Should I fuse my battery?

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Turbotd04   100 mW

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Should I fuse my battery?

Post by Turbotd04 » Nov 30 2019 6:11pm

Just built a 48v battery and just wondering if I should put a fuse in line just after the battery and if so what fuse? Thanks
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Balmorhea   100 W

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Re: Should I fuse my battery?

Post by Balmorhea » Nov 30 2019 8:14pm

In the past, I've used maxi blade fuses for values up to 80A. They are cheap, and so are the holders. For what it's worth, I don't use a fuse protected pack at the moment, even though I built my own pack.

Determine your pack's maximum current rating, and use the next higher value fuse for short circuit protection. And carry a spare.

john61ct   10 MW

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Re: Should I fuse my battery?

Post by john61ct » Nov 30 2019 8:22pm

put close to the pack

sized to protect the wiring from there going out

AHicks   100 W

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Re: Should I fuse my battery?

Post by AHicks » Nov 30 2019 8:23pm

I think a fuse or possibly a circuit breaker, would be part of a good plan. Make it easy to access and no bigger than necessary - based on the size of the wire in use.

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Rassy   10 MW

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Re: Should I fuse my battery?

Post by Rassy » Nov 30 2019 9:27pm

I always use a 48V DC circuit breaker at the battery. It also serves as a handy on/off switch and handles the external spark when you hook the battery up to the controller which happens as the capacitor draws a burst of current. And thus there is no need for any anti spark circuit to keep from damaging your connectors.
Last edited by Rassy on Nov 30 2019 11:41pm, edited 1 time in total.
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fechter   100 GW

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Re: Should I fuse my battery?

Post by fechter » Nov 30 2019 11:21pm

Does the pack have a BMS? Most BMS boards offer some protection against short circuits.

Automotive fuses made for 12v may work or they may start arcing and catch everything on fire. A "proper" 48v fuse is stupidly expensive and quite large. I like circuit breakers as you can get them designed for DC and pretty decent current ratings. I've seen people use "fuseable links" which is just a short piece of smaller gauge wire that will burn before the other wiring during a short. The fuseable link is placed somewhere that won't be a problem if the wire burns.
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Turbotd04   100 mW

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Re: Should I fuse my battery?

Post by Turbotd04 » Dec 01 2019 12:38am

So if im running 48v with 750 watt motor thats 15.68a, I would be better off getting a 20a circuit breaker over a 15a wouldn't I?

AHicks   100 W

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Re: Should I fuse my battery?

Post by AHicks » Dec 01 2019 9:01am

Turbotd04 wrote:
Dec 01 2019 12:38am
So if im running 48v with 750 watt motor thats 15.68a, I would be better off getting a 20a circuit breaker over a 15a wouldn't I?
If you have a capable controller, you could be stuffing a lot more than 15.68a into a 750w motor. For instance, if you have a 35a controller, at wide open throttle the limiting factor will be either the battery's ability to deliver amperage, or the controller's ability. In either case, a LOT of power.

Personally, my plan is to restrict available power with the controller settings, then it's easy to pick a safe fuse size. Problem is not all controllers will have that ability.

My point is, to prevent having to constantly change that fuse, you don't want it to be the bottleneck between the motor and the battery.

Balmorhea   100 W

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Re: Should I fuse my battery?

Post by Balmorhea » Dec 01 2019 3:06pm

Turbotd04 wrote:
Dec 01 2019 12:38am
So if im running 48v with 750 watt motor thats 15.68a, I would be better off getting a 20a circuit breaker over a 15a wouldn't I?
The motor has a 750W rating, but in fact it will burn up as much wattage as the controller can stuff into it. The controller has a maximum current rating, which may or may not be printed on it. You want the fuse to be higher than that value.

Since the battery pack has a current rating, and since it's the thing being fused, it makes sense to use a fuse just higher than the pack's rating. That way it will open the circuit in a serious fault condition, but not under ordinary operating conditions.

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MAXIMUM_AMPS   10 mW

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Re: Should I fuse my battery?

Post by MAXIMUM_AMPS » Dec 02 2019 10:33am

I would use a fuse, BMS or not. Under a dead short circuit, it is possible that the BMS's FETs can go into an avalanche condition, where they are stuck ON and the BMS chip's FET driver cannot turn them off.

https://www.infineon.com/dgdl/Infineon- ... _00-EN.pdf

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motomech   1 GW

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Re: Should I fuse my battery?

Post by motomech » Dec 02 2019 10:27pm

The fuseable link is placed somewhere that won't be a problem if the wire burns.
6 M/M bullet connectors make pretty good fuseable links w/ a 48 Volt system.
Don't ask me how I know that.
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goatman   10 W

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Re: Should I fuse my battery?

Post by goatman » Dec 06 2019 9:44pm

motomech wrote:
Dec 02 2019 10:27pm
The fuseable link is placed somewhere that won't be a problem if the wire burns.
6 M/M bullet connectors make pretty good fuseable links w/ a 48 Volt system.
Don't ask me how I know that.
I vapourised a xt60 once at 52v/14s and was left with a cooked bms, white flash burn on my hands under all the black soot and a bunch of black soot on the wall. a fuse is cheaper than a bms.

I don't use a bms or a fuse on my packs, theyre wired for a bms so ill use a bms to balance if needed . I consider the xt60 the fuse. each of the packs can do 100 amp continuous though.

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