Battery capacity

EFOIL.builders Electronics (ESC, remote, batteries)
#1

Hello everyone,

I started a built and obviously one thing I needed was batteries, but I’m just really surprised by the results I’m getting. This is what I did:

  • Install a power measurement unit
  • Put the propeller in a bucket
  • Run it for as long as I can

The battery is a 6s4p 22ah FiJiLa, I selected these because they are supposed to have a higher rated continuous discharge. But the truth is that it seems more like they are on the low 5ah.
They basically run for 1h to 1h10 at 3.5 to 4 amp (80w drawn).

One thing I noticed is that as soon as the battery is under load the power drops significantly (maybe one volt).

Anyway the question is, am I doing something stupid, or the product is crap?

Many thanks,
Joss

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#2

I would not worry too much about the 1V drop.
Which voltage do you discharge them to? I would call a 6S pack empty at 15V to 18V under load.
From the last image I see 22.3V at 5Ah? At 22.3V each cell is at 3,72V so by far not discharged all the way.
What do the batteries weigh? That is usually also a good indicator for how much capacity they have.

#3

Hello,

There is a BMS integrated so I had to give it a couple of minutes of charge to see what’s the voltage: 21.1, so about 3.5v. Seems aligned with this chart I could find: https://promodeler.americommerce.com/askJohn/Care%20and%20feeding%20of%20LiPo%20packs/LiPo-Capacity-v-Voltage.jpg

But even if it could go lower, I think battery discharge is highly skewed on the end, so it would not be that much more capacity.

Weights with BMS, cables and cover is about 875g. Good point seems on the low side…

#4

To be honest, I would remove the BMS, at least for discharging. Otherwise all the discharge current needs to go over the BMS. The VESC can be programmed to shut off if the battery voltage is low. For charging you may keep it.

Calculation with the weight:

Stated by Manufaturer: 22Ah*21,6Vnom=475Wh
At assumed 800g cellweight (75g BMS&cover) that would be 600Wh/kg
Good cells have around 200-250Wh/kg
So from that, your battery should have max. 200Wh or 9Ah

#5

If it does 1h at 80W then that’s 80wh, it will be nowhere near 200Wh

Even counting 10% left at 3.5V per cell and 1h10 at 80w to be on the conservative side, that’s 80x70/60x1.1=103Wh

Unfortunately the battery is a scam.

#6

Yes but I have a simple esc, not a vesc. Main reason is because box is very small I wanted something that’s in the water to avoid any heating in the battery box.
Obviously in the future I may waterproof a vesc, but time and the amount of things to do are just competing… Next steps is once the battery is solved to build the remote you designed :wink:

#7

Yeah that’s what I thought, obviously I’m a little new to this field so I wanted to make sure I’m not missing something obvious.