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1 or 2 Batteries?

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CharF
Deckhand

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USA
13 Posts

Initially Posted - 09/29/2010 : 18:14:43

I just replace one of my two bad deep cycle batteries. Should I replace both or is it ok to have one good and one bad battery?

I had shore power hooked up, battery charger going, and when I disconnected one battery, the 12 volt lights in the cabin went dead. Is this normal? I think I flipped the battery switch to other positions and that did not bring the lights back until I reconnected the new battery. I was wondering if I might have a wiring problem.

Private Dancer
4018 - SK, STD rig

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/29/2010 : 18:37:18

Definitely replace both batteries at the same time. If you don't you will kill the new battry way before it's time.

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Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 09/29/2010 : 19:09:51

Or take the other battery out until you are ready to buy another battery. As Gary said, if the battery switch is in the "All" position or the batteries are permanently connected in parallel the good battery will drain into the bad battery. It only takes a few total discharges to destroy a battery.

Edited by - Dave5041 on 09/29/2010 19:12:56

CharF
Deckhand

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USA
13 Posts

Response Posted - 09/29/2010 : 19:33:29

They are hooked up to the battery switch and I only select one battery, never all.

I will buy another battery tomorrow and hook it up Friday night.

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Voyager
Master Marine Consultant

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USA
5378 Posts

Response Posted - 09/29/2010 : 21:30:35

Best bet, make sure they're identical. Also, check to see that you can run each device independently from both batteries. To test, disconnect each one in turn, then test each device (radio, lights, depth meter, etc) with the 1 - 2 - both - off switch in the appropriate position.

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Stu Jackson C34
Admiral

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844 Posts

Response Posted - 10/01/2010 : 09:54:51

You only need two identical and/or new batteries if they are in the same BANK. A bank is two or more batteries connected TOGETHER. If you use 1 or 2 but not both, then just buy one.

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Voyager
Master Marine Consultant

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USA
5378 Posts

Response Posted - 10/01/2010 : 20:52:33

Stu - wouldn't you want to charge both from a single charger, or from the engine? You'd use the BOTH switch setting for that. In that case the regulator wouldn't know which battery to charge - the new one or the old one. Or, you could recharge one at a time.

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Stinkpotter
Master Marine Consultant

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Djibouti
9081 Posts

Response Posted - 10/02/2010 : 04:47:36

I was under the impression (perhaps false) that if two batteries are being charged in parallel together, the charging is proportional between them based on their degree of discharge. That would suggest that as long as one is not so bad as to overtax the charger (for example, from an internal short), the other should be safe during the charging process and they'll end up with roughly equal states of full charge. No?

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Voyager
Master Marine Consultant

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USA
5378 Posts

Response Posted - 10/02/2010 : 07:31:23

Dave,
I guess I oversimplified my statement.

It's a little more complicated, but with the same result if both batteries are about of the same age and condition.

Without a charger, when the two batteries are connected together, the one with the greater voltage immediately starts charging the one with the lower voltage through the "internal resistance" of the two batteries, so if one is badly discharged, there can be considerable currents.

Next, if a charging current is applied from an external charger, the lower battery takes most of the current, but since the combined battery voltage is less than the charger voltage, current flows into both batteries, but a lot more goes into the more discharged one.

There's no problem if you have two similarly aged batteries at different levels of discharge (for example: one is at 50% and the other at 90%), but if you have one "old and tired" battery and a new battery connected to the charger, the old one may not allow the new one to reach the highest "float" voltage (stage 3), so the presence of the older battery will rob life from the newer one.

If the old battery isn't too bad and can still be fully charged and will hold its charge, then you're fine. But if I have two unequal batteries, I would not leave them paralleled unless on the charger or for short periods, as the older one might tend to self discharge faster than the newer one, and thus discharge the new one as well.

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CharF
Deckhand

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USA
13 Posts

Response Posted - 10/02/2010 : 08:53:39

I think I would have been okay with just one good battery. But if I do happen to have a wiring or charger problem that I am not aware of, I think it is best to replace both batteries at the same time with the same battery type. So, I bought another just like my first one. After buying the first battery, they gave me a 10% coupon. So the second battery was cheaper.

Thanks for everyone's input. It helps to hear what others are thinking, whatever their viewpoint is: right or wrong. ;)

Now I am going to hank on a jib and sheet some wind! Happy sailing.

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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.