# Deciding on battery for Robot

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1. Oct 12, 2016

### CasVS

Hi All!

I am a Dutch university student and I have an assignment to make a Search & Rescue Robot. Because we have to work with a budget, we have decided to make our own batteries (using AA penlights).

Now while calculating the required capacity of the battery pack, I encountered some problems. My first approach was to calculate the power the vehicle needs to fully function by using the simple formula P = U*I and doing this for every single electric motor and add all these for a total power requirement. This would be for example 2.8 W and calculated the amount of Joules I would need for the entire rescue operation (approximately 6100 Joules). Though when calculating the battery pack (1.5 V and 2700 mAh) I found that I only needed two batteries in series that would give me 8100 Joules ( 3 V * 2700 mAh = 8100 Joules/hour)

Though this seems very unlikely. I think there is something very wrong in my reasoning (I never calculated it in this way before maybe that's why . Could someone tell me whether my approach is valid, and if not, help me to find a strategy to approach this problem.

2. Oct 12, 2016

### Staff: Mentor

The number 2.8 watts for a rescue robot sounds far too low. 280 watts sounds closer. Recheck that calculation.

Edie: could it have been 2.8 kIlowatts?

3. Oct 12, 2016

### rbelli1

When using alkaline and zinc-carbon dry cells you will not get anywhere near the stated capacity with a one hour discharge. If you need the full capacity in one hour you will want lithium cells or a rechargeable type.

BoB

4. Oct 12, 2016

### CWatters

Your calculation is basically correct but as others have said.. check it really is 2.8W and 6100J. That seems very low.

5. Oct 13, 2016

### CasVS

Thanks for the reply! I should have mentioned it will be a small scale robot (30 x 30 cm) and we use small motors.

We use a total of 4 motors, two of them have a rated voltage of 1.5 V and a max current of 0.500 A, and the other two have a rated voltage of 12.0 V and max current of 0.143 A.

According to MY calculations the power needed would be: 2*(1.5*0.500)+2*(12*0.143)=4.932 W
For a 45 minute rescue operation I should have (in a worst case scenario) 4.932*45*60=13,316.4 J

does this make more sense? I still should only use 4 batteries for this application which still feels small in my opinion.

6. Oct 13, 2016

### CasVS

I forgot to mention that I am dealing with a small scale robot of 30*30 cm. I now calculated that with the motors it uses approximately 5 W (Simple P = U*I). This would mean I only have to use 4 batteries, which feels to little for me. Maybe I did something horribly wrong when calculating the Power consumption of the motors as I just took the rated voltage and multiplied this by the amperage under max load.

7. Oct 13, 2016

### Staff: Mentor

OK, when you said rescue robot, we thought that it was big enough to carry victims.

An AA alkaline battery stores 3.9 watt-hours of energy which is 14000 joules. An AAA aklaline battery stores about 5900 joules. I found those numbers with Google. You can do the same for other kinds of batteries.

However, not all of that energy is useful for motors because the voltage drops as the energy is used up. Below is a typical discharge curve for alkaline batteries. You need to find out the minimum voltage for those motors.

L

Is the rescue environment cold? If so, Lithium batteries perform better in cold temperatures than some other battery types.

Shelf life, size, weight, temperature range, rechargability, cost and other factors influence the choice of battery. You have not provided us many of your requirements.