Ask SAIL: Serious Battery Drain

Author:
Updated:
Original:

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 sailmail@sailmagazine.com

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.

Got a question for our experts? Send it to sailmail@sailmagazine.com

March 2016

Related

IMG_8287GR16Mykonos

Cultural Charters: Mykonos

In last month’s column, I covered the amazing mix of cultures that have called the Dalmatian Coast home over the centuries. Croatia cruising is like a smorgasbord of intertwined centuries, and the islands are a movie set. A little farther south, though, you’ve also got Greece, ...read more

cookinglead

Cruising: No Oven? No Worries

Many cruising boats, especially smaller ones, don’t have a conventional oven. But that doesn’t mean that you can’t have all the baked foods you want, from bread to brownies to breakfast rolls to casseroles and even a roast chicken. All it takes is the right bit of gear and a ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com Take no chances  This is my stern with the engine running slowly in gear against the lines. We all know that when we’re charging batteries this lets the engine warm up thoroughly. However, I have a ...read more

ZK-Seaboot-900

Gear: Zhik’s Seaboot 900

A Better Sea Boot Following up on its successful ZK Seaboot 800, Zhik’s Seaboot 900 was created in partnership with team AkzoNobel and Dongfeng Race Team, the latter the overall winner of the 2017-18 Volvo Ocean Race. Designed for serious, long-distance offshore racers and ...read more

01-LEAD-FP-Astrea-42-Gilles-martin-rajet---Navigation

Switching to Solar Offshore

No sensible bluewater sailor would consider setting off on a long cruise these days without some means of generating power other than by burning fossil fuels. The good news is that solar energy is becoming less expensive by the day, making it an obvious answer for providing the ...read more

190812-Tiwal-Video-600x

Video: Tiwal Cup 2019

Who says you need a superyacht to have fun? It would be hard to imagine having a better time on the water than these sailors recently did racing aboard a fleet of Tiwal inflatable sloops. ...read more

GEMINI-BATTERY-PASSAGE

Experience: No Batteries, Big Problems

After sailing on Puget Sound and in British Columbia for over 30 years, my wife, Jeri, and I retired to Florida, bought a house on Punta Gorda and worked hard to get it straight. The walls had been painted, furniture bought and assembled, belongings packed, transported and ...read more