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I want to make a charger for two LiFePo4 cells in series. As I'm aware, charging cells in series may be problematic, due to slight differences on each cells, that may charge one and not the other, being this situation getting worse with time.

I was looking specifically for a IC that would assist the charging, despite having too many external electronics and/or microcontrollers doing the job.

I came across Microchip's MCP73223, which contradicts the need for balancing, claiming to be a model to control 2 cell in series charge.

So, since this IC seems to fit perfectly on my application, the questions are:

  1. Is the need of balancing that critical that even charging only two cells in series will have great repercussions on the charging along time? Why?

  2. If the question for the first question is yes, why would one of the big IC manufacturers develop and sell a chip that induce us on doing something technically wrong?

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"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

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    \$\begingroup\$ Thanks for the reply, but this acually doesn't answer my questionsa at all, only states that it's a bad idea, which I let you know I'm aware on the questions. Rushing to answer sometimes is a bad idea too, as you'll notice this particular chip does not have the third pin you mentioned, thus making this answer quite off topic, since I want to know why would Microchip manufacture this. Maybe it's because it's not that of a bad idea. And if you would please enhance "bad idea" with some technical info, tests of some sort, so I would get more quality here than I got on google. Thanks! \$\endgroup\$ – Sergio Sep 22 '14 at 16:14
  • \$\begingroup\$ They produce it because it's cheap. You notice the use of the phrase "Low cost" a number of times in the data sheet. It's a Bad Idea™ because one of the two cells can end up with low voltage and not fully charged, and if that voltage gets too low it can't be recharged at all. So you get reduced battery life but reduced production costs. As it says in the data sheet - ideal for toys ;) \$\endgroup\$ – Majenko Sep 22 '14 at 16:18
  • \$\begingroup\$ They produce something that's technically wrong but writes "low cost" on the data sheet, so we interpret "it's a bad idea, don't useit"? I don't think so. Again, some techical info or tests on the impact of serial charge (and agian, I'm only specific taling about two cells) would be nice, but I can't find them. \$\endgroup\$ – Sergio Sep 22 '14 at 16:23
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    \$\begingroup\$ "Technically wrong" would be if it caused the batteries to explode. This is only "wrong" when your expectations differ from the intended results of your device. It is perfectly right and acceptable for certain situations and environments. Those situations and environments may not match your requirements, in which case the device is not the right one for you to use. \$\endgroup\$ – Majenko Sep 22 '14 at 16:33
  • \$\begingroup\$ Ok. That would answer the second part of the queston, although in my personal interpretation is a matter of opinion, not a absolute truth. Thanks. The question turned to the microchip's chip, while the real issue for me is, for charging only two batteries in series, how can I predict, measure (or anyone already done it somewhere I didnt find) the real impact? I mean... or dozens of cells, I get the critiallity. But for 2 cells that will ocasionally turn on, how could I know/predict the impact? Because depending on the level of impact I may use this solutuion anyway. \$\endgroup\$ – Sergio Sep 23 '14 at 16:02

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