Quick Battery jumping question

moboy3201

Recruit
Joined
May 19, 2009
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4
I'm sure this has been answered somewhere before, but I'll ask it anyway. I have a 12 volt which I use for my start up lights and accessories (which I rarely use) and then I put in a deep cell which I only run my bilge pump off of. I believe I can run a jump wire from one to the other so when I am underway, I can simutaneoulsy charge both batteries. I have not installed a battery switch yet and will be doing so in the future, but I need to be clear on the jump wire. I've heard it is negative on the deep cell to positive on the 12 volt? I'm just paranoid about the bilge running out of juice if I can't get to the boat for a week or two and we've had strong storms. Thanks.
 

TheMan_90241

Cadet
Joined
Mar 15, 2008
Messages
17
Re: Quick Battery jumping question

So long as both batteries are rated at 12v, then you can run them in parallel. You do this by connecting the Positive from battery #1 to the Positive on battery #2. Do the same for the negative terminals as well. This will configure them to run as one larger battery. Theoretically, they will both charge together at the same rate. However, there are a few things to consider.... Issue #1 - Initially, the batteries will equalize. In other words, if you have one battery that is charged to 12 volts and the other is only partially charged to 11 volts, after connecting the two in parallel you most likely will only have a 11.5 volt charge available until they both get fully charged. Issue #2 - Since they are connected together in parallel, they will also be drained at the same rate, leaving you with no "spare" or "reserve" battery to use once they have been discharged. This can be solved by installing the battery switch. Issue #3 - If you are using them in parallel and the batteries are of two different kinds (i.e. #1 is a standard 12v automotive battery and #2 is a deep cycle marine battery) you may not get the long lasting amp-hours from the deep cycle marine battery that it is designed for, because the standard automotive battery is discharging at a faster rate. Therefore, the standard battery is constantly trying to equalize with the longer lasting deep cycle battery.

In my opinion, bring the two cables necessary to "jump" the two batteries together in parallel only if the need arises to start your motor. Buy and install the battery switch as soon as possible.

I hope this helps. Maybe someone else can chime in and correct any of the technical issues I may have overlooked.
 

H20Rat

Vice Admiral
Joined
Mar 8, 2009
Messages
5,204
Re: Quick Battery jumping question

I'm sure this has been answered somewhere before, but I'll ask it anyway. I have a 12 volt which I use for my start up lights and accessories (which I rarely use) and then I put in a deep cell which I only run my bilge pump off of. I believe I can run a jump wire from one to the other so when I am underway, I can simutaneoulsy charge both batteries. I have not installed a battery switch yet and will be doing so in the future, but I need to be clear on the jump wire. I've heard it is negative on the deep cell to positive on the 12 volt? I'm just paranoid about the bilge running out of juice if I can't get to the boat for a week or two and we've had strong storms. Thanks.

you REALLY don't want to have wires that you move while underway. You are creating sparks and risking starting a fire.

Anyway, unless you need 24 volts, you NEVER want to connect a positive to a negative. Always leave the negatives grounded together, no matter what. Then if you want to charge the second while underway, connect the positive terminals. Also keep in mind if you have an outboard motor very few have charging systems that will be able to charge 2 batteries. Most are just a stator/flywheel charging system that is good for maybe 10 amps.
 

bustermin

Banned
Joined
Apr 13, 2009
Messages
315
Re: Quick Battery jumping question

if you connect positive to negative, you will turn the system into a 24volt system. this is how trucks and heavy equipment batteries are wired.
 
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