HomeBrewBattery

Just as I was getting the battery system nicely set up I moved house and had to dismantle it for transport. It’s been sitting as separate components for the last 3 months, and over the Easter long weekend I have re-assembled it to a functioning state.

The MPP Solar charge controller logs the solar generation and the electricity output from the inverter, and apparently I’ve used 200 kWh of my home-harvested off-grid solar energy! I’ve reset this to get a good understanding of how it performs at my new house.

One of the slowest things in my original installation was getting the solar panels up on the roof. This involves fiddling with roofing tiles and maneuvering the panels over awkward roof edges, and planning the process deterred me many times. I now have to attach the panels to a different roof, and I’ve been told it will be trickier because terracotta tiles crack more easily than the cheaper concrete ones. Space on the ground is an exciting upside of our new property, and so I’ve got things up an running with a temporary panel installation at a thoroughly accessible altitude.

You’ll notice 8 panels in this array, which is more than the 6 panels I bought at the start of this project. This is a second string that I picked up for a bargain over the summer, and I’m testing them before committing the effort to get them on the roof. In fact, I am playing with the idea of making these into a “solar carport” over this section of driveway.

Although its taken a lot of time to build some of the “infrastructure” of my battery installation, I have been steadily building new packs. With the full 6 solar panels up on my roof now, and a bit more storage capacity in the battery, it was time to make the bottled sunlight do more work.

I have installed a small fuse box to make it easier to distribute the 230V AC to different places. The original power point in my bedroom which has been hard-wired to the inverter output is now just one of three distribution circuits. The second one goes to the kitchen, and is now powering the fridge and internet modem/router.

The third goes to my basement “shed”. As soon as the battery is large enough to deliver a sustained 1.8kW load, this powerpoint will be used to charge my electric car from the sun! For now, it is useful in that location to charge workshop and garden tools. This is close to where I take the power for the chargers I use in cell testing and characterisation, so I’m also close to being able to run that whole process off-grid. Even these few extra outlet points open up so many exciting new ways to benefit from my solar battery system.

Late last year I bought some used 18650 cells that had already been somewhat tested. This has dramatically increased my “good” cell count in recent weeks, and I’ve been able to put the second pack halves on the first good string – taking my operational battery to about 1kWh in capacity.

In the design process I’ve dropped from 20 cells per full pack down to 18 so that the packs are short enough to fit double deep in my battery cabinet (keep an eye out for a design discussion post coming soon). Obviously this helps reach the pack count sooner, but processing pre-tested good cells really slowed my accumulation of “poor” cells.

Well, on the weekend I simultaneously reached the next half-pack set of good cells (this is how I construct the packs so it makes sense to count in this quantity) and the first complete half-pack of poor cells. When I get these soldered up the modular design of my battery will be visible.

This evening I plugged my laptop into grid-power at home for the first time in more than a month. I’ve been putting my homebrewbattery to work in a test challenge to see if I can use stored solar energy to charge all of my tech devices. For about 5 weeks I’ve been using my solar-charged battery exclusively to charge my laptop, phones, tablet, cameras, and headtorches. In that time I’ve charged almost 5 kWh into the battery from my poorly-placed solar panel – so I’ve saved more than $1!

To make this challenge more achievable, I wired the battery inverter into a power outlet inside the house. Since my inverter is a small cheap one that is more intended for caravans, it has a noisy fan and I didn’t want to leave it running continuously. I used a power socket with an extra switch, that was wired in to remotely turn the inverter on and off. That part of the project worked superbly. …

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My rudimentary logging has been working pretty well, and I’ve accumulated a bit more than a week of continuous solar charging data. It hasn’t been the best time to do it – after months of drought we’ve had some seriously cloudy (and rainy) weather! Despite this, I’ve been able to put 963 Wh of solar-charged energy into my battery over the last 8 days.

Frustratingly, on October 7 my logging script hung and didn’t exit at the end of the day – so I lost that day’s data. Still, 963 Wh is 1 kWh when stated to 1 significant figure. While it’s tempting to say that I’ve saved a full 24.22 cents, that’s probably a slight exaggeration! Since my logging data is coming from the charge controller, it is measuring what is going in to the battery. If there are any self-discharge losses in the battery itself (I think this is negligible – data coming soon), or any conversion losses in the inverter (almost certainly around 10% losses) then this ~1 kWh of charged energy will correspond to slightly less “useful” energy in my devices.

Still, its probably a solid 20.99 cents or so. Only another 2400 weeks to pay off the solar panels and charge controller at this rate! Yet again the conclusion is obvious: I need to increase my battery size.

When I logged the second solar charge last week I had the computer plugged into mains power and the homebrewbattery inverter was switched off. There was basically no load on the battery, and that was visible in how flat the battery voltage curve was when the charging was finished. Today I raised the stakes and left the battery powering the laptop that was performing the logging. On top of this it was a rainy day with substantial cloud cover. Would the charging keep up at all, or would the laptop drain the homebrewbattery too far? …

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I wasn’t sure whether the small 0.1 A charging current yesterday afternoon was due entirely to the shade on the PV panel or whether it had something to do with minimum input requirements for my PCM 60x charge controller. On paper, the 265 W solar panel should be able to charge my 500 Wh battery in two hours or less.

I left it all connected while I was at work today, and checked it this evening. The battery voltage has risen from 26.1 V last night up to 28.3 V! That’s quite close to the 28.6 V cutoff that I programmed into the charge controller, which suggests that the solar panel essentially gave me a full charge. How exciting!