I'm putting together a project to power a water pump in my pond using solar panels. The pump is rated at 3.5-9v (1-3w).
This will be connected to a 6v 12ah deep cycle battery which will be charged by 3 x 9v (3w) solar panels in parallel. The battery will also be used to power the arduino using a 3.3v voltage regulator.
There will also be a mains backup which will turn on if the battery falls below 50% charged. I will monitor the battery voltage using an analog pin. Once turned on, the mains will continue charging until the battery voltage is at 100%.
Switching on mains will be controlled from a pwm pin connected to a mosfet (IRF520N). I'm looking to protect the battery by changing the charging rate via pwm when the battery reaches around 80% charged as I understand charging the battery at too high a rate above a certain level is bad for it.
I have a few questions:
Will I need a blocking diode on each solar panel or just one in front of them all? Depending on this answer, what size diode(s) should this be?
How can I establish what the battery voltages should be at which the charging rate should change and turn off?
Will the 3.3v that the arduino is powered by be enough to control the mosfet effectively?
Thanks to anyone who can answer any of these questions.
" 6v 12ah deep cycle battery"
I am to assume the battery is a lead acid. If so,
it can take a fast charge up around 14.6v. Then a maintenance voltage should be around 13.5v.
Not knowing your requirements, I may be off track but, I have used solar water pumps without grid power. When the sun shines, and the small battery is to proper voltage, I turn on the pump. When the voltage goes lower than threshold, I turn off the pump. If the pumping times is not critical, that is pretty efficient.
Yes, it will be a sealed lead acid battery. Are the charging levels you mention definitely for a 6v battery rather than 12v? I was thinking it would be more like 9v fast charging and 7.5v for maintenance.
I want the pump to be always as I would like the potential to keep fish in the pond which would require constant water circulation.
Thanks for confirming. Can you also let me know what your threshold levels were for the battery being fully charged and empty? How did you establish what these should be?
I would turn off the battery drain, when it fell below 10.5v.
If the battery had been low (<11v), I would let the charge to up to about 14.6, for about 2 minutes, then drop it back to 13.6 range.
Note, I am still using 12v battery reference, so divide by 2 for 6v
My program will work a bit differently to yours but, using your ranges, I'll let the battery drop to 5.3v and turn the mains on at 7.3v. When the battery voltage reaches 6.6v drop the mains charging to 6.8v. When the battery voltage reaches 7.3v turn off mains charging until battery voltage gets to 5.3v again.
Have you noticed any degradation in your battery? My main concern is that at some point the battery will never get to the fully charged voltage if it wears out, and the mains will be on constantly.
"I'll let the battery drop to 5.3v and turn the mains on at 7.3v"
I would suggest not trying to let the battery drop to 5.3, That is the point I would turn off the load (pump). It is a bit different if you have a charger from the power grid. In that case, I would try to keep a trickle charge of about 13.6 volts all day and all night.
In my case, with only solar, I had to decide when to stop draining my battery (night time).
If you want long life from your battery I think 5.3v is much too low. I have 12v batteries (200Ah) and I would not like to see them go below 11.6v which would be 5.8v for a 6v battery and I try to ensure they don't go below 12v.
For long life it is also necessary to ensure the battery gets a long charge at regular intervals - maybe once a week. It will not be fully charged the instant the charge voltage reaches, say, 7.5v.
You must also take into account that the correct way to measure the voltage is to do so after the battery has been disconnected from everything for 12 hrs or more - which is rather impractical. If you don't leave it to settle it will show a lower voltage when discharging and a higher voltage when charging compared to the "settled" voltage. And the extent to which it reads lower or higher depends on the rate of discharge or charge.
A battery with a larger capacity might be a good idea. And keep in mind that the real capacity may be only 50% of the sticker capacity.
I want the pump to be always as I would like the potential to keep fish in the pond which would require constant water circulation.
This will be connected to a 6v 12ah deep cycle battery which will be charged by 3 x 9v (3w) solar panels in parallel.
Even with 9W of solar, you will be eternally mains charging the battery.
9W is a laboratory measured specification.
From 9W you will be lucky to average even 3W a day, the sun don't shine 24hrs, clouds, angle of the sun etc etc.
Is the main issue with letting the battery voltage drop too low that it would impact on battery life? If so, this is what I'll have to weigh up against my main aim of having the system run without mains for as long as possible. Initially I wanted to have it run without mains at all, but I do need it to run constantly.
By comparison, I currently have the same pump connected to a 6v 1ah battery which is in turn connected to a 12v 10w solar panel!! All connected in parallel with no controller. On a sunny day in British summer it will turn on around 8am and turn off around 8pm at various rates depending on intensity of sunlight, but I'm not bothered about flow rate as it's mostly decorative. It runs constantly and will continue running for around 2 hours once the sun has gone low. It has been connected like that for around a year and it seems to be working fine and the battery looks ok.
I figured that with my next effort a battery with 12 times the ah and more suitable solar panels could get the pump to run all day and night on a sunny day and would be kinder to the battery, especially if used with a controller. So I've never been too concerned about battery degradation to the nth degree, but it would be nice to optimise where possible. I was thinking if I added mains backup, on cloudier days it may get near a float charge during the day but would generally be on a cycle of the mains charging Turing on and off every 12 hours as the battery charged and discharged fully.
With that in mind I need suggestions for charging thresholds that align with this idea. What would be the impact on battery life? I don't really know how long they're meant to keep working for. If it lasted for 2 years I'd be happy.
Ok, but the problem is if you dont know how much power the pump consumes over a 24 hour period , you cant figure out how much Solar panel capacity is needed, unless you grossly over engineer the project.
eg , you are going to increase the battery capacity by 12 times, then increasing the Solar Panel capacity by 12 times would guarantee that the pump runs all the time , but is a horribly inneficient way to get the desired result.
Isn't that something that can be worked out by looking at the rating of the pump ie 3.5 - 9v (1-3w). Since the battery is 6v, I'd guess the pump would be functioning in the middle of this range ie 6v (2w). Is that 2 Watts per hour?
