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snellings avatar image

What size shunt for chassis battery?

I plan on connecting a BMV-712 to my motorhome's chassis battery. Given that this battery bank has significant draw from the engine and generator starter and the batteries are typically rated at upto 1000 CCA, I'm wondering if I need a shunt larger than the standard 500A. If so, how do I determine what size is needed?

BMV Battery Monitor
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Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@snellings, ideally you would use a Hall Effect meter capable of measuring inrush current to determine how many amps are actually drawn when starting your motorhome, but as those are generally expensive, the simplest solution is probably to just figure your max cranking amps and then get the next-highest-rated shunt. In your case, since you mention 1kCCA, you could get the 1kA shunt (article no. SHU102050200) or, to be on the ultra-safe side, the 2kA shunt (article no. SHU202050200).

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Thank you. If I have two 12V batteries connected in parallel, would that double the size requirement or does the two batteries merely provide additional capacity (Ah) without added amp draw?

Well the two connected in parallel would increase your draw potential, but the actual draw is determined by your starter, not the batteries it's connected to. Realistically I doubt that your starter is ever going to draw more than 500A anyway, but depending on the engine it's starting I could be wrong. Is there any way you can look up the specs of the starter installed on your vehicle? That would narrow down your requirements quite a lot. Personally I suspect you'd likely be fine with the stock 500A shunt and almost certainly be fine with the 1kA shunt... but I don't want to say that for sure and then find out later you have a rare 2kA starter...