Akupakk

Fast Charge Battery Pack for small traction applications

board image here

Green Mantis started development in June 2019 based on the previous generation BMS hardware called Means of Energy Storage.
The development goal was to have a 1.5kWh battery pack which could be fully charged in roughly 30min and which would work down to -40°C. It uses 12 40Ah LiFeYPo41 cells from ThunderSky Winston (Datasheet, Importer).

pack image here

Project History

Initial progress went smoothly of the development, but in the end a combination of change requests from the customer and badly communicated changes in the expected shipping date, unusable boards by a local PCB manufacturer and a silicon bug in the LTC6811 BMS IC proved fatal to the project and the customer jumped ship the day before delivery.

Cells

Due to the long lead-time for the cells, we ordered them sometime in July 2019 when some new stock arrived at the importer.

A fresh crate of 40 cells from GWL
All 40 cells well and accounted for

Balancing as it should be
Thermal image while balancing cells

The aforementioned silicon bug also lead to the production of four forbidden lithium pillows, due to the LTC IC not taking the command to cut off charging to some cells.

 
The bloated prototype pack
Bloated cells

Fortunately, due to the chemistry of the cells, they're inherently safe and don't heat up enough for to cause thermal runaway even when mechanically damaged.

Assembly

The pack during assembly
All 40 cells well and accounted for

Since then it has become a proving ground to future projects for small DC grid applications and we expect to finally release a version of mantis to the public which won't lead to bloated batteries.

Lessons learned

The most important takeaway of this project was to communicate clearly that change requests will lead to initial deadlines not being met. There's only so many working hours one can throw at a project before returns are diminished. Also unexpected manufacturing defects and unknown silicon features can quickly eat away at a project plan.

Original Specs

  • 12S1P, 40Ah Cells, 1.5kWh Capacity
  • 120A quick charge
  • -40°C lowest expected working temperature
  • 20A/12V auxilliary power supply
  • Green Mantis Battery Management System
  • 2x Restless Raccoon programmable, bi-directional DC/DC converter
  • Frame made from 2020 extrusion, an aluminium base-plate and acryllic side-panels
  • ~21kg total weight per pack.
  • Packs should be easy to handle and replace (handles to lift and carry them)

  1. Those are similar to normal LiFePo4 but doped with Yttrium. Characteristics include a higher end-of-charge Voltage and an extended temperature range for charge/discharge, though the vendor is a bit light on details. But the work is likely based on this paper. ↩︎