Starter through shunt (what size?)

[jwag956]

Looking to replace older xantrax BMC with Victron. We have 2 house/starter 8D batteries (i.e. older style not dedicated starter batteries). I want to place a BMC on each battery since we alternate between them (there is a battery switch 1, 2, All). To get correct info it would seem that all loads - including starter go through the shunt (via the negative). Victron default shunt is 500A - I am having a hard time figuring out just how many amps my starter motors take - these are for Ford-Lehman SP135 engines.
Or is it not considered a good idea to run starter through a shunt?

 

[SteveK]

good idea? This is the first time I have read here anyone has considered that.

 

[Delta Riverat]

No.

 

[av8r]

The 135 manual does not list starter draw but the 120 manual specifies a starter running current of 650a and a locked-starter draw of 1050a. A 1000a shunt would probably be ok, but I’ve never heard of it being done.

 

[DDW]

Unless you crank your engines for minutes at a time, there is no need. While the current during cranking is very high, the duration is low and the amount of energy is low. Also, the software in the monitoring device will not correctly account for the energy lost, so ignoring it is no loss in accuracy.

For example 650A for 10 seconds is 1.8 AH. There are many sources of error in the monitoring system larger than that.

 

[jgwinks]

I wonder when they say it's a 500A shunt, does that mean that more will damage it or is it just not capable of reading more? I suspect the latter, I run my twin Perkins through mine and never had a problem with it. And like DDW said, if its not reading the starter draw it wouldn't really matter.

 

[Turtle Blues]

I use an inexpensive monitor with dual hall sensors. No shunt installation just slip over the cable.
Available on Amazon for less than $100

 

[jwag956]

Unless you crank your engines for minutes at a time, there is no need. While the current during cranking is very high, the duration is low and the amount of energy is low. Also, the software in the monitoring device will not correctly account for the energy lost, so ignoring it is no loss in accuracy.

For example 650A for 10 seconds is 1.8 AH. There are many sources of error in the monitoring system larger than that.

Thanks - your reply is a great way to think about the entire system. And no - if I am cranking more than 5 seconds I know I have some air leak in my lines.