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NOTE:

• Please read the DISCLAIMER page.
• Even small battery packs can store substantial amounts of energy and be hazardous. If you are considering building something similar, please buy an off the shelf piece of equipment instead and use it in accordance with the manufacturer’s specifications.
• This project was designed for specific NiCd and NiMH cells from a specific vendor and is not relevant for any other cell type, size, brand, or chemistry

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Why?

This is sort of a companion to the NiCd/NiMH Discharger. Back in the early 2000’s when I was flying electric RC planes I built each of the projects. LiPo’s were new and too expensive for my tastes so I flew with NiCd or NiMH battery packs. I was lucky to have a steady stream of used NiCd and NiMH sub-C engineering samples since I was doing hardware and firmware work on a battery management system at my day job. That made them an even more attractive choice, even if their energy density wasn’t cutting edge.

Smart MCU-controlled fast chargers were still fairly new at the time and expensive. Since I had this surplus of cells, my approach was just to have a handful a packs available then fly through all of them. A C/10 charger that could charge up several packs the night before flying was more useful to me than a fast charger that others were often using to charge packs at the field. Especially since those fact chargers were pretty aggressive on charge rates, and I’d really prefer to charge my packs slowly, and once they cooled down. During flight they could get pretty warm since full throttle on my planes could be as high as 30 amps.

What does it do?

Anyhow, I built this simple 3 output trickle charger to simultaneously charge multiple NiCd or NiMH packs. No smarts, just power resistors to provide a quasi-constant current source at a low charge current. The manufacturer handbook for the cells I was using recommended charging (from empty) at a C/10 rate for 10-12 hours. To prevent overcharging excessively, I always made sure packs were empty before charging (see NiCd/NiMH Discharger) and simply used the charger in combination with an old school outlet timer.

Perhaps a bit conservative, but even small battery packs can store substantial amounts of energy and be hazardous – please see the DISCLAIMER page and the NOTE section at the top of this page.

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New life for a 5.25″ bay

The project was built in an old IBM external drive case I found at a surplus store. From googling the part number, it looks like the enclosure was for a single 5.25″ 1.2 MB 1/2 height disk drive. This project is pretty old, but even in the early 2000’s, 5.25″ high density disks were already obsolete. But their plastic enclosures were a perfect fit for this project. It even had molded in cooling vents.

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Dissection

Regarding basic operation, each output had a switch to select the charge current, an ammeter to measure it, and another switch to select between charging or monitoring. The ammeters were junk from some surplus place that I reworked with new shunt resistors to rescale them to measure the current range I wanted. I even printed custom scales for the new current range.

I also added an LED voltmeter and rotary switch to monitor the pack voltage of any of the three outputs. Numerous fuses were added for good measure, and it looks like there are power diodes on each output to prevent current from back-flowing from the batteries if the input voltage supply is off. The only component in the bottom half of the case is a fan for cooling the power resistors, which was probably overkill. Power came from an external 24VDC regulated power supply.

Afterward

I’m not sure I would do this again, but with the excess of free cells I had, and the high cost of chargers as RC electric aircraft were just becoming a thing, it made sense at the time. It was fun to build and I got a lot of good use out of it. My Kyosho T-33 ducted fan (like this one) and my Multiplex TwinJet were a blast to fly and every flight was powered by packs charged by this project.

Catapult-style bungee launching was tricky but definitely my favorite.