"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

The only reason to float the battery is to buffer the charge/discharge requirements.

The float voltage chosen will depend on how fast the recharge should occur and what SOC needs to be maintained. There was a study of calendar ageing vs SOC (Journal of The Electrochemical Society, 163 (9) A1872-A1880 (2016)) which showed a 6% loss of capacity over 9 months for SOC between 80 and 100%, 3% loss for SOC from 40 to 70% , and no loss at 0% SOC . If the 40 to 70% range was used, there would be no worry of overcharging and not much worry over recovery rate.

In this case a float voltage of 3.315V might work well but I would have no confidence that I was working up against the 70% SOC desirable for energy storage. In my case, I want to keep as much energy on hand as I can and since there is no difference in ageing from 80 to 100% SOC, I want to push the 100% and overcharging becomes a worry. I use the variable source solar PV and the load is various things in my house. I looked at some fractional C charging rates which I posted here. For floating, the green curve in fig. 2 is most useful (You will see that a higher voltage is required for charging, then you can drop back to float). For my purposes a float of 3.40V seems to be a good compromise between responsiveness and risk of overcharge. One of the A123 cells specifies a float of 3.45V which is higher than needed for my QH cells (https://www.lifepo4-batteries.com).