DIY Electric Car Forums banner

EV Calculations

5177 Views 8 Replies 4 Participants Last post by  mattW
Here are a couple of simple calculations that a person can use to estimate range.

range[km]=250 x capacity[kWh] / (mass[kg]^0.6)
range[miles]=250 x capacity[kWh] / (mass[lbs]^0.6)

Obviously these calculations do not take wind resistance, drive train efficiency, rolling resistance, etc, but comparing these results to results from sites like may give a good idea of what range to expect from various EV configurations.

Here is another good site for graphs and formulas of this type.
1 - 1 of 1 Posts

· Registered
207 Posts
Hmm, I think we have a lot to discuss concerning tachometers.

First of all, tachometers, as you are probably already aware, requre some kind of frequency-to-voltage conversion to work from a spinning shaft/ignition pulse train/etc.

Your experiment that 1.5 volts DC sends the tach meter movement to about mid scale is a good start, but simply connecting the meter movement to any source of signal derived from a rotational generator is not going to give you what you are after. Even if the match-up was corrected via a variable resistor, you would still be looking at a reading that was non-linear with the speed of the shaft.

A petrol engine tachometer takes a pulses train from the ignition system of the distributor and converts the frequency of the pulses (which is directly proportional to the engine speed) and converts it to a DC voltage/current to run the meter.

The optical sensor tachs do the same thing, except that the pulses are generated by an optical interrupter.

On some diesel engines, a sample of the three phase AC power from inside the alternator is supplied to the V/T converter .There are "black boxes" that can do the conversion and will program for rough calibration of a petrol tach from a variety of frequency generators. See this page for a $60 converter that does this. The generation of the pulse train is up to you.

I can show you how to build a complete tachometer using junk box parts costing $10. All you need to do is couple a disc, gear, or similar serrated interrupter to your motor shaft. A small AC generator of some sort would also work. It's possible that the circuit could be cobbled together to sense motor commutation spikes, but I've never fiddled with that.
1 - 1 of 1 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.