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Topic: SOLVED (more or less): NiMH batteries: current and charging (Read 2 times) previous topic - next topic

JimboZA

Jul 01, 2012, 07:41 am Last Edit: Jul 01, 2012, 07:20 pm by JimboZA Reason: 1
@Mods: Might not be the right forum... move if you like  8)

On advice from the forum I bought a 9.6V NiMH RC battery pack like the attached pic (GP brand, 9.6V, 1600mAh)... the Voltage Monster in our MotoMama's L298 is ravenous, and so 9.6V will likely give me the 6V I need on the 298's output.

My question is in two parts:

Nowhere can I find what current these 9.6V packs can output (as opposed to the 1600mAh of energy it holds). It says here that these guys power all sorts of RC devices, yet mine won't even turn my motor. (Motor does run (albeit slowly) thru the 298 with 4x AA= 6V in, given the 298's voltage drop) so I know the circutry is ok.) The motors I have are tiny little toy motors, 250mA (except for 1A at stall) so I can't believe this 9.6V pack wont turn them. The battery reads about 10V out of the box... my question: does that actually mean the battery is fully charged? Can a battery show full voltage but not be charged in terms of energy,  and thus not be able to supply power? Is there some circuitry required external to the battery to turn it on?- remembering that my meter shows it's "full"?

Secondly, the chargers the shop had cost about double the cost of the battery, and I didn't have enough arms and legs to buy one, but he sold me a wall wart which he said is ok to use as a charger. Is that true?- a wall wart presumably just sits and produces volts all day long, and this one can output an amp. Is it really ok to use as a charger which I would assume needs some kind of way to switch off when the battery is full? Supplementary question to that one: the voltages it outputs bear no resemblance to the switch settings, but I guess that's unloaded. 6V setting gives about 9, 9 gives 12 and 12 gives about 14 iirc. So assuming I can use the wall wart to charge the battery, should I chose 9v on the switch (supplying an unloaded 12ish) or or 6 on the switch supplying an unloaded 9 or so....

I know that's lots of questions all in one there guys, but they're all kinda linked.

All help greatfully accepted!

PS, when I hook my 298 up to the aforesaid wall wart the motors work like a charm.

PPS, I was in two different RC shops yesterday, and nobody in those places seems to know their amps from their elbows  :smiley-yell:
Roy from ITCrowd: Have you tried turning it off an on again?
I'm on LinkedIn: http://www.linkedin.com/in/jimbrownza

DuaneB

Hi,
The staff in hobby shops are notorious for being as opinionated as the are wrong and they are generally opinionated.

Other than that, 10v would indicate your pack is fully charged. The battery pack looks more like a receiver pack than a power pack. Power packs are generally 7.2 volts and can easily get a race car over 60km/h. Power packs usually show a C rating, this can be used to calculate thier ability to deliver current, capacity * C rating = current.

If your pack is a receiver pavk which it looks like, it may not be able to deliver sufficient current.

Why dont you use a multi meter to see how much currentbis being drawn when you run off the walwart and how much is being drawn against the battery, this should eliminate or confirm the battery current as the problem.

Duane B

rcarduino.blogspot.com





Docedison

Jim, Never assume... Charge them overnight before you use them... At least and they may well deliver a little more after a few charge discharge cycles...

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

JimboZA

#3
Jul 01, 2012, 08:04 am Last Edit: Jul 01, 2012, 08:11 am by JimboZA Reason: 1
Thanks for that so far guys....

But Duane, what is a "receiver pack"? That link I posted said they use these to run RC machines. And Alas! my el-cheapo meter is volts and ohms only, no current!

Edit.... I'm guessing receiver as in the radio receiver in the car?

And Doc, is it ok to charge from a wal wart, and if so what output voltage should I set the thing to?
Roy from ITCrowd: Have you tried turning it off an on again?
I'm on LinkedIn: http://www.linkedin.com/in/jimbrownza

Nick Gammon

I am no battery expert, but I doubt it is a good idea to shove a wall-wart onto the battery. Apparently they do not respond well to overcharging, so they may get hot, perhaps very hot, if you overcharge them. Also, according to the book I have here, harmful overcharge can occur if a partially or fully charged battery is charged on a charger with a fixed timer.

According to this, you should be able to get 5C peak from a NiMh battery (that is, use all its power in 12 minutes) but the best result is 0.5C (so in your case 800 mA).

From what you describe, I'm a little doubtful they are fully charged. NiMh batteries self-discharge at approximate 30% per month.

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