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Location: Green Bay, WI | You could indeed just hook up more than one 12-volt battery in parallel, and the engine wouldn't know any difference (neither would the accessories you are powering). However a potential issue is that if one discharges, then so does the other. Say your alternator doesn't charge anymore--then both batteries will simply continue to power the circuits until BOTH go dead.
The "right" way to do it, would be to install an Automatic Charging Relay (ACR), that isolates each battery, but still allows each to be charged by the alternator on the engine. Check out the information at this link:
http://bluesea.com/category/2
...and I think you'll see what I mean.
Keep in mind that an ACR is *not* absolutely necessary--it's simply an added measure of protection against both batteries going dead. When the voltage of one of the batteries goes below a certain point, then that battery is isolated from the circuit and a low-voltage light illuminates (on certain ACR models). But this might be overkill for most applications, to be honest, and I have seen guys simply put another tray and battery next to the start battery, just to have it available if needed. If the one battery supplies the minimum cranking amps required by the engine, then all is good. Of course you'd have to keep the spare battery charged yourself, but it will serve the same purpose in the event that the main unit dies...you will just have to disconnect the dead unit and hook up the fresh one.
If you are interested in learning more about the ACR thing, just let me know and I can point you towards more information on the web.
EDIT: RE-reading this post, I guess I should clarify one thing I said. If two 12-volt batteries are wired in parallel, then the volts in the entire circuit will be 12. However the current drop across each 'leg' will be additive, because the load (the engine starter) is being picked-up by both batteries. So in the case of a dual-battery configuration, the cranking capacity of each battery is essentially additive, and thus then engine "sees" much more cranking power from the power source (ie; both your batteries in parallel). For you this means the engine may turn over noticeably stronger, depending of course on the capacity of the existing battery you have there now. If it's more than sufficient for the engine's needs, then you might not notice ANY appreciable increase in start performance. But if it's borderline, your engine will like seem to "jump to life" when you turn the ignition key.
It's a small point, but one worth making: Parallel-wired start batteries can indeed make a difference, but don't expect to see a huge performance increase if the existing battery you have is of the correct size (capacity), and is properly maintained. I think that the main issue for most guys, is simply having another battery available in case the main start unit goes down. And for owners of the Mercury Verado engines, having strong, steady power is crucial--as the engine is very dependent on the proper voltage for normal operation. You can check out this issue in the library section of the Verado Owner's Club, if you want to know more.
http://www.veradoclub.com/smf/index.php?board=6.0
TB
Edited by tcbetka 4/22/2009 10:41 AM
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