Battery discharge rating

[Old School]

I am currently awaiting the parts needed to assemble a brushless powered (56102 730kv, 300 amp ESC) TFL 1330 mm Outerlimits (same style hull as the gasser Patron Saint). I believe that the batteries will need to be placed either side of the motor to achieve a satisfactory centre of gravity (around 28%). Most likely I will run 10s (possibly 8s) with two 5s batteries in parallel each side (each 5000 mah 35C/70C) and then series to the ESC giving 10,000 mah (?) at 10s. Locally, 5s batteries higher than 35C continuous discharge seem unavailable at my usual outlet. Given the configuration with multiple batteries would the 35C discharge be an issue, please?

[NativePaul]

It depends on what you want from the boat and how realistic the labels are on the packs you want to use.

Personally most of my boats use 25C labeled packs, I have some sprint racing and SAWS boats that I get as high a real C rate as I can afford for, but I want about 6 minutes runtime for most of my boats which is only 10C, and most of the 25C packs I have tried are comfortable operating at 10C.

If you want more power and shorter runtime it pays to do some research to make sure you aren't picking packs labels "True 150C" that are realisticly <20C packs.

You have a 300A ESC how hard are you willing to load it? Are you going to be pulling 300A average with no overhead and spikes way above 300A or are you leaving some overhead for reliability and pulling 150-200A average to keep your current spikes to about 300A?
An average of 300A is 30C on 10,000mAh, and I don't know of any 35C labeled packs that I would trust to actually supply 30C, if you want to keep your peaks under
300A and have averages in the 150-200A range though you should be able to find 35C labeled packs that are capable of putting out the 15-20C that you will be pulling.

Check out the RCGroups battery section where the best in depth testing is done to see what the specific 35C packs you are thing of will actually handle.

[Old School]

Paul,
many thanks. The ultimate aim is a sport boat around 45 mph - 50 mph.

[NativePaul]

I doubt that you will be using an average of 300A then, but without knowing what boat it is going in or the specific batteries it is still impossible to say with any certainty.

My lightweight race boats are faster than that at only 10C, so unless you have a very big/heavy/draggy boat or the battery maker is lying unusually heavily (they all lie to some extent) I suspect that you will be OK.

What does the boat weigh? I use a rule of thumb of 500W/kilo for a very free running stepped hull to get into the mid 50s, and that should get most planing hulls to your target.

[Old School]

Paul,
the boat is a TFL 1330 mm Outerlimits (now called Patron Saint) blank hull (ex-gasser, although this is a factory new hull) that I will be fitting a 56102 730kv motor into. Undecided at the moment whether 8s or 10s. The motor is limited to 41 volts at roughly 5300 watts. I predict that all up weight will be around 7 - 8 kg.

[NativePaul]

For an example of the calculation you have 10s which should give you an average of about 37V during your run, multiply that by the average current of say say 150A to allow for ESC overhead and reliability and you get 5550W, divide that by 500 and you get 11 so that is enough to power an 11kg boat well.

To come at it another way, weigh your boat if it weighs for example 20kg multiply 20 by 500, then divide that result by your voltage which in your case would make 270, that is how many average amps you will use to push that boat with good performance. In this case you would be pushing the ESC pretty hard.

Working out the C rate used is pretty easy as you have 10,000mAh you can just divide the amps by 10, so 15C for my 11kg example and 27C for my 20kg example.

[Old School]

Paul,
many thanks. I think that I should have done an electronics degree.

[NativePaul]

The closest I got to that was high school physics.