A circuit to use 2 single cell li-po / li-on batteries in series and charge them in parallel
I was looking for an option to create a circuit with a switch, so I can use two single cell li-on Nokia cell phone batteries in series, but charge them in parallel through a USB charger. The idea is to be able to install the batteries in a small robot, where they will not be easily accessible to charge with my regular li-po battery charger. I have a couple of these simple but effective battery chargers lying around, but they can only charge a single cell. After some digging around, I found the following circuit and was able to successfully to put it together and test it. You need a “double-pole-double-throw” (DPDT) switches. It is a switch with six pins, essentially two single switches with a common button / slider. The two sides of the switch are not connected and control two separate circuits. So here is what you need:
- The two single cell batteries
- A single cell battery charging board
- Double-pole-double-throw switch. Two single ones wired together will work as well, you just need to turn switch both at the same time.
And here is a quickly drawn diagram of the connection. When the switch connects the upper two sections (pins 1 to 2 and pins 4 and 5 on the other side) the load is switched off, the batteries are connected in parallel and can be charged with a single cell charger. When the switch is the lower position (connecting pin 2 to 3 and pin 5 to 6) the batteries are connected in series and to the load.
Last, but not least,a safety warning: Li-po and Li-on batteries are dangerous and can explode and catch on fire if shorted, damaged, or over-charged! Also, this circuit does not properly balance the two single batteries. You should only use identical batteries, ideally from the same batch, so they have the same capacity and discharge rates.
This is brilliant, perfect for my application, thank you!!!