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Normally you want to measure the current going into the battery banks because when the amps taper off, you know the battery banks are charged up. The alternator will be supplying current for both charging the batteries and for electronics, lighting, pumps, whatever and this includes charging the engine battery bank and any engine electronics. There are usually two battery banks, one for the engine and the other for everything else. With the engine running, usually a relay connects everything together so all banks get charged. So, the problem is finding a place where all the current flows together into one cable. If all the negative cables are brought together at one point and the shunt is connected to that point with the other side of the shunt goes to a common ground, usually on the engine since the starter on a diesel uses many hundreds of amps, then all the current is measured for both banks. Otherwise, if you want to have a shunt on the positive side, you have to choose one bank or the other. However, maybe you want to know how many amps are being pulled from the bank that supplies everything but the engine. When the engine is shut down, knowing the amperage draw will tell you how long you can go without recharging if you calculate amp-hours that are being discharged and know how many amp-hours the batteries can supply. In this case you would put the shunt on the positive cable going to that bank of batteries that supplies the buss and alternator relay connection. You could also tell when that bank is nearing full charge because the amps on the charging side of the meter would taper off. A voltmeter could be used for monitoring both banks, maybe with a switch between banks to you can measure resting voltage (no charging or current draw for at least six hours) to determine how charged up each of the battery banks are. However, be aware that a relay that connects all banks together while the engine is running to charge both banks will show the same voltage on both banks. The relay ties both banks together. The resting voltage varies depending how the state of charge for the battery banks. Trying to describe all this is difficult. A picture would be better. Check out of the library, Boatowner's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems by Nigel Calder
Last edited by LakeSuperiorGeezer; 01-06-2011 at 03:04 PM.