Discharge a 1-3s (or more) LiPo battery to storage voltage. This uses an ATtiny85 to sense voltage and control a high-power LED to drain a LiPo to 3.85v for storage.
This circuit is for one cell of a LiPo battery. It can be repeated in parallel for an arbitrary number of cells. Thus, the circuit runs on up to 4.2v (for a freshly charged LiPo cell), down to around 3.8v (storage voltage, at which the load is turned off).
Circuit
Parts List (one cell)
- ATtiny85 (the internal "1.1v" voltage source must be calibrated)
- 8-pin DIP socket
- High-power load LED (Luxeon Rebel)
- 1N4001 diode (> 0.5A, reduces voltage to the load LED)
- Status LED (red)
- 82Ω resistor (for status LED)
- 68kΩ resistor (voltage divider, must be calibrated)
- 15kΩ resistor (voltage divider, must be calibrated)
- 150Ω resistor (for MOSFET)
- N-MOSFET (to control load; for example from SparkFun; or a transistor with high DC collector/base gain hfe such as TIP120)
- Heat sinks for the MOSFETs (clip-on requires some room to the sides; other shapes allow different arrangement)
A right-angle male-male header can connect the circuit to the balance JST connector of a LiPo battery.
Gallery
I built a 3s discharger, and used a mint tin with holes punched in it as a heat sink for the LEDs (attached with thermal tape). Depending on how far open the tin is, this can provide a desk light, reading lamp, or night-light while discharging.
I mounted the discharger on perf board, and left room to add dischargers for more cells if I get batteries that need it later.
