Making a Charger

[pbjoe02]

I think my emags charger is screwed up. I really don't want to spend $80 on a new one, so could I take a 22 volt power supply, attach some alligator clips, and charge the battery that way?

[MoeMag]

[pbjoe02]

The emag battery isn't all that special, I know because I made one. It's just a bunch of AAAs in a series. What I want to know is if you can charge a nimh battery straight off a power supply that has more voltage that the cell can put out. If I can, I would need to be careful when charging and moniter the battery so it wouldn't overcharge. The emag charger is special because its a trickle charger and increases the voltage of whats put into it.

[MoeMag]

[pbjoe02]

Forgive me. I took you as being a mindless teenybopper.
Sorry

No problem, I actually am in fact a teenybopper, I'm 14, just pretty far from mindless
Anyways, the emag charger puts out about 24 volts, so I think my 22 volt wall power supply shouldn't be a problem. Unless someone here gives me a good reason not to, I think I'll be trying this tomorrow. There shouldn't be any problems as long as I don't let the battery get hot.

[Pyroboy597]

Don't over charge it. How many amps does your power supply put out, and how many amps does the regular charger put out?

[pbjoe02]

pyroboy, I don't know either of those things, however if I monitor the battery I can tell if its charged, and as long as I don't let it get hot it won't overcharge. I've read it takes a good 30 min after it heats up for damage to occur so I'm not worried, just gotta be careful...

[athomas]

If you can use a current regulator of some sort, that would help. It would also protect your powersupply. 12.6 volts is the absolute lowest the battery voltage(BV) without having a dead cell. Measure the powersupply voltage (PV). Series Resistor = (PV - BV) / 500mA . This should give you lots of protection. Make sure the resistor is a high wattage one (~5 watts). With this resistor in series with your power supply and battery, the maximum current draw is limited. As the battery charges the differential will close and the charge will decrease a bit. Monitor the battery with a digital voltmeter as it is charging. You will see a gradual increase in voltage. When you monitor a decrease in voltage that exceeds .25 volts, then the battery is done charging. You will also notice an increase in heat if left to charge past this time. The battery will probably take about 1 - 2 hours to charge depending on how dead it is.

[pbjoe02]

IT WORKED!!!!!

Charged it for about 1/2 hour, it started to get warm so I unplugged it. 2000+ solenoid clicks later, and its still goin strong. Needless to say I am extremly happy, since this saved me $80.

[shades]

It sounds as if your input current is too high. If you continue this type of charging you ARE going to shorten your batteries life span costing you an additional $80 for a battery soon.
Here is a Link to charging NiMH batteries. C = your batteries mA/hr

[pbjoe02]

I have no idea what the input current of my power supply is, but it didn't sound like that big of an issue in that link you gave me as long as i keep the temp low, which I did. Of course that was talking about normal chargers though, and my power supply could have a much higher current. Also, if your thinking that its too high of a current because it charged so fast, the link said batteries will heat up a little when charged quickly, and my battery just heated up a little so it might've not been fully charged. I'm not too worried because I built a 900mah battery pack for 30 something, so I've got plenty of battery life to loose and not a high cost to replace it

[athomas]

As long as you keep an eye on it while charging, you should be good to go.

[pbjoe02]

Cool, thanks, I defintely will.