A Modest Power Supply Project

[Zermelo]

I see there are already accounts within these pages of several, very accomplished power tank projects, hence the title of this piece. The other reason that this was a ‘modest’ project is the intended purpose: servicing a SkyWatcher Star Discovery 150i – an EQ6 it is not.

But I thought it might be worth documenting in case anyone else was in a similar position.

Context

We have recently upgraded from an entirely manual first scope to the 150i, which has tracking, goto and wifi control. The default power option is an internal 8xAA battery pack delivering 12V. Unwilling to keep buying alkalines, I’ve resorted to NiMHs, but these give only around a total 10.4V fully charged, and that’s on warmish summer evenings. Opinion suggests that the mount’s goto accuracy can be impaired if the voltage drops, so I wanted to get off the internal batteries. As an alternative, the Star Discovery mount has a 12V power input socket, and I had already bought a cable with the standard car cigarette lighter plug at the other end.

Apart from powering the mount, the other requirements are currently uncertain, but possibly include an electric dew heater or 12V hair dryer for the same purpose, which I assume will probably connect to a cigarette lighter socket. Also, given that the scope control relies on a phone app rather than a handset, a USB charger outlet would be useful for emergencies (and other gadgets might well also run off USB). The one thing we won’t be doing is running a laptop all night, which is what seems to really burn the juice.

Choices

So we needed, at minimum, a 12V power source with a cigarette lighter socket, and ideally a second socket and one or two USBs. The offerings from astronomical suppliers seemed expensive, and often included gadgets we didn’t need. Standard car batteries were on the heavy side, and would need adapting and connecting. Car jump starters were a bit cheaper than astro kit, but were optimized for short, heavy loads. So I decided to have a go at making a bespoke power pack, where I could choose the physical size, capacity and outlets.

The basis would be a smallish sealed lead-acid battery. I settled on 9AH as a reasonable compromise between weight, cost and capacity, but the design allows this to be doubled if necessary. I considered buying a ‘deep cycle’ battery more suited to the observing usage pattern, but decided on a cheaper, standard model for alarms, UPSs, etc. Time will tell if this was false economy.

For the casing, I used a 5 litre plastic crate from the Really Useful Box range.

I chose these because (a) they are quite robust as boxes go, and (b) they are more nearly rectangular than most. The 5 litre box comes in two versions – I bought the ‘XL’, the only difference being a taller lid. The battery will live at one end of the box, but there will be a similar space at the other end for another if needed.

For sourcing the electrical outlets, I found a number of sellers on eBay and Amazon that offer essentially the same range of instrument panels that run off a 12V supply, for installing in cars/motorbikes/boats, at reasonable prices. In particular, I found that a 2-by-2 panel of these instruments just about fitted onto the side of the plastic crate. I ordered one of these sets that included a cigarette lighter socket, twin USB charger, LED voltmeter and on-off switch, though I ordered a separate, second lighter socket to substitute for the switch.

All of these sockets and instruments seem to be standard sizes and can be swapped in and out of the panels. The sockets also come in weatherproof versions that have integral rubber plugs, which is ideal. I thought the voltmeter might be useful to show how the battery was dealing with the load (an ammeter would also be useful, but I haven’t seen one that would fit this panel). Usual caveats apply to ordering online, there’s a lot of variation in vendor reliability. And a caution about the voltmeters – red now seems to be the hardest colour to find, most seem to be bright blue (not ideal).

Build

I chose the particular plastic box because the battery fitted lengthways across it quite nicely, but I knew I’d need to do something to stop it slipping sideways when being carried. My first thought had been to glue in some stout plastic sheeting to create internal compartments, but these boxes are polypropylene, so good luck trying to get anything to stick that. Instead, I decided to create a separate internal frame that would slip inside the box and wouldn’t need to be glued to it. This would create compartments for the batterie(s) and also stiffen the sides where the outlets would go.

I had a piece of 6mm ply that was about right – thick enough to do the job, but it still fitted around the battery. I cut out four pieces to create the frame, all of which are 10cm deep (I would have reduced this a bit if I had not been using the 'XL' version of the plastic box, as there's less head room). As the box isn’t quite a cuboid, the longer ones are actually trapezia (or trapezoids if you’re reading this from the US) measuring 28.5cm and 29.5cm on the shorter and longer lengths. The shorter cross-pieces are 15.2cm long. The other pieces are softwood I cut up to brace the joints; there’s no way 6mm ply is going to be joined at right angles without some help.

At this point, I used the instrument panel facia to mark out the position of the socket and screw holes on the long side of the plastic box, and cut them out:

I then used the holes to mark the positions onto one of the long frame pieces and cut those out. With hindsight I think it might have been better to cut the frame holes, assemble it and then mark and cut the plastic box. I did find it quite fiddly – you need to be careful with the frame, because the outlet holes have to be big enough for the sockets, but if you make them too big there won’t be enough for the securing nut to purchase on later. The holes in the box aren’t so critical because nothing depends on them and they get covered by the facia. Also, assembling the frame inevitably shifts the positions of the holes slightly, causing them not to line up any more.

Anyway, in the order I did it, the pieces were assembled in stages, measuring up to ensure there was enough space in the end compartments for the batteries. The softwood pieces form the corners for the ply, so you need just one good 90 degree corner on each one. I used PVA, panel pins and some very small screws (pilot holes to avoid splitting). A picture frame clamp proved useful to keep corners square.

When the clamps were removed, the frame was checked for fit inside the box and I gave it a couple of coats of paint. It won’t win any beauty competitions, but it won’t be seen.

The four devices can now all be inserted, passing through the facia, the plastic box, and the frame, and secured from the other side with the large plastic nuts (careful not to cross-thread). I added some rubber O rings of the right size that I had, so that the nuts didn’t take the paint off; large neoprene washers would be even better. These devices will hold the panel securely as it is, but the four screws that are supplied for the facia are worth fitting (and also for aesthetic reasons). Right sized pilot holes required so as not to distress the ply. I found the screws to be too long by about 6mm, so cut off about 3mm and then as I screwed them in I held a small rubber grommet against the inside of the frame to receive the tip and brace it still further (a small fibre tap washer would work well) but actually it bit surprisingly well into the ply.

All four of the devices in the panel need 12 volts, so they need wiring in parallel, both positive and negative, which needed some thought. One option would have been terminal block strip, but I wanted to use fairly chunky cable as I wasn’t sure how much current might be drawn in future, and that didn’t lend itself well. Instead I created a couple of multiway connections using ring terminals connected by small nuts and bolts – I used locking nuts and spring washers to keep the joints tight – these were all bits I had lying around. The instrument panel kit came with spade connectors and insulators. The result is quite compact.

An in-line fuse holder is a must – I managed to short out the battery terminals fairly quickly. At the moment I have a 2A fuse fitted (I put a multimeter on the 150i mount and measured the current at about 0.6A on maximum slewing speed) but I’ll review that according to future use.

With the cabling attached, the box looks like this:

For now, the other battery compartment is empty, but I used “piggy back” terminal connectors on the battery leads so I would need only to run two wires across the top of the box to add a second battery in parallel.

And here it is all working:

The lid overhangs a bit, so provides some extra rain protection for the panel.

It cost about £40 altogether, half of which was for the battery.

 

 

 

Edited July 15, 2020 by Zermelo