LiFePO4 Battery Charger & Management Circuit

[dweeb4]

I have a A123 battery (2300mAH) I want to charge in a reasonable time (3-4 hrs).

I have the battery, charger & a circuit board in a small box. I've tried the LM317 CC/CV circuit for charging but the problem I've found is the heat dissipation inside the box & it's possible long term effect on the circuit board. The external PS can be either 9V or 12V - so there is probably too much heat dissipation in reducing the voltage. I can't reduce this external PS.

I've decide to use a buck converter based board on the MCP2307 - these can be picked up cheaply & with a bit of modification can be adjusted from an output voltage of 3.3V to 3.6V (the max voltage for LiFePO4 charging). These boards are not reducing current however so I need to implement a high voltage sensor cut-off. The MCP2307 has an enable pin that when high turns on the regulator.

So I wanted to implement a simple comparator based voltage sensor that would turn off the MCP2307 when the battery reached 3.6V. I also wanted one that signaled if the battery fell below 3.0V. I have LM393 comparators in stock

So I guess I need a voltage reference like a TL431? Has anybody got a simple circuit using these parts?

My other slight concern is that the MCP2307 outputs about 2A current while charging so if I turn it off when the battery reaches 3.6V what percentage full is the battery?

Edit: Actually, this may not be of concern as these batteries are meant to be able to be charged fully in 15mins or so at high C

 

[dweeb4]

OK, so I came up with this - I'll need two comparators (one LM393) & TL431:
- I'll use the 2.495V reference voltage of the TL431 as the ref input to the comparator.
- I'll use a voltage divider to bring the battery overcharge 3.6V threshold down to 2.5V - this will be fed in on the - of the comparator, the 2.5V ref goes into the + pin of one comparator. So when the battery voltage reaches 3.6V the comparator output goes low - this is connected to the enable pin of the MCP2307 which will turn it off.
- I'll use another voltage divider to bring the battery low voltage threshold 3.0V down to 2.5V & feed this in on the + of the comp & 2.5V ref on the - pin of the orther comarator. This will turn on the enable pin.

Does this sound correct?

I'm not sure how to connect up the TL431 just as a reference voltage?