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

[37PackardMan]

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.?

[Matt Harwood]

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.

[Bloo]

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 September 23, 2022 by Bloo (see edit history)

[Frank DuVal]

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.👍

[prewarnut]

...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).

[alsancle]

I run dual optimas in all my cars.

 

However, Ed and I have been strategizing about this. And my new plan is to have two, but wired with one reserve. One battery is more than enough to crank over the starter. The bigger issue is when they die and you need to be jumped.

 

 

[edinmass]

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 September 23, 2022 by edinmass (see edit history)

[alsancle]

The problem with a Duesneberg is those 3 electric pumps will run the battery down a lot faster than the ignition alone would.

 

I like the idea of flipping a switch to go to the backup battery.   That way you can alternate them and keep them both charged.

[Frank DuVal]

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?

[Marty Roth]

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.

[Hans1]

Charging batterys connected in parallel with one low or old battery and one newer one can leave battery in best basic condition under charged, and damaged. I seem to reclall a class many years ago.

 

 

[Frank DuVal]

In parallel the voltage is the same across both batteries. Each can draw a different charging current to satisfy its needs.

 

In series, this is when the problem happens, as each battery has the same current flowing through it, so one will be unhappy.

[37PackardMan]

I appreciate all the input.

Many thanks!

 

Bob

[Restorer32]

Optima batteries are all the rage and are good batteries but other suppliers such as Interstate have batteries using the same technology as the Optima but with higher cranking amps. Just saying there are other options available.

[vintagerodshop]

I have had many prewar large cars and have tried a variety of battery options. Unfortunately no perfect solution exists and the optima of today are not nearly the quality they were ten years ago.

 

Two Optima batteries in parallel is not your best option. As you said you have one weaker and one better. The weaker one will pull the other one down and will mask the condition until you have a no start situation. @edinmass suggestion if you can fit two a good optima will be more than enough. The second battery if you occasionally maintain or swap to a primary battery will always be there to support you in your time of need.