Monday, 4 February 2013

Even further down the rabbit hole... An Arduino solar regulator

So, I'm accepting my batteries are dead. I will need new batteries. This brings about two questions:

  1. What batteries do I go for this time, and how many?
  2. What can I do to prevent these batteries from dying prematurely?
I've been umming and ahhing about going for 6v Trojan T-105 'golf cart' batteries for a a while now. In fact searching around, I actually found a thread I wrote when my last battery set died. I also found a great rant on batteries and solar panels that makes a lot of sense. See the last posts.

In the thread that I wrote on the SBMCC 2.5 years ago I also wondered about point 1 above:
Of course, then the question is.... what to replace them with  Another set of Exide Portapowers? Pretty cheap (£0.90/Ah), and lasted 3 years. Or step up a price bracket and go for Elecsol EL125's (£1.04/Ah) which are slightly higher capacity, but come with a 5-year warranty as opposed to the 2 year one of the Exides. 
Or even go a bit more exotic and go for Trojan T-105s (6v wired in series) would be £1.09/Ah. I think they might be a bit too tall for my battery space though, and as 6V, I'd have to go for 2 or 4 of them... and I kinda wanted around 330Ah.
So based on the last two posts, I think I'm going to go for a pair of 6v Trojan T-105 batteries. Wired in series they will give be 225Ah (C/20) at 12v. This smaller than my current bank of 345Ah, and my previous bank before that of 460Ah. But my plan is to get another one (or two) solar panels on the roof.

But this brings back an even bigger question about solar regulators.... can my current solar regulator charge at the higher 14.8v? Answer: No. It is a Stecca PR3030 and charges as 14.4v for flooded lead acid batteries.

I originally bought the controller as it had a nice State of Charge display on it with a bar graph and smiley faces. Yeah, I'm a sucker. It also has the ability to measure the current in/out of the batteries and keep a running total of Ah put in to the battery and Ah consumed. There are however two big drawbacks that I only discovered when I got in and started to install it many years (5? 6?) ago.

  1. The nice pretty State of Charge display can only work if solar is the only charging source. As I also charge from a Sterling B2B charger (from the van engine alternator) this mode can't work. So I have to put it in 'voltage' mode, which just gives a plain old voltmeter and no smiley faces :(
  2. In order to count the current going in/out, you have to wire all of you load negatives via the controller. This was a pain, and has resulted in un-necessary cabling and voltage drops. The van is metal bodied, and the body is grounded so in many cases I don't need a return wire back to the battery as I can ground the the chassis/body nearby.
So given all that, I'm not really getting much out of this controller. This battery monitor I'm building will give a much better reading and log of current going in/out of the battery bank and its voltage.

So... the next obvious question.... should I get a new/different solar charge controller?

....or actually.... can I BUILD one? ;) Given that all the controller has to to is take a voltage that may be significantly higher than the battery (17-19v) and drop it down to a suitable charging voltage (14.8v for bulk) and hold it there... couldn't I do this with an Arduino? How about the Arduino that I will already have connected to the battery bank doing the monitoring? Mwahahahahah! A cunning plan is forming and I'm heading yet even further down this rabbit hole of an experiment.

Luckily... as with much of the Arduino world (and why I love it do much) I can stand on the shoulders of others. A quick search around and I found someone already did something like this:

And yes, it is as simple as it looks. It just switches on the current from the solar panel to the battery (via a relay, or transistor) and switches it off again when the battery reaches the right voltage (e.g. 14.8v). Pretty simple.

EDIT: Or I might want to use something like a P-channel power MOSFET such as this with very low resistance across it. Someone has a great blog on using a MOSFET for a solar controller. Not quite as simple as I originally thought, but should be still easy enough.
EDIT 2: Or even better, use a shunt regulator system such as this.

This also has another benefit. One potential problem I have with my current setup is that the solar regulator is not that close to the batteries, it is up at head height and near the solar panel. This means there is potential voltage drop between the panel and the battery. Especially as the cable between the two also carries the load current as well. By putting the solar regulator next to the battery I minimise the voltage drop between the controller and the battery (or rather, I am able to sense the battery voltage at the terminals not several meters away at the current controller's location). It doesn't matter too much if there is some voltage drop from the panel itself as that is going to potentially be at a higher voltage anyway so can 'afford' to lose a few tenths of a volt.

No comments:

Post a Comment