Newbie seeking more advice

Evening all.

Ive spent the last couple of weeks researching for my problem, and whilst I have learnt a lot Ive also managed to confuse matters at the same time. I'm hoping someone maybe able to help me clarify a couple of things.

The pack I wish to build is purely to extend the range on my Diver propulsion vehicle(DPV) speed is not an issue. I currently have a 25V 512 Whr cell.

with the room I have available I think 18650 cells are the way forward and after much reading I think panasonic 18650B 3350mAhr are the right cell.

Now then as has been advised on a seperate thread go big. However this project isn't quite that simple. As I add cells not only do I add mass but I remove air(buoyancy) and my DPV becomes negative, ie sinks. this is extremely undesirable. Whilst it is possible to add buoyancy this requires a structural change and it complicates the matter further.

This is basically the reason for going self build rather than buying a ready made unit. I want to start with 7s13P, which should weigh in about 42/4300g after its put together, but be slightly smaller than the existing unit, and hence be more buoyant. If this works ok I can then add, or if needs be remove a parallel set.

The question Im asking is therefore, would you agree with my choice of cell?

I'll purchase a spot weld module if need be, so therefore weld vs solder?

Would one of you experts be willing to give me a few pointers in layout/design once I have all the bits and pieces? For this to work I think its going to be 2 layers of cells around 150mm dia. with a bms in between. Im thinking I can therefore encase the whole thing in a 150mm pipe and make waterproof, in the event of a catastrophic leak I wont kill the cells.

Many thanks

Simon

That is an interesting project. Do you know what kind of amps you will be pulling? Speed may not be an issue as you stated, but what about thrust and power? Those cells you highlighted are not that high in continuous discharge rate. What are the dimensions available for our battery?

Thanks for the interest.

Attached is a pic of the open DPV. The existing pack is 170mm dia and 165mm tall. Ive been through the manual and on the manufactures website but cannot see the current rating I'm afraid.

I could strip the m/c and see if the motor has any marking? would you think this likely/useful? Its within the unit in a separate watertight housing and not an easy job to get too but happy to try. All gear prepped for diving tomorrow so not realistic to strip tonight but could do tomorrow aft. In the mean time I shall continue to search the internet for a rating and hang off ordering cells for the time being

Many thanks

Simon

getting specs off the motor won't help. what you need is the nominal and peak amp draw from the motor under normal use. those are dictated by the controller and by it's use. You may be able to find out what the controller's max amps are before it cuts power, but the amps in use need to be measured.

And for that, you need a tool something like this. RC Inline Power Meter
You'll also need to rig up a way to read it. it will record your peak amp draw, but you need to see what you're pulling in normal use.

Those cells you chose can handle up to 2C, allegedly. However, at 2C they only have about 2000mha capacity down to 3.3v. They are cells better used for things like radios and camp lights, where the power draw is very low. There may be better choices, but you need to know what your power usage looks like before choosing a matching cell.

As for Solder VS welding, Welding. Hands down. These cells have a delicate vent system built into the end that can be damaged or destroyed by the heat soak from a soldering iron. welding generally has less heat soak and is less risky. If that vent system is destroyed, the cell will not work. but if it's damaged, then the cell may not be able to stop it's self from exploding should something go wrong.

MMMMM

Cant see a way of measuring that under load. I'll ponder for the day something may occur. Im assuming that a no load indication is pretty useless?

S

Check out if the amp limit is written on the bms, itll give you some idea of the systems Max draw.

Thanks guys. I can get one of these:

https://www.amazon.co.uk/VIPMOON-Precision-Consumption-Performance-Backlight/dp/B01M05Q1U4/ref=sr_1_2?keywords=rc+inline+meter&qid=1562430297&s=gateway&sr=8-2

Pretty easy, and I think I've thought of a way of testing it. Did a 2hr dive today setting kit up, needed something to think about.

Watch this space it'll prob take a few days to get a meter and rig up something to test the unit on, but I'll hopefully have some info before the end of the week.

Thanks again

S