Rechargeable NI-MH batteries

[onestep]

After going through about a zillion Alkaline AA batteries this past year, tonight I bought an "Energizer NI-MH Battery Charger". It comes with 4 rechargeable AA batteries rated at 1.2 volts & 2500 mAh each. In addition to these batteries I allready had 4 NI-MH batteries at home; 2 are 1.25 volts & 2200 mAh, 1 is 1.2 volts & 2300 mAh, and 1 is 1.2 volts & 2200 mAh.

Question - can I mix batteries of different mAh (2200/2300/2500) within a device or must all batteries be the same? Also, same question concerning the 1.2 & 1.25 volt batteries.

Onestep

 

[NH_Mtn_Hiker]

Hey DougPaul, where are yoouuu?

 

[DougPaul]

Question - can I mix batteries of different mAh (2200/2300/2500) within a device or must all batteries be the same? Also, same question concerning the 1.2 & 1.25 volt batteries.

Hey DougPaul, where are yoouuu?

You go off and try to get a bite to eat, and someone yells for you...

Don't mix:
* technologies
* capacities
* ages
* states of charge

Any of the above can cause the weaker cells to become reverse charged and damaged.

When I buy a bunch of AA NiMH cells, I label, use, and charge them in pairs (or triples for AAA cells) to try to keep them matched. You could also randomly choose from a matched set of cells if you top off the charges on the bunch that you are about to use.

Basically, all NiMH cells are a nominal 1.25V. The actual voltage will depend on the state of the cell, temp, and recent useage/charging history.

Doug

 

[Pamola]

um, no offense to dougpaul, but doesn't bill b work for duracell?

 

[DougPaul]

um, no offense to dougpaul, but doesn't bill b work for duracell?

Not offended. It isn't like battery info is a secret known only to me. A significant amount of what I post on batteries can be found in http://www.batteryuniversity.com.

I have no idea if bill b works for duracell.
(edit: Bill PMed me--he does.)

Doug

 

[bill bowden]

Nickel metal hydride batteries

dougpaul is certainly correct about not mixing batteries with different chemistries or different capacities. Just to make the situation even more complex, you shouldn't mix batteries charged on different chargers. some chargers (the fast ones) can sense when the battery is approaching full charge (by a temperature rise) and shut down the charger) while others (the real cheap ones) will continue to to charge forever and depend on chemistry inside the cell (usually referred to as recombination or as a shuttle) to keep the battery working well.

All batteries work worse in very cold weather but lithium batteries suffer less loss and are a little lighter.

 

[arghman]

technical quibble:

Don't mix:
* technologies
* capacities
* ages
* states of charge

Any of the above can cause the weaker cells to become reverse charged and damaged.

Definitely don't mix them -- but if you are using only 2 or 3 batteries in series, it is unlikely that any will be reverse charged unless you keep the thing you are powering on until the total series voltage reaches 0 (and then the weaker ones lose).

Some cells would be emptier than others, but if none of the cells are discharged completely, there shouldn't be any damage. The only way one of them would be damaged is if the weakest cell is overdischarged -- at which point you would see the voltage start to drop, but not as much as if all cells were equally weak, and devices which have undervoltage warnings or auto-off circuits would be fooled into thinking that they could still run a little longer & pull the weak cell's voltage down below where it should operate. That would be bad. In the unlikely scenario that you absolutely have to mix them in small series stacks, be conservative & don't use them for very long.

We had an application where I work, where we had to use a bunch of batteries (30 or 48 or 60!) in series, and matching / equalization methods were rather important to get any significant battery life.

 

[DougPaul]

but if you are using only 2 or 3 batteries in series, it is unlikely that any will be reverse charged unless you keep the thing you are powering on until the total series voltage reaches 0 (and then the weaker ones lose).

Agree that small numbers of cells in series are easier to manage than large numbers.

The guaranteed no-reverse-charge limit for N cells in series is (N-1)*(single_cell_voltage). And, of course, if the voltage goes below 0, then at least one cell must be reversed. Between these two limits, you can only guess what has happened. In critical situations, you have to access each cell individually so you can tell (and/or control) what is happening to each cell (such as in arghman's application).

My 2-cell Garmin GPS, shuts down at 2V, well above the guaranteed no-reverse limit of 1.25V for NiMH.

Some battery technologies are more sensitive than others. For example, lithium ion rechargable cells can be destroyed by a single instance of reversing--and can catch fire if defective or abused "in an appropriate way". My laptop battery brings out all intercell connections so that the system can monitor each cell at all times.

Doug

 

[Peakbagr]

I received the suggestion from Doug Paul to start using rechargeables in my GPS.
Now that its getting cold, can anyone comment how well rechargeables NiMH's do in the cold? Relative to Alkalines? I'm guessing that can't do as well as Lithiums.

Thanks,

Alan

 

[David Metsky]

NiMH do better than Alkalines in the cold, but worse then Lithium-Ion or Lithiums. That's a general rule. NiMH are better for digital cameras because of the discharge rates. That is true for any temperature. I'm not sure how GPS work, but in general NiMH are good in cold.

-dave-

 

[DougPaul]

David is mostly correct: NiCads, NiMHs, and lithium-ion rechargables are good to ~-20F and lithiums (non-rechargables) are good to ~-40F.

Ref: http://www.batteryuniversity.com/index.htm

Digital cameras draw high currents (.5-1A ?) for short periods of time. Alkalines do this poorly at any temp. NiCads, NiMHs, and lithiums all do well in this service.

GPSes tend to draw ~100mA for long periods of time. All of the above battery technologies are fine in the warm, scratch the alkalines if it gets cold.

Headlamps typically draw 20 -- 700 mA (low to fairly high currents).

I frequently hike with NiMHs as my primaries and NiMH or lithium spares any time of year for my GPS, camera, and headlamps. Or if I am saving weight, I'll use all lithiums.

(Notational note: 1000mA = 1A.)

Doug