My super-simple wannabe plug-in hybrid.

Background

I’m the proud owner of a 2006 Honda Insight. The 1st generation Insight was the first hybrid available for public sale in the US, and was also a pretty interesting design. It has covered rear wheels, the lowest Cd of any production car to date, and was made in the same factory as the NSX and S2000 with (supposedly) some of the same technology that made the NSX such an amazing handler (aluminum tub chassis).

Problem is, it’s 9 years old now, and the NiMH battery pack is on it’s way out. I’m partly to blame for that, letting it sit for long periods of time without driving to recharge the pack. As a result, some of the cells have high self-discharge and the pack becomes unbalanced after about a week of driving, causing the IMA light to go on and the pack no longer charges or assists during driving.

Solution

The solution is to replace the pack, the bad cells, or to top off the charge and “re-balance” the pack using a trickle charger. This is generally considered a stop-gap measure, as the trickle charge does nothing to remove the Nickel dendrites that are likely causing the self-discharge, but it does prolong the usefulness of a dying pack, and a number of folks swear by theirs as a means to keep driving the car.

The general concensus is that a trickle charge of about 350mA is sufficient to charge a pack that isn’t too far gone, while keeping self-heating to a minimum and enabling the cells to balance.

There are a couple ways to do this, many involving using a low-voltage constant current driver to maintain the 350mA limit, while stacking PC power supplies to get to the pack voltage of ~160-180V. Unfortunately, the pre-made ones run about $350, and while that’s not terrible, they’re also not super easy to install.

What I did

So here’s how I came up with a charger that integrates well, is hidden, and is most likely not going to destroy my car. Maybe.

I was browsing Digi-Key one day for work, and realized that there were pre-packaged constant current drivers for LEDs. Nothing surprising there, as LEDs prefer to be driven with a constant current. However, what I hadn’t realized previously was that there were drivers for long strings of LEDs, who’s combined series voltage drop ended up in approximately the same range as the NiMH pack in the insight. I decided on this guy: Nicely packaged CC LED Driver

Next step was installing it. I soldered a 6-foot pigtail NEMA plug to one end, and about 28" of 22ga stranded wire to the other, with tinned ends. I ended up mounting it to the little corner plate that’s next to the grocery tub: Installed

Which solved a conundrum I had about installing the wires if I mounted the charger to the large battery compartment cover.

Another hole into the fixed part of the compartment, a rubber grommet and some fiddling with the terminal coming out of the sense resistor later, and I had a stealth grid charger.