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Old 03-20-2009, 06:58 PM   #1
Roostertail
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Default how to hook up two batteries parallel

Wanting to have more battery life so I can run my boat lights all night. I only have one 12v battery and am afraid I might not have the juice to start the engine in the morning. If I get another 12v battery how would I hook it to each other.
Thanks

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Old 03-20-2009, 07:01 PM   #2
proguide
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Default Re: how to hook up two batteries parallel

why not have one batt. to the motor and one for everything else?? to hook them together wire the plus to plus and neg to neg that will still give you 12 volts
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Old 03-20-2009, 07:12 PM   #3
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Default Re: how to hook up two batteries parallel

you can get a battery isolator.
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Old 03-20-2009, 07:39 PM   #4
Bonneville Dan
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Default Re: how to hook up two batteries parallel

Yes, use an isolator, or an A-B switch (e.g. Perko). Wiring two batteries in parallel wastes some amount of power, since the two batteries do not discharge at exactly the same rates, so one will actually cause some discharging of the other. The diodes in the isolator prevent one from drawing down the other. Better yet, an A-B switch doesn't suffer the drawback of the slight voltage drop and wasted power due to the diodes in the isolator. Either is simple to wire, and make sure connections are tight and wire gauge is sufficient.

Unless you're really interested, I'll skip the details of Ohm's Law, voltage drops, diode nonlinearities, battery charge/discharge inefficiencies, wire and contact resistances, etc. If so, let me know, or read up. There's a great book at Hillsboro's Main Library (the new one on Brookwood) about 12V battery systems that even taught a crusty old electrical engineer like me a few new things.

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Old 03-20-2009, 07:48 PM   #5
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Default Re: how to hook up two batteries parallel

A - B switch is a great Idea, make sure they will both charge when running the motor.
When I have a bad batterie in my camper I replace Both batteries at same time, so I have the same batteries together. Unlike batteries will not work very well together, they have a problem with life inthat the weaker one will draw down the stronger one.
This is also true for my diesel pickup, They take 2 Batteries to start in cold weather & they better both work.
I have made it a habbit & been told by batteriy shope this is a good practice to follow.

Good luck
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Old 03-20-2009, 09:10 PM   #6
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Default Re: how to hook up two batteries parallel

RT,
I would go wirh the Perko switch or a continuos duty solenoid. One problem with isolators is they use diodes. Diodes do great for blocking current in one direction but have resistance in the other. This can cause a problem when it comes to the charging part. If you are using a trolling motor to charge you may not get much to the batteies in the end. My 8 horse Honda I believe puts out 5 amps. Run it at idle trolling, split that in half and then try to push that thru some resistance and you don't end up with much getting to the batteries. The switch leaves your starting battery alone and the assesories only draw off the auxilary battery.

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Old 03-20-2009, 09:15 PM   #7
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Default Re: how to hook up two batteries parallel

http://www.allaboutcircuits.com/vol_6/chpt_3/3.html

However I think you want one cranking battery and one house so you can always start the motor
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Old 03-21-2009, 12:18 AM   #8
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Default Re: how to hook up two batteries parallel

I have dual batteries in the rear of my boat and one under the dash for the electronics. The batt's in the rear are wired ***. to ***. and neg. to neg. as stated by proguide earlier. What this does is gives you 12 volts but bumps up the cold cranking amps. I also had a battery switch (PERKO) installed, so I can switch between one or two batt's. the one under the dash is on a seperate circuit for the electronics. A Perko switch is exactly that, a switch. It is not an isolator. It's a switch. You can switch from one batt. or two, and when your done for the day you can switch it to off so there is no draw on your batt's at all. Just the of a Mechanic.
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Old 03-21-2009, 05:19 AM   #9
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Default Re: how to hook up two batteries parallel

This is what I did in my rv I use two 6 volt t105's hooked *** to neg then *** from the first & neg from the second gives 12 volts so if you want a lot of reserve you could copy that if you want to stay witha straight 12 volt deep cycle system connect *** to *** then neg to neg & you will have a striaght 12 volt setup.
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Old 03-21-2009, 06:03 AM   #10
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Default Re: how to hook up two batteries parallel

Quote:
Originally Posted by TTT boys View Post
I have dual batteries in the rear of my boat and one under the dash for the electronics. The batt's in the rear are wired ***. to ***. and neg. to neg. as stated by proguide earlier. What this does is gives you 12 volts but bumps up the cold cranking amps. I also had a battery switch (PERKO) installed, so I can switch between one or two batt's. the one under the dash is on a seperate circuit for the electronics. A Perko switch is exactly that, a switch. It is not an isolator. It's a switch. You can switch from one batt. or two, and when your done for the day you can switch it to off so there is no draw on your batt's at all. Just the of a Mechanic.
Yes, the variety of Perko switch I like does also have the "both" setting for higher cold cranking amps in a pinch (ONLY helpful if both batteries are good, otherwise the weak one sucks power from the strong one), in addition to the off position in case you have some accessories, gauges, or faulty circuits drawing power when the boat is not being used. There shouldn't be much draw when all is off, but one of our grandkids climbed into the boat one day and flipped a few switches on the dash, which I didn't discover right away. Could have had a dead battery at an inconvenient time if I hadn't discovered it. The Perko OFF position prevents that possibility.

I would not recommend leaving the switch in the "both" setting except in an emergency to start the boat. Several possible problems can occur including the strong battery/weak battery situation discussed above, as well as being tough for the charging system to try to charge two batteries at the same time. If they are not at exactly the same charge levels or the same cell types in the same condition, one will get more charging current than the other, and in any case neither will get the full charging current from the alternator/regulator. While running the boat, I switch between A and B part way though the fishing day to make sure both batteries get some charge time on each trip. The switch also provides an easy way to assure sure both batteries are capable of starting the boat while you're still at home or at the dock.

Good luck,
Dan
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Old 03-21-2009, 07:00 AM   #11
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Default Re: how to hook up two batteries parallel

Use a switch. If you hook up the batteries without a way to isolate them, when one of the batteries reaches a full charge, it will tell your voltage regulator in the charging system that the system is fully charged, and the second battery will be less than full. with a switch, you can always fully charge both batteries. I've used isolators too, but like he said, the diodes allow wierd things to happen, and are complicated.
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