Measuring amps. 120 watt panels.

Submitted: Wednesday, Feb 22, 2017 at 14:42
ThreadID: 134341 Views:3279 Replies:3 FollowUps:4
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One for the solar gurus. I am getting 14.2 volts at battery but only 0.09 to 1.02 amps. Does the system have to be under load to measure output in amps? Trust I have explained ok as I find this subject difficult to grasp. Cheers
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Reply By: Ross M - Wednesday, Feb 22, 2017 at 15:21

Wednesday, Feb 22, 2017 at 15:21
That means the battery is charged and doesn't need more current, the regulator detects the batt voltage is at the required maximum and electronically turns the power off from the panel. The amp flow will flicker a bit as the reg continually checks if the batt voltage is same or less. If less and loaded, the amps will flow as panel voltage is applied until it again gets to the regulated voltage.

If battery volatge was around 12, the reg would turn ON and deliver the full ability of the panel to the batt. That makes it charge up till regulated level again.
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Follow Up By: Member - Kevin S12 - Wednesday, Feb 22, 2017 at 23:30

Wednesday, Feb 22, 2017 at 23:30
Thanks Ross, another peice of the puzzle falls into place. What I am trying to do is measure the output in amps to understand the period of time an 80ah battery will last running a 40 liter fridge. I guess the answer is calculated at the rated output of the panel.
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Follow Up By: The Bantam - Saturday, Feb 25, 2017 at 10:49

Saturday, Feb 25, 2017 at 10:49
Kevin the thing you MUST understand is that there are many variables ........ a battery is not a bucket of water, a solar panel is not a tap and there will be no hard and fast simple figures.

The output capacity of the pannel will vary with the amount of sun it recieves.

The capacity of the battery will vary dependfing on how heavily it is loaded.

The charge current of the batter will vary greatly even with a constant voltage ....... beginning at a low state of charge, the battery will accept high charge currents ...... as the battery " fills with charge" the charge current will progressively drop till practically nuthin'

Oh yeh ... the average current drawn by the fridge will vary dramaticly depending on how the fridge is used and what the ambient temperature is.

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Reply By: Iza B - Wednesday, Feb 22, 2017 at 15:30

Wednesday, Feb 22, 2017 at 15:30
What is the voltage at the battery terminals after disconnecting and resting for a couple of hours? If the battery is fully charged, then very little current will be going into the battery. Voltage as Electromotive Force (EMF) responds to a voltage differential. Think most simple alternator systems here. Most (no need for splitting hairs here) put out around that 14.2 volts to bring a starter battery up to about 13.6-8 at rest in a fully charged state. The smarter alternator systems can be regularly seen to start out at that 14+ volt then drop to around 13.8 when the smarts realize the starter battery is fully charged. Solar regulators typically miss out on the smarts that come in the more modern alternator systems.

Another example can be seen in the better DC to DC chargers. My D250S puts out about 20 Amps right up to the point that the inbuilt smarts see the battery as fully charged then the box drops way, way back to not much at all.

AnswerID: 608801

Follow Up By: HKB Electronics - Wednesday, Feb 22, 2017 at 16:06

Wednesday, Feb 22, 2017 at 16:06
Actually your 250 puts out 20A until the battery terminal voltage rises to about 14.4V (its maximum allowed voltage) at this point it swaps to constant voltage mode, this occurs when the battery is around 80% - 90% charged dependent on the size of the battery. As the battery continues to charge the charge current will gradual reduce as the internal voltage of the battery rises. The charger will continue in this mode till the current drops top less than a amp and then switch to float charge.

Regarding alternators, most alternators have no idea when the battery is charged, they simple maintain a float voltage, the voltage may reduce overtime but this is due to temperature compensation.

ECU controlled systems are different, the ECU determines the charge state of the battery and controls the alternators output, generally the voltage will be around 14.2V@24C and then reduce to a very low level once the ECU is happy the battery has reached a "reasonable state of charge"

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Follow Up By: Iza B - Wednesday, Feb 22, 2017 at 21:51

Wednesday, Feb 22, 2017 at 21:51
Pretty much what I said, without the complications. I was trying to show a bit of empathy for the original poster who did say he did find the whole thing a bit difficult to understand.
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Reply By: Malcom M - Thursday, Feb 23, 2017 at 11:55

Thursday, Feb 23, 2017 at 11:55

Volts are pretty much a constant value until such time as your supply item can no longer deliver the requested current, at which time the volts value drops.

You can only measure the large current by requesting a large current.
ie the difference between a 100 and 200 watt lightbulb.

You can get cheap inline power meters that will show you instantaneous current either being sucked out of the battery or put back into it

If you want to measure for fridge theoretical run time from your batteries then you need a system that includes a low voltage cut out plus an elapsed time counter.
The idea is to run the fridge from your fully charged battery and measure the time taken from switch on till the low voltage cutout disconnects the fridge. The only variables would be temperature and how often you might open the fridge.
AnswerID: 608832

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