Friday, 21 March 2014

Volts and amps

I'd quite like a display on my bike that shows me volts and amps as I ride along. Volts tells me how much I've depleted the battery; amps tells me how much power I'm pulling at any given moment. I already have volts; amps presents a problem.

The problem is this. With the ordinary sort of ammeter, I'd need to run a six foot, rather thick cable from the battery to the handlebars, and back again, in order to transport up to 40 amps to the meter. That's not good. But there's a better way.

I found a meter on Ebay that for the price of a pint, show volts and amps on one display; 0-100 volts and 0-100 amps. And I don't need the heavy cables. Here's how it works.

To measure volts, only a very tiny current needs to travel, so I can use fine wires. To measure amps, I need a 0.75 milliohm resistor in series with the load (usually called a shunt). Then the meter measures the voltage drop across that shunt; that voltage drop is proportional to the current (using Ohm's law, V=IR), so it can calculate and display the current. So I ordered the meter, and a suitable shunt.

I've had the meter for a week; the shunt hasn't arrived yet. But I realised yesterday that I needn't wait, I can make my own.

Wire tables tell me that 10AWG wire has 0.9989 ohms per 1000 feet. That's very nearly 1 milliohm per foot. So my 0.75 milliohm shunt will be nine inches of 10 AWG. I got a short length of 10 AWG wire, and soldered it up. I connected up the meter, put my standard load of 10 light bulbs on, together with another ammeter, and switched on. I then needed to adjust the point of contact on the 10AWG wire so that the two meters read the same. Viola!

The meter will go on the handlebars, and four wires will go to the back of the bike, That will connect to my switch/fuse/wattmeter assembly via a plug-in connector; it will also connect to a small battery that is needed to power the meter. I looked around my workshop for a suitable six feet of four wire cable, and decided to use a bit of ethernet cable. They have eight wires, so I have four spare wires I can use for something else; I'm thinking maybe the USB power connector for the PDA.

All I need now is for my order of umpteen EC5 connectors from Hobbyking to turn up, and I can finalise the wiring. It all sounds so good, I've ordered two more of the meters, for the other bikes. At £3 each, it's hard to say no.


  1. Your bike is a means of transport from cache to cache. You've got a fairly good idea how long your batteries will last, and you inevitably carry more than you will require for any one outing. Do you really need to faff about with adding more and more monitoring stuff to it?
    You appear to be spending more time fiddling with the bike(s) than you are actually using it/them for the purpose you bought them.

  2. One of the main purposes of a bike, is to spend time fiddling about with it. I greatly enjoy faffing about with bikes. And I especially enjoy coming up with an idea that no-one else has had.