02GF74
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| posted on 7/9/09 at 09:43 AM |
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Calling RC people - where to buy 7.2 V NiMH batteries cheaply
I bought a pair of 7.2 V NiMh off the bay and well, they were cheap and chinese and aren't holding their charge, yes I should have known better
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Not used NiMh batteries before but do they rapidly self-discharge? I would have expected that I can fully charge them, leave them for a week or two,
and they have more or less the full capacity. Usually for RC models they would be used immediatley so maybe that is how they are?
Is that correct or do they sefdischarge quite quickyl?
Anyways, assuming they are bad, I am looking for one or maybe 2 batteries, obviously highest capacity (3800mAH would be enough) but ones that you
have used or brands you can recommend but do not cost the earth.
Must have Tamiya type connector.
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RickRick
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| posted on 7/9/09 at 09:55 AM |
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the higher capacity cells do have problems with self discharge, caused by interal shorts which is why my 6 racing packs are kept on a tekin battery
nurse, which checks and tops up the voltage at 7.2 volts
Is Lipo batterys an option i've just got 2 packs and i'm very impressed.
what about second hand http://www.oople.com/forums/showthread.php?t=28590 go for the gp3300 as these are least susceptable to self discharge
i'm sure he'd put tamiay connectors on for a few quid more
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BenB
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| posted on 7/9/09 at 09:58 AM |
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Even using reasonably NiMHs I've always found self-discharge a big problem....
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balidey
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| posted on 7/9/09 at 10:00 AM |
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02GF74, how cheap were they as I need a very cheap one. Infact if you are selling let me know a price.
I just need a couple of cells for an old cordless screwdriver.
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02GF74
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| posted on 7/9/09 at 10:11 AM |
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a quick search o nthe web reveals indeed that NiMH are quite bad for self-dicharge
so what I am seeing is probably normal.
re how mcuh? can't recall but probably similar price to
these ones
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Mr Whippy
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| posted on 7/9/09 at 10:16 AM |
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What kind of charger are you using?
if you are seriously looking for optimum charging then you need a delta peak charger. I suggest you go on one of the r/c car forums and see what
everyone is using, though you tend to get what you pay for.
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MikeRJ
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| posted on 7/9/09 at 10:19 AM |
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NiMH do self-discharge, and the higher capacity ones tend to be worse for this.
If you want low self-discharge then you need to look at the newer technology cells e.g. Sanyo Eneloop, Panasonic Infinium, Vapex Instant etc.
However, I don't know what the availability of sub-c sized cells is if you need them.
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wicket
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| posted on 7/9/09 at 10:26 AM |
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I use Uniross Hybrio batteries in a camera. They are NiMh with some sort of modified chemistry and they hold the charge brilliantly. Definitely
available in AA & AAA sizes, not sure about other sizes though.
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scudderfish
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| posted on 7/9/09 at 10:31 AM |
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I bought a 4000mAh pack from http://www.modelsinmotion.co.uk/ in Stevenage for £25
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02GF74
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| posted on 7/9/09 at 10:56 AM |
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quote:
Originally posted by Mr Whippy
What kind of charger are you using?
I am using Maplin charger:
hmmm it is supposed to:
* Quick charge with auto cut off
* Built-in auto shut-off to prevent overcharging
but with my batteries connected, they get rather hot so I suspect the charge is not doing that. the heat apparantly is bad for NiMhs
some info. about NiMhs:
One of the problems with NiMH cells is that they self-discharge over a relatively short period of time.
All cells will loose their charge over time, even if they are not used, but this is a particular problem for NiMH cells.
Typically it might be expected that a fully charged cell might self discharge over a period of a few weeks.
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bigandy
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| posted on 7/9/09 at 11:21 AM |
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Try here:
http://www.component-shop.co.uk/
I've always had cracking service from them, and the batteries that they sell have always performed well.
Nimh cells are always going to have quite a high self discharge rate, unless you look in to the new generation cells like Vapex instant or Sanyo
Eneloop. These cells will normally only be available in the AA and AAA sizes though...
If you are using the Sub-C sized packs to power electric models though, I would want to be charging them just before I use them. The packs will
perform much, much better hot (well, warm!) off the charger!
Andy
Dammit! Too many decisions....
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Mr Whippy
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| posted on 7/9/09 at 11:34 AM |
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Hmm yip thought that was the kind of charger you were using.
Problem with those is they over heat your battery pack and then one of the cells gets too hot and dies. Then you have a very dodgy pack and hardly any
capacity. Only doing fast charging will also decimate the capacity to the point where they'll only hold a few minutes charge. You can only fast
charge a few times before needing to do a slow trickle charge, usually overnight. I do that every time I’m going up to the flying field so even a tiny
600ma transmitter pack will last at least 5hrs in the controller. If I fast charged it every time it would last about 10-15 minutes, worse is the
voltage would drop off like a stone.
I bet if you took the shrink wrap off the pack you’d find white stains where the overheating cells have been venting. That’s why I have mine covered
in clear heat shrink so I can see what’s going on inside. It’s also why all the r/c planes flight pack get recharged out of the plane otherwise they’d
cook.
Do yourself a favor and buy a good delta peak charger, which can cycle and discharge too, might cost you £50 but it will make the most of you
batteries.
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RickRick
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| posted on 7/9/09 at 01:58 PM |
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Hmm i think your charger might have done more damage than good. it must be a constant charge, not a pulse or flex chrage, and with a delta peak
value of 5mv per cell max
i use one of these
http://spintec.nl/ICC.shtml
and store my cells on
http://www.teamtekin.com/batterynurse.html
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