Battery discharger

[BrianG]

Hey, I was looking at some battery dischargers and found them to be a bit expensive for the "smart" ones.

I don't like the light-bulb types since there is no shutoff. Plus, the discharge rate changes as the battery is discharged. As the battery gets weaker, it can no longer light the filaments. Unfortunately, the resistance of a lightbulb filament is quite a bit lower when it is not lit. So, at the weakest state of a battery pack, it pulls the most current (or tries to) because of the low bulb resistance.

So, I was designing a solid-state design (using transistors and heatsink) that would discharge at a constant current no matter the charge level and then shut off at a pre-determined cell voltage level. I don't know an easy way to make it smart enough to know how many cells the pack has so there is a switch to select the number of cells (5, 6, 7, or 8 cells). There is another switch that selects the charge current (5, 10, 20, and 30A). The cutoff voltage would be set for 0.8v/cell (this value can be easily fine-tuned). There is even a discharge LED and a "done" LED. It works good "on paper", but was wondering if there truly is a benefit to discharging packs before charging them again? This project will only cost about $50 to build - the most expensive part being the heatsink, but I want to make sure it's worth it.