Quote:
Originally Posted by Rick TYler
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6.5 amp time 110 volt is about 700 watts for an 18" mower. You could probably get by with a 1 kW continuous DC motor. Like the others have said though, this is going to be a power hog. Unlike an FRC bot or drill, this is near coninuous use for however big your lawn is. You will probably want something on the order of a 1 KwHr pack if your yard requires 30 minutes to mow (many batteries you do not want to drain down too low). When sizing a battery, you need to verify Power and Energy. KW-Hr is energy, and KW would be a power rating. Most batteries do not specify it that way though. Since batteries have a relative constant voltage (12 volt will go between 15 and 10 volts), the instead like to use amperage.
"Battery power rating is measured by two standards. The more popular of these, cold cranking power, determines the amount of current (amps) a battery delivers for 30 seconds at zero degrees Fahrenheit while maintaining a minimum terminal voltage of 7.2 volts. The higher the number, the stronger the battery. Think of those old Diehard commercials, with all those non-Sears batteries giving up the ghost in the cold and snow, and this will perhaps give you a visual image.
The second standard is called reserve capacity rating. This is a warm weather rating (80 degrees Fahrenheit), which estimates the amount of time it takes the terminal voltage of a fully charged battery to dip below 10.2 (or 1.7 volts per cell) at a continuous discharge rate of 25 amps. The rating is expressed in minutes. For example, a rating of 120 means the battery will run for two hours (120 minutes) before ceasing to function"
http://www.edmunds.com/ownership/tec...5/article.html
Roughly speaking this means the battery could deliver 25 amps * 12 volts or 300 Watts for 2 hours, or in our case 700 watts for about 50 minutes.
If you are using lead acid, you will want a deep cycle battery. this has thicker plates and is less likely to give out on you. Most new cheap car batteries are substantially lighter. In lead acid rechargeables, this is bad. If you deep cycle one of these, consider them junk. The plates are really thin and get ruined during a deep cycle.
This sounds like a fun project (do i hear robot mower...). If you are merely looking for a fun DC Power electronics project, look into E-boost bikes. These are fun and you can pick up a cheap bike at a garage sale.