Ask SAIL: Serious Battery Drain

Publish date:
Social count:


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


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.

Got a question for our experts? Send it to

March 2016



The ICW North Bound Migration Begins

As the northbound migration begins, we are getting some early reports on conditions along the ICW. The overall impression this spring is that after the damages caused by the hurricanes, the winter storms have apparently not made too many additional changes. There is even some more


Charter: Historic Croatia

Heaps of history—that’s not usually what comes to mind when you plan a sailing charter, but if you like a bit of culture mixed with your cruising, Croatia is the place to go. Caught between two worlds, (the whitewashed laid back vibe of the Mediterranean and the brash demeanor of more


Gear: Pan-Pan man-overboard Locator

There He Goes!The Pan-Pan man-overboard locator won a Pittman award for 2017 as a great idea, and now it is in production as the Weems & Plath CrewWatcher. It’s a two-part system that employs a smartphone app to locate a small personal beacon that triggers automatically should more


SAIL 2018: Reader's Photographs

Are you out there sailing, cruising and living the sailing life? If so, we’d love to see it. Send your sailing photos to sailmail@sailmagazine.comAnd don’t forget to sign up for our free eNewsletter.Check back for updates!This was taken from half way across the 26 mile crossing more

Landing Page Lead

The Volvo Returns to the Southern Ocean

Since the Volvo Ocean Race’s inception, the Southern Ocean has made it what it is. And no part of the race says “Southern Ocean” like Leg 7 from Auckland, New Zealand, to Itajaí, Brazil. The 7,600-mile leg, which starts this Sunday, is not only the longest of the event, but far more


SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comTeak deck paradise  I had a call recently from the man who replaced the deck on my Mason 44 five years ago. He was worried about the way people are wrecking their teak decks trying to get the green off. more