NiMH AA Batteries? Can someone explain?

[DandyDon]

We used to see these sold by Sea Life with 1100 mAH I think it was. Now we're seeing 2000 to 2750 mAH sold at Walmart, and pretty reasonably priced. Some have recognizable brand names, some don't. Fully charged, I wonder how these compare to the one-time use Lithium batteries...?

I see chargers that claim a quick charge of 4 hours, but isn't slower charging better?

The NiMH batteries are also touted as having "no memory," and I'm sure it's much less than the NiCDs, but doesn't it help to drain them fully now and then...?

Anyone really understand and know these things...??

thanks!

 

[under water]

Hi Don,
Here is a useful backgrounder on batteries. http://www.flashlightreviews.com/features/batteries_explained.htm
and
http://www.flashlightreviews.com/qa/batteries.htm
although I'm not sure how 'current' the info is.

 

[do it easy]

Nickel chemistry batteries like to be charged quickly, but they don't like to be overcharged, whcih generates heat and shortens their useful life. A nickel battery in good condition will be able to accept lots of juice quickly without generating much heat until the bitter end.

The nickel chemistry memory is a hot topic. Some say it doesn't exist, some say it does. Some say that NiMH have less memory than NiCd.

I say, regardless which battery and/or myth you subscribe too, please dispose of all batteries properly.

Here's a good link with lots of pictures and small words. http://www.batteryuniversity.com/index.htm (just kiding about the pictures ans small words )

 

[onfloat]

It's all about the chemical makeup of the battery. Some batteries can be destroyed by draining them completely. Some can tolerate a higher charging rate. It's best to follow the manufacturers' instructions.

 

[DandyDon]

(just kiding about the pictures ans small words )

You tease.

Ok, interesting links, lots of discussion. Anyone got a simple story?

 

[do it easy]

Short and skinny- I like rechargeable batteries over non-rechargeable hands down, but I also get a kick out of charging them.

The fast chargers are fine, just be sure to get one that has a multiple shut off features (temperature, negative delta V, etc.) This basically prevents the batteries from being cooked. NiMH like fast charging, but they don't like heat. Some chargers allow you to independantly recharge 4 or 8 or more NiHM batterries at the same time, so that each battery gets the optimal charge.

Go with the NiHM! (just my opinion )

 

[Charlie99]

The fast chargers are fine, just be sure to get one that has a multiple shut off features (temperature, negative delta V, etc.)

Perhaps a bit of useless trivia, but the delta V change on NiCd batteries is much greater than on NiMH at the same charge rate. Many battery chargers use a zero delta V or even a slight positive delta V for NiMH. If you have a charger that has a NiCd/NiMH selector switch, in most cases that's all the switch controls.

 

[do it easy]

Perhaps a bit of useless trivia, but the delta V change on NiCd batteries is much greater than on NiMH at the same charge rate. Many battery chargers use a zero delta V or even a slight positive delta V for NiMH. If you have a charger that has a NiCd/NiMH selector switch, in most cases that's all the switch controls.

Yup- I think the main cutoff with NiHM is temperature since the NDV is so small that it is hard to detect, but the temperature is easy.

 

[bperrybap]

You tease.

Ok, interesting links, lots of discussion. Anyone got a simple story?

Simple story:
My wife has an IkeLite PCa. Great light but eats batteries.
(6 at a time) I put in a set Lithium batteries. As soon
as we turned on the light, poof, the bulb blew.
I'm not sure if it is related to the batteries, but it might be
as Lithium are capable of pushing lots more current than
Alkaline and the Ikelite overdrives the bulb. I wouldn't
be surprised if it actually depends on the voltage droop
that occurs in traditional akalines to keep from blowing the
bulb.

So rather than trying it again ang risking blowing another bulb,
I thought I'd save money by using 2400 mAh NiMh AAs.
At 45-50 minutes into dive, light was dimming same as with
Alkaline AA batteries.
At 60 minutes into the dive, the light, while dim but
still in use, exploded underwater.
Nice loud "pop" and blew the lens top right off the light.:11:

Since then I've read that NiMh batteries start to off gas
lots of hydrogen when the voltage gets below .9 volts and
there is a heavy load.
Because of this, I'll never use NiMh on any underwater light
except the UK LED lights. Those ligts turn the LED way
down (to around 10% brightness) to reduce current draw
on the batteries when the the voltage drops below 1 volt.
It kind of forces you to switch to the backup.


Another simple story:
Several of the high amperage AAs (2200-2400 mAh) are about
1 mm bigger in diameter than a standard AA.
1 mm may not sound like that much, but it is enough that
they won't fit in some devices.
Also noticed this same problem with the AA to C adapters.

--- bill

 

[Charlie99]

Now we're seeing 2000 to 2750 mAH <NiMH rechargeable> sold at Walmart, and pretty reasonably priced. .... Fully charged, I wonder how these compare to the one-time use Lithium batteries...?

There are many different lithium chemistries. The ones intended as drop in replacements for alkaline/NiMH/NiCD (1.2-1.5V sort of range) are typically Lithium Iron Disulphide (LI/FeS2) chemistry. A typical lithium battery of this type is 2900mAh. Hard technical data is a bit hard to get ahold of, but it appears that the voltage stays up closer to 1.5V for much of the discharge curve, meaning that you will get more power out of a 2900mAhr Li/FeS2 battery than out of a 2900mAhr NMH where the average voltage is more like 1.3V. I haven't run the tests, but my ballpark guess is that if you were to compare a 2900mAhr AA lithium to a 2400mAh NiMH in a camera type usage you might see the lithiums last about 40% longer (15% advantage from voltage output, 20% advantage of 2900mAhr over 2400mAhr. 15% + 20% = 40% or more correctly 1.15x1.2=1.4). Not enough to get me to use lithium primary cells.

The only true advantage of the lithium is the very long shelf life, and the much lighter weight than for alkaline or NiMH.