Let's Make Robots!

Working out battery requirements

Hi,

I'm putting together a shopping list of parts for my second robot but I come from a programming background and my electronics knowledge is still in its infancy, so I'm not sure how to work out what batteries I need.

In my last robot I was able to power two lego motors and my Arduino from 8 x AA rechargeable batteries; but this was simply because the lego battery pack came in that configuration and I tried pluggin it into the Arduino and luckily it worked.

I'm now leaving the lego behind and want to run two of these motors:

http://www.coolcomponents.co.uk/catalog/product_info.php?cPath=46&products_id=230

And one of these servos:

http://www.coolcomponents.co.uk/catalog/product_info.php?cPath=46&products_id=235

Plus my Arduino board and a couple of Sharp IR sensors.

Now I think I'm supposed to be looking at the current supplied by the batteries, and the current drawn by the devices, is that right?

The motors say 40mA free run and 360mA stall - does this mean I have to use the larger value when I'm calculating?

The servo datasheet says 8mA IDLE and 150mA NO LOAD RUNNING - is this the same?

Can't seem to find any information on the Sharp IR or the Arduino, which now leads me to wonder if I've got all this wrong anyway?

The AA batteries I have state 2500mAh on them which I assume means mA per hour.  They also have "7h - 500mA" in small, so does that mean they're 500mA batteries?

How do they work when you add them all together? Do 6 then produce 3000mA?

I feel even more confused now than before I started writing this post!

Any advice would be greatly received, or even just a pointer to somewhere I can read up more on what I'm trying to figure out.

Anthony

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I know that feeling!

"Stall current" means "this much current will flow under full voltage while you hold the wheel fixed". It is not likely to happen a lot and when it does no amount of cells will prevent your supply voltage to drop. Which is a good thing because that drop will prevent stuff from burning.

On the other hand a voltage drop will probably starve your computer. It will reset. Al this will happen so fast that you did not even notice it. Now here is the real problem. The tried and proven strategy is to keep two supplies: a very stable one for micro processor and another one that need not be stable for motors and servos.

Caution about servos: they are a combination of motor plus computer. They will burn from too high a voltage. No more than 6 V please. But the occasional undervoltage will not affect their performance much.

Two packs of four AA cells will probably suit your project best.

Finally some practical advice on your batteries (I prefer to call them cells). They are fine! The number 2500 indicates a theoretical capacity that allows you to draw any current for any amount of time as long as those two numbers multiply to 2500 mAh. Within practical limits of course.

The second pair of numbers (7h 500mA) is more practical and probably more useful. It defies the theory and says that running them for 7 hours at a 500 mA current is feasible. Even if it would multiply to 3500 mAh. Lesson: cells give more if you take it more slowly. The inverse is also true: if you take it fast, they will not give you much. Think of the extreme: if you short a cell, it will give you a short burst of heat. And then it dies.

Thanks a load Rik, that clears a lot up.

I think what you're saying then is: a battery can give as many amps as your circuit will try to take, but if it's too high it might cause the voltage to drop and the battery to run out fast?

I guess two batteries is the way to go then.

I've seen on other posts, people recommending powering an Arduino with a 9V square battery - will this be suitable if I'm running my motors from 4xAA?

I assume I can link the power and ground connections of my servo to the 4xAA (with a resistor to lower the voltage?), connect the two batteries' grounds together and then connect the servo's other wire to a port on my Arduino?

Sorry about the bombardment of questions, but I'm very close to ordering my parts and I'm very excited!

Anthony

I cannot advice on Arduno power requirements. The block battery gives 9V but only very few Amperes. Only suitable for micro controllers and nothing else.

I am a Picaxe luser. All our boards come with a pin arrangement that allows to feed seperate power supplies to motors and servos. I do knot know how the Arduino lusers solve that. Keep asking around. Tying the grounds together is always a great idea.

Go ahead and orer your stuff. You can never buy enough battery clips, holders, voltage regulators.

Just don't try to reduce voltage through a resistor. That is ineffective, inneficient and wasteful. Just get the voltage right by putting the correct number of cells in series. Or search for "power regulation" and "voltage regulator".