Connecting two 12v car batteries in parallel
Steve Kuo
I was wondering if I could add this to my car's exsiting 12v system by
placing it in parallel, thus providing twice the available current. Is this
safe to do? Possible problems that I can think of include:
- what if one of the batteries fail and causes a short circuit
- too much charging for the alternator to handle
- batteries voltage and internal resistance differ slightly
Is this done in reality? I've heard of trucks and boats having two batteries
using an A-B-both type switch.
Steve Kuo, sd...@oakland.edu
Bob Haar
|>I looked in my garage and found an extra 12v car battery just sitting there.
|>I was wondering if I could add this to my car's exsiting 12v system by
|>placing it in parallel, thus providing twice the available current. Is this
|>safe to do? Possible problems that I can think of include:
Safe? as safe as any battery work - acid spills, hydrogen expolosions, etc.
are possible, but you will be reasonable safe if you take precautions
during installation. After installation , there should be no particular
psafety concerns.
|>- what if one of the batteries fail and causes a short circuit
|>- too much charging for the alternator to handle
|>- batteries voltage and internal resistance differ slightly
All these are real possibilities, but he last is the most likely
to give you difficulties.
|>Is this done in reality? I've heard of trucks and boats having two batteries
|>using an A-B-both type switch.
|>
Two battery systems are often used in commercial trucks and in RVs and boats,
but for different reasons. Trucks often use different two different
voltage systems 12/24 or 12/48, that are electrically independent. the
higher voltages are used for starting the diesel engine and the 12v
system is used for electronics, accessories and lights (light bulbs
don't last as long at higher voltages).
RV's often used two 12v batteries, one for normal vehicle operations
and one for running the TV, air conditioner, etc without effecting
the primary battery. This is so you can still start the RV's engine
even if you run the secondary battery down. Typically, both
batteries are charged by the normal alternator/regulator system
but their outputs are isolated by a diode package. You can buy these
at RV specialty stores.
Steve, you didn't say why you want to have the second battery. This
is the crucial point in determining if it makes sense.
If you want to power accessories (including high power audio) without
draining the operational battery. In that case, you are doing the same
thing as the RV setup. Get one of the isolation units and wire up your
accessories through a separate fused feed. The biggest challenge is
likely to be finding a place to mount the battery.
If you think you need more electrical power for normal operations,
you've got some other problem. During normal operation, the battery
is not providing electrical power, it is all coming from the alternator.
If this isn't the case, something is wrong. (loose belts are an
often overlooked cause.)
If you want an extra battery for more starting power, you are also
looking in the wrong place. If your car is in good shape, the regular
battery should provide more than enough power. Winters in Rochester, MI
are not that that severe. I've lived in Minnesota where if you want
your car to start in the winter, you plug in a block heater. If your
current battery isn't making the grade, either you need a new battery
or you have some other problem.
If you want a reserve in case you leave the lights on, think about it
more carefully. If you connect the two batteries in parallel and leave
headlights on all night, both batteries will be dead. You will just
up the night for a longer time. Is that worth it? What is a better
scenario is to have a backup power source that you connect after
you discover that the primary battery is dead. You can do that
with the RV isloation unit and still keep both batteries charged
normally. The you need some way to switch over to the backup. Moving
cables would work. An easier solution is to buy one of the batteries that
has a built-in backup with a switch. I have never used one, but I
have seen them advertized.
Robert Haar InterNet : rh...@gmr.com
Computer Science Dept., G.M. Research Laboratories
DISCLAIMER: Unless indicated otherwise, everything in this note is
personal opinion, not an official statement of General Motors Corp.
Larry N. Osborne
>I looked in my garage and found an extra 12v car battery just sitting there.
>I was wondering if I could add this to my car's exsiting 12v system by
>placing it in parallel, thus providing twice the available current. Is this
>safe to do?
>Is this done in reality? ...
I can't answer the safety question, but is _is_ done. I used to run
an agency in the mountains (such as they are) of central Pennsylvania
which used three 10-ton trucks. We used them to haul stuff, and then
they would stop and operate lights and minor electrical equipment for
relatively long periods of time off the batteries. They were Fords
adapted by Gerstenslager, and had two batteries connected in parallel
with simple cables. The cables were the same guage as the ones
running from the front battery to the starter, and were about six
inches long. I ran the place for five years, and the trucks were over
five years old when I got there, so if problems were going to develop
they had a good chance. Like most low-budget operations maintenance
(other than obvious saftey related stuff) had a low priority: the
batteries weren't the same brand, or even the same capacity. No
problems. Worked fine.
And when the temperature dropped below -30 or so we'd hook them in
series to start the damn things (which I _don't_ recommend!)
--
osb...@uhunix.uhcc.hawaii.edu (preferred) | Larry N Osborne
osb...@uhunix.bitnet | SLIS, 2550 The Mall
fax +1 808 956 5835 | University of Hawaii at Manoa
or via W.A.S.T.E. | Honolulu, Hawaii 96822
Tom Bruhns
(as part of a very good answer to a question about car batteries)
>Safe? as safe as any battery work - acid spills, hydrogen expolosions, etc.
>are possible, but you will be reasonable safe if you take precautions
This reminds me of something worth relating if it helps folk be more safe:
Just in case someone may not believe in hydrogen explosions in car
batteries, here's what happened to me one morning: I started the
car, backed out of the garage, the car stalled. I turned the key
to restart it, and instead of starting, it went BANG! I thought at
first someone was playing a trick on me with one of those practical
joke fireworks things you can hook across ignition systems. A bit
of smoke was coming from under the hood and all. Well, I opened the
hood, and about half the battery case was gone, in little pieces
around the engine compartment. I figured what happened was one of
the cell-to-cell interconnects was weak, and when starting current
flowed through it, it got hot and ignited the hydrogen. That's
about the only reasonable explanation for the strength of explosion.
Fortunately I was where I could easily turn a hose on it to wash away
and dilute the acid; it did leave a stain on the paint in one place
on the fender. -- An urban legend this is not--it happened to me,
not a friend of a friend...