24V Lithium-Ion Battery Build for Solar Power Station

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!

Battery build in process, showing the spot welder

I could also have made a 48V portable system to match the powerwall, but I figured it would be less compact and less appropriate to directly power portable equipment. Also it would need more/larger solar panels to reach a supply voltage higher than the battery (to use standard charge controllers). 

Note on nomenclature: lithium iron phosphate (LFP, LiFePO4) cells are also technically a type of lithium ion, but when people say “lithium ion” they generally mean NMC. LFP cells have the advantage of less volatility, but lower density (volumetric and gravimetric). 

Note on lithium ion usage for 12V systems: since the typical cell operating range is 3-4.2V, a 3S system would give 9-12.6V and 4S gives 12-16.8V. This might be fine for some specific 12V loads, but inverters generally have an input range no wider than 10-15V (unless you splurge on a Victron). So no matter which you choose, there is 20-40% capacity loss by not being able to take full advantage of the high or low end of the voltage range. 

Components

My available cells were Samsung INR21700-50E. Each one with a capacity of 5Ah (18Wh) and max continuous discharge rating of 10A. To fit perfectly in the toolbox I was planning to dedicate to this project, I chose a grid of 7S18P. With 18 cells in parallel, that’s a total of 126 cells, 2268Wh. That’s bigger than all but the most expensive portable power stations!

Comparison of cell sizes: from left to right, 21700 (21mm dia x 70mm long), AAA (10.5mm x 44.5mm long), and 18650 (18mm dia x 65mm long)

I need cells... lots of cells.

Key components to build a battery (this is what I actually used), in addition to the cells, of course:

• Cell holders
• Nickel strip, 0.15mm ladder-style
• Insulating rings (bonus safety insurance)
• Daly 7S 60A BMS with Bluetooth and display
• 8AWG wire (this is a good brand; unfortunately not what I used)
• 10AWG butt connectors (my 8AWG wire fit in these but maybe because my wire wasn't as thick as claimed. Same brand also makes an 8AWG version.)
• Kapton tape
• Big pack heat shrink 
• Spot welder

For spot welding, I inherited a Sunkko 709A. It works well enough for 0.15mm nickel with the settings maxed-out. If I was buying one fresh for light-duty use, I like the ones based on super-capacitors rather than LiPo pouches (which seem to wear out quickly), though I haven’t tried one yet. The DIY community seems to swear by the kWeld for more serious usage. 

And make sure that the nickel strip and the cell holders chosen have the exact same spacing. Mine were slightly off, but I was just able to make it work across 18 cells.

Let’s build it!

Left to right is 18 cells in parallel, top to bottom is 7 cells in series: 7S18P

First ever spot welds. I'm sure they can be improved but they will work!

Soldering the main positive and negative connections before spot welding

For the main positive and negative connections, the easy way is just to solder a wire on the corner; many commercial packs even do that. But for way better current-carrying capacity, I can solder the wire all long one side of the nickel strip ladder BEFORE spot welding it to the battery. This avoids excess heat into the cells. Then fold it over to be compact. 8AWG wire plus effectively two layers of nickel the whole length of the battery makes for lots of ampacity.

BMS monitoring/balance leads along the side

Completed battery assembly with layers of Kapton tape to avoid shorts

BMS attached and measuring the total capacity with an electronic DC load.

Working solar power system! Renogy Rover solar charge controller (black box on left) and MPT-7210A boost charger (green box on bottom) for charging other batteries/bikes.

I kept the solar power system running with the battery on my bench top for 6months until I finally sealed it up and finished the power station. I thought maybe I would need to tweak something but no! Ultimately the construction is straightforward.

More about the full portable system in the next post. 

How would you improve this battery?

Cheers,
Mike

Load Sharing Between Parallel Lithium Ion Battery Packs

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.

Showing 4A flowing from the power supply (top) to the DC load (middle). Fluke meter (bottom) is in-line and agrees. Clamp meters (right) are always a little off; this one is decent.

How to build a DIY powerwall using parallel ebike batteries

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?

Completed 48V parallel battery system showing 16 installed packs (of 20), charging at 18.6A

What's the difference between Ikea Bror shelving and workbenches?

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.

Fully assembled, expertly modeled, but poorly organized Bror storage system

How to get the most power out of the MPT-7210A boost charger

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.

The MPT-7210 boost charge controller

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.

DIY portable solar generator power station

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). 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!

Completed solar generator power station in a milk crate. Battery, charge controller, inverter, charger, 12V outputs, USB ports, and zippered pouches for cables and accessories

Fixing a broken solar-powered motion light

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.

Success! Battery-powered light now running from a wall USB charger

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-$25.

(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 each, shipped to my door. Wild. That means negative externalities, unfortunately.)

Sad, dim solar light on during the daytime where it can't help anyone