Difference between Ni-MH & Ni-Cd batteries

[lcm1947]

I was reading about Ni-Mh batteries and noticed they state you should run the battery all the way down before charging where the Ni-Cd says that they should be recharged as soon as you notice a loss of power. I wonder why they differ like that? Anybody know anything about it?

[LCHIEN]

I was reading about Ni-Mh batteries and noticed they state you should run the battery all the way down before charging where the Ni-Cd says that they should be recharged as soon as you notice a loss of power. I wonder why they differ like that? Anybody know anything about it?

NiMH batteries are the newer and better technology.

Neither type should be run down completely, i.e. draining with a flashlight overnight, this leads to cell reversal and kills the pack. Cell reversal occurs because not all the cells in the pack have precisely the same capacity and one will keep sending current when the other is out of current, this current thru a depelted cell is the wrong direction and it irreversibly changes the chemistry.

it is sometimes recomended to discharge the cells before recharging. More about that in a minute but the proper way to do that is to simply use the pack until the motor or light is observed to slow down or dim rapidly, this siginfies that the weakest cell has run out of current. Once it does this don't keep trying to use the tool!
Unlike primary (alkaline) batteries, rechargeable cells drop off quickly in voltage when the current is depeleted where as alkaline cells run on at diminished voltage and current for quite a while.

As for discharging cells before recharging, it has been widespread belief that cells build a memory, if repeatedly used to a level and then recharged ulitmately the cell will memorize that and you will not get usage beyond that time. NiMH are supposed to be much better in this regard than NiCad but present day batteries are much less prone to this according to some reports I've read.

I personally never run my batteries completely dead and no longer try to discharge them before recharging.

[gerti]

NiCds have a memory effect, meaning if you constantly run them a little and then recharge them their capacity will diminish. NiMHs are much better in that respect, while they still have a memory effect it is negligible for most applications.

NiCds have a slightly higher peak power delivery, but their voltage drops as the charge diminishes. NiMHs have a relatively constant voltage, that drops rapidly near the end of a charge.

Or to put it in simple terms: NiCds need some care to live a long productive life, while NiMHs don't.

[cabinetman]

You can use NiCd, but they won't last as long between recharges. The voltage is the same -- 1.2V for both NiCd and NiMH -- but voltage is not the issue. Energy density is what's important, and NiMH batteries have about 3 times that of NiCd. A typical AA NiMH may have a capacity of 1800-2500mAh (milliamp-hours) per charge, as opposed to around 600-800mAh for a AA NiCd. NiMH cells are less prone than NiCd to the so-called "memory effect," where the capacity of the cell diminishes over time. One charge from a NiMH cell will also provide more energy than a similar alkaline cell, but an alkaline cell has more energy than a NiCd. Digital cameras use lots of electrical energy, and you will likely be disappointed in the performance of NiCd cells. And if you use alkaline cells, you will be going through lots of them. Buy some stock in Duracell. Smiley

NiMH cells have a fairly high initial cost, but they are coming down in price. You can find packages at electronics stores with a charger and 4 cells for around $20. Buy extra cells so you'll have a second set to swap out when the first set needs to be recharged. By the way, DON'T EVER try to recharge NiMH cells in a charger not designed for them. Unlike NiCd and some other types of rechargeables that can be kept on a "float" charge, NiMH cells must have the charging switched off when they are fully recharged. NiMH chargers have a sensor -- so-called "charge termination" -- that stops the current flow to the cells once they are 100% charged, This is to prevent overheating when the charger and batteries are left plugged in.

This was an answer to the Nicad vs. NiMH battery in another forum by wwatty.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[DaveS]

One annoying thing about NiMH batteries is that they will lose a charge over time, even if not used.

So, if you take a fresh battery off the charger, put it in the drill (or camera, or whatever), let it sit there for a couple weeks, the batteries will be dead.

I have NiMH's for my drills, and I just combat this by always keeping one in the charger (which trickle-charges), and just flip them when I start working. It only takes my charger 45 minutes to charge a battery, so, as long as I move slowly (not usually a problem), I can make it last until the next one is charged.

The post from cabinetman seems odd to me - as my NiMH charger *must* be trickle charging the batteries, as they are never flat when removed from the charger (unlike the one sitting onthe bench next to it, or the one on the drill).

[just4funsies]

The annoying tendancy for NiMH batteries to lose power on the shelf has caused some manufacturers to go to the more pricey Lithium-ion (L-ion) batteries, because they hold a charge much longer in an unused state, although their active performance is sometimes not as good. A lot has to do with how you use your tools. Also, most of the better charger units have both conditioning cycles and trickle charge functions, meaning it is less crucial whether you completely discharge before recharging, and you can safely leave the batteries in the charger to keep at top level.

Although opinions may vary on which type of battery and manner of care is best, most will agree that there is no "battery utopia", and that there are upsides and downsides to all types, and none will last forever.

Some rules:

1) It won't charge as fast as you want it to.
2) It won't keep a charge as long as you want it to.
3) It won't work as hard as you wish it would.
4) It won't weigh as little as you wish it did.
5) It won't be as small as you wish it was.
6) It won't cost as little as you wish it did.

[lcm1947]

Thanks everybody for the info. Interesting to say the least. The reason I asked to begin with was due to the fact that I was comparing some impact drivers this afternoon ( yes I just had to try one out ) and noticed the difference between the manufacturers battery types. The one I decided to actually get was the 12 volt Hitechi model WH12DAF2. I read some tests last night in the 2005 Tool Guide and while the one Lowe's had is not the exact one the article rated "Best Overall" it must be the replacement for it so figured it might be a sleeper. Anyway I sure didn't want to spent $179.00 for any of the top brands ones so thought I'd try it for $100.00. Lowes had a 10% discount through today to all military personal and vets plus on this particular tool a $25.00 mail rebate on top of that bringing it to the $100.00 from it's regular price of $129.97 and it came with 2 batteries of course like everybody else. Believe it or not the more I looked at it the more that at first ugly crappy tiger green & black grew on me. I actually think it's pretty neat now and it is certainly difference looking. Anyway thanks for the info and I guess I'll find out first hand how bad or good the Ni-Cd's are. I've had my Makita 12 volt drill/driver with the Ni-Mh batteries now for 6 1/2 years so have something to compare it against. Oh the charger charges the battery in 30 minutes which I really liked. Only reason I didn't go with the Ryobi is due to that 1 hour charge time plus the Home Depot had sold out of the chargers and the whole Ryobi rig would have cost me like $120.00 anyway by the time you buy the charger and two batteries.