battery-chargingparallelseries
I'm wondering how I could/should charge the 18650 batteries used in the following circuit. I want to charge them via USB in a power bank case like this.
But I probably won't be able to use the existing charging circuit because I'm using the batteries in series to get more voltage.
simulate this circuit – Schematic created using CircuitLab
simulate this circuit – Schematic created using CircuitLab
Here's one circuit you could use to switch from series to parallel. Check that both switches are break-before-make or there could be sparks. The diode protects the charger if the batteries are switched into series. This will waste a little power, but it should still leave about 4.3V which is plenty for charging the cells in parallel. You could get a bit fancier with a 5V relay if you want, which would mean no external switch is required at all. I wouldn't bother with mosfets because they would increase the component count unnecessarily and require a higher gate voltage than you have at hand for high-side switching.
Your circuit doesn't do what you want it to - both batteries are permanently wired in parallel, as is the charger. The diode does nothing.
You probably don't want to charge them in series from USB anyway, because you would need at least 5.6V (1.4V/cell x 4 cells) but you only have 5.0V available. Charging in parallel can be done efficiently with a switch-mode current regulator, or just a resistor to limit current (and perhaps a blocking diode to prevent discharging into the USB port when not powered) if you don't mind a lower charge rate.
Best Answer
Use a boost converter to make the higher voltage.
This also allows you to control how much current is getting dumped onto the batteries. These kind of batteries need to be carefully monitored while charging and discharging if you want to get full capacity and long lifetime. A microcontroller directly controlling the switch of the boost converter is probably the electrically simplest way to achieve that.
There should also be a way to measure the voltage of each cell separately, and optional current shunts around each cell. The micro watches the voltage of each cell, and performs cell balancing by enabling the current shunts across any cells above the average voltage, then shut down charging altogether when the cells reach the maximum safe voltage. The micro can control the charging current since it is controlling the switch of the boost converter. It can either measure the current and perform control that adjusts the PWM duty cycle to get the desired current, or it can compute the current open loop by knowing the voltages and the inductance.