Dual battery setup using different batteries

Electrical and electronic topics for small boats
pcrussell50
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Dual battery setup using different batteries

Postby pcrussell50 » Fri Jul 24, 2020 1:46 am

I have a boat that uses one battery. As that battery ages but is still serviceable, I would like to transfer it to my 1999 Alert 17 (Special Service classic smirked Montauk), as a second battery. It has had two batteries before, has a Perko OFF-1-BOTH-2 battery selector switch, and a simple old “battery combiner” that is about the size of half a pack of cigarettes (I don’t smoke so that’s a rough guess). But I think I read somewhere that these are dependent on both batteries being more or less the same capacity and state of health.

Is there a combining device that is suitable to run and charge two 12V batteries that are in different states of age and health?

-Peter

Jefecinco
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Re: Dual battery setup using different batteries?

Postby Jefecinco » Fri Jul 24, 2020 8:03 am

If you are unable to find the combining device needed for your application an alternative would be to acquire a small jumper battery to back up the ageing battery on your single battery equipped boat. I've been using one for many years to avoid the added expense of a two battery system while maintaining the ability to restart an engine if the single battery becomes discharged.

I've used the device three times to aid other boaters and it performed flawlessly. If the electrical demands on your single battery boat are modest the small jumper battery may fit your immediate needs.
Butch

jimh
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Re: Dual battery setup using different batteries

Postby jimh » Fri Jul 24, 2020 9:34 am

All the automatic combiner relay devices that I am familiar with for charging two 12-Volt lead-acid storage batteries simultaneously from one 12-Volt charging current source just connect the two batteries in parallel when the device actuates. The “science” or the “feature” that these devices offer as being advantageous is WHEN they self-actuate, which is the element of those devices that makes them “automatic”, that is, they control themselves.

Typically an automatic combiner relay (ACR) actuates to add a secondary battery in parallel to the primary battery only after the terminal voltage of the primary battery has been raised to a particular voltage threshold. The threshold voltage is typically chosen to be above the normal resting voltage of a fully-charge 12-Volt lead-acid battery, and typically the threshold will be around 13 to 14-Volts.

The basis for this design is to only add the secondary battery after the primary battery has been fully recharged. Only then is the secondary battery added in parallel.

If two batteries are in parallel and being charged, the concept of how the charging current will divide between them is somewhat murky. The proposed outcome is the majority of the charging current goes to the secondary battery on the basis that its state-of-charge is lower and thus its terminal voltage is lower. But I am not aware of a really good discussion about exactly what will happen, because the primary battery can now be a source of current to charge the secondary battery, just as if you connected them in parallel without an external source of charging current.

The ACR devices really do nothing more than you would do if you moved the OFF-1-BOTH-2 switch to BOTH from 1.

Some ACR devices use captive cables which are intentionally designed to add a small resistance to the parallel connection in order to act as a current limiting device.

Can these ACR devices work with two batteries that are not identical? Yes. No two batteries are really identical unless their construction, age, pattern of use, temperature, electrolyte, and state-of-charge are identical. That seldom occurs, and is particularly not going to occur with an ACR because it waits for the primary battery battery to be fully charged before adding the secondary battery.

While the ACR device is not perfect, it is better than having to move the battery selector switch by hand.

jimh
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Re: Dual battery setup using different batteries

Postby jimh » Fri Jul 24, 2020 9:42 am

As an alternative to an ACR device, the charging of two batteries from one charging current source can be managed non-automatically by just moving the battery selector switch to either the “1” or “2” position, and then ALL the charging current goes to the single battery that is connected. The only drawback is the operator has to do this manually, and there needs to be some regimen in place to be diligent in setting the switch to an individual battery.

pcrussell50
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Re: Dual battery setup using different batteries

Postby pcrussell50 » Fri Jul 24, 2020 10:52 am

Thanks guys. Fantastic info, Jim. Looks like I can use my existing ACR too and won’t need to buy a new one.

-Peter