Batteries in parallel questions

[John Kogel]

I'm installing a 1.5 volt digital clock mechanism in an old clock radio. The 120 volt Telechron clock still works but it keep lousy time and sounds like a hive of angry bees. It's worn out.

When I've done this before, the radio had to be opened up when the battery went dead. This time, I'm installing a remote battery holder on the back of the radio. The standard battery holder for AA's is a 3 volt pack,two batteries in series. I've modified the contacts now so the two batteries can go in parallel.

Will two batteries in parallel last about twice as long as one battery?

When one battery dies or loses voltage, will it draw down the other one, or will it just ride along until the other one dies naturally?

[Marc]

The discharge cycle will remain balanced. It's the charge cycle that isn't dependable. That's why systems with multiple rechargeable batteries generally use series connections.

Engine starting batteries for locomotives use series connections. 1 1/2 volts is a common size. Some of them weigh well over 100 lbs despite a voltage of only 1 1/2 volts. 24 volts would take 16 of them, for example. Massive starting power, reliable charge cycle and long life.

Marc

[kurt]

Curious then....

Why do mfg's. indicate one shouldn't mix up batteries (putting a new with old partially discharged)? I've done it in desperation a few times, and it seems they die really quick.

Any explanation, or am I imagining things?

[Marc]

It's not a good idea to parallel batteries in varying state of discharge or of different manufacturer/model, etc.

It's only ok if both are in exactly the same condition, age, model, etc.

What you heard from the battery manufacturers is good advice.

One of two engine starter batteries in my diesel pickup blew up when a mechanic tried to start the engine after working on it. He insisted one new battery was all I needed. I insisted on two because they were in parallel.

Marc

[John Kogel]

Curious then....

Why do mfg's. indicate one shouldn't mix up batteries (putting a new with old partially discharged)? I've done it in desperation a few times, and it seems they die really quick.

Any explanation, or am I imagining things?

Batteries are almost always in series to get more voltage, so those should not be mixed. I'm trying for longevity with 2 in parallel.

[John Kogel]

It's not a good idea to parallel batteries in varying state of discharge or of different manufacturer/model, etc.

It's only ok if both are in exactly the same condition, age, model, etc.

What you heard from the battery manufacturers is good advice.

One of two engine starter batteries in my diesel pickup blew up when a mechanic tried to start the engine after working on it. He insisted one new battery was all I needed. I insisted on two because they were in parallel.

Marc

That is right, the two batteries give you extra cranking power, but still only at 12 volts. And yes the gas given off by lead/acid batts is explosive. Which is why the new ones are sealed, no more adding water.

Thanks, I think new AA's in parallel will work fine. I will include a note that one is fine too.

[Marc]

Curious then....

Why do mfg's. indicate one shouldn't mix up batteries (putting a new with old partially discharged)? I've done it in desperation a few times, and it seems they die really quick.

Any explanation, or am I imagining things?

Batteries are almost always in series to get more voltage, so those should not be mixed. I'm trying for longevity with 2 in parallel.

You can mix batteries in series with no consequence greater than running one of them down before the other.

Marc

[John Kogel]

Curious then....

Why do mfg's. indicate one shouldn't mix up batteries (putting a new with old partially discharged)? I've done it in desperation a few times, and it seems they die really quick.

Any explanation, or am I imagining things?

Batteries are almost always in series to get more voltage, so those should not be mixed. I'm trying for longevity with 2 in parallel.

You can mix batteries in series with no consequence greater than running one of them down before the other.

Marc

I suspect a low battery in a series string will draw current from the good battery(s). But only when the device is switched on. When I get a break from inspections, I might do some tests.

I've decided to avoid confusion and have a single battery holder for the 1.5 volt clock. The battery will last about a year and the radio sits there looking good.

[Marc]

Curious then....

Why do mfg's. indicate one shouldn't mix up batteries (putting a new with old partially discharged)? I've done it in desperation a few times, and it seems they die really quick.

Any explanation, or am I imagining things?

Batteries are almost always in series to get more voltage, so those should not be mixed. I'm trying for longevity with 2 in parallel.

You can mix batteries in series with no consequence greater than running one of them down before the other.

Marc

I suspect a low battery in a series string will draw current from the good battery(s). But only when the device is switched on. When I get a break from inspections, I might do some tests.

I've decided to avoid confusion and have a single battery holder for the 1.5 volt clock. The battery will last about a year and the radio sits there looking good.

You're kidding, are you? How can the currents through two batteries differ if the batteries are in series?

Marc

[John Kogel]

I'm not saying there will be a difference in current between the weak and the strong, but that a weak battery in the string will cause an increased draw on the strong battery, The strong battery will try to charge the weak one. That is the surmise that needs to be proven or dispelled, because it has not be proven false, at least not to me. I would measure the current as a way of determining an increased flow of electrons thru the weaker battery, or maybe measure an increased voltage drop in the strong battery.

Maybe I would compare the voltages as better way of measuring the difference. 2 strong batteries under load, then measure voltage remaining compared to one strong and one weak under the same load for the same time period. If you are correct, the strong batteries will all display equal voltages after the trial. Simple.

[Marc]

I'm not saying there will be a difference in current between the weak and the strong, but that a weak battery in the string will cause an increased draw on the strong battery,

No.

The strong battery will try to charge the weak one.

No.

That is the surmise that needs to be proven or dispelled, because it has not be proven false, at least not to me.

The math is proven to every freshman EE student in Circuit Analysis I.

I would measure the current as a way of determining an increased flow of electrons thru the weaker battery, or maybe measure an increased voltage drop in the strong battery.

What planet are you from?

Maybe I would compare the voltages as better way of measuring the difference. 2 strong batteries under load, then measure voltage remaining compared to one strong and one weak under the same load for the same time period.

I think you're not kidding.

If you are correct, the strong batteries will all display equal voltages after the trial.

They won't.

Simple.

Indeed.

Marc

[Douglas Hansen]

warning - thread drift...

When DC solar panels are installed in series and one panel is shaded, the entire string acts as though it were shaded. In other words, 50% shade on one panel in a string of ten reduces output close to 50% for the whole string. I'll wait for Marc to fill in the math...

Douglas Hansen

[Marc]

Ha. Actually, I wouldn't have the foggiest idea about solar panels. Never studied them. There's mathematical equivalents for motors, transformers and batteries but solar cells...i dunno.

Marc