After building my first semi-portable solar power station and then DIY powerwall, I decided it was time to take my battery building to the next level. I had access to good lithium-ion cells from cancelled ebike projects, but what to make?

Design Choices

A more compact 12V system would be a natural choice, but standard nickel manganese cobalt (NMC) lithium-ion cells are not a great choice due to their voltage range (see note below). A 7S system (seven cell strings in series) works really well with 24V inverters and standard low-cost solar equipment (including charge controllers and automotive-style fuses). 7S it is!

Now that my giant DIY powerwall has been built using parallel battery packs, the question came up about how current is shared between the different packs. Most packs are the same, but the second style of packs has a different capacity, different 18650 cell model, and different number of cells in parallel. What does this mean for current sharing between these packs as they discharge? Let’s find out!

Going into this, I suspected that the current flow from each pack would be proportional to its capacity, because if a pack is rated at X number of amp-hours, that current all needs to be delivered over the course of discharge. While in the end this was correct on average, what I did not expect was the amount of variation within the discharge curve.

To find out how the current is shared between the packs, I took one of each type (13s5p/15.4Ah and 13s4p/12.5Ah), charged them up fully, and put them in parallel. Using a DC load to discharge tells me very accurately the total current coming out of the packs, and a current meter in line with one of them lets me know the fraction coming from each pack. (The two packs must combine to total the current being drawn by the DC load).

Test setup

First, confirming that the current measurements are accurate—they are.

After building my first portable power station, I kept going deeper into understanding how battery, solar, and inverter systems work. Combined with access to a whole pile of barely used (yet on their way to recycling) ebike batteries, I decided to give them a second life and build a DIY powerwall out of them. The term comes from the Tesla Powerwall which is a wall-mounted battery designed to provide backup power to a whole home. What can we make ourselves?

It was time to reorganize my garage to make room for a new running treadmill, and the Ikea “Bror” line of shelving, drawers, workbenches, and storage solutions caught my eye. But from the website descriptions it was really hard to see just how sturdy certain items were, and why the workbench-style units cost so much more than the shelving. This is what I learned! I reviewed the Bror here because I couldn’t find any reviews that covered this type of info.

In normal solar power systems used to charge batteries, the solar panel voltage is higher than the battery voltage, and a traditional charge controller can then be used. Sometimes, you want to charge a battery that has a higher voltage than your panels, though, such as an ebike battery with a small solar panel. For that you need a boost charge controller or boost converter. Very few are available, and the most common seems to be the MPT-7210A "MPPT" charge controller. It can also be used to turn any DC power supply (even a free cast-off laptop charger) into a battery charger.

I say MPPT in quotes because the MPT-7210A does not actually do Maximum Power Point Tracking. But it is still a valuable tool -- it is basically a programmable boost converter that has a setting for the minimum input voltage to use. That way, as it ramps up power and the voltage of your solar panels is pulled down, it does not just keep increasing current and collapse the input voltage to zero like a normal boost converter would do, but instead stops when the voltage is pulled down to your setpoint. See my previous post on DIY power stations for more discussion on how MPPT technology works.

So that we could improve general preparedness and be more comfortable during power outages, I decided it was time to buy or potentially put together a portable power station like is so popular for camping/outdoors and emergencies these days (such as from Jackery or Goal Zero; see portable power stations at REI). They are also awkwardly called "solar generators" because they can be charged with solar panels, even though they are really just batteries with some outputs. I suppose the solar panel is the true generator.

I realized this was actually a perfect opportunity for DIY: commercial systems are quite expensive, and DIY would be achievable since the components are so modular. Can be as simple as battery, inverter, and charger. Let's do it!

I love the solar-powered motion lights I have at my house but recently one started coming on during the daytime, then by night barely had any battery left, so something was definitely not working right.

For fun I decided to open it up with my daughter to explore, and found that to my surprise, the inside was delightfully modular! Ripe for hacking. Maybe other models are similar inside? Let me know in the comments, please. I used the MITAOHOH MT-SL-82-4P which comes in a 4-pack for a shockingly low 20ドル-25ドル.

(In general, it still blows my mind that any company can put products like this together for so cheap -- LEDs, solar panels, batteries, connectors, screws, wires, motion sensor, circuit board, and injection molded housings for 5ドル each, shipped to my door. Wild. That means negative externalities, unfortunately.)