Could you tell us from what faults you are trying to protect which equipment? Are you trying to protect your esc and or bms?
Do you know how much time your esc can handle 240A?
I don’t see a problem with 7.5A fuse on charger port. Negative wire? Sure. I can only share my theory on why the negative wire is a popular choice. Perhaps @b264 can shed some light on a technical reason for fusing the negative wire on the charge port.
To reiterate I think there is a decision about what our main protection strategy is:
Fuse nothing accept charge port and cross our fingers. Assure the ultimate chances of maintaining brakes in emergency situations. Esc bms and wiring are our fuse now and very possibly will catch on fire if there is an accidental short, reverse polarity, fault etc.
Fuse based on protecting for only short circuit knowing the fuse should not blow even during demanding overloads. The bms and esc will not have any additional protection and very well could be destroyed and burn down during a fault. There is protection here should a fault turn into a short circuit such that the batteries should not deliver insane amounts of power into a fault which could lead to the batteries themselves burning down. There is still very much assurance that emergency braking functions will work and not be hindered by your fuse.
Fuse is sized to protect equipment such as esc. If the esc, bms, Regen is overloaded, the fuse would open hopefully protecting this equipment. There is the possibility that during demanding situations the fuse would open and you would lose brakes and or acceleration.
Im mainly trying to protect my esc since I reversed the battery polarity and fried a focbox last week. But that was due to applying the connector to the battery in the wrong way. Now I just want to make sure the esc is protected from getting fried. I have 10s2p battery with a bestech charge and discharge bms and im not going to open up the battery any time soon. I think I just need to place a fuse between the battery and the esc. If the chance of loosing power is too high I’d rather avoid using a fuse however. I fried my esc due to my own error afterall so im not sure the probability to fry components while riding is without a fuse is high enough to outweigh a potential loss of power while going downhill. (my first build hasn’t had any shorts or breakdowns in almost a year untill i shorted the esc by my own mistake. I ordered a esk8.de vesc, they mention “a few seconds of max 240A” on their website.
Thanks for your help so far! very interesting topic aswell!
Yes, either side will work but I think it’s slightly better to fuse the negative side for a few reasons.
First, it’s easier. BMS already all work with a common positive and the negative sides are all switched. Making “all” positives just connect together, including the battery positive, is easy and leads to less wiring errors.
Second, if you fuse the negative wire, it gives you access to BAT+ for troubleshooting. If you also use a loopkey with a bypassed BMS, that gives you access to BAT- for troubleshooting.
So, for example, if I need to check battery voltage or if I need to test an ESC using my board as a power source, it’s easy:
Based on a 30q internal impedance of 26 milliohms we can determine the available short circuit current of 3.6/0.026=138A. This is confirmed by the Samsung factory short circuit test of around 135A with a 10 milliohm load.
Series connected groups do not increase short circuit current. The connection resistance of each series connection would actually decrease the SCC but we will assume 0 ohms.
Parallel groups would multiply the SCC for each P group.
The wiring from the battery we will consider although in this case it will be somewhat negligible. Let’s say 10 milliohm for your 12 awg wiring.
Putting it another way to calculate SCC of the pack is:
36v nominal / (10S*0.026ohm/2P+0.01ohm) =
257A available Short Circuit Current
The fuse I linked above is rated to interrupt 1000A at 58V
That said, you originally suggested an 80A fuse. The Littlefuse BF1 series 80A fuse would be a good choice. The 100A would be fine as well. If you’re concerned with more sensitive protection for your esc, I would pick the 80. If you are concerned with losing control of your board during demanding loading but still want some protection to your esc I would pick the 100A.
I will add this BF1 fuse with a 58V 1000A interrupting rating would be good for 30q packs up to 12S7P. Although I would add in some safety factor as a rule of thumb and say each 30q P group has 150A SCC. This would limit it to a 12S6P.
I noticed Littlefuse has a MEGA 70V fuse with a interrupting rating of 2500A. This would be good for a 16S16P 30q. Very nice how much
I would not recommend it unless it has a DC voltage rating.
That said it is better than no fuse and will likely work for you. The 3,15A is good for a 2A charger.
You could consider getting a fuse greater than 4A should you ever wish to use a faster charger. A 5A fuse would still be fine for a 2A charger in my opinion.
Figured I’d ask this here, as @b264 seems to be the guru for anything battery related on here haha.
So I’m likely to be getting a 2A charger, so what rating of fuse would be best to put on the negative line? I’m guessing either 5A or 7.5A? Only reason I’m considering 7.5A is in case I end up upgrading to a 5A charger in future, but if that means its not going to do me any good with the 2A charger I’m starting with then I’ll just change fuses when the time comes.
Actually will probably use a circuit breaker instead of a fuse, something like this
Also, I did consider putting a high-amperage fuse in between the vesc and the bms as another line of protection, but it seems people are saying it’s not worth it as if it goes wrong or trips unexpectedly you can lose braking (which could be worse than damaging the board). This sound about right? I’d been considering something like a 100A fuse but no point if it’s not doing anything helpful.
What do you solder to it. I have bullet connectors on my plug and the females coming off the battery. I feel like I could just put bullet connectors on each end and and on the other ends leave the wire bear to solder to the fuse then heat shrink everything.
