Smart battery charger

I am trying to build a smart charger that will charge a Ni-Mh 2/3a 1200mah 9.6v charger. By smart I mean that it will stop when the battery is full, then if it drops below, say 9.4v, it will charge it to that again. Most of them also have a discharging button that will bring the voltage down before charging it.

For the charger part I found this website
http://www.elecdesign.com/Articles/Index.cfm?AD=1&AD=1&ArticleID=1823

Which details how to make a charger that will charge a 13.3v battery. Of course, mine is 9.6v, so it would have to be modified. The current "dumb" charger for the battery just gives it 12v @ 250ma.

The smart charger also is powered off 220vac, which is not good because I live in the US. IS there a way to make it work off the 12v 250ma charger? Also, the circled components (the overlay thingy wasn't working), what are they? the no/nc one might be a switch, and the RL1 one could be...a relay?

Picture of Smart battery charger
Rapt0r910 years ago
I've actually been looking at building something similar for a robot controller. First off, the site where you got the schematic used 13.3 volts as their finished battery voltage. You can adjust the maximum voltage via the variable resistors. So you can use this schematic to charge basically any battery because the maximum and minimum voltages are fully adjustable. If you want this to be an AC line powered charger, then you should adapt the transformer to suite your local voltages. You need that transformer to step down the high voltage of the AC power to a lower voltage for charging your batteries. The transformer can be adapter via the number of windings to do this. This means that you should keep the rectifier and the capacitor so that the AC power will be inverted to DC. If you really want, you can also include a voltage regulator which will ensure that the output of you rectifier and capacitor is a constant voltage instead of a bumpy line. Apart from that, this seems to be pretty simple to do. You just need to be able to se the two variable resistors to the maximum and minimum voltages of the battery and ensure that the AC to DC inverter is properly built for your local voltages. Once that is finished, you should be abe to use your charger.
NachoMahma10 years ago
"IS there a way to make it work off the 12v 250ma charger?"
. Probably not. The drawing calls for 60W (15 x 4) on the transformer output and your charger puts out 3W (12 x .25). Wouldn't be real surprised if the idle current of the charger in the drawing is that much or more (voltage dividers aren't very efficient).
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. "what are they? the no/nc one might be a switch"
. You guessed it! NO = Normally Open. NC = Normally closed. Indicate position of contacts when a switch/relay is not energized (what's normal about being off, I don't know, but that's the way it works). The center contact (dotted lines) flops from the NC position to the NO position when RL1 is energized.
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"and the RL1 one could be...a relay?"
. RL1 is, indeed, a relay. It controls the NO/NC contacts. It is energized by transistor Q1 and D1 is a clamping diode to absorb the energy of the collapsing field when RL1 is de-energized (helps protect Q1 from the voltage spike).
. The power supply is made up of 3 parts: transformer, rectifying bridge, and ripple capacitor. If the transformer is centered-tapped, you may be able to modify it to use 110-120VAC. The rectifier bridge is most likely a chip and not four discrete diodes. The output of the bridge will be very "lumpy" and C1 smooths things out a bit.
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. As far as I can tell, this charger only looks at voltage; using VR1, VR2, and R1-R3 to adjust it's output via the Q1/RL1 circuit, but I don't fully understand it.
. I'm pretty sure you want a charger that takes into account charging current.