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Leaving Batteries At 70% Charge For Several Weeks

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sethp001
Mainsheet C-25 Tech Editor

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814 Posts

Initially Posted - 03/11/2017 : 09:33:50

Last week, I discovered the shore power cable needed replacing, so I didn't reconnect it when we came back to the slip. The batteries were at 70% charge when we left. I was not planning to return to the boat for another two weeks, but my new shore power cable arrived. The boat is slipped an hour away from my house.

If I leave my batteries at 70% charge for three weeks total, am I likely to cause significant reduction in their life, permanent sulfation, reduction in capacity, etc.? I know there will also be some self-discharge. In summary, should I go to the boat ASAP to plug in the shore power to charge the batteries?

Seth
"Outlier" 1987 Catalina 25 SR/SK/Traditional Interior #5541
"Zoo" 1977 Morgan Out Island 30
"Nomad" 1980 Prindle 16
"Lost" 1988 Catalina Capri 14.2 (sold - yay!)
"Marine Tex 1" Unknown Origin POS 8' Fiberglass Dinghy
https://whichsailboat.com/2014/07/27/catalina-25-review/

islander
Master Marine Consultant

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3992 Posts

Response Posted - 03/11/2017 : 09:56:39

Interesting question Seth. Although leaving batteries under charged does hurt them I have never seen a chart showing a time limit. I would say the shortest time is better so given the opportunity to plug in is the best advise I can offer. Might also be a good time to think about a solar panel? The sun never stops working and it's free.

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Edited by - islander on 03/11/2017 10:02:28

islander
Master Marine Consultant

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3992 Posts

Response Posted - 03/11/2017 : 13:37:31

quote:
Leaving them in a discharged state for any length of time will damage them and lower their capacity.

. Seth, This is from WM and is all I could find on the subject.

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Akenumber
Navigator

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247 Posts

Response Posted - 03/11/2017 : 13:57:00

I have a place in the desert with solar an 2 gp 31 batteries. The solar got turned off accidentally for a 6 month period with the batteries totally dead and they came back to life. They were about a year old though. In my opinion a low battery condition seems to affect older batteries more than new. I had two 31 batteries on our boat that sat low all winter and did die, but were probably 6 years old. As long as they are newer they should be fine.

Ken
San Diego
84 C25 SR/FK 4116
The KRAKEN

Voyager
Master Marine Consultant

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5231 Posts

Response Posted - 03/12/2017 : 07:47:06

Sulfation is the enemy. When batteries at below full charge (12.75VDC) the surface of the lead plates will begin to turn to lead sulfate. This is an irreversible process which reduces the ability to recharge the battery and increase the internal resistance of the battery limiting the amount of current available and the length of time you can draw power. It reduces the Amp Hours of the battery.
Sulfation can be partially reduced by over-charging the battery.
Net-net: an ounce of prevention is worth a pound of cure. Prevent your battery remaining discharged for any stretch of time.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Stinkpotter
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Djibouti
9013 Posts

Response Posted - 03/12/2017 : 09:44:14

How old are the batteries? If they're getting a little long in the tooth, I'd weigh their depreciated value against the nuisance/cost of making the trip.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Dave5041
Former Mainsheet Editor

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3754 Posts

Response Posted - 03/12/2017 : 17:44:51

I think the main reason that new batteries appear to handle extended discharge time or depth is that they initially have a lot of capacity that we try not to use. Older batteries have less reserve and loss of a significant percentage of it becomes noticeable on what would previously have been a 50% discharge. Life expectancy can only be determined in controlled comparisons, not anecdotal reports. We have to depend on published data for that information, and all the data says its bad.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle

sethp001
Mainsheet C-25 Tech Editor

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814 Posts

Response Posted - 03/12/2017 : 21:52:32

Thanks everyone for the replies. Due to your advice, I made the trip tonight to plug in the new shore power cord. The batteries are new as of last spring, hopefully they'll still seem that way this summer.

islander
Master Marine Consultant

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3992 Posts

Response Posted - 03/13/2017 : 12:00:46

I'm sure they will be fine. Now you can sleep at night

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

sethp001
Mainsheet C-25 Tech Editor

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814 Posts

Response Posted - 03/19/2017 : 10:20:13

quote:
Originally posted by islander

I'm sure they will be fine. Now you can sleep at night

I did sleep better!

bigelowp
Master Marine Consultant

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1736 Posts

Response Posted - 04/01/2017 : 07:12:05

My guess is that during the season my batteries are never charged 100% and most likely are in the 60-75% range. I am on a mooring and the only recharge is via the outboard alternator. As I sail off/on the mooring more often than not my recharge is when the wind dies and the iron penny is put into service to peddle back home. Even so my batteries lasted eight years. While they were on a winter trickle charge I never checked water levels. So, IMHO 70% charge for a few weeks should not cause problems -- and is a whole lot better than some of us maintain them!

Peter Bigelow
C-25 TR/FK #2092 Limerick
Rowayton, Ct
Port Captain: Rowayton/Norwalk/Darien CT

Voyager
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5231 Posts

Response Posted - 04/01/2017 : 20:49:14

I'm on a mooring (referred to as a floating dock) in Milford Harbor CT and I charge my battery using the 12A alternator on the Honda 8HP engine. Even so, with a Group 27 battery @ 90AH, if I discharge it 1/4 of the way, I still have to replace 22.5AH.
This would take about 2+ hours of engine time. Efficiency is not 100% so it would take as much as 3 hours.
I rarely motor for that long so my battery would remain under charged much of the time.
To remedy that, I have a 20W solar panel with a regulator that delivers about .75A for 8 hours on a sunny day. This is about 6AH per day. In the course of the week it'll completely recharge the battery.
If I know that I need to use the boat sooner I also have a second solar panel that doubles the rate of recharge. I don't think I could manage the battery without solar power.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

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