Need help understanding / picking fuse for 24v 250w motor batteries

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borentz   1 µW

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Need help understanding / picking fuse for 24v 250w motor batteries

Post by borentz » Sep 28 2020 2:20am

Hi I'm upgrading the 2 batteries wired in series of my electric scooter to 12v 18ah batteries. My scooter has a 24v 250w motor and controller, and it currently has 12v 7 ah batteries with no visible fuse.. I'm confused on what fuse to get for the batteries to protect everything. Do I even need a fuse(s)? How do I pick the right one? I tried searching around online, but I still don't understand.

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Dui, ni shuo de dui   10 kW

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Re: Need help understanding / picking fuse for 24v 250w motor batteries

Post by Dui, ni shuo de dui » Sep 28 2020 2:32am

A fuse isn't absolutely necessary, actually a breaker would be better since you can re activate it instead of it just burning in case of problem like a fuse would do.
Your bike has 24V and it states 250W of power, so 250/24 =10.4Amps. In which case I guess a 16Amps breaker should be enough. You want it to activate in case of short circuit, so much higher than your current rating in normal use.
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AussieRider   100 W

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Re: Need help understanding / picking fuse for 24v 250w motor batteries

Post by AussieRider » Sep 29 2020 7:31am

Hi Borenz, welcome to the forum.
I reckon the best mistakes to learn from are other people's, so here's mine.
When I built my first Ebike, 250 watt 24 volt, I calculated, as Dui did above, that current draw would be about 10.5 amps & started with a 15 amp fuse that blew pretty quickly. Tried 20 amp that blew under heavy load, long steep hill. 25 amp did the trick.
But,
I used glass fuses. Long term, they didn't blow but heated up so the solder in the end caps melted & went high resistance. Fuse looked ok & tested ok on the ohm meter but didn't flow enough current. Had me baffled for hours! I changed to a blade fuse & haven't had a problem.
You're talking about 24v / 18ah, I assume you're using SLA. Put the fuse between the batts, + of one to - of the other.

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kdog   10 kW

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Re: Need help understanding / picking fuse for 24v 250w motor batteries

Post by kdog » Sep 30 2020 6:24am

Ill second aussieriders views. Fuses that are only marginally above the controllers rated output get hot, melt stuff and often don't die. They just make trouble. You want it to waltz in a long hill, or sustained high speed. So think double your Max controller output (15a?) as a starting point. Most countries can put out more than their rated amps for a short period.
A short is going to be hundreds of amps so even a 60amp fuse would be fine. Get the auto style blade ones start at 30a, and change as you see fit...and those glass tube style ones are crap.

John in CR   100 GW

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Re: Need help understanding / picking fuse for 24v 250w motor batteries

Post by John in CR » Oct 02 2020 7:51pm

kdog wrote:
Sep 30 2020 6:24am
Ill second aussieriders views. Fuses that are only marginally above the controllers rated output get hot, melt stuff and often don't die. They just make trouble. You want it to waltz in a long hill, or sustained high speed. So think double your Max controller output (15a?) as a starting point. Most countries can put out more than their rated amps for a short period.
A short is going to be hundreds of amps so even a 60amp fuse would be fine. Get the auto style blade ones start at 30a, and change as you see fit...and those glass tube style ones are crap.
+1 I'd say at least a 50-60A fuse or circuit breaker. It's there for a worst case event...a short in the power mains, and almost any battery will deliver crazy high current in those events.

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