The NiMH fast charger is a compact and sturdy fast charger for NiMH batteries, based on the Maxim 712 IC. Other conditions to meet are selectable number of battery cells to load, and solid mounted battery pack.
The basic “Typical operating circuit” provided by the Maxim datasheet was implemented to maintain simplicity. Some components were added as described to set certain values concerning charge time and number of cells.
Indication of “Power-on” and “Fast charge” is available with LED’s
For fixed operation a DIP switch is foreseen to select the number of cells to be charged. Common battery packs used have 4 or 5 cells. Pins PGM0 and PGM1 are connected to different other pins of the IC selecting the right cell count
For more flexible operation, the DIP switch can be omitted, and instead have a toggle switch installed to the outside of the case. That way one can easily switch the charger to 4-, 5- or 6 cell charging. 6-cell is a nice addition with both PGM0 and PGM1 in open position, as 6-cell battery packs are coming into practice.
The charging timeout was fixed to 264 minutes including Voltage slope detection for charge termination. PGM2 and PGM3 are therefore connected to BATT-.
The charging current is defined by the Rsense resistor value over which the MAX712 will maintain a constant voltage of 0.25V
The TH-D7 battery pack was reverse engineered to create a solid milled aluminum mounting block for attaching the battery pack to the charger.
This aluminum block can also act as cooling rib for the power transistor.
The milled block is what makes this charger specific for the TH-D7 model. All electronic circuitry is off course universal to every NiMH/NiCd battery.