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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Theres a widget here
http://www.capable.ca/rcstuff/fastchg.htm
thats looks like it will do a reasonable job and still be cheap.
It works on three-level temperature sensing to control the fast-charge rate and charge termination (instead of delta-peak-voltage detection), and I believe the circuit could be significantly simplified by replacing that mess of comparators and resistors with a simpler pic-axe micro and some code (if I ever get some time to tinker with it.. I'm snowed under with work again at the moment
The catch is, it only does 7 cells in series, because, like all hobby pack chargers, its designed to run off a car battery, and to charge a battery with the same voltage (or higher) as the battery you are powering it from requires some expensive hi power switchmode circuitry to step the voltage up.
Likewise to run it from 240v. the hobby chargers dont, because hi-current 240v stepdown circuits and transformers arent cheap. If someone can find me a cheap way of coming up with about 20-30v nice regulated and filtered power at 10amps of current, then I could make a micro-controlled circuit that worked like that one, but simpler. but coming up with the power source is hard to do without $$ _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Thu Oct 21, 2004 8:53 pm |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Remember that annoying law of 2 out of 3..
Cheap - Fast - Good
Pick any *two* in combination and the 3rd is auotmatically excluded
There is no cheap and simple way (that I know of) to change 240v AC power into 12v DC power at high currents and still have good rectification and regulation (which are very important when charging batteries).
The hard parts are the main transformer and the filtering capacitors. Both very expensive components when you want 10 amps or more of current. $100+ for the transformer and $50+ for the filtering caps. Add another $50 for the high current rectifiers and regulators, and another $50 for hardware (case, heatsinks, terminals etc) and you can see why high current 12v power supplies are expensive.
Car chargers arent really suitable, since they typically made very cheaply and have crude rectification, non-existant smoothing and poor regulation. (thats how they make them cheap).
It doesnt matter that much for a wet-cell car battery where over-charging just results in the gassing-off of some of your water (which can be topped up) and the unsmoothed output just heats up the water a bit. But for a SLA battery, the gassing of electrolyte is *bad* since its non-replaceable, so regulation is important, and an un-smoothed output with significant ripple on it does nasty things to the cells chemical structure resulting in a gradual but constant and permanent decline in the storage capability of the battery.
If you want to save a few $ on a charger, and dont mind tinkering with regulators and such, I suggest watching out for a 2nd hand (or dead) *big* car charger (10amps plus) going cheap, then junk its output circuitry (you really only want its transformer and case etc) and replace it with some quality filtering and regulators. Takes a bit of work, but will probably save about $100 over the cost of building one from scratch. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Sun Oct 24, 2004 11:37 am |
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