Too Busy For Words - the PaulWay Blog

Sat 7th Jul, 2012

The lightbulb moment

Tony, the other person in Canberra building an electric motorbike, came over last night. We were both keen to try out the new 150W DC-DC converter I'd got recently to augment my auxiliary battery when supplying power for the headlights (permanently on, at the moment) and other systems. Each headlight is about 55-60W, with one for main drive and two for high beam. Having crashed the bike in part because the light was just not strong enough, I wanted to kick it up a notch and make sure I had enough light to drive safely.

So, we clipped off the strange proprietary connector on the DC-DC converter, fitted a pair of Anderson connectors for the 120V end (to match those on the 120V fuse box) and a pair of female spade lugs for the 12V end (to match the fittings on the auxiliary battery wiring. We plugged it into the battery pack and all that happened was that the green LED came on. That was all we were expecting; sure enough, a multimeter showed it was outputting 12.5V on the output stage. This obviously wasn't impressive enough for us.

Tony had the bright idea of plugging it into the headlights of the old bike, still in their boxy plastic housing. The high beam worked. The high beam side of the dual-output lamp worked. The low beam worked - but showed us a very interesting thing. The output was noticeably dimmer than the high beams, even though it should only be 92% as bright (barely noticeable).

We pulled the bulb out and tried a couple of other experiments. We plugged it into the small 12V battery, and it did the same thing - low beam was definitely dimmer than the high beam. And a careful examination of the actual bulb showed that the filament was damaged - it looked rough and slightly irregular compared to the smooth, regular coil of the high beam - and there was a small amount of soot on the inside of the bulb near the low beam coil. So it was definitely physically damaged. Even the small battery would still give full output brightness if the bulb had been working correctly.

This made me feel a bit better about the accident. I was still riding beyond the range of the headlights, and it's still my fault. But it wasn't that the bike electrics were incapable of driving the bulb. If I'd noticed, and turned on my high beam, I would have been fine. I'd charged the auxiliary battery in the bike the previous night; it definitely would have given me full range of vision if the bulb had been working. I feel a lot more confident that the bike will give me full range of light, even if the traction battery or DC-DC converter dies and I have to run on auxiliary battery alone.

I'd also started to wonder whether I should put an external switch, or full DC relay, on the auxiliary 12V systems - the input from the battery charger, the output to the battery monitor, and the DC-DC converter. If the DC-DC converter is draining 150W from the traction battery pack continuously, it'd last about 48 hours from full charge to dead; I didn't want to find out that the converter had killed the $4000 battery over a weekend. But we realised that once the DC-DC converter has brought the auxiliary battery back up to 12.5V, the load on that circuit would be quite low - the DC-DC converter simply isn't going to be able to push 12.5V against a 12.7V auxiliary battery. It might be worth putting a suitable 15A diode in series with the lead to the battery, just in case the DC-DC converter doesn't have back current protection, but I kind of doubt that that'd be left out.

Of course, disconnecting all the auxiliary electrics is a good thing to do if I'm going away for a while anyway. The auxiliary 120V system connects from the battery to the fuse box via an Anderson connector that I can pull out. It's easier and less prone to Catch-22 problems than a relay.

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