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.

Mar 112017
 

So, having knocked the regulation on the LDOs down a few pegs… I am closer to the point where I can leave the whole rig running unattended.

One thing I observed prior to the adjustment of the LDOs was that the controller would switch in the mains charger, see the voltage shoot up quickly to about 15.5V, before going HOLYCRAP and turning the charger off.

I had set a set point at about 13.6V based on two facts:

  • The IPMI BMCs complained when the voltage raised above this point
  • The battery is nominally 13.8V

As mentioned, I’m used to my very simple, slow charger, that trickle charges at constant voltage with maximum current output of 3A. The Xantrex charger I’m using is quite a bit more grunty than that. So re-visiting the LDOs was necessary, and there, I have some good results, albeit with a trade-off in efficiency.

Ahh well, can’t have it all.

I can run without the little controller, as right now, I have no panels. Well, I’ve got one, a 40W one, which puts out 3A on a good day. A good match for my homebrew charger to charge batteries in the field, but not a good match for a cluster that idles at 5A. I could just plug the charger directly into the battery and be done with it for now, defer this until I get the panels.

But I won’t.

I’ve been doing some thought about two things, the controller and the rack. On the rack, I found I can get a cheap one for $200. That is cheap enough to be considered disposable, and while sure it’s meant for DJ equipment, two thoughts come to mind:

  • AV equipment with all its audio transformers and linear power supplies, is generally not light
  • It’s on wheels, meant to be moved around… think roadies and such… not a use case that is gentle on equipment

Thus I figure it’ll be rugged enough to handle what I want, and is open enough to allow good airflow. I should be able to put up to 3 AGM batteries in the bottom, the 3-channel charger bolted to the side, with one charge controller per battery. There are some cheap 30A schottky diodes, which would let me parallel the batteries together to form one redundant power supply.

Downside being that would drop about 20-25W through the diode. Alternatively, I make another controller that just chooses the highest voltage source, with a beefy capacitor bank to handle the switch-over. Or I parallel the batteries together, something I am not keen to do.

I spent some time going back to the drawing board on the controller. The good news, the existing hardware will do the job, so no new board needed. I might even be able to simplify logic, since it’s the battery voltage that matters, not the source voltages. But, right now I need to run. So perhaps tomorrow I’ll go through the changes. 😉