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Battery Specification Questions

chadmichael

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Joined
Feb 26, 2022
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I have a Big Battery 170 Ah 12v OWL, spec sheet here.

Here are salient points from my battery specs and system build...

1) Specs say this has a built in 300 A safety fuse. As I've seen on some tear down videos, this fuse is behind the negative terminal. I understand this to be a sort of last resort safety protection against a very high draw (short circuit? other?)

2) Specs for the BMS say Max Charge and Continuous Discharge are both 175 Amps.

3) Specs say Max Peak Discharge 350A (over 6 seconds)

4) Following Will P's 400 Watt mobile build recipe, we have a 200A fuse bolted on to the battery's positive terminal (sized according to my inverter )


QUESTIONS

1) Why do I need the 200 A bolt on fuse since the battery's own built in fuse is there? What scenario does this fuse protect against?

2) How does the battery every discharge at 350 A for 6 seconds when both the bolt on fuse and the internal fuse (300A) would seem to positioned to cut off the current at a much lower level?

3) What scenario is the internal 300 A fuse built to protect against? And why is it on the negative when we put the external fuse on the positive terminal?
 
1) The fuse used in your application protects the cable and whatever is connected to that cable from overcurrent if a fault occurs.
Only when you know the maximum current rating of the cable and the expected current draw can you select the fuse needed. The 200 amp fuse used in the demonstration may not be correct for your application.
2) Some applications may need a short term current draw, the external fuse would be selected to be suitable. The internal fuse will, assuming it has been selected as a suitible type, take a higher current for several seconds before failing, there are 'slow' and 'quick' blow types available.
3) Failure of the over current protection in the BMS where high current is flowing from the battery positive to negative. The construction of the battery and position of the BMS may have Influenced this position.

Mike
 
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BMS is usually the first line of defense on overload current. It is electronic switch so it can fail and should have a secondary backup like a fuse or mechanical circuit breaker in system. Normal fuse just blows based on melting metal, so reliability is fairly high. Fuse design specification must be selected based on system voltage, so it doesn't continue to arc over the blown metal gap.

All fuses or electronic overload switches have a time duration factor. They will open the circuit after a designed amount of overload time. There are fast blow and slow blow fuses that allow a longer amount of overload time before blowing.

One thing that you have to factor in is surge current situations. One that can be a particular problem is charging large capacitors on the battery DC input of inverter. Some BMS's will trip their over-current protection when inverter DC input is first applied. You definitely don't want your $20 fuse to blow for this situation. On a high wattage inverter this can be a couple thousand amps but only lasts less than a millisecond in time. You do not want to subject breaker contacts to this surge current as it will degrade contact surfaces of breaker giving breaker higher series resistance.
 
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