5 AH NiCd single cell charger

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DICKEYBIRD
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5 AH NiCd single cell charger

Post by DICKEYBIRD » Sat Jun 08, 2002 11:22 am

I was given a couple of Anton Bauer 10 cell camcorder packs that I took apart and kept the good cells. I have a constant current charger that will charge a single cell at 500 mah but was designed for at least 4 cell packs and gets very hot charging 1 cell. These cells are unbeatable for heating the glow plugs for my model aircraft engines.<p>Does anyone know of a cheap source for a single cell 500 mah charger or a simple circuit that could be built up from R/Shack parts inexpensively?

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Externet
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Re: 5 AH NiCd single cell charger

Post by Externet » Sat Jun 08, 2002 12:54 pm

Hello.
The simplest way is to use a 22 ohm / 10 Watt resistor in series with any 500mA capable 12 V source as your car cigarette lighter. A 12V/~6W automotive lamp bulb should work too.
This will yield about 500mA charge.
If the battery gets hot, increase the resistor value to 33 ohm or more until does not; or a less Watt automotive bulb.
Do not leave the battery charging for more than 10 hours. Heating the charging cell will decrease its life.<p>Miguel
- Abolish the deciBel ! -

DICKEYBIRD
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Re: 5 AH NiCd single cell charger

Post by DICKEYBIRD » Sat Jun 08, 2002 1:59 pm

Thanks Miguel, I'll give it a try!

bobsRAC
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Re: 5 AH NiCd single cell charger

Post by bobsRAC » Tue Jun 11, 2002 2:58 am

Dallas Semiconductor (http://www.dalsemi.com) sells (and samples) battery charger timers. This is not necessary if you simply want a manually timed constant current source. The easiest constant current source I've used in the past is the LM317 (sold at Radio Shaft as a 3-term variable voltage regulator). This device comes in a TO-220 with in, adj, and output pins. The device regulates the voltage from the out to the adj pin to 1.25v. A simple (and quite high impedance) current source can be made by placing a resistor between these pins (out and adj). The value of the resistor can be determined by the following formula:<p> R = 1.25 / I<p>where R is in ohms and I in amps.
In the case of a 500mAh battery, I beleive a 50mA charging rate for 10hrs is suggested for long life (although you can abuse that to some extent). Based on 50mA, the needed resistor is:<p> 1.25 / .05 = 25ohm<p>which is close to the standard value 22ohms (red red black). The power dissipated in the resistor is:<p> P = (V^2)/r = (1.25^2)/22 = 71.2mW<p>So, a 1/4 watt 22ohm resistor and a 317 ought to do the trick. I believe the 317 is not a low-dropout device, so you will need to supply:<p> Vbatt + 1.25v + Vdropout ~= 2.5v + Vbatt<p>You'll also want some room to spare because the battery will charge at a higher voltage that it retains off of the charger (just as an automotive battery charges at 14.something and rests at 12.something). So, for a 10-cell, you might want to charge at:<p> (1.2 * 10) + 2.5 + Vheadroom ~= 18v<p>Just to clarify, the 317 acts as a 2-terminal device once you add the resistor. The Vin terminal is connected to Supply+, the adj terminal is connected to the Battery+, the 22ohm connected between the 317's Vout and adj, the battery- connected to Supply-.<p>Image

DICKEYBIRD
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Re: 5 AH NiCd single cell charger

Post by DICKEYBIRD » Tue Jun 11, 2002 5:15 am

Hi bobsRAC,<p>Thanks for your learned response! That's more like what I'm looking for. The batt I'm needing to charge is actually a single 5 Amp Hour NiCd so it needs 500 mah to charge at the C/10 rate. Do you think the LM317 will handle that? If so, what value resistor do you think I'll need. Sorry, my electrical theory is lacking!<p>Thanks!

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Re: 5 AH NiCd single cell charger

Post by DICKEYBIRD » Mon Jun 17, 2002 11:45 am

'Nuther query for you learned electronic types:<p>In my quest to charge the above mentioned 5AH NiCd cells, 1 at a time, I ran across a website with a bunch of wall warts of various ratings. I have charged NiCds before with wall warts that had the wrong voltage/amperage ratings and as long as I verified with a meter that the final charge rate was close to correct for the cells I was using, they seemed to charge up fine and nothing ever caught on fire.<p>If I assumed that the charging voltage should be about 1.5 volts and a 500 mah charge rate for a single 5AH cell which would be about .8 watts, would a 100 mah @ 9 VDC wall wart (.9 watts) work? Or would the huge difference in voltage cause something to burn up? I found wall warts at that rating for less than $4.00 so it wouldn't cost too much to give it a whirl.<p>I have my flame suit on so if this is a really stupid question, fire away and I will take my punishment! :)

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Re: 5 AH NiCd single cell charger

Post by josmith » Mon Jun 17, 2002 1:23 pm

"constant current charger that will charge a single cell at 500 mah" Are you saying that the cells are 500mah or that the charger is 500 ma conatant current? For the former you would want to charge at around 50 ma. <p>Whatever the current is if it's ok for the cell you have you could use resistors in series with the charger to simulate extra cells. Figure 1.2 v per(simulated) cell divided by the constant current in amps to get ohms. Use current squared times resistance to get the wattage you will have to dissapate. The resistor will get rid of the extra heat and your charger will still be constant current.

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