Jun 252017

So, having got the rack mostly together, it is time to figure out how to connect everything.

I was originally going to have just one battery and upgrade later… but when it was discovered that the battery chosen was rather sick, the decision was made that I’d purchase two new batteries. So rather than deferring the management of multiple batteries, I’d have to deal with it up-front.

Rule #1 with paralleling batteries: don’t do it unless you have to. In a perfect world, you can do it just fine, but reality doesn’t work that way. There’s always going to be an imbalance that upsets things. My saving grace is that my installation is fixed, not mobile.

I did look at alternatives, including diodes (too much forward voltage drop), MOSFET switching (complexity), relay switching (complexity again, plus contact wear), and DIY uniselectors. Since I’m on a tight deadline, I decided, stuff it, I’ll parallel them.

That brings me to rule #2 about paralleling batteries: keep everything as close to matched as possible. Both batteries were bought in the same order, and hopefully are from the same batch. Thus, characteristics should be very close. The key thing here, I want to keep cable lengths between the batteries, load and charger, all equal so that the resistances all balance out. That, and using short runs of thick cables to minimise resistance.

I came up with the following connection scheme:

You’ll have to forgive the poor image quality here. On reflection, photographing a whiteboard has always been challenging.

Both batteries are set up in an identical fashion: 40A fuse on the positive side, cable from the negative side, going to an Andersen SB50/10. (Or I might put the fuse on the negative side … haven’t decided fully yet, it’ll depend on how much of each colour wire I have.) The batteries themselves are Giant Power 105Ah 12V AGM batteries. These are about as heavy as I can safely manage, weighing about 30kg each.

The central harness is what I built this afternoon, as I don’t yet have the fuse holders for the two battery harnesses.

The idea being that the resistance between the charger and each battery should be about the same. Likewise, the resistance between the load and each battery should be about the same

The load uses a distribution box and a bus bar. You’ve seen it before, but here’s how it’s wired up… pretty standard:

You might be able to make out the host names there too (periodic table naming scheme, why, because they’re Intel Atoms) … the 5 nodes are on the left and the two switches to the right of the distribution box. I have 3 spare positions.

In heavy black is the 0V bus bar.

This is what I’ve been spending much of my pondering, doing. Part of this harness is already done as it was installed that way in the car, the bit that’s missing is the circuit to the left of the relay that actually drives it. Redarc intended that the ignition key switch would drive the relay, I’ll be exploiting this feature.

Some time this week, I hope to make up the wiring harnesses for the two batteries, and get some charge into them as they’ve sat around for the past two months in their boxes steadily discharging, so I’d be better to get a charger onto them sooner rather than later.

The switch-over circuit can wait for now: just hard-wire it to the mains DC feed for now since there’s no solar yet. The principle of operation is that the comparator (an LM311) compares the solar voltage to a reference (derived from a 5V regulator) and kicks in when the voltage is high enough. (How high? No idea, maybe ~18V?). When that happens, it outputs a logic high signal that turns off the MOSFET. When too low, it pulls the MOSFET gate low, turning it on.

The MOSFET (a P-channel) provides the “ignition key switch” signal to the BCDC1225, fooling it into thinking it is connected to vehicle power, and the charger will boost as needed. The key being that the BCDC1225 makes the decision as to whether the battery needs charging, and how much charge.

By bolting together off-the-shelf parts, we should have something that I can source replacements for should the smoke escape, and there’s no high voltages to deal with.