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Idea and implementation of a simple means of controlling battery power consumption

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Probably the first thing the owner thinks about after buying a trolling motor for a boat, or, say, installing an electric kit on a bike, is how far you can safely sail/drive away without having to go back on oars or pedals. The same fate has not passed me by, so when I got an electric boat motor a few years ago, I immediately had the idea of implementing an ampere-hour flow control from the battery powering the motor in a simple, yet more or less accurate way.
I must point out at once that we will talk about the method of controlling the ampere-hour flow by integrating in time the instantaneous discharge current, and the control methods connected to the measurement of EMF or battery voltage under load, with all their advantages and disadvantages, will be left out of the brackets.
One more thing – the idea was implemented a few years ago, when it was not yet easy to order powertrain condition monitoring devices on ebay (e.g. for electric cars). Now they exist and can be ordered on ebay or aliexpress, but are still quite expensive. At the time I had no choice, so I decided to make the device myself.
Instantaneous current decided to measure by the classical principle – the voltage drop across the measuring shunt (a piece of supply wire) with a known resistance. Initially I decided to use a microcontroller for time calculation (integration) and display, but I was afraid that I would have to make indicators, buttons and enclosures for it all.
And then it dawned on me – there is a ready-made device that can greatly simplify the task – a bicycle computer! Indeed, it is only necessary to convert the current into a pulse frequency (in the range perceived by the bicycle computer), and instead of the instantaneous speed (km / h) we get the current consumption current (A), and instead of the distance traveled (km) – just the consumption in A * h – that is what we need! The other functions of most bicycle computers we do not need much, but, for example, the trip time can be useful in the boat, and the maximum speed corresponds to the peak current consumption (so, for curiosity). In the end – we connect a fully charged battery to the boat motor, reset the path for the trip on the bike computer – and go! Watch the mileage in km – when it begins to approach half of the stated battery capacity – then it’s time to go back.
I was so excited by the fact that I could use an off-the-shelf display and integration tool, that I immediately built a voltage-frequency converter with the then popular ATTiny15. The measuring shunt was a piece of wire with a resistance of 1 milliom. With the resolution of the built in ADC in tiny (including software connected differential preamplifier) device got the ability to measure currents with a resolution of 0.5A in the range of 0-50 Amps, and without any external circuits (except for the 5V supply voltage regulator). I quickly built it with a case and a shock sensor board from an alarm system: Idea and implementation of a simple means of controlling battery power consumption
The conversion factor of the device was programmed as 1Hz/A. In order for the bicycle computer to count the ampere-hour as a kilometer traveled, it had to be programmed to such a wheel circumference length that its 3600 revolutions (i.e. pulses from the converter) would give a distance of 1000 meters. That is, by 278 mm.
Rough calibration of the device was done by changing the solder point of the measuring lead to the shunt, and fine calibration by changing the calculated length of the wheel circumference in the bicycle computer. The idea was tested when riding a boat on the lake, was fully functional, but over time, the electric walk (I’m not a fisherman) with the need to drag the motor and battery, gave way to a simple paddle, and the device was abandoned. I remembered it at the request of a friend who recently bought a beautiful full-rate Li-Fe-Po battery for his electric boat motor.

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