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LiFePO4 pack monitoring - "fuel gauge" IC?
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Clear as mud:
I intend to develop a battery monitoring system for a friend's project.  He will have 4 or 5 large lithium iron phosphate cells in series to power his stuff for three days or so, and he wants to be able to monitor the charge level, polling the battery monitor from a nearby USB port.  Also the monitoring circuit should provide a signal when the battery gets empty (maybe full too).  I'm looking for suggestions regarding initial design decisions such as what IC to use to monitor state of charge.

I've read a little bit of background material, but haven't really started.  My friend suggests that we probably want to go with a chip with LiFePO4 fuel gauge functionality built in, for increased accuracy and decreased development time.  He suggested the BQ78350-R1 from TI, but I am not sure.  That chip is intended to work with a companion chip they are calling an "analog front end," to monitor the actual voltage level on each cell and provide balancing to the pack.  But it's designed for small cells.  The ones my friend intends to use are large ones, rated for a maximum charge and discharge current of 130 amps.

So it seems I would have to build a custom circuit to take the place of the analog front end chip, and I'm thinking it might be better to start off with a different IC altogether.  But the other thing I am thinking is that the balancing may only matter during charging, and perhaps if pack balancing is not needed during discharge, then my circuit doesn't even need to have that functionality.  Apparently my friend already has a charger with cell balancing built in. 
NiHaoMike:
The same BMS chipset will work on both small and large cells. It's just a matter of selecting the correct shunt resistor and MOSFETs.
Siwastaja:
If you want to have cell-level measurement and balancing, using an off-the-shelf chip may be easier because of the complexity of level-shifting or isolating things if you do your own.

For coulomb counting, you can do it with your microcontroller direclty; these chips are often more complex to use (and total black boxes which still need configuration to work properly), than just doing it yourself.

dt = prev_time - cur_time;
charge = current * dt;
if(voltage >= high_voltage_limit)
{
    charger_enabled = 0;
    charge = full_capacity;
}
if(voltage <= low_voltage_limit)
{
    load_enabled = 0;
    charge = 0;
    full_capacity -= charge/10; // adjust full_capacity a bit each time so it gets closer to the actual measured capacity
}

soc_percentage_out = charge/full_capacity*100;


Do note that TI (or originally BQ) is notorious for totally saturating and dominating the market with unusable and dangerous piece-of-shit lithium-ion management ICs. Some are all right, some have caused a lot of grieve. Beginners like to design them in with blind trust. I have stopped using all TI li-ion ICs completely and if I won't find something else, I full-custom it, understanding the risks in doing custom design, but seeing that trusting a TI li-ion chip is a bigger risk, at least for me. Your mileage may vary.
Daixiwen:
Cell balancing is only needed during charge. If your friend already has a charger with balancing included, and if the battery pack already has a protection circuit against over and under voltage at the cell level, then your circuit doesn't need to monitor individual cell voltages, it can be enough to monitor the current and the full pack voltage. In that case the TI chip is overkill. I too have bad experience with TI battery monitoring chips. They are a lot more complex to use than what you initially think when reading the datasheet, and full or gotchas and black undocumented magic.
To monitor the current it's best to use an actual coulomb counter or a very high resolution ADC, especially for long discharges. Some microcontrollers specialized in battery monitoring, such as the ATMega406 have a high resolution ADC specialized for current and state of charge monitoring, but you will need to write your own software.
If you are not afraid of using TI chips maybe the BQ34110 would be an easier solution. They say it supports up to 32A but you can support more with some scaling. I have never used it but it seems a bit easier to use than most of the other fully integrated offerings they have. It uses a coulomb counter so I would expect it to be more accurate than the Mega406 over long periods.
Clear as mud:
I've been reading the eevblog forum long enough that I've heard people complain about TI integrated circuits not always working as they are supposed to.  And my first thought about the particular chip I mentioned was that it was overkill.  So, I'm leaning towards the advice to use a simpler coulomb counter IC.  Something with 8 pins, like the STC3100 from ST Microelectronics.

But I also need to measure voltage on each cell, because my friend is not buying these as a pack, just individual cells, so we need to build our own solution for making sure the cells don't go undervoltage.

So I'm thinking I'll use a coulomb counter IC, a microcontroller with A/D built in to monitor cell voltages, and a serial to USB converter chip to get the data out.
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