Sunday, October 18, 2009

Last-In, First-Out: Charger / Balancer

I normally don't wrap-up projects in reverse order from when I start them, but the charger/balancer is going to be an exception for a few reasons. One: It works okay as-designed, since it's really simple. Two: It is still annoying to use and in all honestly I probably won't.

The point was to solve two problems with charging a medium-sized lithium iron phosphate battery pack. The biggest problem is the lack of small, inexpensive constant-current/constant-voltage power supplies that also plug into the wall. (Think laptop adapter size.) Thanks to the wonderfully cheap Meanwell PLC-series LED supplies, this was easy to solve. I got a 100W version, which can put out 2.65A (regulated to this current at anywhere from 30-36V), or about 0.6C for my battery pack. This means a < 2-hour charge, which is pretty good for a very portable charger that plugs straight into the wall. There's just one small problem:

Why are my ears ringing at night?

For whatever reason (cheap electronics) the Meanwell LED supply goes into a very noisy switching routine in constant-current mode, and the frequency of switching happens to be about 1,700Hz. Fun fact: The human ear is most sensitive to frequencies around 2kHz and three times more sensitive to this frequency than to middle C (262Hz). Basically, if I wanted to torture someone with a particular tone, this would be it. The good news is that you can tell when it switches over to constant-voltage mode, because the 1,700Hz fades away leaving only a faint and relatively pleasant high-frequency voltage regulation circuit.

The second part of this project was the balancer. The balancer was a very cool idea. It used the LTC-1440, a comparator with a built in voltage reference, to detect overvoltage and trigger a power draining transistor. Said power transistors were 44H11s, which are very solid NPN transistors with h of well over 100. The base circuit is described in more detail in my last post. And, well, it works just fine:

What would I do without MATLAB?

The current starts to flow through the transistor at about 3.55V and reaches a peak value (set by a resistor) at about 3.60V. The peak value in this case was set to just about 2.2A, which is what the Meanwell supply was to be set to. At such a current, the transitor would be required to dissipaye 2.2A*3.6V=7.9W. That's a lot, but not unreasonable with a heat sink and fan:

Baby heat sinks.

The boards stack nicely and a modestly powerful 120mm case fan blows air across them. Tested at 2.2A for over 30 minutes, the steady-state temperature of the transistor's tab settled at about 55ºC. That suggests it could be pushed a bit harder, probably all the way to the 2.65A limit on the Meanwell supply. This would just require changing the 220-ohm base resistor to something a bit smaller.

So what's the problem? (There must be one.) Well for one, something is killing the LTC-1440s. My best guess is it's the fan or the 12V switching regulator that powers the fan, or some combination of the two. With the fan disconnected or running from a separate 12V supply, the problem does not present itself, even after multiple connect-disconnect cycles. The symptoms are: only the circuits connected to the top three or bottom two cells fail, and they stop producing a reference voltage or an output after some small number of connect-disconnects. Could be momentary overvoltage, made possible somehow by the switching regulator connected across the whole cell stack and depending on which side (high or low) makes contact first.

I really don't know. Normally that would bother me, but as I was testing this thing I realized that I also don't care. The biggest problem is that it's too damn big. The whole portability aspect is gone, and forget about taking it through an airport (or out in public in Boston) with all the circuitry and wires. Not much I can do about that...a cell balancer needs as many wires as there are cells (plus one). And the transistors definitely need heat sinks and a fan to balance at any reasonable current. So forget it! For now anyway, I can live without it.

"But how do you keep the cells balanced?" I don't. Every once in a while I might drain or charge ones that are way off with a power supply.

"But how do you keep the cells from overcharging?" I feel like if I explain the method, somebody will try it and blow up their cells... It involves setting the charger CV so that, worst-case, no single cell can exceed a certain voltage. For example, if the pack is at 33.50V, and the highest cell is at 3.37V (measured), I set the charger to 33.50 + (4.20V - 3.37V) = 34.33V. This is more than enough voltage overhead to put it into CC mode, where it sits happily for a long time before I have to readjust the CV based on a new worst-case measurement. If none of that made sense to you, you should probably invest in a user-friendly charger/balancer. :)

Anyway, back to other things:

Going to the SMMA 2009 Fall Technical Conference in two weeks. Mildly frightening, but I am looking forward to the immersive motor experience. Definitely a post in the works after that.

The 3ph Duo controller is DONE and WORKING and I am starting the long process of writing it up. An extensive post will follow.

The Epic Axial Motor progress.

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