I'm making an AUV that requires 15-20 minutes of operating time. I'm using 6 of these motors (Blue Robotics T200 Thrusters) for propulsion; I'm looking for 6S lipo batteries (22.2V) but i don't know how to calculate the capacity (mAh) I need to run my vehicle. Is there any formula that allows me to calculate the required parameter?

Parameters required: Battery capacity, Maximum underwater operation time @ given load.


1 Answer 1


You don't gave all information that are needed for such calculations. However, the technical details of the Thrusters give a peak consumption of 350 Watts each. So you have 6 of them, makes overall 2.1kW/h.

Lets assume your Batteries have the following details: 6S 22.2V 3300mAh. Therefore the amount of energy the Batterie stores is: 22.2V*3.3Ah = 73.26W/h.

Moreover assume that you will drive only 25% full-speed in Average, 2.1kW/h * 0.25 = 525W/h.

Let's calculate the runtime under these assumptions: 73.26W/h / 525W/h = 0.139542857h * 0.6 = 0,083725714 minutes runtime.

So if you want to archive 20minutes operation time reaarange the formula: 525W/h * (1/3h) ((== 20 minutes)) = ~158Wh / 22.2V = ~7.1Ah

So you need a battery which have at least 7.1Ah capacity at 22.2V, assuming an average load of 0.25%.

Please consider, you will have a peak-load of 25A * 6 = 150A. This means the given battery (7100mAh) needs to support a minimum discharge current of 150A/7.1A = 21.12C. If you don't respect these you battery might begin to burn, causing harm to your vehicle and the Environment".

Moreover you don't gave information about the consumption of any electronics inside the AUV, therefore this are not in the scope of this post.

  • $\begingroup$ Electronics inside the AUV have a separate power source, so I didn't really mention it. The weight of the bot will be negligible i.e we'll be balancing the buoyancy so that it has 0 weight. Please tell me if any other information is required. $\endgroup$
    – Sandesh
    Commented Dec 11, 2017 at 7:53
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    $\begingroup$ I rearranged the equation to give you a rough approximation for 20 minutes. I assumed a typical load of 25% based on my AUV experienced. I already assumed you vehicle has a neutral weight in water ;) $\endgroup$ Commented Dec 12, 2017 at 6:53
  • $\begingroup$ Sorry to bother you again, but will I need some form of cooling mechanism for the lipo? It's in a closed environment (obviously 😅) so I'm a bit worried about the heat generated. Prior experience with quadcopters tells me that I should do something about it, but I'd like to know your opinion on this. $\endgroup$
    – Sandesh
    Commented Dec 13, 2017 at 3:43
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    $\begingroup$ It would be great to have this question as an additional question, because it's unrelated to the original one. However: From my experience additional cooling even with mobile i7 CPUs with LiFePo4 cells it is not needed. However I Have had much bigger cells and used much less discharge levels. I recommend to have a few temperature sensors. The heat dispensation depends on the hull material (AlMg3 in my case). So an estimation is again really hard to make. $\endgroup$ Commented Dec 13, 2017 at 8:55

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