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NiCd/Nimh Battery Tech Info
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timmeh
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Just a question for someone i know.

Is there some way of makeing a safe charger for drill bats that is dirt cheap?

Something that will charge them quickly.
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Post Thu Oct 21, 2004 7:31 pm 
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Spockie-Tech
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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 Confused

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 $$
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Post Thu Oct 21, 2004 8:53 pm 
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timmeh
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Thats pretty easy to follow.

BUT! my friend needs a 12v charger Sad

Is there a way to make a cheap charger for sla batterys?

Other wise theyl have to use a car charger Very Happy
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Post Sat Oct 23, 2004 10:33 am 
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Knightrous
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Car chargers are all we've been using on ours, Andrew just modified them a little bit so they charge at 13.8v max
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Post Sat Oct 23, 2004 10:42 am 
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timmeh
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how do u do that?
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Post Sat Oct 23, 2004 10:54 am 
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Spockie-Tech
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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 Crying or Very sad

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.
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Post Sun Oct 24, 2004 11:37 am 
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Valen
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i wonder if the output of an AT/X psu could be hacked up to 13.8V or whatever a SLA's constant voltage charge voltage is
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Post Mon Oct 25, 2004 1:13 pm 
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Spockie-Tech
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They can, and there are lots of articles about how to do it on the net, but its a bit of a kludge in most cases.

You need to load the 5v rail up with a big resistor or a lamp to keep the other rails steady, and in some cases you need to rewind transformers and things. it seems to depend on what model of power supply you get.

Its an option, but a bit hit-and-miss as to whether you will get what you want from it or not, and theres lots of HV stuff inside a switchmode, so not the sort of thing you want to encourage young guys without much electronics experience to be tinkering aorund with in search of a cheaper solution.
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Post Mon Oct 25, 2004 1:32 pm 
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Valen
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no, i wouldnt really attempt it either
i was just wondering
SLA charger from jaycar is about $60 or so
given that your SLA's are going to be totally rooted after roughly one event anyway (as glen/cobra has found out) if you want the cheaper option go a high current car charger for cheap then sit on the battery with a multimeter.
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Post Mon Oct 25, 2004 3:00 pm 
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Glen
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never really thought of it that way...

i guess seeing as though i only got one event out of super cheap slas if your using them i guess monitoring the slas with a car charger would be cheaper.

but however ive gotten about 3 odd events and 1 hour of testing out of the diamecs. pretty pleased with them albeit one is leaking.
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Post Mon Oct 25, 2004 6:09 pm 
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Glen
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http://www.robowars.org/forum/album_pic.php?pic_id=332

those are the battery packs im making at the moment. just trying to see which setup works best really. dont think there is any difference in how a staggered or how an in line pack would perform.

im going to solder them all with 12 gauge wire between the cells with just the standard 60/50 rosin core and use my 15w soldering pencil (no gun here..) with a standard tip.

also they have 6mm lexan spacers between them, to hold them all in place laterally. and when i build them for good ill just hot glue them all into there holes.

unsure about what to cover them up in.. really i think ill just pack the top and bottom in tape lol.

oh yeah and ill be having a 90mm fan on the top of each pack blowing air into the packs.

any crippling problems with any of that you can see?
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Post Thu Oct 28, 2004 9:16 pm 
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timmeh
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Sounds good to me.

Y dont you have the fan on the side blowing air threw and that way you can tape up the top and bottom of the packs leaving both ends open and that way your makeing a tunnell for the air to blow threw?
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Post Thu Oct 28, 2004 9:50 pm 
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Glen
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id like to do that but mounting a fan horizontally to the packs would be next impossible. i was also going to try and setup the fan to suck the air out in one of the packs to see which one works best as well.
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Post Thu Oct 28, 2004 9:54 pm 
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Valen
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you shouldnt need to solder to the cells
use the solder tabs on the batterys
thats what we did
and solderd between those tabs.
soldering to the tab is ok (thats why they put them on there)
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Post Thu Oct 28, 2004 11:40 pm 
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Totaly_Recycled
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I did another check on those cells im testing for reverse polarity they are both still siting at 0 volts and the ones either side have droped to around 1.1 volt each so far

Post Sat Oct 30, 2004 9:17 pm 
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