replacement for NI CADS in cordless vac.

Forgot to say that it is a 7.2V 6 cell pack.

I've had a similar experience with a similar dustbuster-type vacuum. Unfortunately, a new set of batteries didn't cure the problem for all that long. Within a year of occasional use, the vacuum's run time was down to a minute or so... the batteries were pretty badly fried.

I believe that a slow-rate (C/20 or so) charger would be safe enough for use with NiMH cells, although it'll take a day or so to completely recharge a fully-drained set of batteries.

It's probably not ideal - just as it isn't idea with NiCd cells. Leaving either type of battery on constant charge (other than a very low-level trickle charge) seems to be a good way to shorten the life of the cell/battery.

Optimum charging of NiMH batteries requires more "smarts" that these devices tend to have, I'm afraid. I believe that the preferred approach is a fairly rapid charge (C/1 to C/3 or so) with an automatic cutoff when the battery terminal voltage stops rising, or the battery temperature increases, or (as a fallback) based on a timer. It's then a good idea to apply a very small trickle charge - maybe C/100 - just enough to overcome the battery's tendency to self-discharge.

I would *NOT* attempt to charge any sort of lithium battery with your existing charger. Most lithium batteries require very careful charge control... I don't know if the LIPO chemistry is less ticklish than the older lithium formulas, but there's a real danger of causing a fire if you mis-charge certain types of lithium battery.

I'm sorry to say that I suspect that the answer is "yes"... the damage may not be immediate but I believe that leaving NiCd batteries on constant charge is likely to greatly shorten their lives.

The sub-C cells are probably good for a few amp-hours. 600 mA would recharge them overnight. Unfortunately, unless the charger has some sort of smart charge-control chip in it, the wall-wart will probably continue dumping a fairly high amount of current into the cells even after they are fully charged, and they'll heat up. You might not get more than a year of use out of them before they, too, will be cooked.

I'm tempted to see if I can whip up some sort of simple circuit to install in my dustbuster cradle... maybe an 8-pin PIC with an ADC to act as a charge controller, and a PNP or P-MOSFET pass transistor to control the charging. It ought to be possible to sense the presence or removal of the vacuum, hit it with a refreshing charge when it's plugged back in, and shut down the charge (or switch to a trickle or pulsed maintenance charge) after a suitable number of hours.

Below is not the *best* approach, but a very good one, and better than the "standard" dust-buster type charger. It uses a constant current of ~C/10. When terminal voltage is near, it reduces to something like C/50. This example "steals" current from the regulator when the battery voltage rises to about 7.4, reducing the charging current from ~ 125 mA to roughly 20 mA:

Ed

Nicads are lower impedance than NMH. I can't say for sure if this matters in your case.

I have used

I commend you for not dumping the vacuum in the trash. I used to rebuild my electric razor, but Philips has the battery management down to an art that the shaver lasts so long you might as well upgrade.