SERIOUS BATTERY DRAIN
Q: While sailing on a friend’s boat we noticed what we thought was excessive voltage loss overnight. The vessel has five 8D conventional flooded lead-acid batteries for the house bank and one 8D AGM battery for the starter. After running the onboard generator for some time during the day and including what the solar panels put in, we would start the night with close to 13 volts on the monitor. Then in the morning we would find the monitor reading 12.1 or 12.2 volts. During the night we would run the autopilot, refrigerator and freezer, VHF radio, running lights and chartplotter. The radar was on standby most of the time. We were able to lower the loss somewhat by turning the freezer off overnight.
My question is in regard to our attempt to find the cause of what we thought was excessive voltage loss by searching for a ground fault. We did this by disconnecting the positive lead from the battery and connecting the leads from a volt/ohm meter between the positive battery post and the disconnected positive cable. On the house bank with everything shut down we got a reading of zero. However, with the start battery we got a reading of 12.48 volts. After that we disconnected all the negative cables, one by one, from the start battery, and each time we got the same reading, even after all the negative leads were disconnected and the battery switches in the off position. Would a ground fault give the results we found?
Carl Updyke, via firstname.lastname@example.org
NIGEL CALDER REPLIES
In theory, with the voltmeter between the positive post and the positive cable and nothing connected to the negative terminal on the battery you will not, and cannot, see voltage. There is simply no circuit back to the battery negative. Where a ground fault would show up is if you have the negatives connected with everything turned off (or pieces of equipment disconnected) and you still saw voltage, indicating that there is a path to ground that bypasses all the switches and equipment.
Assuming your meter is reading correctly, what I think may be happening is you have some kind of a connection between the positive terminal on the house batteries and the cranking battery (maybe voltage sensors attached to the battery posts), and although I am finding it hard to visualize, you are actually measuring from the house battery positive to negative. If you disconnect the house battery positive and the reading goes away, then this is what is happening.
On the broader question of why your batteries are discharging so fast overnight, given you nominally have around 1,000 Ah of capacity (at 12 volts) it would take a substantial ground leak to pull this down. It is more likely that you have a problem with lost capacity, either through generalized sulfation (which may be recoverable with an “equalization” charge) or one or more failed battery cells.
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