I am building an ebike from scratch, I have a 1 kwh 16s8p battery from used A123 cells, 52 volts. I don't understand if I need to fuse both the positive and negative supply wires with a double pole breaker or if I can just fuse the positive one with a single pole one? If I am drawing over the desired amps, why do I need to cut both high and low sides from battery?
Double pole or single pole breaker
- Thread starter gsorter
- Start date
AGS
100 W
- Joined
- Feb 13, 2023
- Messages
- 295
It depends upon how much room you have and some would say don’t even bother with one. Remember that the job of a breaker is just to protect the wiring to prevent it from catching fire.
I fitted a double pole breaker mounted on a din rail and it takes up a lot of space.
This one is a 125 amp dc breaker connected to a Sabvoton 72100.
I fitted a double pole breaker mounted on a din rail and it takes up a lot of space.
This one is a 125 amp dc breaker connected to a Sabvoton 72100.
Last edited:
If you cut the circuit right at the output of the battery casing, then there's no path for current to flow. Doesn't matter if it's B+ or B-.
If you cut the circuit at some other point in the wiring, then whatever wiring is between that point and the battery casing can still have a fault and potentially short to the other battery wiring, such as in a crash, etc. For this event, it doesn't matter if you cut both wires because that length of wiring is still vulnerable on both of them.
So cutting both B- and B+ doesn't buy you anything regardless of the point at which you do it.
You should still also have a fuse (preferably inside the battery, right at the cell block output, either B+ or B-; I use the B+), so that if anything happens that shorts wiring even at the battery case exit (crash, etc) it will blow and still protect the cells from a short that could cause a fire. (don't count on the BMS doing that in an emergency event--complicated devices can fail at the worst times, simple ones like fuses rarely do if correctly sized, placed, and installed).
The fuse should be sized for a higher current than the system will ever normally see; a direct short across the battery terminals would cause such a high current that it would blow rapidly anyway, probably before wiring/etc even noticed it was heating up.
But look at the datasheet for the fuse you want to use to make sure of that; they all have a time vs current curve to say how long it takes at a certain current before it will blow.
If you cut the circuit at some other point in the wiring, then whatever wiring is between that point and the battery casing can still have a fault and potentially short to the other battery wiring, such as in a crash, etc. For this event, it doesn't matter if you cut both wires because that length of wiring is still vulnerable on both of them.
So cutting both B- and B+ doesn't buy you anything regardless of the point at which you do it.
You should still also have a fuse (preferably inside the battery, right at the cell block output, either B+ or B-; I use the B+), so that if anything happens that shorts wiring even at the battery case exit (crash, etc) it will blow and still protect the cells from a short that could cause a fire. (don't count on the BMS doing that in an emergency event--complicated devices can fail at the worst times, simple ones like fuses rarely do if correctly sized, placed, and installed).
The fuse should be sized for a higher current than the system will ever normally see; a direct short across the battery terminals would cause such a high current that it would blow rapidly anyway, probably before wiring/etc even noticed it was heating up.
But look at the datasheet for the fuse you want to use to make sure of that; they all have a time vs current curve to say how long it takes at a certain current before it will blow.Thanks for your well researched answer. I understand some recommend a 2 pole breaker because you really aren't grounding nor should you to the chassis in this type of floating setup, and need to fuse both B+ and B- sides. I also understand grounding to chassis at high dc voltages like 50+ could be dangerous to the rider. I am not an electrical engineer, or grounding expert, and just assumed cutting one side would be sufficient
