Measuring Voltage and Current on electric bicycles, scooters, motorcycles or other electric circuits
In any electric circuit - whether it's an electric bicycle, scooter, etc, it's useful to know how much power the gizmo is consuming, especially when it's running off a battery pack. On battery powered vehicles you need to know the voltage sag under load, and how much power has been consumed. That's to avoid taking the battery voltage too low, and to avoid over-discharging the battery pack. Your riding range, the distance you can travel, is determined by how low you take the battery pack. The battery pack will be damaged by either of those conditions, ruining your investment in an expensive gizmo. Hence, what we need is a display to monitor voltage, amperage, and kiloWatt-hour consumption over time.
Many gizmos - cell phones, tablet computers, electric cars, and other modern battery powered gizmos - take care of monitoring voltages and consumption, rendering the data into a simplified battery gauge. But on the do-it-yourself vehicles like our electric bicycles or scooters, battery management might not exist. Where your cell phone or electric car battery management is good enough we can just use the thing and not worry about low level details, the typical DIY electric bicycle/motorcycle/car needs to do so. You, in your garage, might not be able to build battery protection circuits that ensure the battery won't be damaged or explode. It's up to you to monitor and manage the battery pack status yourself. Of course you want to avoid damaging the battery pack, and therefore want to know how far you can ride, how much energy you can safely draw from the battery pack, etc.
A typical display is wired in-line between the controller and the battery pack. You need to ensure it's turned on/off when the vehicle is on/off. The display will show volts, amps and watts. In some cases the unit will measure speed by counting wheel RPM (meaning you must program in the wheel diameter), and then show you a watt-hours/mile calculation. If the unit is installed where "weather" (rain) can penetrate, by all means ensure it's properly weatherproofed.
How much battery capacity remaining? How far can you ride/drive?
For an electric bicycle, electric scooter, electric motorcycle, or any other electric vehicle, the range is determined by a) how aggressive a rider/driver you are, b) the kiloWatt-hour capacity of the battery pack.
Battery capacity is measured in kiloWatt-hours. and consumption is measured in Watt-hours per mile. Our page on Electrical basic measurements and values explains these further.
Watt-hours per mile consumption is higher the faster you ride/drive. This is a simple fact of the physics of moving through the atmosphere - to go a higher speed means more energy to push through the air.
Approximate driving/riding range for any electric vehicle is simply:
range = kiloWatt-hours / kiloWatt-hours/mile
You can see this with an appropriate display. Try riding/driving at different speeds and watch the Watts required at one speed or another. See that going faster increases the energy consumption -- Watts. You'll also notice that over time the battery voltage decreases, and that for high Watts consumption the voltage sags. Understand these effects and you'll get a sense of how far you can ride/drive.
You'll also notice something odd about the voltage. Your bicycle might have a 36 volt battery, but when fully charged it will read over 40 volts. The "36 volt" number is what's called a nominal voltage. The venerable lead-acid car battery is sold as a 12 volt battery, but when fully charged is at 13.2 volts or so. The nominal voltage is not the fully charged voltage, and it isn't even the fully discharged voltage.
A competent battery vendor will give you information about nominal voltage, the maximum safe voltage, the minimum save voltage, and so forth.
Recommended display characteristics
Weather proof: Any vehicle, even a car, is exposed to the elements. That means the unit must be safe from moisture from being rained on, or fog, etc.
Voltage/Amperage range: The display must be compatible with reading the voltage and amps that will be present in your vehicle. The manufacturer will tell you the voltage/amp range it will display.
Installation: The display has to be wired into the circuit between battery and controller. How will you accomplish that task? Also, for a bicycle/motorcycle, the display has to be mounted securely to the handlebars.
Numerical display: Many "fuel gauges" for electric vehicles are nothing more than a series of LED's. This doesn't tell you anywhere near enough information. Further, they may have been designed for lead-acid batteries and won't work to read the capacity of a lithium battery pack.
Speedometer: This is a nice-to-have because it can tell you watt-hours per mile consumption.
I've got a couple specific product recommendations below. AliExpress has products from several manufacturers, and these searches are useful:
The "Watt's Up" WU100 measures Amps (up to 100A), Voltage (up to 60v), Watts (up to 6500W), Amp-Hours, Peak Amps, Peak Watts, Minimum Voltage ... It is built into a plastic housing that might be water-proof. It has two pairs of leads and is meant to be wired in-line between a power source (Battery) and load (Controller). When turned on it displays the values in real time.
Many people wire these into their electric bicycles, mounting them on the handlebars as a dashboard.
Buy: "Watt's Up" RC Watt Meter & Power Analyzer WU100 Version 2, R/C ELECTRONICS WU100-B Watt's Up Watt Meter Blue RELP0100, http://www.amazon.co.uk/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK, http://www.all-battery.com/wattsuprcwattmeterandpoweranalyzerwu100version2.aspx, http://www.batteryjunction.com/wuprcwamepoa.html, http://www.batterystuff.com/solar-chargers/t-01003.html, http://www.hobbyking.com/hobbyking/store/__6380__watt_meter_power_analyzer_watts_up_ver_2_.html
The Cycle Analyst is an excellent dashboard display for electric bicycles, electric scooters, electric motorcycles, and even electric cars. They show amps/volts/watts/etc and can be connected to the wheel to show speed and watt-hours/mile figures. It can track the watt-hours consumption per ride by simply resetting the display every time the vehicle is turned on. It tracks consumption over time by averaging together the watt-hours etc per every time the display is reset.
It comes in several varieties - some have a shunt meant for low power vehicles like electric bicycles - while others use an external shunt and can be used with higher power vehicles like electric cars.
This is very similar to the Watts Up meter shown above.