New Electronics Project – Power Box V2

Posted by under Electronics, on 20 June 2013 @ 10:41pm.

Just a quick note before I begin, the Peugeot 306 I mentioned in my last post was sold last week. I managed to make back what I paid for it so it was no loss for me at all besides the time and worry spent over it. Still, you live and learn from your mistakes. There is one thing I have to note however. I sold the car through Auto Trader, and as soon as I signed up, less than a few hours later I had interest in the car. This was great, but then you read the e-mail and realise it was a scam. I got 2 e-mails both sounding very similar; A person needs a car for their son/daughter, but they are going away for 6 weeks and can’t view it. They will send payment via PayPal and they’ll pay the costs, and have a ‘shipper’ collect the car once payment is made. It all sounds perfect. What people don’t always realise is that they will then open a Paypal complaint and claim their money back, and there is little you can do to stop it. Auto Trader recommend you ignore these types of e-mails. I actually edited my advert to say phone calls only and cash on collection, as the scammers won’t call you or deliver the cash, so they ignore the ad.



The (incomplete) circuit for the powerbox v2. Left: Arduino. Right: 2x 5v @ 3A Switched Mode PSU’s.

Right, onto something more interesting. I decided earlier in the year to make version 2 of my power box (v3 if you include it just being a box). This one will be updated to include a digital voltmeter based on the Arduino. This also allows me to put in various other things on the screen too such as current draw, solar charge current, temperature, etc. I’m also upgrading the USB sockets from an LM2575 to the LM2576 which is a jump from 1A to 3A output. I’ve decided to split the 3A between 2 USB ports (perhaps 3, I don’t know yet, most likely 2 though) so that it can handle tablet PC’s and more modern phones. Some tablets won’t charge unless they get at least 1.5-2A of power and you never know, someone might want to charge one.

I’m also going to wire up the inverter to a proper socket this time and have a power switch on the front so you don’t have to mess about with opening the box to use it. This was a massive pain and had the risk of it being wired up incorrectly too.

The biggest change however is that I now have a 10w solar panel to charge the battery when we’re away. It is small enough to fit in the box but powerful enough to offset the use of the battery. It won’t fully charge it if we’re using it, but it will help stop it going flat as quickly. Last year we flattened it in the week (it was as dead as dead gets the day we left). This damages the battery if you do this so I would prefer to keep it topped up by use of solar. Here is a bit of maths…

The battery is 90Ah (so you can draw 1A for 90 hours until it’s dead, theoretically).

The solar panel, on a good day will output 3Ah (0.6A x 5 hours good sunlight) which is about 36Wh, plus perhaps a bit more when it’s indirectly in sunlight. I intend to roughly calculate this with the arduino’s solar input current.

Assuming a phone battery is 5Wh, I can charge roughly 6 phones per day from solar alone without using any battery power. This is assuming the solar panel does output 0.6A for a minimum of 5 hours and the system is 100% efficient.

Of course, it’s never perfect and when you want solar power the sun goes in, so these figures will likely never be reached. I won’t know for sure until it’s tried out for real.

Regardless, it’s better than having no solar panel at all. Over the course of the week, I could put 20Ah of power into the battery from the solar alone, and if we use what we used last year then it’s a win win situation. I do of course have use of the car if it does go flat. A 15-30 minute blast on the jump cables will sort it out no problem.

Anyway, I’ve been ordering parts for the project this evening so I can hopefully get it completed before camping this year. If not, I still have the old faithful box from last year instead which works just great. I’ll keep the blog updated with developments on this as I go along.