Update 4/21/08: Click here for the latest wiring schematic.

A few weeks ago, I’d posted my simple charging schematic that I’ve implemented in my Artoo. It allows me to switch between normal usage or battery charging. I’m not an electrical engineer by any stretch of the imagination, and my wiring diagram wasn’t very elegant, but the setup worked fine.

Bob Ross pointed out that I really didn’t need to isolate the ground connections in the system while charging. This would save me one relay, and power to run it. He was kind enough to redraw the simplified circuit for me, and also came up with an improved design which allows me to run my droid (low load systems) from an external 12VDC power supply while the batteries charge.

Now that I have my new 60A relays I’m hoping to wire this up over the weekend, but thought I’d share it here first.

[PDF Version]

Thanks again Bob 🙂

I’m using simple 12V SPDT automotive relays in my droid to control switching of power between normal operation and the charging system.

Typically automotive relays are 30A/40A, but I felt I needed something a little bit more substantial. I found these 60/80A relays at SkyCraft for less than $5, which is very reasonable. I’d tried to find something local but came up blank.

The only catch is the terminals are 3/8″, instead of the more normal 1/4″, luckily I found some at Pep Boys for a couple of bucks. I’m hoping to work on getting my electronics re-installed this week and i’ll swap these into my setup at the same time.

It does look like I may end up trying running the Pittman Dome Motor at 24 volts as a test going forward. I can’t see adding a gear at this point in time.

This leads me to my next problem, my battery sub-system is all 12 volts. I’ve had no need for 24 volts in the body as my main drive motors the NPC-2212’s can only run at 12 volts, and everything else is stepped down from 12 volts using Dan’s distribution board.

I know how easy it is to create 24 volts from two 12 volt batteries, which I have and can do with what I have on hand, but configuring in series and then tapping off for 12 volts adds a level of complexity for charging the batteries w/o removing or rewiring each time. There’s also the problem of one battery draining faster.

For those unfamiliar with creating 24 volt from 12 volt batteries, this is a really good site explaining how and some of the problems.

One of the problems he talks about is charging unbalanced batteries in parallel and something called a “Battery Equalizers / Balancers” which can fix this problem. The unbalance comes from the 12 volt tap on a 24 volt system, and the two batteries discharging at different rates.

I’m hoping someone has tried an equalizer in their droid before. I’ve found a lot of information on equalizers, mostly large clunky boxes for RV’s and buses, but there are smaller version out the like this one.

What I’m hoping is to use the 3 12 volt 7amp batteries I’m using right now in parallel, and then combine two of these in series for 24 volts. For charging have a switch/circuit to disconnect/isolate them from anything in the droid and reconfigure just in the parallel state to charge all 3 batteries together.

Tall order I know without causing a short circuit or fire 🙂