555 Battery Charger which won first place in the 555 Design Contest Utility category. Using a 555 is a rather clever way to get two comparators and a Set-Reset latch in a single 8DIP package, which is needed for the high and low trip points. The major difference between my design and Mike's is that instead of using a relay like him, I use an LM317 as a constant current source to limit my batteries charge rate.
This charger only implements the constant current stage of your standard lead acid battery charge curve, since that is when most of a battery's capacity is recharged and is much simpler to build than multi-stage controller logic. I appreciated the minimalism of only using a 555, where a three stage charge really only makes sense with a microcontroller. If I did use a microcontroller, implementing all three stages of the charge cycle would be much easier:
- Constant current until the battery reaches 14.2V
- Constant voltage at 14.2V until charge current falls below a set threshold.
- Float the battery at 13.4-13.8V indefinitely.
Once built, calibrate the two trim pots feeding the 555 so that the charger kicks in at 11.5V and shuts off when the battery reaches 14.2V. The button pulling pin 2 low forces the charger into charging mode until the battery reaches 14.2V again. Forcing the charger back into standby mode can be accomplished by pulling pin 6 to 5V, but I opted not to include that.
Ideally, I would have a much better heat sink on the current regulator, but 317s are almost indestructible. When it starts to over-heat, it simply folds back the current and charges the battery longer and slower, which is fine for my application. Not shown in the schematic is the switch which doubles the current sense resistor to 3Ω for a slower charge.
I primarily use this to recharge my small SLAs with a 18V laptop power supply, but used the same connector as my 10W solar panel, so solar charging is an option in the future. I'll probably play with that at some point.