My previous post focused heavily on battery chemistry and sizing. As I looked at the available systems I thought the LifePo4wered/18650 would be perfect: it comes with a battery cell, it handles USB connection and charging, and I can use the raw battery output to power the Geiger counter. (It is not available to order, but if you talk to the seller they may be able to work something out with you. They were exceptionally helpful to me.)
In this case, though, the /18650 is the same as a /USB: it's just the USB connector and charging IC--the lack of output regulation that I was excited about comes at a cost: no overdischarge protection. If a LiFePO4 cell is drained too far, it simply stops taking a charge.
This doesn't sound so hard: set up a low-on-resistance FET in series with the raw battery output, then set the gate to turn off when the voltage falls below a certain threshold. This last part could be done with the AtMega on the Geiger counter board, since I'm pretty sure at least one ADC and one I/O pin are free and some creative wiring could get the AtMega to turn itself off reliably while the USB could override it to turn everything back on.
I felt that solid-state relays are overkill (typical current ratings), so I did not consider them much. Some ICs that do the switching are called power switches and readily come with on resistances as low as 30 milliohms, but they don't come in maker-friendly modules that I could find rated for less than 20A. I could design a really small board for one, I could reconsider SSRs, or...
I bought the LifePo4wered/Pi+. It does overdischarge protection and larger battery cells, all in a nice shiny package. I currently plan to reprogram the AtMega to use some of its spare I/Os to talk to the MSP430 on the battery management system, though I may reconsider getting an SSR module and using the /Pi+ with my Pi.
I like to think of things as block diagrams. Here's how I ended up seeing the /18650 in terms of a full battery management system:
- Battery cell
- LiFePO4 18650
- Battery charger
- IC (noted in schematic)
- Battery overdischarge protection
- Battery output regulation
- None (only needed to match sink characteristics)
- External power source
- External power sink
- Pololu 5V regulator