> > That's getting pretty heavy. Thanks for the rule-of-thumb. > > What do you mean by "balancing"?
Since you asked.. ;-) One of the perennial problems of on-road and other large EV drivers is that these vehicles are powered by very large strings of batteries. Now, for whatever reason, all batteries are not created equal - tiny variations in manufactoring will make some batteries be 'full' slightly earlier than others, and have slightly different internal resistances. If you're running flooded/lead acid, this isn't that big a deal, because you can balance your batteries by overcharging them - the batteries that are completely full will just split their water into hydrogen and oxygen and vent it, while the un-full batteries keep charging. This isn't really good for the batteries exactly, but it doesn't kill them very quickly and it's certainly cheap. Just don't forget to water occasionally. [Note: Some people disagree and say that 'equalization charges' don't damage, and are possibly even good for, batteries. As with all things battery related there are a wide variety of opinions. This is just mine.] If on the other hand you're running sealed batteries - because you hate watering batteries, or for a number of other reasons, some personal, some technical, you have a problem. Because your batteries will slowly get further and further out of balance until eventually some of them are being massively overcharged and venting more-or-less irreplacable electrolyte while others are still not completely full. A series string of batteries is only as good as the weakest cell in said string, so when the one that is emptiest empties completely, even if all the others still have plenty of charge, that's where you stop driving. Enter cell balancing - which can also be used on flooded batteries, but is generally considered more cost than it's worth. In general, you have two types of systems - additive and subtractive. Additive systems add power to the emptiest battery, or to several of the emptier batteries, while subtractive systems take power away from the fullest battery, or the fullest batteries. I can go into gory detail here if you want, but I think you'll find that it's all been covered in the archives, so I'll just summerize the options I know about. PowerCheqs: Currently made of unobtanium, but you may be able to buy a used set. Have a few distressing tendancies, including a tendancy to drain packs completely dry and to catch fire on occasion. Not my favorite balancing system. $50 a battery, last I looked. Lee Hart battery balancer: Relay based system. Never commercially produced, but you can get plans from Lee (I think for free). Additive system, props up the lowest battery with a 30A DC/DC. Not sure what the per-battery cost is. Rudman Regs: Available in a wide number of versions, including soon the mk3 microproccesser based network driven version. Subtractive regulators. Like $35-40 a battery last I looked. It should also be noted that if you want battery balancing on the cheap and are willing to do a bit of physical labor, you can also just take your battery pack out every month or two and put it all in parallel and charge it for a while. Yes, this really does work, though it is a fair amount of work. I'm sure I left out some things, list greats please feel free to add additional comments. S.
