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:

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).