# solar panels in parallel

#### denison

Joined Oct 13, 2018
59
Hi everybody, I have a 7.2ah sla battery. I use this to operate my remote electronic lock on my gate. This is a 12 volt lock. Working current start state =1.2A persistent state = 120ma.
I got a 12v solar panel to charge the battery. My first one was a 10 watt. I found this was not enough to keep the battery charged enough to keep the gate locked all night. I added 2 20w panels giving me 50w in total. These were connected in parallel.They still did not keep the battery charged.
I downloaded a tutorial which said that the panels should have the same nominal voltage but they don't have to have the same amp value.
I recently disconnected the 2 20w panels leaving only the 10w. The gate stayed locked during the night for the same time as when the 2 20w were connected.
The tutorial makes no mention of power ratings when connecting in parallel. So my assumption now from the results is that the output of the full 3 panel array follows the lowest rating panel 10w.
Can anybody confirm that this is correct?

#### nsaspook

Joined Aug 27, 2009
6,719
You can add current sources (loaded down solar panels in this case) when they are connected in parallel between the same nodes. This assumes the load is actually capable of drawing that total current. The beginning state of charge of the battery is just one of the wildcards here as is the charging current and voltages from the various configurations. Too little information.

#### oz93666

Joined Sep 7, 2010
731
By persistent state 120ma do you mean this is current drawn from the battery 24Hrs / day ?? that's about 40WHrs /day (accounting for losses) which maybe too much for your panels to deliver , depending on shade from trees etc ... On an overcast day 30W of panels will deliver less than 40WHrs

The tutorial is correct , power ratings don't matter , it's the voltage , just connect all panels in parallel.

You say your panel is "12V " what does that mean ??? Hopefully 17V ... what does the label say ??? Are all your panels identical?? Where are they positioned ? any trees ? , Do you just feed the panels into the battery with no regulator??

#### Ylli

Joined Nov 13, 2015
877
Your gate draws a continuous 120 mA to stay locked??? I would expect the lock to be engaged with zero current and just need current to unlock it.

#### gerty

Joined Aug 30, 2007
1,269
Also .. What is the age/condition of the battery.. Sounds like you have a Maglock, the current draw seems appropriate for that..

#### denison

Joined Oct 13, 2018
59
By persistent state 120ma do you mean this is current drawn from the battery 24Hrs / day ?? that's about 40WHrs /day (accounting for losses) which maybe too much for your panels to deliver , depending on shade from trees etc ... On an overcast day 30W of panels will deliver less than 40WHrs

The tutorial is correct , power ratings don't matter , it's the voltage , just connect all panels in parallel.

You say your panel is "12V " what does that mean ??? Hopefully 17V ... what does the label say ??? Are all your panels identical?? Where are they positioned ? any trees ? , Do you just feed the panels into the battery with no regulator??
All panels have a nominal voltage of 12 volt. A linear regulator is set at 14.7v. The allowed cycle voltage of this battery is 15v. The 10w panel has a pmax of 0.572a at 17.5v. The 20w panel 1.12a at 18v.
I have just changed the regulator to a pwm type of my own design for better efficiency. Battery still not holding lock all night. This is on a single 10w panel. Only getting 14.3v from the panel and 0.42a. The pwm is set to 14.27v.
I will now try a single 20w panel.
I am convinced now that power does matter now from the results I have given. I believe all panels are only delivering 10w total which is the power rating of the 10w panel.

#### nsaspook

Joined Aug 27, 2009
6,719
Rough rule of thumb:

Calculate the needed Ah to keep the lock working all night (or however long the energized period is). Multiply that Ah by 2 for the needed battery Ah size. Multiply that Ah by 0.25 for the needed A from the Solar panels to keep a good battery recharged with typical sunlight variations.

#### Audioguru

Joined Dec 20, 2007
11,251
You show no schematic.
Do you have a diode in series with each solar panel to prevent them from discharging the battery when in the dark or even when only one of them is shaded?
Do your solar panels "move" to always face directly at the sun?

A guy had a field beside a busy road but he did not want to pay a high price for mains electricity so he lighted a large advertisement with a battery that was charged with a solar panel. The charge did not last long enough (no diode?) so he added a windmill generator that the battery made spin all night long (again, no diode?).

#### denison

Joined Oct 13, 2018
59
You show no schematic.
Do you have a diode in series with each solar panel to prevent them from discharging the battery when in the dark or even when only one of them is shaded?
Do your solar panels "move" to always face directly at the sun?

A guy had a field beside a busy road but he did not want to pay a high price for mains electricity so he lighted a large advertisement with a battery that was charged with a solar panel. The charge did not last long enough (no diode?) so he added a windmill generator that the battery made spin all night long (again, no diode?).
I have a diode at the battery to stop reverse current flowing into the panels at night. Solar panels don't move but they are approx. horizontal and the sun is now during the day almost directly overhead.
The reason the lock works during the day is the 10w solar panel input and the battery apparently is charging enough for the lock to operate a little during the night but not all night.
Battery voltage this morning was only 8.6 volts. Not enough to operate 12v lock. I have taken battery out and am charging it with a 12v source. The current into the battery is 0.72 amps. It could be a faulty 7.2 a/h sla battery.
What is the best way to check the condition of the battery? It would be several years old now. My 10w solar panel was putting in 0.42 amps at around midday which seems a fairly good charging current.

#### denison

Joined Oct 13, 2018
59
Rough rule of thumb:

Calculate the needed Ah to keep the lock working all night (or however long the energized period is). Multiply that Ah by 2 for the needed battery Ah size. Multiply that Ah by 0.25 for the needed A from the Solar panels to keep a good battery recharged with typical sunlight variations.
thanks. I will do that.

Joined Feb 20, 2016
3,259
I would think your panels and battery are no way near large enough for that current draw.
Also, the battery needs to be big enough to run the lock for a couple of days with no charge, as that will help cover cloudy days.
Running a battery down to 8.6 V will kill it pretty quickly to.
I have a couple of 5KW arrays and today. at midday, the power is only 600W each. It is often lower that that.
MORE panels is good

#### oz93666

Joined Sep 7, 2010
731
I have a couple of 5KW arrays and today. at midday, the power is only 600W each. It is often lower that that.
MORE panels is good
Something sounds not right there .... It's raining here , my panels still delivering 45% (Thailand) ... the very lowest I've seen on the most dark overcast day is 18% midday

Even UK latitudes It should be better than your figures .... Stats show a static 1KW panel will deliver an average 3KWHrs/day (Europe)

So TS 30W should deliver 90WHrs/day (low panel price means nobody uses tracking on panels these days. The gains are very small and the cost and maintenance astronomical)

Joined Feb 20, 2016
3,259
These are grid connected so there will be a bit of a loss in the inverters. I do not know if the reading is from the solar input or inverter output.
I just checked an they are up to 1153W and 1147W at the moment. (1:20PM, Southern Australia.)
Even so, I think the setup for the gate is too small.

And why is it a powered on lock?
Unless it is designed to swing open on power loss, like some fire doors swing closed on power loss as a safety precaution, I'd look at changing the lock sense.
And yes, no tracking is needed nowadays. Just add more panels. A lot cheaper!

#### Sensacell

Joined Jun 19, 2012
2,544
Yes it sounds like an expensive and complex wild goose chase to build a solar farm to keep a lock locked.

There are many other intermittent power consuming lock technologies that could run for a year on a pair of D cells.

#### Bernard

Joined Aug 7, 2008
5,357
a good fully charged 7.2 Ah battery should operate the gate for 2 days, if not battery is weak.
Best to make your own solar cell measurements, both open circuit & short circuit. Best operating
point for cell is about 75% of OC V.

#### MrSoftware

Joined Oct 29, 2013
1,667
If your current requirements are from the spec as opposed to a real reading, then take an actual current reading at the lock so you know what your real draw is. Compare that to the battery capacity to know if your battery is actually too small. Also check the spec on your battery to be sure that the 7.2Ah capacity isn't calculated with an end voltage that is too low to operate your lock. If they rate it 7.2Ah, but the second half of the power comes under say 10V, then your actual usable capacity might be much less than you think.

Also that type of battery usage is considered deep cycle, and it will lead to a relatively short battery life. Consider a lock that requires no power to remain locked.

Thirdly charging a battery is a lossy process. A lot of the power will be lost to heat, especially with lead acid chemistries, so your panels need to give you significantly more energy than your battery will hold. Also if you don't have one already, look for a circuit that makes more efficient use of the power that is available. Here is one for Lithium batteries as an example, look for one that is compatible with SLA: https://www.sparkfun.com/products/12885

#### denison

Joined Oct 13, 2018
59
If your current requirements are from the spec as opposed to a real reading, then take an actual current reading at the lock so you know what your real draw is. Compare that to the battery capacity to know if your battery is actually too small. Also check the spec on your battery to be sure that the 7.2Ah capacity isn't calculated with an end voltage that is too low to operate your lock. If they rate it 7.2Ah, but the second half of the power comes under say 10V, then your actual usable capacity might be much less than you think.

Also that type of battery usage is considered deep cycle, and it will lead to a relatively short battery life. Consider a lock that requires no power to remain locked.

Thirdly charging a battery is a lossy process. A lot of the power will be lost to heat, especially with lead acid chemistries, so your panels need to give you significantly more energy than your battery will hold. Also if you don't have one already, look for a circuit that makes more efficient use of the power that is available. Here is one for Lithium batteries as an example, look for one that is compatible with SLA: https://www.sparkfun.com/products/12885