What Else Does this Circuit Need? (Lithium Charger)

The schematic below is my draft attempt at modifying this popular single cell Li-ion charger based on the MCP73833 (http://www.adafruit.com/products/259) to be able to charge multiple cells in series at once without disconnecting them by just putting a separate copy of the circuit on each cell. Also on each cell will be a protection circuit on the load side to protect from undervoltage/overcurrent such as this one http://www.all-battery.com/ProtectionCircuitModulePCBfor3.7VLi-ionBatteryPack-32088.aspx.

My final Li-ion pack/charging requirements include:

  • 4S 14.8V (only 2S shown in the schematic)
  • Built with 14500 800mAh Li-ion cells
  • 2-3A total output
  • 2 feet between cells because of weight distribution needs
  • Be as small/light as possible

I intend to have the ‘Power Out’ and ‘Power In’ jacks shown carrying the current in parallel from something like a 5V 3A wall wart between each circuit for each cell. The output after the protection circuit will then be put together in series with the next one over the 4 cells to create a single 14.8V pack. At the end of the pack will be 4 wires, +/- for the charging input and +/- for the 14.8V output (and up to 8 different STAT pins going to my micro).

Is this a proper approach? I’ve lurked forever and read lithium charger posts here until my eyes bled but none seemed to fit my unique distributed circuit and 4S needs. If there’s a better approach I’d love to hear more. Also because I’m a member of the popular group of much better coders than EE types my burning question really is this: What do I need between each of the charging circuits to isolate them while they’re charging? Intuition tells me that I need either a diode or some sort of rectifier or something between the independent charging circuits.

In any case I want to be as safe as possible working with lithium as I am fully aware of the dangers particularly with regard to overcharging. I appreciate any help anyone can give me because I’ve spent a LOT of time reading everything I can find on lithium charging and have finally decided that I need to ask for help. Thanks in advance.

It would seem to me that, as long as the OUTPUT ground is not shared (and the circuit is OK with isolating input from output grounds), you don't need anything more than the regular reverse-current-protection diodes.

The theory here is that the charger puts out a certain voltage between the outputs and makes sure the current is OK. The voltages across the outputs, summed together, just make for a voltage ladder, very similar to how the batteries (when delivering power) sum together.

Then again, I'm also one of those CS guys, although I did grow up on Phillips EE experimenter sets, and took EE classes as electives in college ;-) I'd love to be told right or wrong by someone who does this for a living!

Why do you want to charge them individually? That would imply they are not being drawn down at the same rate, which would not be good. I think you'd be better with a chip that is designed to charge & monitor several cells in series.

http://www.maxim-ic.com/datasheet/index.mvp/id/3036 for example.

@CrossRoads, I am trying to use a circuit per cell mainly because i cant find an IC to do anything else. Additionally, because the pack has about 2 feet between cells and 7 or so feet from the first to last cell I had a lot of concern that they wouldn't charge correctly in series with one IC. If I were to try it, can you recommend an IC to do that?

@Jwatte, I didn't think about I until it was mentioned but I really would like to b able to share GND between the charger and cells so the circuit can charge and run from wall power at the same time. I can't say I understand at all what that changes in terms of what else would be needed between the independent charging circuits?

The IC I posted already will do that. It has a couple of shunt resistors to monitor charge current so individual battery temperature monitoring is not required. It passes along incoming voltage so charged batterys are not discharging while you connected for pattern generation testing or whatever. It stops battery draining when they are heavily discharged. You only need 1 wire from charger to battery1, 1 wire from battery1 to battery 2, 1 wire from battery2 to battery3, and then 1 wire back to the charger.

CraigKC: @Jwatte, I didn't think about I until it was mentioned but I really would like to b able to share GND between the charger and cells so the circuit can charge and run from wall power at the same time. I can't say I understand at all what that changes in terms of what else would be needed between the independent charging circuits?

Think about it: If each of the output "grounds" are connected, and each battery has a "ground," then you're shorting out each of the batteries except for the last.

If you 16 gauge electric wire (2 conductor, no ground) from charger to battery1 to ... battery4, then you can get a series circuit that works with the charge-all-batteries circuit suggested above. Use the "live" part to hook the batteries in series, and the "neutral" part to return back to the charger.