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Month: August 2018

Reached first set – 70 good cells!

Reached first set – 70 good cells!

I’ve been interrupted by a family holiday, but just before leaving on that trip I reached the first real milestone for my battery build: 70 cells. This will allow me to make half-packs (10 cells each) and produce a fully-functioning 24 volt battery prototype. In the process I will be able to optimise the techniques for pack construction.

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I’m calling each set of 7 packs a “string”, so that the battery is made up of strings which are made up of packs which are made up of cells. This first string of 70 cells has 0.518 kWh of storage, which is slightly above my estimate of 1 kWh per complete 140-cell string. My initial estimates of achievable cell capacity have turned out to be reasonable.

Just a few of these cells have had their plastic cover damaged by the extraction process, and the orange sleeves are sitting on top in this photo ready to be fitted as replacements. Obviously, some care must be put into the cell arrangement within a pack for visual appeal.

Dividing the 70 cells into packs is less arbitrary, because it is vital to have the packs as balanced as possible. I devised a simple algorithm as a starting point, and it has turned out to be remarkably effective.

  1. Sort the cells by capacity from highest to lowest.
  2. Take the top 7 cells in this list and distribute them in order into packs 1-7.
  3. Take the next 7 cells in this list and distribute them in reverse order into packs 7-1.
  4. Continue down the rest of the list, filling in this “zig-zag” manner.

This means that pack 1 gets the highest capacity cell, but then only the 14th in the list. Pack 7 gets the 7th in the list, but then also the 8th. This process produced packs ranging from 20.5 Ah to 20.59 Ah, and that 4% variation can be handled by balance-charging.

Designing small packs for modular battery

Designing small packs for modular battery

Lithium chemistry cells are not a good match for 12-volt systems, because almost exactly 3.5 cells in series would be required. To get around this, and to reduce the size of copper wire needed to carry high currents, most people run 24-volt systems, or even 48-volts. Since this project needs to start small and bootstrap up to larger sizes, I’ve decided on 24 V.

This means that I need 7 “cells” in series, although of course these will not be single cells but rather large packs of cells in parallel. Many other home battery builders have opted for large packs of 80-120 cells in parallel, and this has the advantage of getting large storage capacity fairly neatly. However, it means that 7 x 80 = 560 cells are required before the 24 V battery can first be “turned on”. I want to quantise the battery into smaller packs so that I can go “live” well before reaching 560 cells.

With cell holders that allow a triangular-close-packing of the cells, I can make packs of 2×10 cells that are attractive and slim-line. Interestingly, a battery of 7 20-cell packs would have about 1 kWh of storage for realistic reclaimed cell capacities.

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There’s also a maintenance advantage of using smaller packs strung together. When the battery is larger, it will be possible to remove one of the 20-cell packs from a set of parallel packs and leave the battery operating (although it will become unbalanced if left like this for long).