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Author: Lachlan

Energy Arbitrage!

Energy Arbitrage!

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It’s been raining quite heavily for 2 days, and this evening the battery was down to only 3% state-of-charge. The lights in most of the house run off-grid from the battery, and so I needed to get them some non-battery power. Luckily I’ve recently connected the grid input to my inverter. It was time to test the “bypass” mode where the inverter feeds grid power through to the loads.

Grid “bypass” through to supply the load, and also charging the batter at 1.71 kW.

It works beautifully, and interestingly in bypass mode the inverter seems to provide a charging trickle of about 1A to the battery even when configured not to charge the battery from the grid. After 10pm when my electricity switches over to cheap off-peak rates I got to test one more inverter capability for the first time: grid charging of the battery! With numerous packs now in my battery build I was able to configure the utility charging current up to the max setting of 60A. This means charging at about 1.7 kW.

The wet and cloudy weather is forecast to hang around for a few more days at least. This grid charging feature means that I can play the merry game of electricity arbitrage: store up cheap off-peak electricity from the grid to use later during expensive peak-pricing times. It’s not quite as good as my own free off-grid solar, but it’s still a way to save money – as well as helpfully “load shifting” to make the grid run more smoothly.

New location

New location

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The MPP Solar charge controller log shows a total of 300 kWh of solar energy was harvested at my old address, providing 200 kWh output. The discrepancy might come from the stored electricity I used at 24V battery DC level before the inverter.

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.

Ground mounting is a convenience shortcut for now.

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.

Expanding utility

Expanding utility

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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.

Three output circuits to expand my off-grid system.

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 “Solar Off-Grid” power point in my shed lets me charge my lawnmower from the sun.

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.

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.

More packs!

More packs!

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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.

More packs coming!

More packs coming!

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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.

Enough good cells for another half-pack string.

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.

63 cells making the first “poor” string of half-packs, ready for colour sorting.

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.

Tech devices as a preliminary usage challenge

Tech devices as a preliminary usage challenge

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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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Surprisingly good pack balancing

Surprisingly good pack balancing

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One of the notorious details of lithium batteries is the need to keep all of the cells in balance. With a string in series, the total voltage can stay within safe limits but individual cell-level voltages can be dangerously high if other cells are correspondingly low. The trick is to keep all of the cells “balanced”, and that is what LiPo chargers have to do actively. Inside power tools and laptops the balancing is handled behind the scenes; for phones there is no need since they use single cells.

The “cell-level” voltages that matter in my homebrewbattery are the packs that contain 10 (soon to be 20) cells in parallel. I divided the individual cells up into packs to try and achieve similar capacities for all seven, but it was not possible to achieve perfection. Even then, the capacities I measured were at a 1 A discharge current – and actual performance of each pack will also depend on other parameters such as internal resistance. I had no way to predict how well the packs would stay balanced after a few charge-discharge cycles.

In the long term I want to get a nice battery monitoring system that takes care of balancing, but my design goals are to start cheap and bootstrap the battery. To keep with the data-focused approach that has proved interesting so far, I decided to simply log the pack voltages occasionally to “manually monitor” the state of balance. My target was to have the packs stay reasonably balanced for a week at a time, and if that was possible then I’d use a hobby charger to do an active balance every weekend. Here’s the data so far:

After nearly a month the packs are still almost indistinguishable from each other in voltage – the variation is within bounds of +/- half a percent! During October 10-12 the packs became a bit more unbalanced, but that corresponded to the deepest level of discharge during the experiment so far (a week of cloudy days and storms made it hard for the solar charger to keep up with battery use). A more prominent imbalance at deeper discharge is not surprising. Imagine a set of buckets of slightly different size that start full of water, so that they are all at “100%”. If exactly the same volume of water is removed from each bucket, then there will be differing amounts of water left in each. When most of the water is gone, there may be a bucket which is essentially empty and one which is still 20% full – the imbalance will appear enormous. Filling the same quantity of water back into each bucket will return them all to 100% full and exactly balanced. This shows up for my battery, since the packs returned to closer balance when the battery charged after those darker days.

It is interesting to ponder what the “safety threshold” for imbalance might be. The PCM 60x charge controller is configured to reach a maximum battery voltage of 28.6 V, which is about 4.085 V per pack. I’m deliberately stopping short of the 4.2 V upper limit for LiPo cells, because this provides a safety buffer and increases the longevity of the cells. If a pack was undervoltage and another was overvoltage by compensating amounts, the dangerous point would be when the high pack went past 4.2 V. This extreme situation corresponds to the packs pushing past an envelope of +/- 2.8%.

This chart is live – and I’m adding it to my statistics page. Follow along with me as I wait for imbalance to creep in – but at this rate I’ll be fine until Christmas!

Almost 1 kWh in a week

Almost 1 kWh in a week

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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.

Solar charging on a rainy day

Solar charging on a rainy day

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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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