question Charging a 2s lifepo battery

[killerkappi]

Hello everyone

I'am planing to make a uController system for my PC case to controll water temps, fans .... So the thing is i'd like to power this system with the pc psu. So this Controller will only have power if the pc is running. But i want to be able to start my pc per bluetooth so i want to make something like an usv. I planed to make a system which charges an lifepo4 2s battery while the pc is running and it will discharge it while the pc isn't running. I will ad an extra uC for the battery management (an attiny 13 i guess) which meassures the cell voltages on the balancer. If one of them is to high/low he will stop charging/discharging. But i'am not sure yet how to limit the charging current cause i don't know the resistance of a battery . And with a multimeter would be the worst idea . So i drew a little schematic. But of course i'am happy about different solutions.

[Doramius]

You may also want to add into your diagram a delay shutoff relay to allow the fan and pump to be maintained for up to 3 minutes after power-off. 
1 - It keeps a flow if you do a simple restart or quick cold shutoff
2 - allows a little extra cooling flow time so very warm liquid doesn't pool at the CPU block when power is disconnected

 

I was also in the middle of typing how to determine internal resistance of a battery when I found this: http://www.instructables.com/id/How-to-measure-the-internal-resistance-of-a-batter/

 

[killerkappi]

The Pump is still gonna be conntroled with the pc mainboard cause i like the software there.(aquastream xt). With no fan connected on the cpu fan header my pc does run all fans for 40 s after shutdown anyway. Also the pump. It looks like it keeps uf the 12v rail for a moment after shutdown. I don't want it any longer than that because i always plug out my pc after i've shut it down. Thanks for the tipp with the internal resistor. Now i remember i've already done that once in my education.

[Doramius]

I hope it works out for you.  Don't forget to post pics and how it turned out. 

[killerkappi]

Another question came up. The loading end voltage of an lifepo4 battery is at 3.6V per cell. in an 2 s situation this would be 7.2V. So now if i want to have 7.2 v at the battery i cant put an resistor before cause that would lower the voltage at the battery. And if i want to make an voltage divider with the battery and the Resistor it would be like cira an 20milliOhm resistor in series with an 200ohm resistor. So the voltage had to be huge to get this voltage over the battery. Or do i read datasheets wrong. I read 4 mOhms in a random lifepo4 datasheet

[Doramius]

Let me check my formulas, but depending on the circuit and battery types, you should not need to put anything in between.  Most I've ever seen is a filter cap if you have AC connected to charge at the same time as powering a circuit.  Overall amperage is more of your concern.  The series batteris are treated as if one battery with multiple cells.  You need to make sure your overall flow is not too much and won't cause your batteries to overheat or damage.  This will still bring your internal resistance into function, as you'll need to add the resistances in series to your overall circuit.  You'll probably want to make sure the gauge of wire/connection between the 2 batteries can handle at least 2 times the voltage of a single battery.  IE: 3.6V+3.6V = 7.2V   Not required, but definitely helps to prevent heat and resistance at point of connection between batteries.