Normal solar garden lights are dim and boring. This project is about building a bigger/better/brighter variant of these. Maximum re-use of already available parts is planned. If there is a way to hack something togehter/use it for something else than the original purpose, this is the route to go.
This project should be done in a few working hours as soon as parts are here, so its done on a few evenings after work. Time spend so far: 4 evenings à 3-4h (most of it documentation/plans).
I write this down to try out http://hac.io, not to show a (relatively boring) project in the first place. Still, this is something others may be interested in.
Sub-Projects (see Project log): - Solar panel mount [100%] - lights [80%] - undervoltage protection [75%] - step down [test circuit works at 20W, needs some theoretical evaluation: 60%] - record U/I chart of unknown solar panel /w electronic load [60%]
just for completeness... yes, temperature drift of solar panel (so MPP voltage) can be compensated by the transistor in the input-voltage-regulation-loop, at least if forward voltage, temp. coeff. and temp. diff. are the same for that transistor and the panel (which is not the case for the real application, unless I hand-pick the transistor and couple it closely to the panel):
A paint bucket, with some modifications, makes up a nice enclosure for the battery and electronics.
A 35Ah battery is held in place with an L-shaped metal part originating from an old old SCSI flatbed scanner power supply. As it is made of perforated metal, it is ideal to mount the electronics on there. The metal part is mounted to the bucket with rivets.
Holes were drilled in the bucket to let moisture out. I don't plan to overcharge the battery but if it does by some reason, the fumes should have a way out, too.
Front-panel-mount-type fuse holders and battery terminals are ordered and should arrive in some days...
Finally, today is a day with sunny weather + some hours of free time :D
Question: At which voltage should I use my (unknown type) solar panel to get maximum power out of it? Is it really that worse if I use it a few volts above or below? Is it worth the effort to use a buck converter?
The battery charge controller should do two things: get as much power out of the solar panel and switch off if the battery is full.
To get maximum power out of a solar panel, it is necessary to use it in (or at least near) the MPP (maximum power point), normally an MPP-Tracker algorithm would do this.
Here is a sketch of the U/I relation of a solar panel and the P/U relation:
Output current stays about constant as long as the voltage on the panel is below a certain point. But as power equals current times voltage, it makes sense to use the panel in the upper range where P is max.
I am too lazy to build an own stepdown controller with custom regulation, mpp tracker algorithm etc (see here for a nice one by a guy who does this in his job, too), so I'll go the easy route based on this concept.
It does no software MPP tracking, but still provides better efficiency than a simple linear regulator (or even a series diode). Plus these LM2596 step-down converter boards are really cheap (2 eur per board) and I got some of them in my parts bin.
This test setup with 2k pot / 47k voltage divider to base of bc547b with 220k pullup works for holding the input voltage above a certain point.
With further modification it fits the application:
The copper board is soldered to GND of the DCDC board and acts as additional heat sink. It took two soldering irons set to 450°C to solder that board sandwich together, but the caps seem to have survived it. The additional board got holes to mount this construction in a housing. I've soldered the in/out wire GND/"-" directly to the GND contact of the switching regulator for cleaner wiring.
The additional NPN transistor is located near the heat producing elements, which is not ideal, due to temperature dependency (ideal would be same temp. as solar panel). The ideal location for this circuit would be near the solar cell. Calculations or maybe a simple simulation have to show if this matters that much.
In normal (unmodified) schematic, the step down converter just does the following to regulate the output (simplified, there may be a nested current control loop, depending on regulator, which I omit here):
...with resulting output current:
With the additional parts, the schematic forms nested control loops:
As long as the input voltage is about a set value, the buck converter just does its normal work and regulates the output voltage. As soon as the input voltage gets too low (because the solar panel can't provide that much current), the outer control loop kicks in and limits the current by disabeling the output:
*ADD MEASUREMENTS FOR DIFFERENT SCENARIOS HERE*
As the MPP is temperatue dependent (a solar panel is just a bunch of odd mutant ninja diodes, so shockley equation applies), it makes sense to add at least some degree of temperature compensation to get out more power:
The forward voltage of a diode changes with some mV/Kelvin (usually around -2mV/K for a silicon one).
As a solar panel consists of a bunch of diodes in series and this temperature dependency applies to every single diode, the temperature dependency of the whole panel sums up to
The circuit sketched above regulates its input voltage to a point depending on the base-emitter voltage of the BC547B on its enable pin. This voltage is generated from the input by a voltage divider, so:
The "undervoltage protection and dusk light switch" has two purposes:
It switches the load off as soon as a critical (low) voltage is reached.
It disables the output as long as the solar panel still provides more than an adjustable voltage (e.g. by day it disables the attached LED lights).
There is no charge controller in here as it is a separate function. There are no fuses drawn here, too. Always use fuses if dealing with stuff that can provide enough power to burn or damage something... Also, there is no (required) flyback-diode for the relais coil drawn.
The circuit is designed with low parts count and bin parts reuse in mind, so it is by no means a perfect by-the-book circuit and may show bad design technique. Think of diode forward-voltage and its temperature dependency, relais hysteresis depending on part variation etc.
Here is a quick sketch of the schematic which shows the working principle:
The output from the charge controller is connected to the car battery by a power diode. A 10 volts zener diode (with current limiting 2k2 resistor in series) drives the base of an BC547B npn transistor. A second 2k2 resistor is in parallel with the base-emitter-path of the BC547B npn transistor. This reduces the sensitivity of the circuit to noise, but also reduces hysteresis (which could lead to oscillation). The base capacitor of a few µF dampens oscillations which could occur by the non-zero series resistance of the car battery, a fuse etc. as the transistor circuit provides negative, phase shifted (relais inductor coil) feedback with gain.
If the battery votlage goes high enough, current flows thru the zener diode and the base voltage of the BC547B npn transistor rises high enough that there is enough base current to drive the relais strong enough to switch on. Load connected.
If the battery voltage falls low enough, base voltage and current of the BC547B transistor goes down so the relais will switch off again. Load disconnected.
automatic dusk light switch
Assuming that the battery is fully charged, there is no or only a low load on the solar panel. This means it provides approx 20V (at least mine does). A solar panel is (within limitations) a current source with current depending on light level.
By driving this current thru a current-sensistive amplifier/switch (BC547B again...) and preventing the relais-driving npn transistor from switching on by stealing its base current, the logic function "if light level is above a certain limit, disable output" can be achieved.
To make the switching point adjustable, a potentiometer can be used to set the base current.
As the load on the solar panel increases (charge circuit tries to kick in as soon as battery voltage drops), the output voltage of the solar panel drops further (now by darkness and load). This mechanism provides positive feedback, so this time no oscillations should occur.
First test testup of undervoltage circuit:
...built on prefboard:
...provides the following Uin (CH1/X)/Uout(CH2/Y) relationship (sorry for the screenSHOT, I had no fat16 usb stick):
Hysteresis is just about 240mV with this circuit, which may be too less to avoid oscillation, based on the series resistance of the battery.
I've decided to leave the dusk sensing out of this circuit part for now, as the LED modules provide this function.