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QUESTION:
I've read a number of comments regarding reliability with various chargers. There is also the question of balancing a battery pack, etc. In lieu of a BMS, does it make sense to use a number of small chargers (i.e., one per battery) instead of a large pack charger? I'm leaning toward individual chargers, but haven't read anything that discusses the problems. If you could point me to something that presents the pros and cons, or if you could weigh in on the issue, I would greatly appreciate it.
ANSWER:
Individually charging each battery really offers each battery the best charge possible, assuming you have a smart charger on each battery. So why is this scheme not more commonly used? There are a number of critical factors. First, and probably foremost, is ultimately cost vs. charging times. Most people want at least a moderately fast charge on their EV. Placing a fast charger (i.e. in the range of 10A or more) on each battery would be really expensive! The other key issue is reliability. Failure rates on cheap components is generally high enough that the probability of having one of the chargers fail in the first year or so is pretty high. If a charger fails, and you don't know about it, you run the risk of driving off with all the batteries full ... except that one with the failed charger! If it isn't already clear, let me elaborate. Let's say your drive the day before pulled the batteries down to 40% state of charge (SOC). Let's say the charger that failed quit after putting in 10%. So the next morning, all the batteries are at 100% SOC except the one that's now at about 50%. You head out on your normal commute, that will again pull the batteries to 40% SOC. You make it to work just fine, since this only takes out 30% (the ill-fated battery is now sitting at it's maximum allowed discharge of 80% discharged, 20% full). At about two thirds of the way back home, the battery is now sitting at 0%. You continue to drive, possibly noticing the vehicle not performing very well (you've now got a huge resistor in the pack!). Cell reversal is imminent, with damage almost certain.
If you find a way, either manually or automatically, to monitor each charger, and you can live with slow charge times, then individual chargers (as long as they are "smart"), is probably a pretty good way to go.
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