Battery Charger

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I recently bought an E-Flite Blade CP radio-control helicopter. It is excellent value for money for someone like me who has no existing radio kit or the skillset required to build a model. The box comes with everything required to fly except for 8 AA batteries required for the transmitter.

My only slight dissapointment is with the battery charger supplied, don't get me wrong, it will charge the batteries, however it will cook them as there is no form of cutoff. The manual states to charge them until they get warm. The charger provides 12v at 300mA or approximately c/2, so it take roughly 2 hours for a full charge, and don't forget you put them on and go to bed!

I designed and built myself a new charger, based upon the MAX712 chip from Maxim. This IC is a charge controller, capable of managing the charge of upto 16 cells at 4c.

The standard E-Flite pack is 8AAA sized NiMH cells with a capacity of 650mAh and the documentation states that the charge current should be limited to 800mA for rapid charge. The MAX712 datasheet has a reference design that works perfectly, I have made some minor modifications to it as I wanted a power and a rapid charge LED. I also (not shown in the diagram) added a fan as the transistor gets quite hot, even with a heatsink.

This circuit has Rsense deliver about 850mA during rapid charge, it also does voltage slope shut off so the batteries are switched to trickle charge soon after reaching full charge. The batteries still get a little warm, but it is much less due to the shutoff.

I have used my Radioshack Model 22-812 multimeter and recording software to generate plots of the battery voltage during charging. They can be accessed (in a new window) here

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Update

I found that this charge rate is too high for the packs, especially as they start to age. I have changed Rsense to provide between 350mA and 400mA and changed the time limit cut off to 3 hours. This has proven to be a better charge rate resulting in a pack with a usable amount of charge in it. At this time the packs are about 6 years old and are probably at the end of their life, the eflite packs don't accept the charge. The ones I built hold about 10% of rated charge. I plan on buying a slew of AAA cells from eBay and building some more packs.