Low Voltage Off Grid Solar

Why do this...

I wanted to do something and installing a large grid-tied system wasn't an option.  I realised that for the most part I didn't need to be able to power the really big energy consumers (Fridge/Freezer, etc) I only need my mobile phone, houeshold VDSL and laptop to work in the event of power failures (or load shedding).  These are all low voltage DC.

Most large solar installations work by taking the 12v (or whatever) stored back to 240v and then you use the power bricks/PSU as normal to get down to the DC voltage.  Each of those 2 stages is only around 80% efficient (source: youtube videos).  Going from 12v DC to XXv DC with a buck converter - also aroud 80% efficient - is more efficient and can be done safely as a DIY project.

Solar Panels

I bought 6 145x260mm 12v 7w solar panels and paid around £6.50ea (in 2022 money).

These claim to deliver 583mA ea in ideal conditions, in reality on grey UK day late morning I see less than that from all 6.  The panels are mounted facing south and inclined (see photo on right)

The panels are isolated from the battery with 1N5817 schottky diodes, 1 diode per set of panels prevents the battery dicharging into the panels and (at certain times of the day) the top panels are in sunlight, whilst the bottom ones are shadowed.  Each diode is rated at 1A, and I've never seen a current flow from 2 of these panels abouve that.

Battery

I have a permanently connected 12v 7AH lead acid battery that I simply float charge.  The panels' voltage is pulled down by the battery if they head much over 13v so this solution does not need a BMS. 

I have previously tested this set up with 1 panel outdoors - connected directly to a battery via a diode - over an 18 month period without issue (until the battery was discharged too far and damaged through an accident).

Adding a battery management system and (lithium based) batteries is a possible future step, or using a USB-C PD capable powerbank.

Load

In addition to the 12v battery I'm also charging 7.4v LiPo cells during the day. These are portable and using a buck converter can power my main distribution Switch and Router at home for up to 3 hrs each.

So far I can pretty much charge my mobile phones, digitial camera and drone batteries without the need for mains electricity.

Efficiency

The "marketing" rating on the panels is 7w.  On a reasonable sunny autum day the 6 panels generate almost 14w (from a possibly total of 42w) so 33% efficiency.

Objectively 10-33% during daylight seems acheiveable.

To measure what's happening the cheapest option I found was a TC66C inserted in to the circuit by wiring up 2 USB C (1 male and 1 female) connectors - there is (obviously) no USB PD protocol, but because I'm around 12v it's perfect!