Where to buy a current shunt

As part of some remetering, I saw the Kisan ChargeMaster or whatever it is called and was intrigued. It's basically just a voltmeter that you can hook across your battery terminals to monitor battery voltage, but also allows monitoring of charging current through use of a thick 0.1 ohm shunt on the negative lead.

Not wanting to spend 150 bucks on a voltmeter and shunt, anyone know where I can obtain such a shunt? I check out mouser.com quickly and didn't find it on a quick search, but it could be there. If anyone knows where these shunts can be obtained, I'd be greatly appreciative.

Why not just use a .1-ohm, 10 watt resistor?

MCM electronics should have it, if not then Newark will.
www.mcminone.com/default.asp

Better yet, here's 50 watts.
MCM 50 watt, .1 ohm

Very snazzy lookin' too

Post edited by: loudhvx, at: 2007/07/16 13:43

Wait, won't I blow that up?
P = v^2/r .... 14^2/.1 = 196 W

Clearly by the negative battery cable the potiential is much less that 14, but how can I be sure what wattage is sufficient.

Say just after stating the batt is pull 10 amps to charge for a few minutes.

10amps by 14 volts is 140 W... will the 50W resistor be enough?

The voltage on the resistor represents 1/10 the current. In other words, if the voltmeter measures 0.3 volts on the shunt, then there is 3 amps through the shunt. (Positive means it's charging, negative means it's discharging.)

The shunt must not be in the line that gets starter current. If the shunt is on either of the battery cables, it will blow up during startup. It needs to be in the line that re-charges the battery, but not in the line that feeds the bike or the starter.

Normally, the shunt voltage will be a 1/2 volt or so. But there is the possibility of much more going through it.

Let's say the battery is getting 5 amps, (or is giving 5 amps). 5 amps times .1 ohm is .5v. The heat dissipation in the resistor will be 2.5 watts. 10 watts may be too small for all situations, however. 50 watts should be fine. It will need some air to cool, too.

The 50 watt resistor is theoretically good up to 22 amps (as long as it's getting a lot of air flow and is heat-sink mounted).

The 10 watt is only good up to 10 amps.

One problem arises if you lose the charging system and have to run off the battery alone. Let's say the bike needs 10 or 15 amps. You will lose 1 or 1.5 volts on the shunt, and this is at a time when you need every volt you can get. A better shunt would be .01 ohm, but it gets more difficult to get any accuracy. That also means you could use a much lower wattage shunt.

I just trust the battery-voltage to tell me what's happening.

Post edited by: loudhvx, at: 2007/07/17 02:24