How to Charge a LiPo Battery

92 Hacking Electronics

(a) (b) (c) (d)

supply to minimum, so as to prevent any nasty surprises. The voltage immediately drops to 11.4V (Figure 5-3b), so we can gradually increase the maximum current. In actual fact, even with no current limiting (turning the current knob to maximum), the current only rose to 580mA and the voltage increases to 14.4V (Figure 5-3c). After about two hours, the current has dropped to just 200mA, indicating that our battery is getting full. Finally, after four hours, the current is just 50mA and the battery is now fully charged (Figure 5-3d).

Figure 5-3 Using a variable power supply to charge a lead–acid battery

How to Charge a LiPo Battery

The technique we have just used on a lead–acid battery using a variable power supply will work just as well on a LiPo battery if we adjust the voltage and current accordingly.

For a LiPo cell, the voltage should be set to 4.2V and the current limited (usually to 0.5A) for a smallish cell, but currents up to C are sometimes used in radio-controlled vehicles.

However, unlike lead–acid and NiMH batteries, you cannot put a number of cells in series and charge the whole lot as one battery. Instead, you have to charge them separately, or use a “balanced charger” that monitors the voltage at each cell separately and controls the power to each.