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Old 08-11-2016, 12:28   #1
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Solar panel controller question.

I currently have 2 x 100 watts solar panels with 20 amp mppt controller. I have four t-105, 6 volt house batteries wired to 12 v., and one 12 volt starter battery. I want to add another 100 watt panel. Will the 20 amp controller handle this or do I need to add another controller, or change to a 30 or 40 amp controller?

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Old 08-11-2016, 12:39   #2
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Re: Solar panel controller question.

The controller instructions should list the max watts the controller can accept
I think My Morningstar MPPT says 300 watts

Look at the manual or look online

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Old 08-11-2016, 12:45   #3
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Re: Solar panel controller question.

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Originally Posted by galynd View Post
I currently have 2 x 100 watts solar panels with 20 amp mppt controller. I have four t-105, 6 volt house batteries wired to 12 v., and one 12 volt starter battery. I want to add another 100 watt panel. Will the 20 amp controller handle this or do I need to add another controller, or change to a 30 or 40 amp controller?

Thanks,
Greg.
I think you are fine, but just by a little. 300 watts at 14.8v (the recommended bulk charging voltage for t-105s) is 20.3 amps. So, at the maximum efficiency, you'll maybe clip off 0.3 amps. In the real world, you'll hardly every see 20.3 amps, so I don't think it is worth it to change the controller. I don't believe the controller is damaged by slight amp clipping.

First qualification is that your controller allows you to set the bulk/absorption voltage at 14.8. Any lower and your amps will increase, but I still think you can live with the slight clipping when in the optimal conditions. If you have optimal conditions I bet you won't have an undercharging problem with or without the clipping.

Let me also ask whether the third panel is the same as the first two? If the optimal voltage is different (Ivoc is the abbreviation?), then the controller will work less efficient. The controller will aim for the average optimal voltage, which will be suboptimal for all three. I think it is recommended that this value be within 0.5v for all panels. Food for thought.
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Old 08-11-2016, 12:51   #4
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Re: Solar panel controller question.

What Marc said - I have a Tracer 2215BN (rated for 20 amp). The documentation says Maximum input is 260 watts (which, at 12 volts, is 21.6 amps). I have 200 watts of solar and have never seen anything close to 20 amps of input. If you have seen your 200 watt bank create 13.3 amps or less then adding another 100 watt would theoretically stay within the 20 amp limit (assuming that's what yours is).

Pretty sure you'll be fine.
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Old 09-11-2016, 09:44   #5
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Re: Solar panel controller question.

Hi Greg: you are going to need to add an additional charge controller for the extra 100 W panel, or replace your existing 20 amp charge controller with one that will handle a minimum of 25A. How I arrived at this conclusion is fairly simple, 300W of solar divided by 12.8 V (battery voltage) equals 23.4 A. You should strongly consider purchasing and MPPT type controller for your replacement unit which will maximize your energy production. It's a good investment for the small amount of additional money. Good luck
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Old 14-12-2016, 10:10   #6
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Re: Solar panel controller question.

It's strongly recommended to install a bipolar DC circuit breaker just before the controller. So my suggestion: install this DC breaker rated at 20A, then install your new solar panel and see what happens. The controller itself may deal with an eventual extra load, you would be wasting some power if your panels exceed 20A, but you'd be safe.
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Old 14-12-2016, 12:23   #7
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Re: Solar panel controller question.

Imho, add another for redundancy. Backup.
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Old 14-12-2016, 12:51   #8
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Re: Solar panel controller question.

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It's strongly recommended to install a bipolar DC circuit breaker just before the controller. ........................
Could you explain what that is? Compared to what? Thanks, serious question.
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Old 14-12-2016, 13:55   #9
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Re: Solar panel controller question.

There will be some (rare) occasions when the output from the solar controller would be greater than 20A with the 300w of solar panels. Most (not all) solar regulators will self protect themselves and limit the output to 20A. The total loss of power will only be slight, although any loss of output is annoying.

There are some concerns about solar regulator relying on its self protection mechanism to avoid damage. Installing a 20A controller for 300w of solar is not something I would recommend purchasing, but if you have an existing controller it is reasonable to use it and hope the lifespan is acceptable.

Check with the manufacturer that the regulator will self protect at 20A. Also make sure the ventilation is adequate.
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Old 14-12-2016, 14:12   #10
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Re: Solar panel controller question.

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Could you explain what that is? Compared to what? Thanks, serious question.
Bipolar circuit breakers cut both the negative and positive supply in responce to an overload in either pole.

A conventional single pole circuit breaker will only cut the positive supply and will only respond to overload in the positive circuit.

Bipolar cuircuit breakers are superior, but are overkill for many situations. While circuit breakers are recomended on the input side by most panel manufacturers they are much less imprtant than circuit breakers between the battery and solar controller. The output of the solar panels is already self limited.
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Old 14-12-2016, 14:58   #11
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Re: Solar panel controller question.

I have 3 100 watt panels which are never(almost) shaded and a 20 amp mppt controller and it works fine. I have never seen more than 15 amps come in from panels.
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Old 14-12-2016, 22:12   #12
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Re: Solar panel controller question.

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I have never seen more than 15 amps come in from panels.
The average output obtained from the solar panels is more important than the maximum output, which will only happen occasionally.

However, the maximum output is an indication of the "health" of the solar system. Problems such as excessive voltage drop from poor connections/wiring or the solar controller not tracking the MPP adequately, will be reflected in a reduced maximum output.

If you are in an area with good solar insolation, your maximum output should be greater than 15 amps from 300w of solar.
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Old 19-12-2016, 16:37   #13
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Re: Solar panel controller question.

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Originally Posted by noelex 77 View Post
The average output obtained from the solar panels is more important than the maximum output, which will only happen occasionally.

However, the maximum output is an indication of the "health" of the solar system. Problems such as excessive voltage drop from poor connections/wiring or the solar controller not tracking the MPP adequately, will be reflected in a reduced maximum output.

If you are in an area with good solar insolation, your maximum output should be greater than 15 amps from 300w of solar.
If his panels are mounted flat and he's in the upper latitudes, he's not too far off the mark. The max. potential output in those situations could be 16 or 17 A, but from what I've seen here in sunny San Diego, nobody gets a full 20 A out of 300w of panels, those are theoretical max. numbers under STC, real world never comes close to those numbers.
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Old 19-12-2016, 17:03   #14
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Re: Solar panel controller question.

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I The controller itself may deal with an eventual extra load, you would be wasting some power if your panels exceed 20A, but you'd be safe.
Can you explain this?
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Old 19-12-2016, 17:14   #15
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Re: Solar panel controller question.

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Can you explain this?
Solar controllers limit their output. Most of them are capable of producing 10% higher than rated output. So theoretically, you might see up to 22 A out of a 20 A controller, realistically, you'll probably see 18 - 20 A out of 300 w of solar panels in real world usage.
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