Second-gen portable power station with DIY 24V lithium-ion battery

Finally all the pieces are coming together! With my DIY 24V lithium-ion battery complete, it’s time to put it all together into a new portable solar power station and retire my original one. The original power station used a lead-acid AGM battery (60lbs alone), so was not very portable. This new one is more than double the capacity, less than half the weight, and cleanly packaged.

The goal for this project is to make a battery station that is always active and collecting power from my solar panels. This means the DIY powerwall can stay dormant most of the time for maximum safety. Once the solar battery fills up, I can dump it into the powerwall all at once with a boost converter (to go from 24V to 48V), or at any time use it to charge up my ebikes (generally also with a boost converter).

The business inside! Solar charge controller, fuse block, and power input/output ports. More details below

Access lids open for normal use of the BMS status and DC-DC boost battery charger

The power station in all its glory

Reminder of what the battery inside is like -- assembled completely by me from reclaimed Samsung 21700 cells. 7S18P, 90Ah, 25.2V nominal ("24V").

I had a random old toolbox that I planned to use, and even sized the battery to fit right inside. Including a solar charge controller, fuse block, and ebike charger, I think I can just manage to squeak it all in. 

Power station schematic / overview planning sketch

Key features and components:

• Input and output:
• XT60 female panel mount
• XT90 female panel mount
• Made myself with a regular anti-spark female connector and this 3D-printed part because off-the-shelf anti-spark female connectors do not appear to exist
• Charge / Output Module, switched by the solar charge controller’s programmable load outputs:
• Boost charger MPT-7210A (see my writeup on performance)
• 2x USB-C 65W PD
• 2x USB-A fast charge
• 2x USB-A plain old ports
• Cigarette lighter socket
• XT60 female
• 5521 DC port
• Battery
• 2.2kWh 24V 7S lithium ion
• with BMS display
• Solar charge controller — Renogy Rover 20A (100V max input)
• Fuse block and Wago connectors for distribution
• Solar panels -- 2x 100W panels that can be folded into a more-portable suitcase -- more on that build in the future

About wall charging:

While my normal usage charges exclusively with solar, it is also possible to charge via a wall charger. I have a dedicated 7S lithium ion charger which can just plug directly to one of the battery ports, but it is very low power (only a few amps). A bench power supply with constant-current feature could also do the same thing.

But a clever and powerful way to charge from the wall is to use cheap/free normal DC supply connected to the solar input of the MPPT. The trick here is that since most power supplies, like a laptop power supply, do not have constant-current ability, if connected straight to a battery to charge it they will just shut down due to overcurrent. The MPPT effectively adds a current limit to a DC source!

When using a relatively "dumb" MPPT like the Renogy Rover, you have to provide a big enough DC supply to max it out; there is no option to set a lower current limit. So for 20A max output (on the 20A version of this MPPT) with a 100V input limit, I can actually put three big laptop power supplies (free from the office IT recycling bin!) into series with each other. If each one operates at 19V and 11A before shutting down, three in series gives 11A @ 57V (627W). With the MPPT outputting 20A peaking at 29.4V (588W), the laptop supplies work their tails off but it works! And the charging is fast for a large non-cooled DIY battery -- 20A on a 90Ah battery means flat to full in ~5hrs (about a 0.22C rate). 

Note that my first-gen system had a PWM solar charge controller which behaves differently, so cannot be used with a standard DC power supply. A PWM charge controller will not limit current until it is in the final stages of charging and the PWM functionality is actually occurring. Until then, it's just a straight shot between the power supply and battery.

Power output ports

Inside the power outlet box that is fed by the load output of the solar charge controller. USB PD, XT60, 5521 barrel, and standard 12/24V automotive port.

View of early assembly into the toolbox. Note the complicated spacing to get the boost charger spaced down to clear the outer lid of the toolbox!

The total weight for the system is 32.2lbs, blowing away the competition from commercial products (the Anker SOLIX C2000 Gen 2, for example, weighs 41.5lbs). Those others are of course more polished and sturdy and include inverters, but I win for Wh/kg.

What more would make this system better? Let me know in the comments.

Cheers,
Mike