# Battery capacity requirement calculation

Elliot,
At BR, when you calculated the minimum necessary battery capacity for the BlueRov series, what thruster duty cycle and mission length did you use? For example, for a one-hour “typical” mission, what average thruster % of maximum did you use? I would guess about 30%.
My current BS-1 project has 4 thrusters drawing an estimated 4 amps each at wide open. A 30%, one hour mission would draw 4.8 amps for one hour, or 4800 mah. My hotel loads will come to about 400 mah. So, the 5800 mah battery that I currently have should work.
I realize it all depends on the mission, but BR had to start with a predefined mission for recommended battery capacity. Right? Have you changed that much over the years?
Thanks,
Richard

Hi @model14,

These aren’t details I’m well-versed in, so I asked internally but haven’t yet received a response.

Best I could find on the forum was this response from 2016, which at least provides a general sense of the testing conditions at the time. I’m unsure whether additional tests in changed conditions have been performed to determine the current run time with the current battery options, or whether the run time estimate has just been kept the same with the understanding that it’s a rough guide and more precise calculations require direct power estimates for a particular use-case.

Since energy usage / battery capacity is an integral quantity, there are many ways to reach the same theoretical result according to different usage patterns. If you assume the thrusters use the majority of vehicle power (which is likely true for most ROV use-cases), you can calculate an “effective constant throttle” level quite easily by dividing the battery capacity (in Wh) by the number of thrusters and the time estimate to get the “effective constant power” of a single thruster,

P_\text{effective} = \frac{E_\text{battery}}{n_\text{thrusters}\times \text{duration}}

which can then be compared to the power usage vs PWM plot in the T200 performance charts to determine the effective throttle level