Diesel RV Club, an FMCA Chapter

Technical => Engine Electrical => Topic started by: skitime on June 03, 2019, 05:38:15 PM

Title: What size shunts for amp hour meter?
Post by: skitime on June 03, 2019, 05:38:15 PM
I am adding shunts to my starting batteries and coach batteries for a dual amp hour meter.  On my gas rig I had dual 500 amp shunts.  How big of shunt would I need for the diesel pusher.  The 2 starting batteries are 950 CCA and the  4 AGM coach battieres are 100 amp hour each. 
Title: Re: What size shunts for amp hour meter?
Post by: Brett Wolfe on June 03, 2019, 06:38:05 PM

Welcome to the DRVC.

VERY unusual to even want a shunt on a chassis battery, since they are not involved in dry camping/running house items.

Hopefully we will have an electrical engineer chime in-- you should be looking at load of starter and intake manifold heater (both at the same time) at the coldest temperatures you may be experiencing. Those are the conditions when you can't afford a voltage drop due to anything, including a shunt of insufficient capacity.
Title: Re: What size shunts for amp hour meter?
Post by: skitime on June 04, 2019, 06:12:38 AM
I always like knowing what the batteries are doing and if there is a drain on the batteries that I am unaware of. I purchaced a pair of 1200 amp shunts and I am installing them but I am not sure that is big enough but there are not many shunts bigger than those that will make sense to install because of the size of them.
Title: Re: What size shunts for amp hour meter?
Post by: ziggyh on June 04, 2019, 08:49:53 AM
The way I understand the use of a shunt is that is it needs to match the amp meter you are using, the internal resistance of the shunt and the meter are used to calculate a ratio or multiplier factor.
The starter current on diesel engines can be from 500-1200 amps plus depending on weather its a direct drive or gear reduction starter and a Brett indicated variables such as how cold the engine is and oil viscosity and condition to mention a few. 

Also a meter capable of measuring 1200 amps will probably not be able the measure a small parasitic load of a few amps which would be enough to drain the batteries in a few days.  The chassis batteries will always have some small parasitic draws depending how many modules need to have some power to retain memory.

You could always monitor the chassis batteries with out having the starter draw pass through the shunt by installing it inline with the chassis disconnect switch if you have one or by installing it between the battery and clean power leads.

A shunt adds a bit of resistance to a circuit and even though it is a small amount it could be enough to create a voltage drop that could affect the voltage available to the starter motor. That could be calculated using ohms law where e=IR for example the voltage drop across a resistance or shunt of .005 ohms with a current flow of 800 amps would be 4 volts that the starter motor does not get. The proper way the do that calculation would require you the know what the resistance of the starter motor is to accurately get what the voltage drop would be but I think you should get the point of the affect of adding resistance to a starter cable.

Another method of measuring a large current would be to use an inductive type of pickup, though it would not add resistance to the circuit the same issue of being able to measure a small drain when having the capability of measuring 1000 amps.
Title: Re: What size shunts for amp hour meter?
Post by: pfhays on June 04, 2019, 04:36:42 PM
I've got two 12 volt 8D chassis batteries and had two 12 volt 8D house batteries but replaced the house batteries with four 6 volt golf cart batteries with slightly greater amp hour capacity.  I have a two 500 amp shunts, one for each set of batteries.  The 8D chassis batteries are 1000 cranking amps and the house batteries are 440 amp-hours.

If it matters, the chassis batteries start a CAT C10 engine.
Title: Re: What size shunts for amp hour meter?
Post by: skitime on June 09, 2019, 07:16:39 PM
I had this meter on my last gas rig for 16 years so I have a lot of experience with it.  Now that it is setup on the diesel pusher, I can report that is seems to be very accurate with the  dual 1200 amp 50MV shuts compared to the dual 500 amp 50MV shunts.  So far I am a happy camper with the setup. 
Title: Re: What size shunts for amp hour meter?
Post by: Lotos-rggp on June 22, 2019, 07:46:01 AM
How's a shunt works:
https://www.ecodirect.com/mobile/product.aspx?ProductCode=bogart%2Dmkb%2D500%2D50&404;http://www.ecodirect.com:80/Bogart-Engineering-MKB-500-50-500A-50mV-Shunt-p/bogart-mkb-500-50.htm= (https://www.ecodirect.com/mobile/product.aspx?ProductCode=bogart%2Dmkb%2D500%2D50&404;http://www.ecodirect.com:80/Bogart-Engineering-MKB-500-50-500A-50mV-Shunt-p/bogart-mkb-500-50.htm=)

Technical Information on shunts is located at:
http://www.bogartengineering.com/support/manuals.html (http://www.bogartengineering.com/support/manuals.html)

And an faq at:
http://www.bogartengineering.com/support/faq.html (http://www.bogartengineering.com/support/faq.html)
I have no association with Bogart but their new WiFi interface looks interesting.
97 Safari Sahara 3550 CAT 3126