Using only a Multimeter...

[FourPoints]

what is the proper way to determine the output in amps, or ah from a solar panel when attached to a battery?

I'm trying to compare 2 panels, one outputs 24 volts, but is rated at 5w, the other outputs 19v, but it rated at 12w.

I can check the volts easy enough and sure enough the 5w panel outputs higher voltage, but how do I test the amperage output to see how much I am actually putting into the batteries?

 

[Timo42]

You could connect your ammeter in series with your panels, that's the usual method.

 

[FourPoints]

lets pretend I don't have one (because I don't), can this be done using only a multimeter?

 

[kloudie1]

the meter will have to have at least a 1 ampere scale, preferably a two ampere scale.. as Timo says, break the hot lead and insert the ammeter set to read amperes (some have different plug holes for amperes) into the gap. Solar panels are strange because they act almost as constant ampere sources.. that is, without a "charge controller" to hold the output voltage at the rated number, they will make fewer watts at 12V than they do at rated.. The 19V rated panel should make more amperes when hooked directly to the battery.. It should be your best power production case..

 

[Timo42]

Multimeters usually have an ammeter function, but iirc you could use a fixed resistor instead of the battery, measure the voltage and use Ohm's law to calculate the amperage. Or spend $20 to get a meter with amps

 

[kloudie1]

Ya really don't have to measure, the 12W one will be the best..
this meter for $4.99 has a 10 ampere scale .. if ya have a Harbor Freight around, ya might want to keep one of these on the boat
http://www.harborfreight.com/7-function-digital-multimeter-90899.html

Remember ya have to put the meter in series with the power lead to meeasure amperes.. that is, all the power goes through the meter.. not like measuring voltage where ya are using a parallel connection.

 

[LloydB]

Another proper test would be a reliability test one of which can be done over the internet, the usual method is anecdotal or documented experience by non-stockholders.

 

[FourPoints]

Ya really don't have to measure, the 12W one will be the best..
this meter for $4.99 has a 10 ampere scale .. if ya have a Harbor Freight around, ya might want to keep one of these on the boat
http://www.harborfreight.com/7-function-digital-multimeter-90899.html

I know the 12w SHOULD be the best, but given the fact that it is the same dimension as the 5w, and visually it is a totally different type of solar cell arrangement, I just want to test this somehow to validate the claim that for a panel that costs only $30 more, it outputs 2.4 times more power.

 

[Bill Roosa]

More is not best, you have to consider what you are trying to do with the panel. Presumably you are trying to charge a 12 volt lead acid battery.
The one with the higher voltage is going to be able to charge you batts when the sun is not as strong more so that the one with lower voltage so over time performance may be better with the higher/smaller wattage panel

 

[Stu Jackson]

If they're physically the same size and you're questioning the difference between the technology used to make them, there have been many published sources about the output of the two different solar cell methods.

 

[JohnShannon]

A solar cell makes about 0.5V at peak power. The panel is comprised of many cells.

The output current is proportional to the individual cell size (and sunshine) the output voltage is determined by the number of cells stacked in series.

A solar panel has a funny behavior. 3 numbers more or less describe it.
Short circuit current
Open circuit voltage
and
Vmpp (voltage of maximum power)

Ideally in your case you want a panel with a Vmpp of say say 14V or so to best match the panel to the battery. It you charge with a 24V panel you will be in the short circuit current area of the panels curve and charge power will be well under rated (at Vmpp) power.

Use the lower voltage higher power panel.

 

[Bill Roosa]

I agree with you JohnS 4 numbers but there are a few kinks to consider. As the panel is exposed to the sun it gets hot and power output drops (aka voltage drops) also on partly cloudy days the voltage drops. This is why you want the max peak output to be in the 19 volt range. That way on cloudy days and really hot ones the voltage will be sufficient to still charge the batts. I believe this is always achieved in the same way, by having 36 cells in parallel.

 

[Zaphro]

This is how I learned about everything I know about electricty and electronics. Looks like it's been update a bit in the last 25 years, but still basic and easy to read for new technicians.

http://jricher.com/NEETS/