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Author Topic: SOLVED (more or less): NiMH batteries: current and charging  (Read 2430 times)
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@Mods: Might not be the right forum... move if you like  smiley-cool

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


* gp tc201 battery.jpg (398.27 KB, 1600x1200 - viewed 35 times.)
« Last Edit: July 01, 2012, 12:20:05 pm by JimboZA » Logged

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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

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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
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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?
« Last Edit: July 01, 2012, 01:11:12 am by JimboZA » Logged

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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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Nick thanks but you lost me a wee bit there... does that mean its best to use an NiMH battery at mA of half the mAh rating? In my case it would be 2 x 250 for two motors so 500mA, not quite the 800 so not optimum..... not that I'm looking for optimum, I'm looking for the bloody wheels to turn!
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It sounds on the face of it that the battery should be OK, you are using under 0.5 of its 1600 mAh rating.

Your model appears to be here:

http://www.smallbattery.company.org.uk/sbc_tc201.htm

According to that:

Quote
This high power NiMH pack is designed for use in a wide variety of remote control R/C models.  This racing pack will power most radio controlled cars, boats, planes, etc and is often used to power the transmitter device. For eample, they are commonly used in For Quickdrive and Prosport cars.

It is really 8 (presumably quite small) batteries in series. The photo you showed said "charge before use".
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In the reply referencing receiver packs... I've never seen a receiver that could discharge a 1.6 AH battery in normal use, He has a 750 mA max load, good enough for 750mAH/1600mAH or about 2 hours of continuous running allowing 100 mA for the control stuff Arduino, Remote or Guidance. I do think that the battery pack being new is not supplying enough current to the load... or it is defective..., That I sincerely doubt as batteries don't fail that way, they just lose capacity prematurely over several months depending on use and care in charging and storage temperatures. Defective ones will usually show in the first several months of use, not out of the box.

Doc
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Yep it does say that Nick, and I neglected to say that it's been feeding on the wall wart since I found it didn't work even tho it read 10V and it seemed not to be any different this morning. (Safety aspect: given the uncertainty of the wall wart's suitability as charger, it's actually outside on the stone floor of the porch. So if it burst into flames it's outdoors, and is lying on a huge stone heatsink...)

I'm concerned about what voltage to set the wall wart to though- assuming the wall wart is ok as a charger in the first place.. I assume the charger (or wall wart) needs to be higher voltage than the battery else the battery will try to power my house  smiley-mr-green. But given the discrepancy in the wal wart switch settings vs the no load voltage it reads, I don't know if the thing was actually charging or not.

This explains the expense of a proper battery charger I suppose.... it takes care of all this crap intelligently and charges the battery nicely!

So my last question so I can stop annoying you guys on a Sunday.... assuming I charge it with wall wart, what no-load measured voltage on the wall wart should I set the switch to to make sure it pushes some vees into the battery?
« Last Edit: July 01, 2012, 01:27:56 am by JimboZA » Logged

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The shape of the battery pack is enough to tell me that it would not fit a performance rc car, and so it is either a receiver pack or a toy car pack - i assume its actually 8 AA batteries shrink wrapped together.

If you look up nimh saddle pack or nimh stick pack you will see the shape of racing batteries, these will definitley deliver the current you need, my lipo racing packs deliver upto 72 amps.

One 'ghetto' test you can rig up is to use several leds connected in series across your power supply, choose the leds so that thier total voltage drop is just under 8 volts and dont forget a current limiting resistor. Connect these across your battery and they should light (remember to calculate the value of the current limiting resistor based on the 1.6 to 2 volts not dropped by the leds).

Now do the same but with the battery connected to your project as well, if the leds go out, your project is drawing more current than the battery can deliver causing its voltage to drop below the 8 volts required for your leds to light.

Its the same approach i have used in the servo power demonstration videosbin the two links in my signature.

Duane B

rcarduino.blogspot.com

Duane B
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Jim, There is a recommended charger that 'nice' little store you bought the batteries from that will sell you a charger designed for that use only... buy one and charge the batteries for 10 12 hours the first time, unless the instructions tell you not to... specifically. and don't leave them discharged for any amount of time, The Nickel electrode will "Grow Whiskers" of Nickel metal through the separator/electrolyte retainer and short out the battery, it only happens in the fully discharged state. They can be "Blown Out" by a high current pulse but they grow again from the same locations quicker on full discharge. Too Hot and Too cold are bad hot the battery eats itself and too cold it cannot deliver full power.... And now you know everything you never wanted to know about batteries... You Can safely charge it at C/10 or about 160 180 ma, current limited, the batteries want to see a constant current the voltage will rise as the charge increases.

Doc
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@Duane, yes I'm pretty sure it's the cheaper end of the RC scale, the toy end. Was just thinking along the lines of the LED test too, when you replied.

@Doc, yep they have the proper charger- but it cost twice what the battery cost!

Still don't know what indicated, no load voltage to set the wall wart to if use it as a charger....
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Lots of details and figures here:

http://en.wikipedia.org/wiki/Nickel–metal_hydride_battery
« Last Edit: July 01, 2012, 02:00:07 am by Nick Gammon » Logged

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Hmmmm not sure why I didn't look there first, I usually do.

It answers one question and that's that the charging voltage needs to be from 1.4 to 1.6 per cell, ie 11.2 to 12.8 for my 8 cell battery.

Leaving aside for the moment the wisdom- or otherwise- of using a wall wart as a charger in the first place, having charged the battery for a while now at 11 or so volts it does actually turn a motor over. So perhaps the battery was in need of charge in spite of measuring 10v to start with (that's what I thought all along) but that charging it with the wall wart too close to 10v wasn't actually driving vees into the battery.
 
So, going to leave it another hour or so and then hook it up to the project.

Thanks for all the help, guys.

Jim
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As I understand it, NiMH batteries pretty much deliver a constant current until they are nearly discharged, and then they will start dropping current.  This is different from alkaline batteries whose current drops as the battery capacity is depleted.  The lack of a drop-off means the simple minded battery meters which measure current won't work on NiMH batteries.   For example, on my digital cameras, the low battery light does not come on until you have maybe 10-20 shots left.  Some NiMH batteries provide an extra contact for things to read the capacity.

Here is a link that discusses batteries in great detail: http://batteryuniversity.com/.
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