For the past few weeks, I have been designing, printing, redesigning, and thinking about cells holders for a DIY powerwall. I decided to go with 8P packs, as those will allow for easy replacement of faulty cells, and if one cells fails, I only have to replace a pack of 8. Each cell will be individually fused at 4A. As for spot welding vs soldering, I go with soldering – especially for the fuse wires, it is much easier, and does not damage the cells. I use a 60W soldering iron with a big tip, and have never had any problems.
I finally started getting the packs put together, and realized that the 350ish cells I was planning on using is much more than I originally thought. I am only using cells over 2000mAh, and each cell has ben tested in my 18650 testing station, according to my process here. I may need to find and strip down a bunch more laptop batteries.
7S will be the configuration for this wall, and I will start out with 6 or 7 packs of 8 cells in each parallel group (7S 48 or 56P). This 7S is ideal for a 24V system. Unlike 12V systems, 24V systems allow the use of common 24V inverters, and DC-DC Buck converters can be used to achieve different voltages. If I had gone with a 12V system, then it would be the classic debate of 3S (9V-12.6V) vs 4S (12V-16.8V). Most 12V inverters have a low voltage cutoff of around 10.5V and a high voltage cutoff around 14V. With either option, over half your battery remains unused. Going with 7S, we can use a 24V inverter. The voltage range of a 24V lead acid system and 7S Li-ion pack are almost perfectly matched, so all the power in the batteries can be used.
I will eventually be building some solar panels to charge this, but for now, I will stick to charging it with a 930W server power supply that I bought on eBay for $25, paired with a 600W converter from Aliexpress. I will also use some high power TP4056 modules or an iMax B6 to top off individual cells when required. The best solution for cell monitoring would be a Batrium BMS system, but those are expensive. Instead, there will be a small voltage monitor attached to each cell with a switch to monitor voltages while charging and discharging, but does not draw power when sitting idle if the switch is turned off.
The whole thing will take up roughly a 1x3ft area, weigh about 30kg, will be able to store around 2.5kWh of energy.
Another idea that might come in the future to complement the solar charging I will be adding to this – attaching a generator/car alternator to a stationary bike in order to charge the power wall whenever you want.
Stay tuned for more updates on my powerwall coming soon!
Project by: Micah Black
Written by: Micah Black