Battery level

This is where you talk about the NXJ hardware related topics such as the brick, sensors, LEGO pieces, etc.

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Battery level

Postby Barry1337 » Mon May 17, 2010 10:14 pm


I'm trying to find out how long my batteries can last with different parameters. Since I'm not a great electrician (and barely know Ohm's law) I have a few questions.

1. What does the getVoltage() function really return? If you read the voltage at time A and B while nothing is attached and do the same while a motor is running, will the results (voltage at time A and voltage at time B) say anything about how much of the battery a motor uses? (I'm trying to find out how much the motor uses at different speeds/power levels).
My goal is to calculate which combination (mostly the motors) will consume the most battery power.

2. In the NXT Hardware Developer kit pdf it says that there is 4.3V output supply for input and output. But how do you convert this to mA? (i.e. how do I know how much mAh a sensor/actuator uses?).
On figure 11 they have a graph that lists max mAh and normal mAh when there is load on the motors and when there is none.
It also lists 9, 5 and 3.3 V but what do these numbers mean?
Above the table it says that the switch mode power supply generates a 5 volt power supply from the batteries and from the 5 volt power supply, another 3.3V supply is generated for the ARM7 processor and the bluecore chip.

If anyone has some experience with the lejos hardware (or electronics in general) or knows someone who does, please let it know.

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Postby s.frings » Tue Jul 20, 2010 2:34 pm

The Voltage tells you if the battery is at a useable level, but it does not tell you how much energy is stored in the battery.

Basically, a battery is empty if its voltage is below 1 Volt. Rechargeable nickel batteries should not be discharged below that level, because they can be destroyed that way (not chargeable anymore).

The NXT hardware does not include current sensors, so you never know how much current is flowing.

The Battery pack has nominal 9V, if you use 6 alcaline batteries, or 7.2V if you use rechargeable Nickel batteries. A set of DC converters and regulators provide stabilized 5V and 3.3V for the electronic parts. However, the motors are driven with unregulated power directly from the batteries.

But all that does not tell you how much energy is stored in the batteries. The best way to measure the power consumtion would be, to attach an external stabilized power supply of 9V to the NXT, and put an ampere meter in the line. Then you can measure the power consumption quickly (9V x measured current in Ampere = Power consumtion in Watts). But you have to use an expensive true RMS Ampere meter, because the current is not continously.

A much more useful method to measure the power consumption would be: Start something with fully loaded rechargeable batteries and wait until the NXT reports low battery state. The do the same with some else condition.

You will get a accurate RELATIVE comparision of power consumption under different conditions. This is much more useful than any calculated number.

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