Ryobi Adapter
They sell these, but they're underpowered. I'm going to build mine at 20V, that's what the 18V battery measures when it's charged.
I'll start with a 24V transformer. I found a MC33167 that might get it down to 20V (rate 5A). LT1270ACT is rated at 10A
this guy used 20 amps. I found this on Amazon, which seems like it would work pretty well.
Deep Cycle
The other option I'm considering is a deep cycle battery. The catch is that I can't get one near 20v, so I need a boost converter. But I can just double the voltage. The deep cycle battery would probably be around $25.
http://www.electroschematics.com/648/555-voltage-doubler/
http://www.circuitsgallery.com/2013/04/automatic-battery-charger-controller.html
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Transistors need to be TO-3 packages.
- Battery $25
- NPN power transistor $2.50
- PNP power transistor $2.50
- Heatsink $3
- Caps $3.75
- 12v adapter $5
- 12v relay $3
I'm also going to get the etching supplies, makes more sense to buy in moderate bulk
- Boards: $12.50
- Ferric Chloride: $11
Updates
I've ordered the parts and I'm still waiting on a few. But I realized a small problem. The dc adapter I bought is for 12v, but the battery actually needs to be charged to almost 15 (at least 13.3v). Meh... I just ordered a new one. Additional $8, plus I have 9 extra sockets (and those sockets are common).
Build Notes
I thought I had 10K pots, but I only had 5k, so I wired two together (on the breadboard). Ordered some 10k.
V2
I got a first pass of this working, but I never got it assembled. I also bought a high power ACDC adapter. For the next version, I'd like to put everything into a single box.
It will have 4 modes: charging, battery powered, wall powered, full off. When it's plugged in it's always charging (or maintaining charge). When it's not plugged in, it's either in battery power or full off.
I have a volt meter, I'd also like to hook that up so that it's always on when plugged in, but I can power it with a button when it's not.
I'm going to run everything off an Arduino. The old version used a relay, I've got power MOSFETs coming, I'd rather use those. They can also be driven with PWM to limit current.
Tests to Run
- Does the volt meter pull any current (sensing) when un-powered?
- Test configs for a full power sensor on the Arduino
- Test configs for a proportional charge.