Last week I worked on a client design including a lithium cell charger. For that design, only a LiPo cell would fit the size requirements. I started by copying the design from the work I had already done on that other project. The lithium cell charger has a reference design in the datasheet, so I implemented it as described in the datasheet and then adjusted values for my application.
Re-using designs can save a lot of time. Not only do you not have to read/understand a new datasheet and reference implementation, prototype level testing has already been done. I haven’t actually done the prototype level testing yet, but I will for the client before I re-layout the programmer.
Then I realized maybe I didn’t want to use a lithium cell for the programmer. The advantage of a lithium cell is power density, and a simple prismatic(3d rectangular) shape. The disadvantages of a lithium cell are fire risks, and high costs in low quantities.
The alternative for a lithium cell is a battery of 3 or 4 NiMH cells. Three cells in series would be 3.6 Volts. Four would be 4.8V. The advantages of NiMH cells are consumer availability and price, very low safety risks, no special requirements for shipping. The disadvantages are more complex charging, very little drop in price for large quantities, cylindrical cells aren’t very efficient for space, lower power density.
I chose to go with a battery of NiMH cells. I feel the consumer availability, price, and easier shipping are very significant advantages for the programmer.
Maximum charge voltage per cell is 1.78V. Multiply that by 4 cells is 7.12V(not optimal for charging over USB port). Multiply that by 3 cells is 5.34V (This is close to the 5V I would get from USB). Being a little under maximum charge voltage will still work but limit maximum charge current. Since I want to charge off of the USB connection, limiting it to 500mA(or 0.5 Amps) or 100mA is a good choice.
After looking at price and complexity of charger ICs, I changed my mind. I will go with the lithium cell and charger. I chose the resistor values to charge at approximately 500mA with a time limit of 8 hours. The chip manages current flow to the system and charging the lithium cell at the same time.
I hope that you have found this blog of value, thank you for reading! I have uploaded the KiCad files to github.com. Use the link under the PI Gear logo to go get it.