Follow up post: Case for 13S20P Li-Ion Battery Pack

This is a follow up post from the post here:

I decided to start a new thread, as this is about the case only.

Originally I built a 13S15P Setup (13 Cells in Series, 15 in Parallel, 45Ah Capacity) but as this got fairly hot with the way I wrapped and protected the battery,
I decided to not only top it up with 5 more parallel groups (additional 65 cells) but to make a professional case that would include the BMS and fans for active cooling.

I also wanted a sort of quick mount, which was a goal that I did not quite achieve as I would have liked to have it, but mounting and taking off the battery is still fairly quick (including disconnecting the four cables probably a 2-min job to mount and another 2 minutes to taking off the battery for charging or transportation.

These are the case parts with three additional fans (80x80mm, 48 Volts) .
The case is from (Germany)
and cost 610 Euro incl. shipping and 19% VAT. The fans were extra.

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First fan mounted onto the backplate. The fan pushes the air out of the case, in the same direction as the main propellers push the air, so this is tons of extra Thrust for flying (just kidding)

The holes are supposed to let the air being sucked in, that the three fans push out of the case.
The holes are positioned exactly where the gap of four cells are and fit the holes of the plastic battery holders:

case assembled, but not wired up yet:

testing whether the cables fit through the holes with the rubber cable protection:


Testing whether the mounting brackets fit and work as designed.

They do! Snug fit, nice and tight.

Wiring complete!

on the top left there’s another set of cables coming out. One that is a close loop, which would allow me to disconnect the three fans from power supply, if needed, without having to open the case again
(not a nice solution yet, but I want to know how the fans perform first, before I decide to add a switch or a potentiometer, or whatever…)
The other cable is a bit longer, it has a button at it’s end, that allows me to turn on and off the battery while I have the OpenPPG on my back.
Turning on the power also turns on the fans, and vice versa when turning off the battery.

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A few more pictures:

The case and the fans add a bit of weight, but when it’s on my back, it feels really good.
The weight is, unlike with a 12 Liter gas tank, much further up, and that makes the whole unit much more comfortable to carry


That all looks really nice! Good job!

Where are you going to mount the low voltage switch?

I have to admit (not that it is hard to admit) that I paid someone I know (Thank you Jörg!) to do the proper planning and drawing.
I just told him how I wanted it, he added some ideas, went through some iterations in the design process of the mounting brackets and then I went for it.

I am not sure what you mean with “low voltage” switch - but the BMS takes care of shutting off the current when voltage goes to the lower limit.

turning on and off the battery manually is done with the button that i wired and tied to the shoulder straps. I will take a few more photos tomorrow.

Here’s a screencast of the BMS’ App, testing the battery WITH the case and the harness on my back.

It took me a few hours and watching that screen cast again and again, until I figured why the battery kept shutting down:

The Voltage Sag when depressing the throttle is so high, that the “low voltage” protection is triggered:

I received some more nickel strips and the 65 additional LG HG2 18650 cells, but I am still waiting for the plastic cell holders/spacers to arrive, to be able to mount and add them to the battery pack.

@GliderPilot any comments on why the voltage sag is so heavy?
Do you think it will change when the additional 5 parallel groups are added, because there is less stress on each parallel group thereafter?

That’s a beautiful case! You should not have any heat issues with this.

I agree… I used the same foamed rubber as padding between the cells and the case, but this time it was not wrapped around, AND I cut out holes in the same position as the holes in the aluminum case.
So the air should be sucked very nicely through in between the cells and keep them from overheating.

I did not do any time measuring, but I did let the props run for a little under ten minutes yesterday, I guess, and the temperature did not exceed 32 degrees celsius.

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I do know that doubling the number of parallel cells will cut the voltage drop in half… so yes, adding any number of rows will help.

How full were your batteries to begin with? If they are 50% charged it’s easy to drop to zero% with full throttle. Full batteries drop to about 50% with four bonkas at full throttle.

There are two reasons for voltage drop. One is the resistance in the batteries. More significantly, as you draw current from them, you deplete the chemical reactants near the plates, and it takes time for new reactants to take their place. When the batteries get warm the recovery is quicker and the voltage drops less. So running it down to 50% during a flight and getting the batteries warm in the process will give you a different result than a cold start at 50% capacity.

Do you have any pics of the battery laying inside the case with bms without top cover on?

even though the weather was great for flying, I used the weekend to upgrade my battery from 13S15P to 13S20P

This is what it looked before:
Jan 20

And this is what it looks like after the upgrade, with an additional 65 cells (what a beast!):

I also took the opportunity to shorten the balancing wires a little bit, to make things cleaner.

For the busbar, I purchased a new set of flat copper material, a bit smaller, with round edges.
I also decided to not solder the wires, but to connect them with nuts and bolts. You can see the bolt that will be used for this purpose on the top of the copper bus bar:

Here’s the negative terminal, connecting to the input of the BMS.
I used some duct tape (probably should have used heat resistant kapton tape, but well… ) to avoid accidentally touching both terminals with the conducting aluminum case.

The thinner wires on top of the array of cells connect with four thermistors to measure the battery’s temperature. I also rewired and shortened those.

Here’s a top view of the battery where you can see the BMS. it has aluminum plates on both sides for heat dissipation. the other side directly connects with the aluminum case with the help of heat conductive material.

You can also see the negative output of the BMS with the positive terminals connected to the bus bar.

Here you can see the three fans that are for cooling the battery… I haven’t tested yet whether all three are really needed, or wether just one or even none at all would work as well.

And another picture of the whole setup, lid still open:


That’s it, mission completed. :slight_smile:


Looking good