self balancing a pack of NiMH batteries

[Zak28]

I read NiMH self balance at C/10 rate.

Can a pack of 8 2.3Ah series AA NiMH batteries self balance if charged at C/10 230mA @ 11.5v ?

 

[BradtheRad]

It's more reliable when all cells are identical and new. I've had radio-control transmitters with this setup. My airplanes had 5 or 6 cell battery packs with one connector to plug into a charger.

I seldom tried to balance them, although I measured voltage on each cell sometimes, to see how they behaved during charging and in use. I suspected the cells would become imbalanced after a while. Eventually I was putting in oddball replacements, despite the fact I started with the best intentions.

 

[Zak28]

It's more reliable when all cells are identical and new. I've had radio-control transmitters with this setup. My airplanes had 5 or 6 cell battery packs with one connector to plug into a charger.

I seldom tried to balance them, although I measured voltage on each cell sometimes, to see how they behaved during charging and in use. I suspected the cells would become imbalanced after a while. Eventually I was putting in oddball replacements, despite the fact I started with the best intentions.

Thats correct and the cells I am utilizing are unused and haven't been thru any cycles.

I guess I might as well charge them well above C/10 rate?

 

[BradtheRad]

charged at C/10 230mA @ 11.5v ?

This means the total pack should reach 11.5V while charge rate declines as cells become full. During and after charging you might measure slight voltage differences among the cells. If they differ by less than 100mV then it's probably all right.

 

[Zak28]

Can NiMH explode/combust from overheating?

I believe there is stored oxygen gas when excess heat occurs.

 

[BradtheRad]

NIMH's are tolerant of abuse up to a point. I had a fast charger which pushed 1A through a pack of little 6 AAA. They heated up so they were uncomfortable to hold. I'm certain that's why their useful life was short. They never exploded or oozed, however.

Nicads are more robust. They can stand being charged 24/7, although surely the charge rate should be less than C/10.

We pay for the convenience of rechargeables. We must give attention to their state of charge. A smart charger is sensible although we need to use it a great deal if we intend to save money.

The batteries that ooze are usually half-used alkalines after I try to charge them regardless of quickly or slowly.

Li-Ion batteries are the kind we hear news reports about exploding and starting fires.

 

[Zak28]

...My airplanes had 5 or 6 cell battery packs...

What voltage did you charge them to? I noticed the xtar VC2 plus universal battery charger charges to the point were they become extremely hot and it apparently charges them using USB 5v as I measured at the prongs.

The discharge curves from the energiser NiMh AA battery always start from 1.4v so the charger charging them to ~1.46v seems inappropriate.

The energizer charger uses delta v detection it seems like the xtar does not do this rather detects current flow which is probably the cheaper way to shut off charging.

Battery datasheet
https://data.energizer.com/pdfs/nh15-2300.pdf

Charger datasheet
https://data.energizer.com/pdfs/ch1hr3_emea.pdf

 

[KlausST]

Hi,

Useful battery knowledge:
https://batteryuniversity.com

Klaus

 

[BradtheRad]

What voltage did you charge them to? I noticed the xtar VC2 plus universal battery charger charges to the point were they become extremely hot and it apparently charges them using USB 5v as I measured at the prongs.

The discharge curves from the energiser NiMh AA battery always start from 1.4v so the charger charging them to ~1.46v seems inappropriate.

When I made the battery pack for my first electric airplane (1994), I tested my collection of sub-C nicads, to find the healthiest ones. They had tabs for soldering. The brands were mixed, with a few from Radio Shack.

I charged them overnight at a couple hundred mA. I didn't watch the voltage closely since the voltage dropped a few volts when running the motor, because it drew several Amperes. So if the motor spun I'd go ahead and fly.

Presently I have many old rechargeables, nicad and nimh. In my commercial dumb charger (100 mA) they rise to various voltage levels. Sometimes 1.7 or 1.8V. It's due to high internal impedance (which often happens to unhealthy batteries). When taken off the charger I read 1.44V practically every time I measure a cell. Within a few hours they drop a tenth of a volt.

1.4V and 1.46V differ by about 4 percent. I can't be sure my own meters are that correct, To test each battery individually meant poking two pins through a layer of tape (so I could read its voltage). This was inconvenient, so I tended to gauge battery performance by watching how long a charge lasted, from one usage to the next. It was imprecise of course.

 

[FvM]

The discharge curves from the energiser NiMh AA battery always start from 1.4v so the charger charging them to ~1.46v seems inappropriate.

Don't jump into conclusions, better refer to a detailed datasheet with charging specification, e.g. this from Panasonic

 

[Zak28]

Don't jump into conclusions, better refer to a detailed datasheet with charging specification, e.g. this from Panasonic

View attachment 149560

Whats weird is the charger claims the cutoff voltage for Ni-Mh/Ni-Cad: 1.40V +/- 0.1V yet it charges them far past that.

 

[FvM]

1.4 V cutoff is inappropriate, you need 1.44 V (@20°C) for slow 0.1C charging and higher values for rapid charging.

 

[Audioguru]

Since you have Energizer Ni-MH batteries, you should also have their Ni-MH manual: https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf

 

[Zak28]

Why does the xtar chager I use claim it cuts charging off at ~1.4v +/- 0.1v yet charges the cells to ~1.45v?

 

[KlausST]

Hi.

Why does the xtar chager I use claim it cuts charging off at ~1.4v +/- 0.1v yet charges the cells to ~1.45v?

Because 1.45V is within the tolerance range of 1.4V +/- 0.1V.
Why do you ask?

Klaus

 

[Zak28]

Hi.



Because 1.45V is within the tolerance range of 1.4V +/- 0.1V.
Why do you ask?

Klaus

Thought it was +/- 0.01v tolerance.

 

[FvM]

The discussion seems to miss the point, a cut-off voltage of 1.4 V will never charge NiMH completely.

 

[Audioguru]

The charging is shown on a voltage graph on battery datasheets: