# Help with solar power system.

Discussion in 'Power Electronics' started by Bearlike, Mar 26, 2020.

1. ### Bearlike

7
0
Mar 26, 2020
Hello all, I need some advice please.
My solar kit:
An old 255watt, 30.2 volt, 8.43amp solar panel.
Batteries x3 MK AGM mobility M45-12 sld m, 12 volt 45ah :Max charge rate 13.5amps
Duel Charge controller PWM 20 amp LTD-D1220d. :Max input from solar 41volt, 260watt

Lets say that it's a blazing hot day and the old panel is working to it's max; will the charge controller pump out 8.43 or 20 amps? Am I right in thinking what ever amps it receives, it gives to the batteries (8.43a)?
I don't want to charge the batteries too quickly or be left with out power; I'll be running 12volt led lighting, laptops and a 12volt satellite tv receiver & 12volt computer screen.
Do you think I have enough batteries?
The problem that I have is two batteries have been used for a year, and the other is new. Which is why I got a duel channel controller. I intended to buy another battery of the same kind, but would it be best if I use the two old batteries on one channel, and buying a larger wet cell on the other?

2. ### Harald KappModeratorModerator

9,668
2,019
Nov 17, 2011
A solar panel operates not from temperature but from solar irradiation. It can produce power on a sunny but cold day and no power on a hot but shady day.

No.
Add 1: the controller has internal losses, power that is not available to the battery.
Add 2: You need to think in terms of power. Looking at voltage of current alone is insufficient. See below:

Assuming "the old panel is working to it's max" which would be 30.2 V, 8.43 A in your case. That is equal to 255 W (rounded numbers). Let's further assume the output of the battery charger is 14 V (to account for a charge voltage higher than the battery's nominal 12 V - adjust numbers to the real values you can measure). 255 W at 14 V means the charge current is 18 A, but: you need to take into account the internal losses of the charger, usually expressed as efficiency. Let us assume the charger is 85 % efficient (again, check real numbers in the datasheet), then the output power available is 0.85 × 255 W = 204 W which results in a max. charge current of 14.5 A. This is more than your batteries are rated for, so you'll have to limit the charge current at the controller to 13.5 A.

A well designed charge controller will take care of that.
A smart move in my opinion.

That depends on the power drawn by your loads and the charge state of the batteries.
Assuming (again) that your batteries are fully charged and you can discharge them to 20 % of their nominal capacity (again: real numbers vary and depend on the type and age of the battery you use). At 45 Ah that means usable capacity is around 36 Ah.
With an average load current of "x A", each battery will last for t = (36 Ah / x A) - result in hours.

Paralleling batteries is problematic if the batteries are not nearly identical. Even if of the same type but different age. Better use the old batteries as long as they last, then replace both by new ones.

3. ### Bearlike

7
0
Mar 26, 2020
Thank you for the valuable info.
The out put of the controller is 14.6 volts with an alleged 95% efficiency.
So I need to down size to a 200 watt panel, or as you say limit the output; as there is no facility to do this with this controller.
Is there a limiter that I could use? Some bolt on black box of tricks?

4. ### Externet

732
154
Aug 24, 2009
Your solar panel may be capable of delivering 8.43 Amperes, IF the 30.2V batteries are discharged enough to accept them; and the 20 Amperes is the designed output capability of your charge controller.

5. ### Bearlike

7
0
Mar 26, 2020
Hello, it's the panel that's 30.2volt, but I know what you mean.
That's an interesting crest you have; Spanish English, with a crown, Gibraltar maybe? Barking up the wrong tree, or just barking. Lol.

6. ### Harald KappModeratorModerator

9,668
2,019
Nov 17, 2011
Do you have a link to the datasheet or manual of the controller? I can't find LTD-D1220d anywhere.
Just that the controller is capable of delivering 20 A doesn't mean it will do so.

7. ### Bearlike

7
0
Mar 26, 2020
The only reason that I bought the 20 amp controller is the restrictions on the solar panel wattage.
As the panel is a 255watt I had to go for the 260watt max, from solar, controller. I really wouldn't want to have 20 amps going in to my batteries, 8 to 10 amp and I'd be happy.

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8. ### Harald KappModeratorModerator

9,668
2,019
Nov 17, 2011
Looks like as described under chapter IV you can set the charge current, although the details are not really clear. You may have to experiment with these settings, using an amperemeter to check the current.

9. ### Bearlike

7
0
Mar 26, 2020
Happy days.
I thought that section was about charge priority between the two batteries. Read it, scratch the head, throw it in the draw.
Thank you so much for your help.