Bypassing the BMS shunt

[SniperGaulois]

It sais on another forum that putting a tiny strand of copper across the shunt bumps the current limit to 30-50+ ampers.

it seems it is basically a resistor so it must be possible to fine tune the shunt to the perfect level to keep the current high while protecting the batteries. are they in parallel? In a spiral? Is it possible to just link 2 or 3 of them? can you put the wires in a diagonal or in a cross shaped at either end or side-by-side-or maybe lengthways, what's the difference?

 

[TylerDurden]

I would advise against modding a BMS shunt.

Controller shunts are regularly filled with a bit of solder, starting at one end, to reduce resistance and raise current limits. Fine tuning is achieved by partially grinding back the solder to get a slightly lower output.

Bridging the shunts with wire is not very effective... the connection needs to run along the shunts with higher conductivity than the shunts themselves.

 

[fechter]

It's not a spiral, the are in parallel. To change the resistance, push two of the shunt wires together and fill in the space between them with some solder. If you want to really lower it, put a piece of copper wire parallel to the shunt wires.

 

[PJD]

You can end the cut-and-try guesswork by measuring the resistance of the shunt by running known amount of current across it and measuring the millivolts drop across it. Then add another parallel shunt resistor of the right length so the resistance is reduced by the percentage you want to increase the current limit.

Since the manganin resistance wire used for these shunts is hard to find, you can use steel coat hanger wire which is less resistive, than the shunt material, but resistive enough for the length to be reasonable. (see the picture below) 12 AWG (about 1.9mm) coat hanger wire (the thinner kind used for the cardboard tube-type hanger) has a resistance of i.25 milliohm per inch or 0.0495 milliohm per mm.

I used this method to raise the current limit on my e-max controllers. My target was to raise the current limit from 70 amps to 95 amps. I hit the 96 amps on one controller and 93 amps on the other.