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Author Topic:   Behavior of Batteries Wired In Parallel
Knox posted 05-14-2008 12:23 AM ET (US)   Profile for Knox   Send Email to Knox  
Hello! I am hopeful that you can advise me as to whether my idea will work or not. I currently have a single dual purpose starting / deep cycle battery in my 21 foot boat. I was considering installing a second identical battery in parallel (not series) so that I can essentially double my amp hours without effecting voltage as running in series would do. I do a bit of fishing and recreational boating where the motor is off for extended periods of time and I run a number of electrical items. I know I can install a dual battery switch with the whole setup that goes along with that, but if this other idea is an option it will get me up and running for a good bit less expense as I already have the second battery.

So specifically, am I correct that this will essentially double my reserve minutes / amp hours? And, with them hooked up in parallel, will my alternator charge them both when I'm running the motor? Otherwise, is this a viable option to get some of the benefit of 2 batteries "on the cheap?"

Thanks in advance!

jimh posted 05-14-2008 09:32 AM ET (US)     Profile for jimh  Send Email to jimh     
Batteries wired in parallel tend to behave more like the weaker of the two than the stronger of the two. When charging batteries in parallel there can be problems if the two batteries are not similar in their state of charge and internal resistance. The charging current may not divide among the batteries as expected. However, if both batteries are nearly identical, paralleled batteries will work, after a fashion.

Most boats with dual batteries use a primary battery distribution circuit that includes a switch in order to isolate the engine starting battery from other loads so that one battery will not become discharged to the point where it can no longer start the engine. Many boats are wired in such a manner as to prevent discharge of the engine starting battery by non-engine loads so that the low-charge starting battery situation is unlikely.

Jerry Townsend posted 05-14-2008 01:15 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Knox - with batteries in parallel - the charging current will be divided in favor of the weaker of the batteries.

Now, if one battery develops a short or bad cell - that problem will drain both batteries - and sometimes in very short order - even with the engine running.

While use of solenoids and relays can help regarding the isolation - the best solution is as Jim mentions - the battery circuit distribution system with a selector switch specifically made for that purpose.

Now, you can have two isolated batteries, that can be simultaneously charged, running from say, battery "A" and if needed, the positive cable can be moved to battery "B". This makes you the switching system and eliminates the need for a switch. ------ Jerry/Idaho

newt posted 05-14-2008 09:20 PM ET (US)     Profile for newt  Send Email to newt     
Your least expensive option is carry your second battery and a set of jumper cables that you probably already own until you decide if you really need a second battery.

On the other hand if you want to permanently install the second battery, you should use a switch. They are not all that expensive and you will need additional battery cables no matter what.

jimh posted 05-16-2008 12:26 AM ET (US)     Profile for jimh  Send Email to jimh     
Adding a switch instead of just hard-wiring the batteries in parallel is only very marginally more expensive. There is the cost of the switch, about $30, and the cost of one additional cable with connectors, about $5.

When you compare $35 to the total cost of ownership and operation of a 21-foot boat, you are talking about a very small increment in your outlay.

In the long run with electrical installations, doing it the right way usually proves to be the least expensive.

Knox posted 05-16-2008 12:35 PM ET (US)     Profile for Knox  Send Email to Knox     
Great information and points gentlemen. Thank you for your time. It looks like I need to go with a dual battery switch. You make a good point that if I install it myself there is little additional cost. Do you know where I can find a simple diagram and simple instructions? I don't think it will be hard but I sure don't want to mess it up!
newt posted 05-16-2008 12:46 PM ET (US)     Profile for newt  Send Email to newt
jimh posted 05-16-2008 08:58 PM ET (US)     Profile for jimh  Send Email to jimh     
Most primary battery distribution switches include a schematic diagram of the suggested wiring.

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