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Old 15-08-2015, 12:03   #1
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Solar Panel Max Amps

Take the following solar panel:

Peak Power (+/- 5%) - Pmax 137 W
Rated Voltage - Vmp 24.0 V
Rated Current - Imp 5.7 A
Open Circuit Voltage - Voc 29.1 V
Short circuit Current - Isc 6 A

Let assume an MPPT genasun controller.

The panel spec show a rated current of 5.7 AMPS.

If my batteries require a charge of 14.8 volts peak what is the max amperage you can expect to see from the panel?

5.7 amps or 9.25 amps (137 watts / 14.8 volts)?
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Old 15-08-2015, 13:11   #2
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Re: Solar Panel Max Amps

Your maximum amps will occur when the battery is discharged, not at the 14.8V level. Say the battery is just under 12V, and under charge it starts at 12V. This would give you 11.4A (or, say 10.8A with 95% MPPT efficiency). As the battery voltage rises, the current will drop.
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Old 15-08-2015, 17:29   #3
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Re: Solar Panel Max Amps

I too would expect some 10'ish figure at 12 to 13 something volts. Tapering off as the voltage gets higher.

Perhaps a very smart regulator can keep up the higher amps in high voltage / high drain scenario, but this would be around 13.4 to 13.8 for a 12V lead acid. (I think)

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Old 15-08-2015, 18:16   #4
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Re: Solar Panel Max Amps

Quote:
Originally Posted by Paul Elliott View Post
Your maximum amps will occur when the battery is discharged, not at the 14.8V level. Say the battery is just under 12V, and under charge it starts at 12V. This would give you 11.4A (or, say 10.8A with 95% MPPT efficiency). As the battery voltage rises, the current will drop.
Even if you have a fridge/freezer, nav instruments, radio, autopilot etc etc running?
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Old 15-08-2015, 20:00   #5
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Re: Solar Panel Max Amps

Quote:
Originally Posted by StuM View Post
Even if you have a fridge/freezer, nav instruments, radio, autopilot etc etc running?
With that much load, the batt voltage will not be rising...

Jim
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Old 15-08-2015, 20:12   #6
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Re: Solar Panel Max Amps

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With that much load, the batt voltage will not be rising...

Jim
OK then - any/some of the above ( say 6 - 8 amps worth)
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Old 15-08-2015, 20:24   #7
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Re: Solar Panel Max Amps

Assuming a completely dead battery - I'm talking theoretical here. I didn't really account for all the variables I suppose.

The gist of my question is - if a panel manufacturer states a maximum rating (above example is 5.7 amps) is that the highest amperage you can expect to get out of the panel, regardless of higher voltages available? Or can you exceed that amp output under the right conditions, and if so, what would be the highest theoretical output you could achieve?
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Old 15-08-2015, 21:50   #8
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Re: Solar Panel Max Amps

With an MPPT controller the amps from the panel and the amps into the battery will not be the same most of the time. The MPPT controller will try to adjust the panel amps (and thus voltage) so as to get the maximum power from the panel. Assuming the batteries will accept this much power then the battery current will be about 90-95% of the panel power divided by the battery voltage (assuming the wire sizes are adequate).
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Old 15-08-2015, 21:54   #9
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Re: Solar Panel Max Amps

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With an MPPT controller the amps from the panel and the amps into the battery will not be the same most of the time. The MPPT controller will try to adjust the panel amps (and thus voltage) so as to get the maximum power from the panel. Assuming the batteries will accept this much power then the battery current will be about 90-95% of the panel power divided by the battery voltage (assuming the wire sizes are adequate).
Then why list a maximum panel amp output?
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Old 15-08-2015, 23:05   #10
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Re: Solar Panel Max Amps

Very OLD Kyocera Panels installed in 2000 4 x 125 watt into a Trace C-40 controller and 2 GellCell 8-Ds (450 amp hours) the batteries were 7-years old at this time. Data came from Trace battery monitor on the controller and Link 2000R battery monitor.

8-amps at 13.2V (105 watts) per panel at 11 AM
9-amps at 14.3V (128 watts) per panel a little after 1 PM

Batteries discharged overnight to a resting voltage of about 12.4 (about 90 amp hours down) at 7 AM.

Latitude 24 N in June (sun directly overhead) Start seeing a small charge before 8 AM and am getting about 4 amps/panel by 9:30 AM.

Not very scientific data but real life for many years.

I don't have the data sheets but I think this it:

Maximum Power: 120 watts
Maximum Power Voltage: 16.9 volts
Maximum Power Current: 7.10 amps
Open Circuit Voltage: 21.5 Voc
Short-circuit Current: 7.45 amps

I do remember testing an open circuit voltage at 19.6V in San Diego sunshine before installing the panels so the datasheet above is close.
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Old 16-08-2015, 04:32   #11
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Re: Solar Panel Max Amps

I like to think of a good MPPT controller as sort of a self-adjusting DC-DC converter. When the panel voltage lis above the battery voltage, and the battery is not full, you will have more amps coming out of the controller (to the battery) than the amps coming in from the panel. The watts on either side are nearly the same (98% or so efficiency with the Genasun). Use a clamp ammeter to measure both sides and check it out, it's pretty cool.
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Old 16-08-2015, 05:12   #12
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Re: Solar Panel Max Amps

Quote:
The gist of my question is - if a panel manufacturer states a maximum rating (above example is 5.7 amps) is that the highest amperage you can expect to get out of the panel, regardless of higher voltages available? Or can you exceed that amp output under the right conditions, and if so, what would be the highest theoretical output you could achieve?
Using you original numbers, Peak Power (+/- 5%) - Pmax 137 W,

Amperage is a calculation of watts divided by voltage,(As on the label,Peak Power (
Pmax137 W Rated Voltage - Vmp 24.0 V Rated Current - Imp 5.7 A, 137/24=5.7 ) So your theoretical max amps are 137w/ battery voltage, very flat battery at 10 volts would be 13.7 amps, battery just on the top of bulk cycle at 14.3 v = 9.5 amps, This is supposing you have full STC conditions (1000w psm, on axis, 25c panel temp) and no losses from controller & wiring,
There is a condition called "edge of cloud effect" when you can briefly get spikes, But nothing substantial for charging,

To add, When you are looking at the label, There is an often missed "number" on there, Temperature Coefficient, Can cost you 20% + important consideration if you run in hot weather, Have a look at this (easier than me typing it lol )http://www.solar-facts-and-advice.co...mperature.html
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Old 16-08-2015, 07:28   #13
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Re: Solar Panel Max Amps

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Originally Posted by zboss View Post
Then why list a maximum panel amp output?
That's so the system designer will know how to specify other components in the overall system. The panel specification of volts and amps tells how to specify the controller. The controller must be able to handle the maximum panel volts and amps (not necessarily at the same time). Also, the wire sizing from the panel to the controller should take into account the maximum panel amps.
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Old 16-08-2015, 09:59   #14
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Re: Solar Panel Max Amps

Very interesting but WAY too technical for me �� but sort of a related question -I have two 135 Kyocera panels and blue sky mppt controller an six very old 6 volt golf cart house batteries Problem is that when boat is left in storage (with all power drains off) the batteries need continual topping up (at least monthly) as water in cells drops a lot--
Couple of questions:is this to be expected with old batteries?. Is the problem likely because of the age of the batteries (7 years+). Will I have same boiling off problem when I replace the house batteries this year.?
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Old 16-08-2015, 10:40   #15
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Re: Solar Panel Max Amps

Foothillsailor:

I have the same battery configuration - live aboard in San Diego with 500 watts of solar panels.

May - Sept I put about 2 ounces of water in each cell once a month
Oct - April 2 ounces every couple months

I've experienced that water consumption with brand new batteries and seven year old batteries.

I had water miser caps on the batteries for years but they did not do much to reduce the need to check the liquid level in the cells every couple weeks. Just the price of inexpensive flooded cell technology.

Is your solar controller float setting adjusted properly?
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