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# Battery Calculations

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#### Anical

##### Newbie level 6
I am working on Line Maze solving robotic car. I am wondering how to select the battery. I talked to my lecturer he said for that I need to calculate how much is the overall power, voltage and current consumption. I did that yet i am not able to understand how to decide which battery i should be able to use. Can anyone guide me on how to choose the battery or the calculations assisting it?

what is the power , voltage and current values you have calculated?

Hi!

If you know the voltage, and the current draw of your robotic car, you need to consider the constant current draw, and then see which batteries actually have a good mA/h rating to match.

Example (from memory, excuse any errors, but roughly correct): I made a naff LED torch that can draw from 20mA to about 200mA or so on some settings.
My preference, due to size, was a standard rectangular 9V battery but it would have lasted 30 minutes at most, 12V (3 x 4.5V) battery instead should last 30 hours, so I opted for those - bulky, but longer lasting. 1.5V batteries AA or AAA have a better mA/h value than 9V batteries, which I imagine is why they are in so many battery operated devices. A tiny camera battery would last a few seconds...

It's good to browse online shops which provide datasheets for the batteries, or manufacturers, or check battery comparison webs, to get a better idea. In my experience it was not always easy to find what should be one of the first things a person would want to know about any battery.
Sorry I can't remember the formula for calculating A/h or mA/h. If I can find it I'll add it later.

Convert load to Watt-hours and decide on t in fraction of an hour, H and then size the battery based on cost and Watt-hours capacity using

power avail = Vavg*Ah

duration ~ Vavg*Ah / average load power [h]
~ Ah rating for similar duration divided by load current.

keep in mind that Ah drops in capacity from 20C to C to C/20 as ratings are often based on 20h loads

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