The first version of my Raspberry Pi outdoor setup ran somewhat successfully for around two months but the two car batteries that I used to bank energy have almost completely died (obviously not suited for deep cycle tasks).

The obvious, simple idea would be to get a deep cycle lead acid battery, but those aren’t that cheap and they weigh 20+ kg. While looking for alternatives I noticed certain power banks offered quite good value for money, being much cheaper than buying individual 18650 lithium cells, without even considering the BMS and other circuitry. So I bought the 30Ah and 26.8Ah versions of the Romoss SW30+ on sale from Aliexpress for around 60AUD, which is only $0.3/Wh!1from a quick Google, the market average is around $150/kWh, and that’s for large scale installations Preliminary testing with a 5W LED over 12+ hrs and extrapolating showed that the stated capacity is quite accurate.2here are the numbers if you’re interested: https://gist.github.com/trishmapow/4cb29de94970f4d592d5d6e1e9a580e2

Now the interesting part is somehow linking both powerbanks, the load and the solar panel to ideally run the load while charging both banks (at maximum power).

Each bank has 2 inputs:

  • 5V 2.1A via Lightning (useless for me)
  • 5V 2A/9V 2A via micro-USB

2 outputs:

  • 5V 2.1A via standard USB-A
  • 5V 3A/9V 2A/12V 1.5A via PD USB-A

and a special bidirectional type-C port accepting 5V/9V and outputting 5V/9V/12V.

Technical documentation doesn’t really exist so after some testing I found out that

  • input via micro-USB accepts anything from 4.5-9.5V, which is important as I want the highest voltage to minimise I2R losses from flimsy cables
  • pass-through charging only works up to 5.5V(!), if the bank is fast charging we get no output (sad)
  • the bank will charge via type-C if both type-C and micro-USB are plugged in (important later)

The (not so) basic idea is thus

           |----> buck conv. 9V 18W -> bank 1
solar panels                             |
24V 200W   |----> buck conv. 5V 10W -> bank 2 --> load (<10W)

I’ll be using 2 LM2596 adjustable buck converter modules; they’re pretty cheap and simple to use. When the sun is out bank 1 will charge at full speed while bank 2 will barely charge at all, as most of the power will pass through to the load. At night, bank 1 will top up bank 2 via standard USB-A out to type-C in (this is not active when bank 1 is charging at 9V).

We’ll see if this over-complicated system works when I get the buck modules (part 2).