This had to do the following:
- Tell me the voltage produced by the Solar Panels.
- Be a battery-state indicator.
- Give me switching control as to which battery is being charged.
- Provide isolation and protection for the high-current circuits.
- Box to put it all in. I got a one from Jaycar - in fact most of this stuff came from Jaycar.
- Voltmeter - full-scale deflection 20v (v1).
- 2 x Changeover switches with a central off position (s1-2).
- 2 x Small green LEDs with mounting grommets (l1-2).
- 2 x 1K ohm Resistors (r1-2).
- 2 x 6A 400V Bridge Rectifier - BR604 (d3-4). I used a bridge here to lower the voltage drop across the diodes - i.e. 2 diodes in parallel for each unit. You connect the lead from the solar panel to both AC pins and the barrery's positive terminal to the + pin.
- Car Battery Monitor Kit - Jaycar Cat. No. KA1683 (v2).
- 2 x Inline 3AG Fuse Holders with 5A fuses.
- Surface-Mounted cigarette lighter connection.
- Various odds and sods:
- Bits of plastic to mount internals on.
- Araldite to stick components to the inside of the box.
- Heat-shrink insulation to cover soldier joints.
- Thick and thin wire - for the high and low current parts of the circuit.
- A piece of clear perspex to make a window for the Battery Monitor Kit LEDs.
- Silicone sealant to stick the perspex down.
- Screws / Washers / Nuts / etc.
- Sealing grommets where cables pass through the box.
- Small cable ties.
The labeled components not in the parts list are:
- p1-2: Solar Panels
- d1-2: Solar Panel protective diodes - these came with the panels (good old IN4001s, if you're interested)
- b1-2: The boat's 2 batteries - the maintenance of which this project is all about.
Put all the bits in the box and solder them together.
OK, a little planning won't go amiss.
First thing to do is build the Battery Monitoring Kit. There are full instructions that come with it and all components are supplied. The only thing outside the capabilities of the average yachty is the suggestion that it needs to be calibrated with an accurate adjustable voltage supply. I haven't got one of those and I wasn't about to go out and buy one. The unit uses LEDs to indicate voltage within a fairly narrow range. There is a pot (potentiometer) in the circuit that is used to set when the first LED comes on. To set this up, you really need to be able to read (as opposed to supply as is suggested) an accurate voltage and this is easily done with a cheap digital multimeter. Here's the circuit:
The components are:- b1: A fully-charged battery - this needs to be fully charged to give you the voltage range you need. A good fat car battery or any DC voltage source up to 15V. While it doesn't have to be a full-on stable voltage source something reasonably well regulated and smoothed should be fine.
- p1: Potentiometer. 1000+ ohm will do it. Much less than this will probably draw too much unwanted current.
- v1: A digital multimeter set to 20V full-scale.
- bmk: Battery Monitor Kit.
All the control stuff I mounted on the box lid. Everything else went on the base. You need to drill, file and otherwise make the right sort of holes for anything that needs to be mounted through the box material. Planning the front panel layout obviously helps. As some of the other components can be a bit bulky, you need to mount them in such a way that the lid closes properly. The Battery Monitoring Kit has LEDs that need to be visible on the front panel. I cut a slit in the lid to accommodate them and made up a little mounting bracket that glued to the inside of the lid so that the the LEDs were flush with the surface of the lid. I then made up a window with some clear perspex and siliconed it over the slit.
Once everything is securely mounted, it can be wired up. I suggest that all solder joints are covered with heat-shrink insulation. Any parts of the circuit that takes high (> 1A) current needs to be in heavy duty cable. All the rest can be in thin wire. It helps if you wire the box and the lid separately and then form any wiring between the two into a harness kept together with small cable ties.
Once assembled, mount the unit near the hatch in such a way that the cigarette lighter socket is near the hatch slide and about half-way down the hatch travel. This is so that the flexi-connection is at its slackest when the hatch is half-closed.
For the connections to the batteries, I led long lengths of heavy duty cable through grommets in the box. These have to be long enough to be led over the inside of the hull down to the battery compartment. The negative is connected to the common earth and the positives are connected to each battery respectively through a 5A fuse holder near the battery-end. The cables are held on to the hull with screwed-down cable ties bases + ties, and I also led the cable down the cockpit ladder secured in place with more cable ties.
Testing the Battery Monitor
Flip the Battery Monitor switch to each of the batteries and the LEDs should indicate the voltage-state of that battery.
What Should Have Been Done Differently
- I originally designed - and fitted - the main battery protective fuses in the box. This is a mistake. The fuses should be as physically close to the batteries as possible for full protection. Not wanting to leave two big holes in the box, I now have 2 fuses on each line. Not really something that needs fixing.
- I should have added an Ammeter into the circuit. As it stands, you can't tell if the batteries are actually receiving any current from the solar panels. While I would like to get a new box and do the mod, the accountant won't sign-off on it. The inconvenient solution is to pull one of the fuses and put the multimeter in line with the battery on its current setting.
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