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#1
Old 06-26-2010, 10:16 AM
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Join Date: Aug 2002
Location: Music City USA
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Can I recharge a deep-cycle marine battery with my car?

I have a 12v deep-cycle marine battery that I use for small electronic devices (like my CPAP) while camping. It hasn't been used in a couple of years, and I want to make sure it is fully charged before taking it on our trip next week.

I don't have a battery charger. Could I plug it into the 12v recepticle of my SUV and let it charge up as I drive around? Would that work?

If not, are there places that will charge your battery for you?
#2
Old 06-26-2010, 11:21 AM
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I looked into just this question a few years ago. Your vehicle's alternator is likely not designed with the capacity to charge both your running battery and your deep cycle at the same time. That said, you can probably get a decent charge on your marine battery, at some expense to your car battery.
I'd be willing to do that a time or two, but in the long term it pays to buy a dedicated charger.

Naturally, I no longer remember what's needed to hook up a second battery to a vehicle's alternator, but this looks to be a useful bit of Googling: "auxiliary battery charging alternator."

Last edited by Squink; 06-26-2010 at 11:23 AM. Reason: where the hell did that apostrophe go, I put it in there somewhere!
#3
Old 06-26-2010, 11:50 AM
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I just did this. Start your car, remove the battery while it's still running and install your marine battery. All that's left to do is drive around until it's charged.

Last edited by imfloating; 06-26-2010 at 11:50 AM.
#4
Old 06-26-2010, 11:55 AM
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In other battery threads; Rick, Gary T. and others have warned against charging rundown batteries with the car alternator method. You need to go the battery charger route. Drop off the battery for testing and charging at a service center. It's cheap and better for the battery and your car. A new alternator/regulator or tow truck add up in expenses.
#5
Old 06-26-2010, 12:02 PM
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removing a battery from a running vehicle is risky. very easy to cause damage to your vehicle.

after your battery has recharged to recover from the starting using jumper cables could recharge a second good battery.

not having using the battery in years it may no longer be good. if it might still be used then it might require better maintenance and charging than connecting to your car. a marine store could have the best chance of evaluating and rejuvenating.
#6
Old 06-26-2010, 12:30 PM
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1, Never remove a car battery from a car that is running. As stated by johnpost, you could cause major electrical damage to your car by doing this. (FYI, you used to be able to get away with this on old cars. Not anymore.)

2. Your OEM alternator is not designed to charge a battery that has been significantly run down, or dead. That's what a battery charger is for. Regularly charging a depleted/dead battery with your alternator will significantly reduce the life of the alternator.
#7
Old 06-26-2010, 01:48 PM
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Thanks for the responses everyone. I last used the battery two years ago so it's probably not completely dead, but it might be worthwhile to spring for a charger.
#8
Old 06-26-2010, 02:28 PM
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You can probably find one for under $20 if you look long enough. Probably even cheaper if you cruise around yard sales for a while.
#9
Old 06-26-2010, 02:58 PM
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with an old manual charger you do have to monitor the battery to not overcharge the battery.

modern electronic chargers can have functions such as monitoring the charge to not overcharge the battery, it also may have a battery rejuvenation mode that could correct some bad battery conditions.
#10
Old 06-26-2010, 02:58 PM
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Quote:
Originally Posted by Skammer View Post
I last used the battery two years ago so it's probably not completely dead, but it might be worthwhile to spring for a charger.
Measure the open-circuit voltage of the battery using a digital voltmeter, and then report back. If it is significantly discharged, and has been in this state for a while, there's a good chance sulfation has weakened or destroyed the battery.

FYI, you should never allow a lead acid battery to be discharged for a long period of time. Due to their thicker plates, deep cycle batteries are more forgiving in this regard, but they can still be damaged if they sit too long in a discharged state.

When storing a lead acid battery, you should keep it on a float charger. Here is one for only $8.
#11
Old 06-26-2010, 06:46 PM
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Join Date: Apr 2010
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Dealing from ignorance

The fact is you have a battery of unknown capacity and charge and it's already two years old. One option is to have a car shop charge it and do a load test on it so you know what you have. If you want to do it yourself, you must/should start with a cheap volt-ohm meter to measure the starting voltage. It would be about 12.62VDC when charged with the top charge removed. Then you would need a charger to get it up to full. Even the cheap ones have gauges and instructions. However, this seems like mostly a one time deal for now, so I suggest just getting it charged by someone who knows and has the equipment.
#12
Old 06-26-2010, 06:49 PM
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Join Date: Apr 2010
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Dealing from ignorance

The fact is you have a battery of unknown capacity and charge and it's already two years old. One option is to have a car shop charge it and do a load test on it so you know what you have. If you want to do it yourself, you must/should start with a cheap volt-ohm meter to measure the starting voltage. It would be about 12.62VDC when charged with the top charge removed. Then you would need a charger to get it up to full. Even the cheap ones have gauges and instructions. However, this seems like mostly a one time deal for now, so I suggest just getting it charged by someone who knows and has the equipment.
You could simply run cables from your car and run the car for an hour. That's the most expensive way as it will overheat your alternator and reduce the life if the battery is very low or defective. If you have jumper cables, bring them along so you could do that in an emergency, and only then.
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