Max713 NiCad/NiMH smart charger.
July 23, 2009
I made a battery charger with some free samples of the maxim max713 chip.
The 713 supports both NiMH and NiCad while the 712 is for NiMH. Both can fast charge up to 16 cells and support V/T, temperature and time out charge cut off. It can charge the battery while still powering the load. If you don`t need NiCad support get the 712, it`s better for charging NiMH safely because of the way it measures the cut off voltage.
The basic linear circuit is fairly easy to make up, you must use the formulas here to find the values for R1 and Rsense though. Do this with the biggest battery pack you want to use.
- Choose how many cells to charge. Minimum Input Voltage = Number of cells x 1.9 + 1.5
- Find out R1. R1 powers the chip. R1 in ohms = (Minimum Input Voltage - 5) / 0.005
- Decide on a fast charging current. Ifast in mA = Battery capacity in mA / Charge time in hours
- Find the Rsense resistor. Rsense in ohms = 0.25 / Ifast in A
- Set PG0 and PG1 to the cell number according to datasheet Table 2.
- Set PG2 and PG3 to set the cut off time according to datasheet Table 3. Cut off should be slightly higher than charge time.
- PNP power dissipation. PDpnp =(Maximum Input Voltage - Minimum Battery Voltage) x Charge current in A Check this against the PNP datasheet. This is wasted heat and depending on your cell count range you will need a heatsink and/or fan.
For my charger I chose up to 6 cells (the picture shows jumpers up to 8 cells but it`s not wired up yet). Fast charge current and Rsense aren`t set in stone because they can change if you charge different capacity battery packs.
- Minimum input voltage = 6 x 1.9 + 1.5 = 12.9v
- R1 = (12.9 - 5) / 0.005 = 1600. I picked the next lowest resistor at 1.2k.
- Ifast = 2500mAh / 2 hours = 1250mA.
- Rsense = 0.25v / 1.25A = 0.2 ohms.
- PG1 and PG0 both unconnected.
- PG2 connected to BATT-, PG3 connected to REF pin. With a charge time of 2 hours, the timeout is the next highest at 132 minutes. There will be losses through heat so it`s fine. Also voltage slope cut off is enabled to turn off automagically when the voltage stops rising.
- PDpnp = (13 - 4) x 1.25A = 11.25W. 2N6109 maximum PD is 40W but it gets lower as it gets hotter. For every degree C above 25 minus 0.32W from 40. If I think it could get up to 60 degrees.. 40W - (60-25) x 0.32 = 28.8W max power dissipation. Well over 11.25W.
Actual charge current above is about 900mA because the fan sucks up a bunch. The jumpers and resistors are really fiddly, I still have to get around to putting the temperature probes on it, changing the jumpers to dip switches or rotary switches, adding 8 cell battery support, and mounting it in a box. The current charge labels on the right are only accurate for a 1000mAh battery but it gives me an idea of what kind of charge rate to expect.