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Preparing to accommodate more packs

Preparing to accommodate more packs

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It has been a long time since I posted an update! All the momentum I had built up with cell testing suddenly crashed when I had to get the proper “modular design infrastructure” ready to handle more than one string of packs. Each task pushed back on a dependency that rapidly expanded the to-do list. The “mainframe” bus-bars really needed the rack system to be a bit complete, and then my battery leads needed upgrading to cope with the coming packs, and then the inverter needed to be mounted more permanently, and so on.

The single-string battery was not really ready for modular expansion.

So it feels as though no progress has been made on the battery. However, looking back at photos from 18 months ago highlights what has changed. Last year I got a second-hand Flammable Materials storage cabinet and mounted it in place. I didn’t have feed-throughs for my battery leads and so the original string of packs was just sitting on top of the cabinet. The inverter was temporarily mounted to a palette leaned against the wall.

It seemed like the right time to put the cells aside and focus on some construction and engineering. Mounting the inverter was mostly straightforward, but there were a range of design decisions that needed to be made for the battery pack rack. It was during this process that I lowered my packs from 20 cells down to 18 – the slightly shorter packs will fit two-deep in the cabinet if I ever get to that many cells!

Here’s a little window into the things that have occupied my battery time. In fact, this installation was all completed some months ago, but the video has been languishing waiting for the simple voice-over.

Designing small packs for modular battery

Designing small packs for modular battery

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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).