possible choice is Littelfuse 0326015.MXP 15A, 125VDC, slow blow, ceramic 6mmx32mm. Sizing(15A): 15A=4h continuous, 20A=1h continuous, 30A=5 to 60 seconds. In reality, those 15A fuses never false-trip at 15A peak and can be used all day long, so sizing is really simple - just pick your peak current and that's it.


If 15A value is too small for your application, there are 20A, 25A and 30A options in the same d=6mm, L=32mm format. You typically want your DC fuse to be as long as possible and sand-filled, in order to prevent unwanted arcing.

Pdf is here: http://www.littelfuse.com/data/en/Data_Sheets/325P_326P.pdf

On my scooter with 20S 50ah pack I use: BMS, fuse and magneto-termic switch.
When I think on how much power I am sitting on, never enough protection.
The nice thing about this fuse is that one can use just as a copper terminal between 2 cells and so it can be easily serviceable:

DVDRW said:

dogman dan said:

You can use an automotive fuse. if one 50 amp is hard to find, you can use two wires, and a 30 amps on each positive wire.

Click to expand...

Cannot find any over 32V DC

Click to expand...

Littelfuse has 58 VDC blade fuses with a 1A thru 40A rating. They also sell different blade fuse holders, for example stackable inline fuse holders with crimp connectors, to allow for easy replacement of fuses.

These fuses and holders are widely available through various sellers like Mouser, Digikey, etc.. Also they are not really expensive with under a dollar if you buy 10 fuse holders and a few dollars for a fuse. (Which you actually don't need to replace ever, if things go right.)

http://www.littelfuse.com/~/media/automotive/datasheets/fuses/automotive-fuses/littelfuse_tac_ato_58v_blade_fuses.pdf
http://www.littelfuse.com/~/media/commercial-vehicle/datasheets/automotive-fuse-holders/littelfuse-fuseholder-fkh-datasheet.pdf

Lots of good suggestions here. However, most of them involve using fuses at a voltage that they weren't really designed for, which I don't really have a problem with but since it's kind of critical that whatever you use actually works when it is needed is there any way to mathematically determine what is going to happen?

My Guess would be to just go by wattage - FE if a 12 volt auto fuse is rated at 30 amps (360 watts) my GUESS would be that it would blow at about 7.5 amps from a 48 volt system (also 360 watts) but from looking at the size of conductors (those skinny looking motor phase wires for example) used compared to fuse conductors, maybe that isn't right at all. If it is then 4 30 amp automotive fuses in parallel would be about right for 30 amps at 48 volts - right? That might not be the most elegant way to do the job, but I could whip that up right now to get safe(er) while I suss out the best way to go. But if I'm wrong that would be worse than nothing, because it would make me THINK that it's under control.

It sure would be nice to get close without a lot of trial and error.

Also, it seems like controllers should have overcurrent protection of some kind built in - do they.

Fuses are only rated by current. Only current heats and so melts the fusible element. Voltage doesn't matter except when it becomes great enough to jump across the gap after the fuse has opened.

Thanks - after a little more digging I see that you are absolutely correct. Any fuse will blow at its proper current rating (unless something malfunctions) regardless of the voltage - but a low voltage fuse used in a high voltage application may allow a long lasting arc that could result in damage or fire, or it could blow in a very violent manner that is less than ideal. However, a high voltage rating fuse will work fine in a low voltage application - it just might be kind of bulky.

I whipped up a regular automotive blade fuse holder to work with my bike, and tried it out this past weekend with a 35 amp 12v fuse which worked fine with my 48v 20a - 13s 4p panasonic 3400 mah battery pack. I'm going to try it with smaller fuses until I find out where it actually blows under normal use then replace the 12v fuses with littelfuse 58v automotive blade fuses - should be perfect. About $5.