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It seems we are getting a slew of people wanting to get crazy run times from their boats. Some of these numbers are just crazy! So I’d thought I would post the formula here and hopefully some folks can crunch some numbers. First, all batteries have their capacity listed on them, usually in MAH, which is MilliAmpHour. It’s easier to use the AmpHour value, since boats use big time amps, not milliamperes. So take that mah number and divide it my 1000. So 5000mah becomes 5ah. That is 100% capacity. If you run your batteries down to that each time, they will fail pretty quickly. You need to leave AT LEAST 25% in the batt, so 5ah Times 0.75 gives a useable capacity of 3.75ah.
Rule 1: Batteries wired in parallel add the capacity of the batteries together, to get the total ah. So two, 5ah batts wired in parallel gives you 10ah and with the reserve, 7.5ah.
Rule 2: Batteries wired in series do NOT add, you are adding voltage instead. So two, 5ah batts wired in series doubles the volts, but the capacity stays the same for each cell. So two, 5ah batts gives 3.75ah.
Now to calculate runtime, you need to find the time ratio number between the stated capacity, which is given as 1 hour, and the desired value. This time ratio will then give you a number that you can use to find out how many amps you can draw for the desired time. Let’s jump into an example:
I want to run 5 minutes with my boat. Getting the time ratio, divide 5 by 60 to get a value of 0.08333 Let’s assume I’m running two, 5.2ah batts in parallel, so I have 10.4ah combined and I can use 0.75 of that which gives 7.8ah So with that size of capacity, if I want to run 5 minutes, my average amp draw cannot exceed 7.8/0.08333=93A. This is a reasonable value...
So for you guys wanting to run 20 minutes with that same battery, which is pretty good sized, you would need to work with an average amp draw of 20/60=0.3333, then 7.8/0.3333=23.4A. I don’t know of too many boats 30” and above that is going to only draw 23A, especially with a crazy speed expectation.
So do the math before you post something crazy, because most of us are just laughing at some of these posts!
Rule 1: Batteries wired in parallel add the capacity of the batteries together, to get the total ah. So two, 5ah batts wired in parallel gives you 10ah and with the reserve, 7.5ah.
Rule 2: Batteries wired in series do NOT add, you are adding voltage instead. So two, 5ah batts wired in series doubles the volts, but the capacity stays the same for each cell. So two, 5ah batts gives 3.75ah.
Now to calculate runtime, you need to find the time ratio number between the stated capacity, which is given as 1 hour, and the desired value. This time ratio will then give you a number that you can use to find out how many amps you can draw for the desired time. Let’s jump into an example:
I want to run 5 minutes with my boat. Getting the time ratio, divide 5 by 60 to get a value of 0.08333 Let’s assume I’m running two, 5.2ah batts in parallel, so I have 10.4ah combined and I can use 0.75 of that which gives 7.8ah So with that size of capacity, if I want to run 5 minutes, my average amp draw cannot exceed 7.8/0.08333=93A. This is a reasonable value...
So for you guys wanting to run 20 minutes with that same battery, which is pretty good sized, you would need to work with an average amp draw of 20/60=0.3333, then 7.8/0.3333=23.4A. I don’t know of too many boats 30” and above that is going to only draw 23A, especially with a crazy speed expectation.
So do the math before you post something crazy, because most of us are just laughing at some of these posts!
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