Friday 26 April 2013

Watts and volts

I'm still planning the build of the new electric bike, and I got to thinking about battery life. With most electric bikes, there's an indicator of how much battery you have left. In my experience, that shows "full" most of the time, and then when the battery is nearly exhausted, it quickly goes through 3/4, 1/2, 1/4 and then fades out. Nearly useless.

I'm thinking, that indicator is actually a voltmeter, with a few LEDs instead of numbers. So I've ordered a voltmeter to use instead. It cost about $2, and it'll show me the voltage numerically, as it declines from 28.6 to 23.5, or whatever, and I think that'll give me a better idea for what's happening.

And then I had another thought - what I really need is to keep tabs on how many watts have been drawn. The battery is 24 volts, 10 amp-hours, which means 240 watt-hours of energy. So I bought a wattmeter ($13) that will tell me the ongoing draw in watts (amps times volts) and the cumulative draw in watt-hours. My idea is, I'll set it to zero when I put in the battery, and the first time I use the wattmeter, I'll note how many watt-hours that battery can store. From then on, I'll know quite accurately how much energy it gives me. I'll also be able to correlate the "watt-hours remaning" with the voltage, so that then the voltmeter will give me more information than just the voltage. The watt-meter will go where the battery is; the voltmeter on the handlebars.

This is the wattmeter that I got. There's something that's on offer for about $2 that they say is a voltmeter and wattmeter, but I had a look, and I think it's only a voltmeter.

I also found a source for 24 volt batteries that are more than 10 amp-hours. The 10 ah batteries are £161 (I've been paying about £200). When I go out on the bike, I usually take two or three batteries with me (depending on how long the circuit is).

Amp-hours    £ cost   kg weight

10            161      5
12            187      5.7
15            213      6.6
20            239      8.1
30            323     11.3
40            369     13.6

That 40 amp-hour battery looks good! OK, it'll weigh as much as the rest of the bike put together, but for getting the bike over obstacles I'd remove it. In my experience, three 10 ah batteries last me the whole day, so 40 should be more than enough! It comes in two possible sizes, 360 mm long and either 100x150 or 80x195.

What will it do to the center of gravity of the bike? It'll make it a bit higher. But it'll lower the center of gravity of bike+me, since it'll be lower than most of me, and I'm most of the weight of bike+me.

The same people also offer a 50, 60 and 80cc two stroke petrol engine conversion for a bike. I *really* fancy that! I used to run a moped, so I know what it feels like. It would give a lot more power than an electric bike, and I'd always know just how much power I have left in the tank (which is half a gallon, enough for probably 80 miles).

Unfortunately, it turns the bike into a machine that can't legally be used on bridleways. And to use it on the road, it would need taxing, insuring and licensing. Legally, it becomes pretty much a motorbike. So it's not a useful proposition for me.

But doesn;t this look tasty!

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