Got a battery in a piece of equipment that drains down overnight? If you're certain the battery is okay--proper levels of electrolyte, with all cells functional--then here's how to pinpoint the "draw."
Tests for draws--or whatever you call undesirable drains on the battery--are conducted on the negative (ground) cable of the battery. If the machine has two or more batteries, find the main battery cable that grounds all the batteries to the machine's frame. Find that battery cable, then make certain the key switch is off and all monitors and peripheral electrical systems, etc. are off.
If the battery(s) drain down in a matter of hours, it means there are a lot of amps disappearing, so any test equipment must be capable of dealing with at least 10 or more amps. There are induction testers capable of handling hundreds of amps. These testers have jaws that open so you can clamp them around the negative battery lead. Once clamped around the negative lead, they magically read the amperage passing through that cable. With the key switch off, an induction-type amp meter should read "zero." Readings of more than one amp confirm an electrical problem.
If the battery(s) go dead over several days, then the drain is probably less than 5 amps and a standard voltmeter with amp-testing capabilities can be used to diagnose the problem. Use the voltmeter's owner's manual to configure the test leads into the HIGHEST amp-load test ports--usually a 10-amp port. With the voltmeter configured to read amps/milli-amps, disconnect the negative ground cable from the vehicle's ground point. Touch the positive lead of the voltmeter to the disconnected ground cable. Touch the negative lead from the meter to the grounding point where the cable was previously attached. With the voltmeter completing the circuit between the vehicle and the battery, if there is a drain/draw on the system it will show up as an amp reading on the meter.
If the drain/draw is in the milli-volt range, it may be necessary to reconfigure the voltmeter to read milli-amps. It's best to always test for amps before testing for milliamps. Using a voltmeter configured for milli-amps to test a circuit with a multi-amp drain will destroy cheap voltmeters and at a minimum burn out internal, replaceable fuses in higher-quality voltmeters.
If placing a voltmeter/ammeter "in series" with the ground circuit of the battery reveals a load of more than 5 or 10 milli-amps when the keyswitch is off, there is a drain in one of the vehicle's electrical systems. Three to five milli-amps of load is generally acceptable.
, combines and trucks have computer systems that must be constantly energized, so there will always be a slight load on the system, even with the keyswitch off. Don't be concerned if the voltmeter/ammeter briefly shows more than 5 milliamps when first connected to the ground cable---computers that are dormant in the absence of battery power briefly use a few extra milliamps to reboot when power is reintroduced. Their power use should decrease to minimal levels within 10 or 15 seconds of power-up.
Once a drain on the batteries has been proven, with the volt/ammeter connected between the ground cable and ground point, locate the machine's fuse block and methodically pull every fuse while watching the amp reading on the meter. When you pull a fuse and the meter reading drops to near-zero, you've identified the circuit with the problem. Once you know which circuit is the villain, methodically disconnect connectors or components of that circuit while watching the ammeter to narrow down and identify the exact location or cause of the drain on the batteries.