This is from a post I wrote a year ago:
If you want to test to see if you have a DC-based ground leak, the test for that is rather simple. The steps for seeing if you have a DC-ground leak are as follows:
First—the preliminary diagnosis test:
1) Turn off all equipment and disconnect any solar panels
2) Disconnect the positive side of the battery banks.
3) Leave the main battery isolation switch turned on for the bank in question
4) Set the meter to VDC mode, range appropriate for your battery bank
5) Connect the meter between the positive terminal and the disconnected cable
The meter should give no reading. If it reads XX volts for your XX VDC system, one of two things is happening.
1) You've left some equipment connected and turned on. This could be a bilge pump, a power feed to a stereo for the radio's memory and clock functions, or a hard-wired fume detector.
2) If you've disconnected all the "hard-wired" equipment and still get a reading, then you've most likely got a ground leak in your boat's DC system.
The Ground Leak Check:
1) Set the meter in Ohm mode and set it to the lowest range (x1).
2) Connect the leads of the Ohm-meter (or multimeter in Ohm mode) to the disconnected positive lead and the negative terminal of the battery.
The meter is now reading the resistance of any circuit to ground that exists in the boat's wiring. The reading on the Ohm meter display can help you identify the cause of the leak.
0-10 Ohms means it is most likely a piece of equipment left on
10-1k Ohms is a low-drain piece of equipment left on, or a serious ground leak
1k-10k Ohms is a minor leak
10k+ Ohms is an insignificant leak
How Big is The Leak?
The ammeter function of the multi-meter can tell you what the current leakage is. If your meter can read up to 10 Amps DC, then you can use it to measure amperage for leaks down to about 1.3 Ohms resistance on a 12 VDC system, or 2.6 Ohms for a 24 VDC system.
To see how big the leak is, put the probes on the positive battery post and the disconnected cable. The meter readings can be interpreted as shown:
<1mA — insignificant leakage
1–10mA — minor leakage
10mA–1A — major leak or some equipment left on
>1A — Usually some equipment left on.
My first question is are you sure you've disconnected all the "hard wired" equipment and turned everything else off. If you haven't then you can't be properly checking for a ground leak. Things like bilge pumps, memory power for electronics, etc are all possible sources of the 12V that may not be "leaks". Did you disconnect all of them?
90 mA is a relatively large leak, and would tend to indicate to me that more likely, you've left something on. It could be as simple as an led indicator light, which you aren't aware of.
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a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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—Cpt. Mal Reynolds, Serenity (edited)
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