# 18650 4S with 150A discharge rate

Hi, I am building a rover with 8 T200 thrusters and I want to make a battery with 18650 cells. I don’t know anything about batteries. I want to make it a 4S cause I want my rover to be power efficient so I don’t want to run my thrusters at 18.5V. So, here is my question how many 18650 cells do I need, how many cells do I need in parallel and is there a PCB which protects the battery like this one. Am I missing any other important details?

Hi jakkala,

As an example, our 18Ah 4S Battery uses 24 Samsung 30Q cells in a 4S 6P (4 cells in seris, 6 cells in parallel) configuration. The Samsung 30Q cells are 3.7V nominal each, with a 3000mAh capacity and 15A constant discharge rating. This means that the total battery voltage is 43.7V= 14.8V, capacity is 63000mAh= 18000mAh, and the discharge rating is 6*15A= 90A. Note that discharge ratings are temperature limited, and this rating is at for a best case scenario in a non-enclosed space when the pack discharge is only cell limited, as noted in the documentation.

If you want to make your own battery, the capacity and discharge rating will depend on the specification of the 18650 cells you use. 150A is quite a lot, and if you wanted to draw that much current from the battery constantly you would also need to make sure the battery construction can also hold up. You would need thick and wide solid nickel (or copper) battery strips, very heavy gauge wire, and a high current 150A rated connector. If space and weight was also an issue, you could also sacrifice some capacity and use higher discharge lower capacity 18650 cells with fewer in parallel. When cells are packed tightly together, cooling can become an issue, so you would need to address this as well.

For an 8 thruster ROV, this is likely overkill. Unless you are running all 8 thrusters at 100% gain full throttle for more than a few seconds and have no other current bottlenecks in the system, you will not need near 150A constant discharge. In our experience, a BlueROV2 with 6 thrusters and 4 lights going full bore tops out at about 60A, due to losses and voltage drop throughout the system. There is a size limit to certain connectors and cables that we can use on the BlueROV2, so we made the trade off in the interest of price and practicality. This means that the voltage drops a bit before it reaches the thrusters, but this is not an issue since the cables in question are water cooled and do not heat up enough to be an issue. In any case, running all 6 thrusters at full throttle for more than a few seconds is not realistic in an ROV. I would expect you would practically top out at about 80-100A at most on an 8 thruster ROV, and you would only need this in bursts.

A built in battery management system (BMS) is nice to have, but not strictly necessary if you are monitoring voltage and current already. I suggest you look into E-bike forums for more information on building your own 18650 pack, they have a lot of information.