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Optima batteries in parallel...what is the result?


37PackardMan
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My 1937 Packard Super-Eight battery case  can cradle two Optima 6-V batteries.

If I install two batteries in parallel, and one is nominal strength and the other is weaker, WHAT do I end up with?

I KNOW that the result is still 6 volts, but what do I actually end up with amperage-wise.

Does the weaker one drag down the nominal one and I end up with the average amperage, or is the result a summation and the weaker battery bolsters the nominal one.?

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In theory, they should equalize at about the same level of charge. The stronger one will discharge into the weaker one. You'll still have plenty of amperage, it just won't be able to crank as long. Two Optimas will deliver plenty of punch to start just about anything, even if they're a little soft. I put two in my '35 Lincoln when I first got it and it was running hot, and it would never start once it was warm. With two Optimas, there was no way for that sucker NOT to start. It kicked over like a 12-volt system, hot or cold. Really impressive.

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What Matt said. Still, it would be highly desirable to start out with 2 in the same condition.

 

50 minutes ago, 37PackardMan said:

I KNOW that the result is still 6 volts, but what do I actually end up with amperage-wise.

Does the weaker one drag down the nominal one and I end up with the average amperage, or is the result a summation and the weaker battery bolsters the nominal one.?

 

All I can tell you for sure is that you would have more amps AVAILABLE than with one battery, unless one of them was really toasted and not doing anything except dragging the system down.

 

As for the actual amps drawn by the starter, less is better. A DC motor like a starter makes maximum torque when stalled. It also draws the most current. So, if it has trouble cranking and chugs down, it draws more amps. The battery will do all it can. It can get really out of hand. The voltage falls. The starter tries to draw more but really can't because the voltage is falling. This can be a downward spiral with a weak battery. This part is confusing. I'd stick with the previous paragraph. A heavily loaded starter will try to draw more amps. If you have more amps AVAILABLE, that is a good thing.

 

 

Edited by Bloo (see edit history)
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Back in the GM 5.7L diesel days, they had two batteries, and typically when it wouldn't start we just replaced the older battery. Not saying that was correct, just saying it worked for those 22.5:1 compression engines. So, two unequal age or capacity Optimas of the same voltage will be fine together.👍

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...What Matt said. The initial process of discharge or equalizing can build up heat in the cables or the battery itself so beware. This sort of thing is going on all the time with solar panel set-ups, generator back-ups, etc and there's mathematics on-line on other sites and forums. As long as the battery type and rating is the same usually no problems. The amount of available current will be less than the summed rating of both batteries as some current is "wasted" feeding the other battery. As I've mentioned in another thread however is the question one has to ask, "do you really want that much amperage available at all times through all circuits?" (e.g. how reliable is all the extant wiring, contacts, fusable links, grounding, etc).

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To clarify, on long distance tour cars, I run two batteries but only one hooked up. In the event of a generator failure that way I have a full battery to run on for a few hundred miles. This occurred last fall on the Blue Ridge Parkway on the Duesenberg tour. The generator failed. The battery slowly ran down while going along on the tour. (I wasn’t in the car so I didn’t see the guage read discharge.) I was right behind and could see the car develop a running problem……low fuel pressure from the factory electric pump no longer keeping up pressure or volume. On the side of the road trying to diagnose the fuel issue I quickly figured out the issue. Shut the car off, disconnected the generator, swapped over the battery cables from the dead battery to the fully charged battery, and we were off in less than five minutes. Finished the daily tour, and after dinner installed the spare generator and swapped out the dead battery for a fully charged spare. Fast, easy, and a minor issue to deal with in the parking lot. 
 

On show cars, I run both batteries as I like the extra bang when cranking. Especially since the motors are new and need the extra power from higher compression. So EVERY car in our collection has two Optimas. They are all set up to run as dual or single with cables that can swap or be installed on a minute notice. Works great, but at the price of Optimas today it is expensive. I use Brillman to make up all my speciality cables. Fantastic quality, fast turn around, and very reasonable. 
 

Along with the dual battery set up, on all our tour cars I carry a spare generator, starter, loaded distributor, fuel pump, ect. Each component has all the tools and hardware to deal with it on the side of the road……..I use cheap Harbor Fright tools in the road kits. That way I don’t need to hunt for tools while on the shoulder. Preparation before the tour is fifty times easier than making things up as you go when your stuck in the middle of nowhere.

Edited by edinmass (see edit history)
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5 hours ago, prewarnut said:

As I've mentioned in another thread however is the question one has to ask, "do you really want that much amperage available at all times through all circuits?" (e.g. how reliable is all the extant wiring, contacts, fusable links, grounding, etc).

Yes. One battery can start a fire. So fix the issues first. Drive happy. 
 

Your receptacles at home are connected to an infinite grid, do you complain?

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We replace our 6-Volt Optimas in pairs - may be a bit more expensive, but how do you put a price on real dependability - or worse, being the guy on the side of the road?

At least as important is the size (and quality) of your battery cables, ends, and clean, non-painted, non-corroded connections !

We generally use Triple-Aught (000) sized stranded copper wires. Yes, they are a bit more pricey, but then quality isn't cheap, and neither is piece of mind.

There are too many 6-Volt cars trying to run on cheap 12-Volt cables, with owners thinking they need to "upgrade" to a 12-Volt system when lightweight cables are more an issue.

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