Non intrusive logging of battery data

I own an electric bicycle, which I bought second hand, and is around 5 yrs old. It uses a big old Li ion battery pack.

I don't know how feasible it would be, but I want to log some data while the battery is cycled, to hopefully allow me to calculate/guesstimate capacity and battery wear.

I'm imagining either an induction coil wrapped around the powercable monitoring draw from the battery, or some arrangement spliced in serial with one battery cable.

It could then just log values read via an arduino to an sd card live, as the bike is ridden.

It would have its own power supply, and be as far as possible disconnected and unaffected by the battery it is monitoring.

Can an arduino get this information?
Would it be any use?

If you just want to monitor current, a Hall effect current sensor is probably your best bet. If there's a current rating marked on the motor that should help you choose one. A Hall effect sensor senses the current magnetically and it puts-out a voltage proportional to the current.

For voltage, you'll need a [u]Voltage Divider[/u] (2 resistors) to knock the voltage down below 5V. (Higher value resistors will waste less power but high impedance/resistance circuits are more prone to noise pick-up so you might need a capacitor to smooth-out the voltage).

And since there might be voltage spikes, you might want to add a [u]protection diode[/u] or two on the analog inputs.

It would have its own power supply, and be as far as possible disconnected and unaffected by the battery it is monitoring.

It makes more sense to use the same battery (as long as there's enough voltage to run the Arduino :wink: ). The Arduino doesn't take much power compared to a motor, but you might want to switch it off when you switch-off the bike.

Thanks for the reply.

The motor has to be limited to 250W legally, so thats easy. The controller circuitry should throttle the current I guess?

Would a voltage divider impact on the voltage delivered to the motor?

No, the voltage divider is a high resistance so it draws a negligible amount of power from the battery, like micro-amps.

Induction coils wrapped around the wire don't work for DC. You really need to break into the wire and add your own current-measuring device. Sparkfun has a good selection of current sensors.

Thanks Morgan. They look just the job.
I hadn't considered that there would be a piece of hardware that readily avaliable

I've got no issue clipping the cable, I'll just put a bullet connector in so that I can take the sensor out easily if needed.

Many thanks,