You can overcharge the batteries but considering how long it takes to recharge them you really must make an effort to do it. Most everything with a USB port has a battery charge indicator on it. It's simple enough to disengage the dynamo and look at the charge indicator. If it shows fully charged just leave the dynamo disengaged so you don't overcharge your device and the batteries in the circuit. You may leave the device connected to the batteries allowing the device to use the external batteries and it's internal batteries. When you see the device batteries needing to be recharged simply engage the dynamo recharging both the device and the batteries in the dynamo circuit. If the batteries in the dynamo are very low or almost completely discharged you can rapid recharge them by engaging the dynamo, disconnecting all devices from the USB port, and open S1 so the LED lights are not on. This will allow 500mA of current to recharge them. Most AA Ni-MH batteries can be fully recharged in 2 hours at 500mA. It's hard for most people to ride continually for two hours without stopping, especially when you need to cross roads and deal with traffic. I keep up with rapid recharge time by reseting the trip timer on my cycle computer and when it reads 2 hours I know the batteries should be completely recharged. The trip timer will stop and restart automatically so you need not try and keep up with how much time was spent at stops. Riding at night requires the LED lights. Usually 120mA is just enough to maintain a charge in the batteries as there being discharged into the LEDs at stops or moving very slow in traffic. My batteries are rated at 1.8 amp hours which will run the LEDs for several hours without the dynamo engaged. One other note about the circuit. The 470mF capacitor across the headlight LED is not required when used with batteries. It's only there in the event my batteries are damaged, lost, or stolen. Having the capacitor allows running the LED lights directly from the dynamo. Of course you can not plug any USB devices into the circuit without the batteries. This would most likely damage the device. Running the LEDs directly from the dynamo will not damage them as they can withstand more current than the dynamo can produce. This will at least allow you to travel at night to your next scheduled stop. Most dynamo lighting systems do not have batteries nor do most riders with dynamos want batteries as they went with a dynamo lighting system to free them selfs from needing to worry with batteries!