Battery discharge is occasionally desirable, though batteries are usually flown down. Weather, change of plans, battery break in and other reasons make a discharger useful. Most chargers offer meager discharge capability with the Powerlab 8 offering about 4 amp, 100 watt capability. Still, not much fun to condition and break in 4 - 6 batteries at that rate, or to come home with 4 fully charged batteries from a rain out. There is also "regenerative discharge" on IChargers and Powerlab, but that implies lead acid battery ready to accept charge. Hopefully we can share a few simple discharge ideas.
I had a few parts setting around including a 12 volt computer fan and shield screen and a few 25 watt resistors and decided to try to make a discharger.
Parts:
12 volt 120mm computer fan with screen
25 watt 20 or 27 ohm resistors
Miscellaneous solder, wiring, and battery connectors.
The copper bars energizing the resistor wires are just 14 gauge solid household wiring stripped and soldered. The resistors are adhesed with silicone rubber to the screen.
Using 6 cells and about 24* volts gives about 1.2 amps/20 ohm parallel resistor loop and then about 28.8 watts/loop. If you use 27 ohm, that is 0.89 amps/loop and about 21 watts/loop. I had 3 of the 27 ohm resistors and wen to to Fry's. They were out of 27's and had 20's so 3 of the 27's were used and 4 of the 20's. Calculated about (4*1.2) + (3*0.89) = 7.47 amps/hr or about 125 ma/minute. Without a fan, the approximate 310F temperature max of the resistors will be exceeded and with a fan, still close.
The fan is 12 volt and 0.35 amps, so that gives about 40 ohms of resistance. A 10 ohm and 33 ohm resistor all in series will drop the voltage to about 12 and keep the fan happy. Of course that adds a bit of amp draw.
I run about 130 ma/minute or about 7.8 amp/hour, not quite 3C for break in of batteries. I run 9 minutes to get just lower than storage voltage of the PL8 charger. I am using batteries charged to about 4.16 volts and normally discharge to about 25%. Obviously fully charged batteries would take another minute or so. If I had a second screen to put on the fan, I might load a couple more resistors to up the load, but don't want to compromise airflow.
Considered adding a timer cutoff for the unit, available 24 volt systems fairly cheaply, as are low voltage cutoffs. I didn't have either and most are Ebay China origin, and it works fine without if you don't leave it running. In practice I never use it without carrying my smart phone with a timer set. It would be easy to over discharge a battery, but the timer works for me so far.
If 5 cells, the resistance to control the fan is adequate, and if 4 cells, the 10 ohm is left out of the circuit. And, yes the wattage will drop with voltage drop, so I might use more resistors for less cells and larger amp cells.
An enclosure of Formica was made about 7" long and a slide fit. That helped airflow and shielded the hot resistors from touch.
I will say simple as it took longer to think about and obtain the resistors than constructing. Not counting letting the silicone dry overnight.
*The 24 volts is an average approximation as the voltage is falling along with wattage.