Charging Dissimilar Batteries in Parallel

[boscoe99]

This has been harder to depict than I had imagined.

On some sites it is stated that two dissimilar batteries should never be charged in parallel. Reason stated is that one battery, or the other, will have an adverse effect on the other battery.

My tests don't indicate this. Photos below show two batteries connected in parallel while being charged. One photo has the large battery fully charged with the smaller battery being charged. The other photo has the smaller battery fully charged while the larger battery is being charged. The fully charged battery has nil current flow while the less than fully charged battery draws whatever current that it normally draws.

Circuit looks like this:


Photo of the two batteries;

[99yam40]

I would think by putting a set voltage across a battery or cell of a battery the battery/cell will take charge as it is needed to get to the proper voltage/charge.
but if you series some batteries or cells, there can be problems if they are not all the same

[boscoe99]

Yes. Being in series is another kettle of fish. Seems to be internet lore that it is unwise to parallel different batteries. That is my story any way until proven otherwise.

Here are some views of the other power supplies. Results with respect to charging batteries in parallel are about the same. The power supply is much less jumpy. Steadier voltage and current readings.

Motor driven automotive alternator;


Motor driven permanent magnet generator;

[99yam40]

well I want to see the permanent magnet generator too.
I am trying to remember what you ended up using, if you told us before.
was it off an outboard?
or maybe a modified alternator?

[boscoe99]

It is an AC Delco 12SI alternator that was modified by the removal of the voltage regulator, brushes and rotor. Rotor was replaced with a rotor having magnets. It only outputs AC voltage. Unconrollled. Voltage rises with the speed of the rotor. Wish I had a variable speed motor. Right now I have to change pulleys to change the rotor speed.

https://mwands.com/wind-turbine-motor-pma

[DennisG01]

Hmmm. Interesting about charging in parallel with one battery "full" and one battery "discharged". Yes, I have always understood that this wouldn't work. Even the battery charger companies (good, reputable ones) that I have called and asked this said the only way to properly to do it is to first completely charge each one individually.

Is there a way to further test your theory by letting them charge and then load testing each one or maybe an even more accurate way of measuring/testing to see that they are, in fact, FULLY charged? Maybe a "before and after" load test (or another type of test)?

[boscoe99]

I have a hydrometer that indicates they are fully charged. Also, based on the resting voltage I believe they are fully charged.

[boscoe99]

With an automatic charge relay in use, or a battery switch set to the both position, are not both batteries being charged a the same time?

[DennisG01]

With an automatic charge relay in use, or a battery switch set to the both position, are not both batteries being charged a the same time?

Well, technically (correct me if I'm wrong), an ACR doesn't charge both at the same time - it determines WHICH battery needs the charge and diverts the juice in that direction.

With a switch, charging in the "both" position would essentially be the same thing your test is doing. But I've always known (or at least I thought I did) that charging on the "both" setting is never as good as "1" or "2".

There's got to be a good reason why ACR's are considered a great update over the standard battery switch?

Note that I'm NOT saying you are wrong - I'm just trying to wrap my head around this since I've "known" the opposite (to your findings) for may decades. A couple quick questions - or maybe more appropriately, looking for "holes" - which, in reality, is a good/proper way to check the findings on something like this - possibly to help prove it!

-- Did you use the hydrometer and check resting V on the battery after being fully charged by itself? In other words, a "baseline" before starting the test.
-- When you checked resting V... was it immediately, or after 'resting' overnight to eliminate a surface charge (or a quick zap of power from a 12V item)?

[boscoe99]

Not sure when I checked with the hydrometer. Need to get a better one.

Yes to letting the batteries rest overnight.

My understanding of the ACR is that it is acting just like a switch. But electrically con*****ed. And only activates when the voltage of either battery is above X volts. To make sure that power is available to charge the batteries.

I might need to get an ACR to double check what I believe. All of my beliefs don't turn out to be correct.

[99yam40]

It is an AC Delco 12SI alternator that was modified by the removal of the voltage regulator, brushes and rotor. Rotor was replaced with a rotor having magnets. It only outputs AC voltage. Unconrollled. Voltage rises with the speed of the rotor. Wish I had a variable speed motor. Right now I have to change pulleys to change the rotor speed.

https://mwands.com/wind-turbine-motor-pma

Oh yea, now I remember you mentioning those things.

Have you run the test long enough to get the low charged battery charged up enough to shed some of the current it was taking in , and see both batteries share to some extent?

or go the other direction with you test and have the big battery low and the small fully charged to see how they react.

and then you could hook up more than 2 batteries all fully charged to see how that goes

[boscoe99]

Yes, I have connected the two batteries in parallel with them being at a different state of charge. The battery will the higher state of charge will transfer current to the lesser charged battery.

Once the states of charge are equalized they act as one battery. One bank if you will. With output being the sum of the two.

[99yam40]

so what is the purpose of this exercise?
as long as the batteries are at the same voltage/charge the the power supply will put the same amount of current into both batteries.

I would think that a larger battery will take more current than a smaller one when the batteries are at the same charge just because there is more capacity that needs to be fed

[boscoe99]

so what is the purpose of this exercise?
as long as the batteries are at the same voltage/charge the the power supply will put the same amount of current into both batteries.

I would think that a larger battery will take more current than a smaller one when the batteries are at the same charge just because there is more capacity that needs to be fed

I read lots of funky information over on the other site. For instance, to be charged in parallel, batteries must be of the same type (flooded lead acid or AGM lead acid), must be of the exact same size, must be about the same age, etc., etc., Otherwise, one battery will either be discharge or will over charge the other battery.

Knowing just a tad of ohms law, that made me suspicious. Furthermore, ACR's/VST/Isolators/Combiners/Etc. are doing this daily with apparently no reported ill effects. So I decided to install some volt meters and amp meters to see what is to be seen. I am not seeing what folks are saying.

Think about it. In your house you have one power supply (wall outlets usually) and many different loads. Clocks, fans, refrigerators, lamps, etc.. Plugging in one device is putting it into parallel with any other devices that are plugged in. One device does not affect the others. Well maybe a refrigerator or air conditioner may momentarily affect the voltage but the system quickly recovers. Why would charging batteries be any different?

My comments relate to when two or more batteries are being charged. Power supply having a higher voltage than either battery. If they are in parallel and no power supply, all bets are off.

Am I missing anything?

[Keithbaja]

Well, YES and no.. in a real life scene such as on a boat with a small, wimpy outboard alternator, undersize cable runs. Ground faults and a host of other possible varible's.. the answer your seeking grass hopper might be more defined so as I can give you the correct answer.. This is my forta and am a Electrical spe******t. Love doing diagnostics and system design. As soon as the two "Battle Born" 100 ah lithium Batterys show up I doing a sweet install. Custermer has cked my refs and had a easy 'energy audit'. More or less open check on my refs for all the required parts.. yes haw.. no cutting corners on three batt solar system with 1kw pure sine wave inverter. The battery company is running behind with high demand since the hs covid scare. And now the solar company that I'm sourching with has offered to higher me in my local market.. I'm already working 7 days. Jeeze. I could hire 3/4 electrical techs but,, there are none
Major crisis. Anyway what exactly are you trying to figure out ??????