NiMhCharger

It is based on the MAX712 integrated circuit. Even if this model has been recently replaced by newer components like MAX846A, it is still possible to purchase is on the web. I chosen it due to the easy way to configure the number and capacity of cells to be recharged, and because it is possible to have it in a friendly PDIP package. The MAX712 provides also a trickly current charge at the end of the main Fast Charge phase, in order to maintain the battery pack always at top of its charge. The load can be directly applied to the battery pack without to disconnect the charger.

Q1 may be any PNP power darlington supporting more than 4A. Four pin of the MAX712 are used to setup the number of cells (PGM0, PGM1) and the max charge time (PGM2, PGM3). That timeout is applied when the Voltage Slope method fails to detect the completion of the charge phase. Normally, the circuit stops the charge when the batteries voltage reverts its growing. An additional temperature sense could be used too but I decided to avoid it to simplify the project. The Voltage Slope added to the Timeout are largely sufficient to safely charge any battery pack, having or not an internal NTC sensor. The MAX712 allows to charge up to 16 cell units of Ni-Mh battery type from C/4 to 4C rate. To charge the Ni-Cd ones, just replace it with a MAX713 in the provided socket. The two circuits are pin-to-pin compatible. To ensure a longer battery life, I decided the lowest charge rate C/4 by poperly sizing the RBx resistors. And I decided the max timeout to 264 minutes, connecting PGM2 and PGM3 both to BATT- pin. About the amount of cells to be charged, I reduced the choices to an even number of units. Acting to a 2x6 rotary switch you can select from 2 to 12 cells (2,4 to 14,4 volts). You could think to use a larger rotary switch to reach a number of 16 cells. The charging current can be choosen varying the Rsense value, by means of a second switch, that inserts additional millihoms to compose a bigger Rsense value. The MAX712 provides to charge the cells at C/4 rate with a current Ichg = 0,25 / Rsense. The values of RB1 to RB12 are indicated in the left table hereafter. The right table depicts the connections between PGM0/PGM1 pins and BATT-/V+ pins to instruct the ic about our pack size.

Use above B/W hi-res flipped version for PCB creation. Adapt its size to 66x39 mm.

During assembly I used a 11 positions switch to select Rsense, sacrifying the lower 300 mAh capacity. Be care to include the Battery “-“ pole to GND pin total resistance (including wires and connectors) in the 0,273 ohm RB1. Use a milli-ohmeter if possible, or organize a measure session with a stabilised power supply and a couple of tester measuring both current and voltage, and add parallel resistors to RB1 up to have RB1 = 0,273 ohm – Connections resistance. I used pieces of a resistive “constantan” wire, measured, cut and soldered on the other switch terminals. Also the resistive wire is available on the web for purchase.