Sunday 2 September 2018

17 weeks - on shadows and cables

17 week update. Weather-wise, we've started to see some signs of Autumn approaching - people have talked of _putting the heating on_, and backpacks tend to contain a mix of sun cream, and waterproofs.

That said, it's still mostly warm, if cloudy, and the sky is having a blue period this weekend, so should be good to get a full charge up. Saturday has been very warm indeed.

However, warmth for us doesn't necessarily make for good charging. The main difference in noticing now is in shadows - it takes longer in the morning for the sun to reach my standard charging slope, and it leaves it faster in the evening. There are also extra shadows passing over during the day - the long fingers of chimneys now reach further than in the summer. And the solar panels seem to perform best when they are completely free of shade - a small patch can drag the power down quite a bit.

The importance of cables

A few weeks ago, I ended up reading about USB charging cables for an evening, which descended rapidly into the basics of electricity and resistance. This was kicked off by seeing some claims of "fast charging ports" on USB power meters (more on these below) - these turn out to remove 2 of the 4 connections within a USB connection, namely the 2 which transfer _data_ instead of _power_.

In cable terms, removing these 2 data wires generally means that there is more physical space for the power lines. Bigger power lines means less resistance from the walls of the cable, which means more _current_ and faster electricity transfer.

This rate of transfer is really important as charging gets harder, as above - we need a faster flow rate to fill the same battery in less time. Not only are there fewer sunny days, each day gives us less charging hours. (And, I assume, the Sun is weaker than in the height of summer?)

Generally, wires are about "28 gauge", but fast-charge cables (usually supplied with tablets with larger batteries) are "24 gauge" - see this blog post for a good explanation. I was about to order some new cables, but checked quickly, and the Syncwire cables I have already seem to do the job. (The Amazon page explicitly says they do full transfer speeds up to 2.4A.) So woo.

Length matters

Today I also realised I have a really short USB cable that came with one of the batteries - as in, 10cm rather than my 1m Syncwire ones. In theory, this should improve the amps too, as less length means less resistance as well.

So I plugged it in and measured it - wow! What a difference! No clouds to affect measurements - the 1m cable was giving me about 0.8A, while the tiny wire pushed that up to about 1.05A, so an extra 25% or more power.

The disadvantage, of course, is that it's harder to arrange your battery so that it's out of the sun, but I find it's usually OK to tuck it behind the panels anyway. I'm going to have to rethink my cables very soon - it really feels like maximum efficiency is the only way to go, with the days getting shorter so quickly.

Bring on the equinox!

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