Hey
I need some basic solar cell questions answered if possible:
what is a good solar cell output (amperes wise) for a robot with motors.
what is a good power storage method? battery (rechargeable 9v 200ma) or some type of capacitor? also what would be a good capacitance for the power storage?
how do i select a battery for a solar cell? am i trying to get one with much bigger capacitance than the solar cell ampere output or what?
preferable a quick charge time and essentially long storage time.
any recommendations would be fantastic. links to products on this site would also be very good.
ALL help will be much appreciated
if i have 6 AA batteries in series (total of 7.2volts and 2850mAh) and a solar panel which is outputting 260mA with 7.5Volts do i need a voltage regulator to stop the batteries overcharging?
iv been reading a piece that is saying that i need to charge the batteries at 10% to stop them from overcharging (using a voltage regulator and a resistor) but as the solar panel is already outputting approximately 10% of the capacity of the battery so is it needed?
i will bare in mind that exceeding the voltage of the solar panel of the battery can be beneficial but what would happen if i was to exceed the amperage? is the benefit of exceeding the solar panel voltage due to voltage lost in the process of getting to the battery?
what would be needed to stop the solar panel from outputting power when the battery is full? how do i select one given the spec of my solar panel and battery? would the battery explode if no device is used to stop the solar panel output?
with, for example, a 15.4 volt solar panel and a 12 volt battery, what would be more of a concern for the motor, the 12v or the capacitance (amperage) of the battery?
basically, what is more important in the selection of the solar panel and battery. the batteries amperage output or voltage?
Excellent. when i mentioned that i read a piece that said i could charge at 10% and not damage the battery i was from REUK (the last 2 links you posted) so i will probably be going with that design.
I have now got all my parts but my batteries came fully charged, to continue with testing i need to see how long it takes to charge the batteries using the solar panel therefore i need to discharge the batteries.
what is the quickest and safest way to discharge these batteries?
i have 8 AA 1.2v 2850mAh batteries which all need to be drained, any recommendations or small circuits i can make to flatten the batteries fast?
i.e. with lots of LED’s or motors?
with the lack of the sun iv been using an over head projector to test the solar panel which is giving me impressive results of around 270mAh and 14ishV. however im a bit worried now because having the solar panel in my room with the light on im only getting 5ish volts and the 8 AA batteries come to around 9.8v, then theres the volt drop across the voltage regulator, so if i was to set up a system in this room i assume the batteries wouldnt charge because i need more volts to be outputted from the solar panel to the batteries, is this correct?
iv thought about taking away half of the batteries to leave 4 AA’s at around 4.8v but im worried about powering components in the future i.e. motors and sensors for the robot.
i know the light bulb isn’t providing much light at all and the panel is about 2 metres away from the bulb but its still better than the sunlight outside at the moment which leads me to worrying.
Im going with the LM317T voltage regulator set up which you suggested previously and that i looked into a lot before.
once these batteries have been charged fully i hope it will take longer to use the 2850mAh capacity than it would to charge, i.e. the batteries never getting in a near flat state as they will be recharged faster than its used. is this possible with a simple circuitry with some small motors and a PIC microcontroller as the capacity is much larger than a normal battery?
any tips or tricks for me in this situation. I’m trying to keep the solar charging as simple as possible, i just want the batteries to keep trickling over so they never get flat.
ok, i now have my solar panel and my batteries.
the solar panel is 12v, 4.3W which makes it 358mA max output. i flattened the batteries 2 at a time using a portable CD player. i should be receiving a battery tester in the post soon so i can confirm that the batteries are flat.
testing the solar panel in the very little sunlight we have here in England at the moment im noticing that im getting very close to the max voltage, getting around 11v, but im getting a very little current output, around 0.016A or 16mA out of what should be 358mA max. now is this normal or is there something wrong?
the weather atm is overcast, no direct sunlight and relatively cold. As most would assume in the south of England at this time of year!
the dc-dc converter looks interesting and maybe something i should look into at later dates but i think it would confuse matters to much at this time. i think i’ll stick with the current configuration (8 AA and the solar panel).
I’ll charge the batteries using the OHP and hope that the little sunlight/light it receives after that will keep the batteries topped up.
as for checking the batteries, how would i go about checking that their actually charged? using a multimeter iv noticed the voltage is the same, regardless of if there flat or not and when checking the current its reading 1A, which i can only assume is wrong from a single AA battery.
Now iv been told that taking readings from a battery in an open loop (i.e. just hooking the multimeter up to a battery like i did to get those previous readings) will give wrong readings but is there anyway to test these batteries when there in my circuit that im wiring tomorrow? i also have a battery tester but i need more information than ‘good’ or ‘bad’.
I assume the capacity of the battery is what we most wish to know here so is there a way i can check that?
I’m having a bit of trouble checking if my voltage regulator system is working. I followed the instructions and schematics from here: reuk.co.uk/Solar-Battery-Charger-With-LM317T.htm
i used a 4.7ohm resistor so that the max current output i should be getting is around 266mA. However the OHP that i use only outputs up to 270mA and I’m still getting that so i dont know if its working or not. i see no change whatsoever using this circuit but i dont know if this is right and when it hits approx 266mA it cuts off?
voltage isnt effected at all using this circuit. it can still go right up to 14v. should it do this?
any ideas on how to test this? i tried using a power supply but i wasnt getting any response from that so i dont know what to do
What happens when you overcharge a battery depends on the battery design but mostly the chemistry (NiMH, NiCad, Lipo, etc). Some will vent gas, others flame.
A charge circuit that monitors the voltage of the battery and does a “trickle” just to keep it topped off. In most cases, the battery pack voltage determines the current % of charge so you can monitor the state of the battery.
Usually we would suggest choosing the power source required by the project, then design the solar charging system around that. Of course, the type of battery chemistry should be considered beforehand due to specific requirements such as high amperage output so this will influece how you approach integration of a solar charging sytem.
So for example if you have 12V motors and a 5V microcontroller on your robot, the 12V battery pack is suggested. You would then regulate the 12V for the 5V device. The solar panels would then therefore need to be ~12V and an amperage that is high enough to give you some extended operational capacity.
Even with today’s advances, most solar robots will not be able to operate 24/7 unless their design is perfect or really simple. You can imagine the robot stopping when the batteries get weak, going into “low power” mode and deploying solar panels
For small low power robots, a solar panel can be used as a “solar engine”, basically a small circuit that stores and pulses power to the motors.
For larger robots, a solar panel is most often used to extend battery life between charges or as the primary charge method.
As much as you can get without weighing down the robot to the point it no longer moves
Batteries and capacitors are both valid energy storage methods. Depending on the specifications of the capacitor, batteries are usually prefered for medium to long term storage (they do bleed however), capacitors for short term storage or accumulation/conditioning.
According to PowerFilm Solar: “In a nutshell, the measured power provided by a solar panel is dependent not only on the amount of sun that strikes a panel but also on the impendence of the load. For any given panel, there is an optimum impedance of the load that allows the panel to provide the maximum power into the circuit. If the resistor is not appropriately matched to the solar panel, the power provided will be greatly diminished. You could look at “Part B” on the following link to see what I mean: web.ew.usna.edu/~bruninga/labsats/EA467-EPSLab06p1d.doc.”
Basically, you want to select a panel that offers a bit more voltage than the battery. Powerfilm has for example a 15.4V panel designed for charging 12V batteries, but a 12V panel might also work. The Amperage will simply determine how fast the battery charges.
In some cases, a lower voltage panel can be used if the voltage is “stepped-up” via a charge circuit between the panel and battery. You really do have flexibility of design.
If you look at residential solar panel setups, the system usually incorporates the panels, charge controller and a battery bank. Often you will see deep cycle lead acid batteries used which are less finicky so battery chemistry needs to be taken into account when designing a solar system. Lithium Polymer batteries can, for example, vent quite violently if overcharged so precautions must be taken to protect your investment if protection circuitry is not present on the battery pack.
You may never encounter a problem depending on the battery chemistry and design. We would however suggest a proper charge circuit that monitors the cell voltage and trickles the power when the battery is full.
Ideally, you can use a small lightbulb to drain the batteries. Lightbulbs found in flashlights work well. Some people use large 1 watt resistors (5-10ohm), that are usually large rectangular resistors. Don’t use regular resistors as they may overheat. The lightbulb approach is probably the easiest. If you are draining at 7.2V, this may be too much for some light bulbs. Maybe use a small DC motor?
One thing you could try, in order to simulate full sunlight conditions, is to place a spot light or table lamp right up against the cell. Beyond this, if you want to obtain the most performance from a solar cell, the impedence of the cell and circuit should be matched. This is more advanced stuff but may interest you. This is what Powerfilm had to say:
This is one of the main reasons many people will design a solar charge circuit that adapts to the input voltage and keeps the output at a suitable charging voltage for a battery pack. Optionally, you could simply have it stop when the panel isn’t outputting enough “juice”.
In this case you would need to “step up” the voltage. We do have step up voltage regulators that may be worth a look.
Changing the battery pack configuration may not be a good solution because then when you get full sunlight the 4.8V battery pack will be getting 14V from the panel which can’t be good for the cells in theory.
You need a way to stabilise the voltage regardless of the source voltage of the panel and provide a stable output voltage to the battery pack. The Keep It Simple method would be to just live with lower voltages when not in direct sunlight. Some power is going into the battery pack in theory.
Battery capacity can be determined by looking at the voltage. If you are not having good results when the battery is not connected, try taking a reading when the battry pack is under load. There are discharge curves to demonstrate this:
If you are using ±5% resistors, it may be off by a bit due to variances in the manufacturing process.
Depending on the actual LM317 you purchased, you can control amperage or voltage. Some are amperage control, others are voltage control. You need to figure out which LM317 you purchased from which manufacturer and pull up the specifications sheet.
