Hi Patrick, I run 3 18650 lithium cells in series. The cells are wired to a controller built into the battery stack. This is used purely to shut down the stack if any cell voltage drops below 2.1 volts. The controller is re-enabled if all cell voltages are above 3V.
I have a 4 or 5 pin din socket mounted on the case, connected to each cell positive, and to the most negative point.  I charge the 3 cells from a bank of usb charger boards.  Each board is fed from a mains to +5v @1 Amp output isolated supply.  Thus all cells are charged at the same time to the voltage set by the charger board(4.1V), and as the cells are individually charged, no balancing is required, and the cells are charged in typically 3.5 hours. I stress that the outputs of the Mains PSU (£1.80-ebay) and the inputs and outputs of the USB chargers (£0.89-ebay) are isolated from each other and floating. They are connected together at the charging plug/socket.  A better design would be to use a single mains supply with 3 isolated outputs.

I can achieve > 7W on 7.0 MHz into a resistive load with 16.6V supply ( 4xlipo cells), But I let the magic smoke out of the BS170 when I had a badly mismatched load.  I get 4.9w at 7.0MHz, 5.05W at 7.8MHz. I suspect the LPF requires minor tweeking, but I haven't bothered. This is at 13.8V

Regards Vin  G4ksy