The batteries I picked out (literally, from the bottom of a crate filled with cruft) for LOLrioKart have been problematic. They’re more or less in hibernation from being stored so long. While I’ve used my multicharger to try and tend to the batteries, it’s simple too low-power to really wake the batteries up.
Charging individual cells was an option, but not something I wanted to repeat 44 times, if not more, so I would try to ‘zap’ the cells to get them to a workable voltage, then charge the entire pack. But, by the time one pack completed (usually after 18+ hours at 6 amps), some cells would have zero-volted again in the other packs. This was obviously not a sustainable activity.
Plus, pegging a charger at maximum capacity for hours is unhealthy for the charger. Alas, after weeks of nearly constant work, my charger died with a little puff of smoke.
I’m not peeved. The fact that it lasted this long near the top end of its ratings means that the cheap stuff has gained some serious ground in terms of reliability. For those interested, here it is.
After that, I decided to pour a little more brute force into the equation. So now I present…
A 2,000 watt variac. A 50 amp bridge rectifier. 10,000uF of giant capacitors. And… well, that’s about it.
I even added little caster wheels so I wouldn’t have to pick it up and drag it around.
It’s a completely unregulated power supply that *sort of* functions as a constant-current supply when I hook it up to something. There’s no auto-cutoff, no temperature sense, no readouts (have to poke at it with a multimeter), and no SCRAM switch, besides a 40 amp circuit breaker (really 2 20-amp units paralleled). It vibrates at 60Hz ominously, and probably raises the level of the Homeland Security Terror Threat Level a notch just by virtue of existence.
And it works beautifully. Throwing the switch on all 4 packs and giving it full crank trips the local 120 volt circuit breaker (20a) within a few seconds. So, to keep things tame, I limit the battery charge current to 25 amps, a healthy 2 electrical horsepower (+a pissed-off pony) at 70 volts.
Note that this is a later pic, where I have added a current shunt (upper left corner of the board) such that I can get an amps reading without dying.
I actually lied in the title. NiCads charge optimally under constant-current conditions, and a great way to terminate the charge is the rate of temperature rise (dT/dt). For the time being, it’s “poke the battery, come back 5 minutes later and poke it again”, and if the answer to “how much hotter is it?” is between “quite a bit” and “alot”, I disconnect the power. This would be easy to systemize with a few thermistors jammed between the cells.
Currently (LOL PUN) though, I have to babysit it through the charge, lest I want to fill MITERS with a cloud of heavy metals and sodium hydroxide.
The bottom line is, the NiCds needed some ass-kicking to wake up. Work on LOLrioKart can now continue – I was losing interest for a bit because of the battery issue, since without batteries, having an Etek is sort of pointless. The goal is to get it at least running by Campus Preview Weekend, because…
What better preview of MIT is there than this?!
-c