Portable phone charger

Hello! Been a while, hasn't it? Sorry about that, but I'm back now. The piano synth articles will continue soonish, I've been having lots of fun getting my old code working in new environments, and I should be able to write it all up soon.

Meanwhile...

I've been playing a fair bit of Ingress lately. It's an alternate-reality game, meaning you have to get off your behind and travel around to places in the real world in order to play the game. Good for exercise, but the problem is that even on my Nexus 4 with its lovely amounts of battery life the game burns through power at a staggering rate, from fully-charged to flat after only 2-3 hours of continuous play.

So I thought it might be worth buying a portable battery charger, essentially a high-capacity LiPo battery with a USB-compatible output. Then I looked at the price of them. Not cheap, not really an impulse purchase.

I estimated that the bulk of the cost would be in the battery itself, and I already have some high-capacity rechargeable LiPos floating-about in the form of RC airplane batteries. Perhaps I could do something with them? It turns out I can!

My RC plane batteries happen to be 12 volts or thereabouts (for the curious, they're 3S - meaning 3 x 3.7V cells in series), which is the same voltage that car cigarette lighter sockets provide. So, if I can hook-up an RC plane battery to a lighter socket, then in theory I can use a car cellphone charger to recharge my phone on the go.

Here are all the parts laid-out:

1. 1800mAh 3S 11.1V LiPo RC plane battery. I already possess a few of these so I didn't pay anything, but a new one is around $12.50 at the moment. 
2. Flexible electrical wire. I chose 14 AWG just because I had some already and it matches the battery's wire, though I don't actually need that gauge, not at the discharge rates we'll be dealing with. I probably used about $0.50 worth of wire, at a guess.
3. A car cigarette lighter socket. Cost: $1.50.
4. A male Deans connector. I use Deans connectors for all my RC batteries so I just need a male plug to connect to the battery. The real thing comes in packs of two for about $3.50, but you can also find bulk packs of cheap knock-offs for much less.
5. A car cellphone charger. This one cost me $5.80, a real bargain!
6. Heat-shrink tubing. I'm not even going to bother pricing this one, for the amount I used it's less than a cent.

Total cost, somewhere in the region of $10, plus $12.50 for a new battery.

Here's how it all goes together, step by step.

Step 1: Solder the two wires to the terminals of the socket. The positive terminal is the centre pin, while the negative is the outer wall. Following convention, I used red for positive and black for negative. I'm not brilliant at soldering, but it does the job.

Step 2: Tie a knot in the wires and slip the end cap on. The knot is there so that if the wire is pulled too roughly it won't damage the solder joints.   

Step 3: Screw the end cap onto the socket, and then use some heat-shrink tubing to protect the place where the wires come out.

Step 4: Cut the wire to a sensible length - I chose around six inches - and then solder the Deans connector to the other end.

Note: don't forget to slide your heat-shrink tubing onto the wires BEFORE you do this, otherwise you'll have to desolder them and do the whole thing again and feel like an idiot. I speak from experience ;-)

Using a hairdryer to shrink the tubing onto the solder joints:

And that's pretty much all there is to it! Here's the whole thing connected-together and charging my phone. I switched-off the flash for this shot so you can see the pretty blue glow from the charger:

A close-up on the phone screen, to show it's really charging:

And that's how I spent my evening.

Going further, I have a low-voltage alarm on its way to me from somewhere in China, which attaches to that spare white connecter dangling from the battery. The reason for this is that LiPo batteries really hate being discharged all the way, it pretty much just bricks them, so you always want to leave some charge in them even when they're "empty". I'll program the alarm with a safe minimum voltage and it buzzes when the battery gets down to that point, reminding me to disconnect. It also has a cute LED readout to display the current voltage so I can see at a glance how it's going. 

The batteries I use are 1800mAh, which is a measure of how much charge they store. If I want to carry more weight and spend more money I can go up to larger batteries, 5000mAh and beyond. But I prefer these because their main use is flying planes where weight is important.

My phone has a 2100mAh internal battery but that's only 3.7V, so because the RC plane battery is 3 times the voltage it's actually equivalent to about 5400mAh at 3.7V, or another 2.5 full charges. As I already stated I can't use all of that power, some needs to remain in the battery to avoid damage. I don't yet know how much that will be, so I'm going to have to wait until I have the alarm to know how much charge I can get out of it. 

My colleague Ken who knows far more than I do about phone chargers, has suggested that this kind of switching power supply is about 80% efficient at converting electricity from 12V down to the 5V the phone needs, and so I expect I'll actually see about 1 to 1.5 full phone charges from a single fully-charged RC plane battery, which ought to give me up to about 6 hours' autonomy in Ingress before I have to go home to recharge, or about 9 hours if I carry two batteries with me.

Of course, my family might have something rude to say about it if I decide to spend all day out playing video games...