"Blind" charging of batteries in series, regardless of chemistry, is a bad idea. By "blind" charging I refer to the treating of multiple series cells as one single battery.
However, blind charging is cheap and simple. If you don't care too much about the longevity of the cells, but you do care about the cost and space, then blind charging of multiple cells is an option.
Most multi-cell chargers don't blind charge. They treat each cell as a separate entity and tailor the charging to ensure proper charging of all the cells in the chain.
You will notice that in the schematics for all the chips of this type that there is more that just two wires connecting the charger to the battery - there is a third that connects to the mid-point between the batteries (or two more when it's a 3-cell charger). That allows the chip to monitor the state of each individual cell in the chain and affect each cell separately.
It's not 100% perfect, but it's a darn sight better than blind charging, and sure beats removing the batteries from a device to manually charge each one separately.
As you blind charge cells in series the cell with the most charge triggers the end of charge, so that cell is always fully charged. The other cell hasn't fully charged, and never fully charges. This is the source of the cell imbalance.
Over time that second cell's voltage drops further and further until it gets to a level where it can no longer be charged back up even if you were to take it out of the circuit and charge it separately. It's now dead.
So while blind charging series cells does cause cells to die over time, it's not a catastrophic (as in the blowing body parts across the room catastrophic) failure, but a gradual diminishing of charge until it's all gone.
You'll notice a key couple of phrases in the data sheet, especially under the Typical Applications section:
- Low-Cost LiFePO 4 Battery Chargers
- Toys
Cheap products where you care more about how much it costs to make them than how good they are. You don't care that the you car's battery will be useless in 6 months - the kid will be bored with it by boxing day anyway. If they really wanted a good remote control car they'd get a real one, not a toy, but of course that'd cost more.
So the bottom line, I reiterate, yet again, is:
COST
Best Answer
The problem is that most of the cell voltages are higher than the A/D (and MUX?) can handle, so you need some way to get it in range. You could just put a voltage divider on each cell tap, but then you have to individually scale each tap voltage and subtract them to get the cell voltages, which is difficult to do accurately.
The solution used in many chargers is an op amp configured as a differential amplifier, like this:-
simulate this circuit – Schematic created using CircuitLab
This example shows one of the identical circuits that would be used to get the voltage on each cell in the pack.
Op amp OA1's output is proportional to the difference between voltages at each end of cell 3. The op amp can only accept a maximum of 6V on its inputs, so R1 and R2 divide the + (non-inverting) input voltage by 3. R3 and R4 have the same ratio so the voltage on the - (inverting) input is also divided by 3, and the equal ratios ensure that differential balance is maintained (so voltage changes on lower cells have no effect). The gain of the op-amp is set at 0.5 by the ratio of R4/R3, so the useful output voltage range is reduced from 1V (3-4V) to 0.5V (1.5-2V).
The circuit shown above works for up to 4 cells, but for higher voltage you either need higher voltage op amps (and Vcc) or a higher division ratio.
This technique is commonly used for up to 6 cells, but I don't know how successfully it could be extended to 16 cells. Theoretically your A/D converter has enough resolution to handle the smaller voltage range, but in practice it may be quite difficult to get the precision you want. You could amplify the signal with another op amp, but then you may run into offset and drift issues.
Another option might be to wire a precision voltage to frequency converter across each cell, couple the output pulses via opto-couplers, and measure the frequencies using a counter/timer in the Arduino. The V/F converters would draw some power out of the cells, but if they have high capacity it shouldn't significantly affect their capacity.
However I would probably go for the much simpler approach of 16 voltmeters. I would just get 3 of these 6S lipo monitor/balancers. Or if I only needed to check the voltages occasionally, I would just get one 6S meter and wire some balance cables onto the battery to plug it into.