Dead batteries on my boat
July 12, 2009 7:09 AM   Subscribe

Came back to my boat after two weeks away and both 12V batteries were dead. (I had accidentally left the switch in the on position.) I need to know what I should do next...

Three part question: 1. As far as I know, the only things drawing power are the little light on the AM/FM radio and the bilge pump. We did get some rain so I know the bilge pump must have run, but still, this doesn't seem like it should have been enough to completely drain the batteries. Does anybody know if it is? 2. The batteries are just 15 months old, but I'm wondering if I should play it safe and just replace them (an expensive proposition) since someone advised me to replace them every two years anyway. 3. If I'm going to charge them, what is the best way to to do this?

I always check the meter while the boat is running and it definitely seemed like they were being charged while I was running last time out.
posted by robverb to Sports, Hobbies, & Recreation (8 answers total)
Bilge pumps can definitely drain the batteries - they're usually high output, high torque devices, plus they might have run continuously if the switch was activated but they'd lost prime.

The best way to recharge the batteries is to remove them from the boat and hook them up to a charger/conditioner.

Replacing them every two years seems excessive unless you're using the boat every day, especially considering the cost of marine batteries.
posted by squorch at 7:22 AM on July 12, 2009

I keep batteries charged up with a solar charger so they are always fully charged. I have 5 years on marine batteries so I would not replace unless the battery has failed. I agree with squorch.
posted by JayRwv at 7:48 AM on July 12, 2009 [1 favorite]

My mum's got a boat. Last time I was visiting her in the UK, every boat near hers had a little wind charger on the back. Like this. If your area gets more wind than sun (see e.g. the UK ;-), that would be a good option for the future.
posted by idb at 8:01 AM on July 12, 2009

Do you always have a handheld VHF with extra batteries aboard? Do you have SeaTow insurance? How far from shore do you go? Is it a sailboat that you could sail back to your dock/mooring if necessary? Do you take your wife/kids out on the boat?

Yes, it's expensive to replace the batteries. But the time to invest in new batteries is before you're 25 miles from shore and you can't get the engine to turn over.
posted by McGuillicuddy at 8:11 AM on July 12, 2009

check the light in the head. happened to a friend. took a while to realize it wasn't the bilge pump.

turn one battery off, one on and use a jump box to start one engine. then turn both batteries on and let them charge a bit. don't turn them both on when trying a jump box, the box doesn't have enough in it to get it started while trying to recharge 2 dead batts.

if the batteries didn't freeze while discharged they're probably fine. seconding solar charger idea, maybe keep a jump box in your car or hooked up to shore power.
posted by KenManiac at 8:32 AM on July 12, 2009

You could have damaged the batteries. One complete (i.e., terminal voltage = 0.0V) can kill some types.

If these were automotive, I'd chance depending on them, but since they are marine, I would definitely think twice before trusting them.

If you think you'll be unable to avoid abusing them in the future, look for an undervoltage / over-dishcharge protection module. It's not as cheap as being consistent, but in the long run, it will save you money if you are absent minded.

You might want to consider making a shut-down checklist and following it religiously. Aircraft pilots do this. Memory is too flaky a thing to count on when safety is an issue.
posted by FauxScot at 8:53 AM on July 12, 2009 complete DISCHARGE (i.e., terminal voltage = 0.0V)

[I should have used a checklist!]
posted by FauxScot at 8:54 AM on July 12, 2009

Thanks for your responses everyone. I had the batteries tested and they had nothing left in them - so recharging would have been a waste. Because they were only a year old, West Marine gave me partial credit so the replacements didn't cost very much. I also picked up a solar charger while I was there (thanks for the tip JayRWV).
posted by robverb at 1:41 PM on July 16, 2009

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