VELLEMAN K7300 UNIVERSAL BATTERY CHARGER/DISCHARGER KIT

• charge current: from 15mA to 750mA (selectable)
• charges both NiCd and NiMH batteries
• usable battery voltages: 1.2V, 2.8V, 3.6V, 4.6V, 6V, 7.2V, 8.4V and 9.6V
• quick charge in 52 minutes / normal charge in 14 hours
• automatic discharge and charge cycle
• dimensions: 3.4 x 3.1"
• THIS IS AN ELECTRONIC KIT AND REQUIRES ASSEMBLY.

Bottom Line: Skip it and buy a far superior ready-made "smart charger" for half the price.

First, the charger is good for single cell NiCad batteries, because it will discharge the battery before charging, thereby overcoming the memory effect. (NiCads tend to develop reduced capacity if they are recharged before being fully discharged repeatedly.) But for a battery (2 or more cells), doing this runs the risk of one cell discharging a little before the others, in which case their discharge ends up reverse charging the early discharger, which ruins the cell and hence drops the battery voltage (by 1.2 volts for every dead cell). So this function is of limited value, applying only to single-cell NiCad batteries.

But there isn't much call for NiCad single cells anymore; they have given way to NiMH cells, which are superior for nearly all applications (and are much kinder as well) to the environment, as they don't spread toxic cadmium when tossed in the trash). Moreover, NiMH cells don't have the memory effect problem, and in fact unnecessary discharge tends to shorten their life. Only a few stores still carry NiCad cells.

So is the charger good for NiMH cells? Alas, it lacks the sophistication of off-the-shelf "smart chargers" that sell for half the price, often with 2 or 4 of the newer pre-charged NiMH cells. (The older NIMH cells self-discharged at a rate of about 1% a day, which meant that even if they topped them off at the factory, there was a good chance they would be dead by the time they ended up in your radio at home. The new ones don't have this problem.)

What is special about the smart chargers is they use a microprocessor and sensors that monitor the battery temperature and voltage in order to avoid overcharging them. Charging too fast and too long are (after letting them discharge completely), the major causes of early loss of cell capacity. The program looks for the "reverse delta" in the cell voltage (after steadily rising while charging, it briefly drops back a wee bit), which is the endpoint for healthy charging. They generally monitor the charging time as well, shutting down even without the reverse delta if it goes on too long.

The Velleman unit does use a timer, but it is the only method of stopping the charge. It is better than nothing, but cannot take into account the amount of charge actually needed; it's like going to the gas station to fill up your tank, and pumping gas for 4 minutes and 32 seconds because that's the average time it takes to fill a tank. You are likely to end up with gas on the ground or with a tank that is empty again too soon. One more thing: if the power to the charger is interrupted, the timer starts all over again.