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Unread 03-31-2010, 08:24 AM   #1
TheGhost
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Dual Batteries

Got a quick question that I'm sure someone like JeepHammer or Mike Romain will know the answer to. If I am running two batteries (12 volts) and one of the batteries starts to not be able to hold it's charge - will that hurt the other battery or cause it to discharge prematurely (the bad battery draining the good battery)? Does low voltage in one battery act as a load on the other battery?

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Unread 03-31-2010, 08:44 AM   #2
jfwireless
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Quote:
Originally Posted by TheGhost View Post
Got a quick question that I'm sure someone like JeepHammer or Mike Romain will know the answer to. If I am running two batteries (12 volts) and one of the batteries starts to not be able to hold it's charge - will that hurt the other battery or cause it to discharge prematurely (the bad battery draining the good battery)? Does low voltage in one battery act as a load on the other battery?
Yes, a bad battery can impact a good battery if in parallel. Do some searches, JH has made serveral posts regarding dual battery set ups.

In general when batteries are in the permanent parallel mode they should be of the same type, size and, age.

Jim
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Unread 03-31-2010, 08:48 AM   #3
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Depending on how your dual battery system is wired will determine whether the bad battery will drain the good battery.
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Unread 03-31-2010, 08:49 AM   #4
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X2^

The weaker battery will impact the stronger one.
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Unread 03-31-2010, 08:49 AM   #5
TheGhost
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OK - Thanks. That's sort of what I thought - just looking for confirmation.
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Unread 03-31-2010, 08:50 AM   #6
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^x3 does the same thing with golfcarts. best thing to do is get a battery switch then you have the option of running one or the other or both at the same time
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Unread 04-01-2010, 12:52 AM   #7
Cutlass327
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Semi trucks run 3 to 4 batteries, and if one developes a bad cell or whatever, it can take the other 3 down to where it needs jump started.
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Unread 04-01-2010, 07:02 AM   #8
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When a battery in a connected string goes bad,
It demands CONSTANT charging...
That will OVERCHARGE the rest of the string, and overcharging results in overheating and warped plates.
---------------------------------------------------

One way to avoid this is with a CHARGE ISOLATOR.

A charge isolator takes the current from the alternator,
And uses a voltage regulator for EACH BATTERY.
This DOES NOT work if you have the batteries wired together in a string...

In your case,
You would want to,

A. Fix the dead battery, probably replace BOTH at the same time with the same age, size, type of batteries,
And use the 'Good' battery in something else...

B. Replace the 'Bad' battery, and wire your batteries for 'Isolation'...
Which adds some wiring, but also gives you tremendous trail redundancy.
--------------------------------------------

If your batteries are wired directly together during operation,
(Even with a solenoid type 'Battery Isolator' that connects them together during operation)

You will want to replace BOTH batteries with the same size, type, age...

An 'Old' battery next to a new battery will cause the same issue, just slower to kill the new battery.
The older battery requires more charging, while the 'New' battery is getting cooked.

If you have a 'Staring' or 'Automotive' battery next to a 'Deep Cycle' battery, you should NOT connect them together except in emergency situations....
(Jump starting, winch operation, ect.)
'Deep Cycle' and 'Starting' batteries are built MUCH differently inside,
And charging the two together will cook one of them.

If you have two different sizes of batteries,
One a larger, higher output, and one a smaller, lower output battery,
You MUST use them isolated.
Again, one will demand more charge, and the one that doesn't need it will cook.
-----------------------------------------------

With a SECOND starter relay (Solenoid) on the fender, you will connect BOTH batteries to the starer when starting.
This gives you TWICE the starter amperage, and your starter will be VERY happy!

I actually had a starter relay quit working, and didn't even know it until I did maintinance/oil change and checked the starter relays individually, finding out one had given up as much as 3 months earlier!
The second starter relay started the vehicle without me even knowing one was not working! TRAIL REDUNDANCY!

This also keeps the batteries ISOLATED when not in heavy/emergency use.

Combined with a charge isolator if you have different size, age, type of batteries,
The batteries are NEVER connected together except when you are lugging with the starter.

A second set of winch relays will keep the batteries isolated until you do some winching, then BOTH batteries will help the winch work,
And go back to being isolated when you shut the winch off.

With a solenoid or manual connector switch between batteries,
You can 'Self Rescue',
Self jump start if the lights or radio gets left on,
You simply connect the two batteries together, and start the vehicle up to charge the dead one back up.

I once had the alternator quit about 20 minutes into a trail day at the local off road park...
That would have normally been the end to my day, and my $20 admission fee...
But I flipped the self rescue switch, both batteries worked together, and it shut off the 'Non Essential' items, like radio, air compressor, lights, ect.
And finished my day, picking up a common alternator and installing it on the way home. TRAIL REDUNDANCY!

It's extra wiring, which people run from like the plague,
But once you understand the principal, it's not hard to do at all.

Last edited by JeepHammer; 04-01-2010 at 07:23 AM..
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Unread 04-01-2010, 08:44 AM   #9
TheGhost
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Thanks for all the insight. I think I'll move this down a bit on the list until I can get 2 new batteries and do it right. I found another JH thread that had the wiring diagrams - be not afraid...for I am Sparkacus!

Thanks to all who responded!
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