Wow, a month gone by without any real progress. I blame the cold weather. Which has finally passed. I hope.
But, I've just run out of excuses. The final pieces to my puzzle arrived yesterday. Not what I was originally planning, but I had kept my fingers crossed and searched daily for a deal on these, and said diligence and patience paid off. No need to continue my home-made version.
Viola! :
A matching pair of 72 volt chargers. Fairly light weight, very efficient, intelligent, run on anything from 100v to 240v AC and are good for 1kw each. For less than I could buy cheap 10a chinese chargers off ebay for, of unknown quality or efficiency. Hooray!
Just tested them out on my LiFePO4 pack. They started charging it up no problem, without being programmed specifically for it. And my BMS will cut off the charge once the battery is full. Once said BMS is finished being installed, I'll find out what voltage the chargers cut off at on their own. There are already a few different charge profiles in them if I need to raise or lower the final charge. I'd like to have the chargers turn off a few volts before the battery is 100% charged (87v) just in case my BMS's ever fail to interrupt it, so as to lessen the likelihood of any individual cell getting over-charged. The chargers appeared to be cutting back current starting at about 82 volts, which is just about right. At a guess, they're set to maintain the battery at a steady 82.8v float charge, which, were these 12-volt lead-acid batteries, would be 13.8v -- exactly what you want for a float voltage for your typical lead-acid battery. And works out to 3.45v per cell for my LiFePO4s, which is almost perfect, IMHO, since that's ~99% charged and falls 0.2v short of the 3.65 max voltage. We'll find out for sure once that BMS is installed.
It looks like my plan to use 72-volt "architecture" in twin to keep costs down is going to work. That's about the maximum you can get small BMS/PCBs, which are a tenth the cost of a professional one good for a 144v system. Same goes for chargers. I could have easily spent $2500 for a BMS and a charger, whereas I've now about $350. Which is about what I was aiming for. Mind you, it's not every day you can find a matching pair of high-quality chargers cheap.
I had also ordered this a couple of weeks ago:
It's a hall-effect transducer, with specifications to match the one that Mechman was using with the mini-motor controller he was using to control the field on the sepex motor he had, both of which are now in my possession. Rather than buy a serial adapter from Kelly to allow me to reprogram the controller, I went looking for a transducer with the same properties as the one he had. It only cost about $5 more to have one that matched rather than a generic one, which is cheaper than having to buy the serial adapter as well. Field control solved.
The weather has warmed up. I now have
all the parts I need to finish this conversion properly. My kid is going to Gramma's house for a week...yeah, I'm totally out of excuses not to get on with this. Wish me luck!
(Woot-woot! I have 2kw worth of chargers!)
