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Model Forum / Radio Controlled / Air Models / August 2006Forming Charge For Small Battery Packs
I've got some 6 Cell, NiCd battery packs of 270 mAh & 400 mAh capacities that I need to charge for the first time. There are different schools of thought on whether or not modern NiCd cells still need to be conditioned with a forming charge, but I do it just because it can't hurt. For the smaller pack, C/10 gives a forming charge rate of 0.0270 A (say 0.03 A), but my charger goes down to only 0.1 A. I've checked a few hobby shops' websites for dedicated slow chargers, but nothing seems available, at least not in the UK, and I don't really want to adapt my Tx/Rx wall chargers to do the job because I just don't trust them to work adequately with a 6-cell pack. I guessed that I wasn't the first person to run into this, so I checked RCU, etc, but I didn't find anything really useful in the old postings. So... how would others do the forming charge for these small packs? On the other hand, is the C/10 rule of thumb still valid for such low capacities? 0.03 A is a pretty low current! Eamon You could always build a simple constant current charger if you can handle a soldering iron, there are lots of circuits out there. The circuit I use has 1 transistor 2 diodes and 2 resistors total. Dave :^) > I've got some 6 Cell, NiCd battery packs of 270 mAh & 400 mAh capacities > that I need to charge for the first time. There are different schools of [quoted text clipped - 15 lines] > > Eamon I use a bench power supply, one of the 'proper' type with a current limit knob as well as a voltage knob and a pair of meters to show you the voltage and current, this is ideal for slow charging batteries. They can be had on Ebay if you keep an eye out, here's the sort of thing I'm talking about 280008720810 in fact I have the same model but bought second hand. Greg > I've got some 6 Cell, NiCd battery packs of 270 mAh & 400 mAh capacities > that I need to charge for the first time. There are different schools of [quoted text clipped - 15 lines] > > Eamon Cheers both for the advice. I'm hoping that a cheap bench power supply comes up on ebay. In the meantime, I'll make myself a const current charger. I'm absolutely not an electronics wiz, but that should be just about do-able by me. Hopefully! Thanks again, Eamon >I use a bench power supply, one of the 'proper' type with a current limit > knob as well as a voltage knob and a pair of meters to show you the [quoted text clipped - 28 lines] >> >> Eamon > Cheers both for the advice. I'm hoping that a cheap bench power supply > comes up on ebay. In the meantime, I'll make myself a const current [quoted text clipped - 13 lines] >>> my charger goes down to only 0.1 A. I've checked a few hobby shops' >>> websites for dedicated slow chargers, but nothing seems available, FWIW - I have a number of 300mAh NiMh packs that I use in small models, and yes, finding a charger that will go down to 30mA or so is tricky. (I prefer to slow charge these) The solution that I found has worked out fine for me - From the local market, or Maplin etc, you can buy an unregulated psu, normally used for toys. They are switchable for providing different voltages, and cost about £3. Wire one of these up to your battery, and try different value resistors in series with the battery pack until you get the right charging current of about 30mA. I can't remember offhand what value I used - but with one of these cheapo psu's, I can slow charge 3 of my 300mAh NiMh packs (8 cells) at 30mA overnight. This url shows the psu that I mean: http://www.maplin.co.uk/Module.aspx?ModuleNo=97200&criteria=unreg%20psu&doy=26m7 hth, cheers Dave > I've got some 6 Cell, NiCd battery packs of 270 mAh & 400 mAh capacities > that I need to charge for the first time. There are different schools of [quoted text clipped - 6 lines] > the job because I just don't trust them to work adequately with a 6-cell > pack. If you have a normal bench power supply unit, those that deliver 0-15v, or even if you have an old CB-type 12v "power pack", then use that, but put a simple bicycle lamp in series with it. You didn't give the battery voltage, but let us assume it is 7.2V. If you had a 13.8V CB power pack, then you need to drop 13.8-7.2=6.6v It just happens that a 2W 6V bicycle lamp passes 300mA. So put the lamp in series with the battery. If the battery is totally DEAD, then use two lamps in series until the battery voltage rises to a safe level, then short out one lamp. During a normal charge, the battery voltage varies by, say, 1V (and that's a LOT) then the voltage across the lamp will fall from 6.6V to 5.6V, which is still only a small variation in the current. In effect you have a constant current charger, without any diodes, resistors, Veroboard, transistors or integrated-circuit regulators. Just a simple bicycle lamp, and you get a free charge indicator. You can also charge from your car battery - that is also 12V. 300mA overnight (12 hours) will take less than 4AH from the battery, and a healthy car battery should normally have a capacity of 10 times this :-) So if you do intend to BUY anything, buy a pair of bicycle lamps. They should cost you about 50p, at the most. Charge faster by all means, but stick to the C/10 rule whenever possible. It is much kinder to all batteries. |
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