12VDC system voltage fluctuations

Batteries, Generators, Solar, Wind and Electronics
Post Reply
User avatar
mike cunningham
Posts: 332
Joined: Sat Dec 20, 2008 11:21 am
Location: Jacqueline, F30 #3, Discovery Bay, California

12VDC system voltage fluctuations

Post by mike cunningham »

I installed a little 12VDC three way USB charger in my boat. The charger has a small voltage display on the front panel of the charger. Similar to below but with three sockets.

https://www.amazon.com/Jebsens-Charger- ... th+voltage

The device works great but I noticed the DC voltage is fluctuationg quite a bit when the AP is drawing signifcant current (3 - 6 Amps). Are such fluctuations in the boat's DC voltage typical or is this indicative of a problem? I recently performed a 24 hour load test on my 11 year old AGMs. The results were consistent with expectations for a 220 AH house bank.

I also have a zantrex battery monitor. The voltage flux do not show up on the monitor but I believe the voltage is smoothed at the Zantrex to mask short term fluctuations. This may have been going on for a long while.

Any thoughts on whether 4 - 5 tenths of a volt fluctuations are typical of a 12 system in ops with AP or pumps running?
Mike Cunningham
Freedom 30 (Mull) Hull #3
Build date...June, 1986 . Freedom Yachts USA, sloop, shoal keel
Gun Mount and pole retrofitted (purchased from a Hoyt Freedom 32)
Yanmar 2gm20F , 1600 hrs fixed two blade prop
e-rud and ocean racing equipment

User avatar
Posts: 219
Joined: Sat Dec 13, 2008 8:40 pm
Location: Lerwick, Shetland Isles

Re: 12VDC system voltage fluctuations

Post by Castaway »

I would be inclined to suspect the accuracy of the readout on the charger, especially if it is 'plug in' rather than hard wired. Other current draws would cause some voltage drop, but if the load is steady, that is likely to be constant. On the other hand, the autopilot is probably stopping and starting frequently, and that would certainly change the current draw (and thus voltage drop) on a moment to moment basis.

If your batteries hold their charge for a reasonable time when not in use, they are probably OK.
Gerald Freshwater,
s/y 'Castaway', (UK F35 cat ketch, centreboard, 1987)
Lerwick Boating Club
Shetland Isles, Scotland

Post Reply