Thursday, October 1, 2009

What's Next: Charger / Balancer

Not ideal.

Charging LiXX batteries is an annoying, time-consuming process and can easily result in a dead battery pack, or in the case of Lithium Polymer batteries, massive destruction. There are two main problems:
  1. They need a constant-current, constant-voltage (CC/CV) supply. You can't just hook them up to an AC adaptor or a variable-voltage power supply. The internal resistance is so low that even a small difference in voltage between the supply and the pack will cause a large current to flow.
  2. Cells need to be protected from overcharging on an individual basis. Unlike lead-acid, they will not self-balance and are not tolerant of temporary overcharing. The voltage of every cell should be kept below a certain value at all times.
There are plenty of solutions to Problem 1. Most lead-acid battery chargers have a CC/CV strategy of some sort. Unfortunately, they only make them in increments of 12V (14V cutoff). So for my target application of a 33V pack with a 36V cutoff, I'm out of luck.

The more expensive option is an adjustable charger such as the Astroflight 109. This is basically a lab-style power supply in a small package. It will automatically monitor and adjust voltage and current, taking whichever hits a limit first as its operating point. So for a fully dischaged, 30V pack, it may only need to put out 31-32V to supply 5A. But as the pack voltage rises, it will adjust its own voltage to maintain 5A until the upper limit of 36V is reached. Fancy. But it also has problems. The biggest of which is that it runs on a 12V supply with no provisions for plugging into the wall. So for $120, you still need an AC adaptor. And it still doesn't do balancing.

Under ordinary circumstances this frustrating lack of good options would be the cue for me to build something from scratch. One idea I had was to use a Dell notebook adaptor as the front-end, since I carry one (or two) of those around on a regular basis anyway. But that still means I would have the build the CC/CV stage AND the balancer. And I don't want to make a big project out of this...I just need something convenient. So I started looking for hybrid off-the-shelf/custom solution and stubled upon this:

It means well.

The Mean Well PLC-100 is a 100W power supply module "suitable for LED lighting and moving sign applications." Clearly, exactly what you need for charging batteries... But actually LED drivers tend to be current-controlled devices, and in fact this power supply does have a constant-current output from 75% to 100% of its voltage rating. So the 36V version can provide a stable, adjustable current of about 2-2.65A from 27-36V. Perfect for a 33V battery pack, which should rarely drop below 27V anyway. 100W will charge the pack in under two hours. The cut-off voltage is also adjustable to between 85-100% of the rating. So the 36V version can be adjusted from 30.6-36V. For different-sized packs, there are versions for 12, 15, 20, 24, 27, 36, and 48V, all with a similar 85-100% trim range.

That takes care of the front-end supply. Now all that's left is the monitoring and balancing part. It would be excellent if this part didn't require a microcontroller or any programming. It would be even more excellent if each cell had its own, independent, floating circuit that handled balancing. Turns out this isn't actually that hard:

The monomer, if you will. (Click for a clearer picture.)

This circuit runs entirely off the voltage of a single battery cell. The core is a voltage comparator, the LTC1440, which has a built-in 1.182V reference and optional hysteresis setting (not used in the circuit above). The comparator check the voltage of the cell, through a 1% resistor divider. When the cell voltage reaches 3.6, it will turn on its output. The output drives an indicator LED and a transistor which bleeds away power. If the transistor can bleed away more current than the charger can supply, it can even be used to balance and the end of a charge. In this case, that would mean a transistor that can sink 3.6V*2A=7.2W. That's a pretty beefy heat sink, but not unheard of.

The boards will look something like this:

Don't ask why it only has five cells.

You can chain them together indefinitely, or use only a fraction of a board. No programming. No extra power supplies. Very simple.

When combined with the Meanwell front-end, the whole thing will be small enough to fit in a backpack and comparable in price to a Li+ charger/balancer/AC adapter combo. I'm doing it more for the convenience of not having to look around for power supplies, but I never can pass up the opportunity to use things for other-than-their-intended purposes.

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