concern

Re: concern

Try just one connected at a time--see if one is producing 90% of the current and the other is only producing 10%... This would be the ultimate test (of course, your battery has to be discharged/load present so that the charge controller will output maximum current.

You could also try bypassing the controller (solar panel+10 amp meter) and see what current each panel/pair can produce into your battery bank.

Another test is to place a current meter set to 10 amp maximum scale and put it across each panel... The short circuit should be near--or a bit greater--than Imp rated current.

Amorphous panels can lose upwards of 20% of their output current in the first 6 months--they are supposed to degrade much less than that after six months--at least that is what the manufacturers claim.

On my GT array--with "good BP" panels with a Grid Tied Inverter system, I typically see about 70% or so of the STC rated wattage (2,500 watts out of 3,500 "panel" rated watts).

I only see 3,000 Watts (~85%) on a very clear and cool day with clean panels--and then only a few times per year.

Panels on a GT Inverter (properly running) is going to be about the maximum power one can get from solar panels. Off-Grid / Battery Charging type are going to get less than maximum unless the controller thinks the battery needs more current (controller settings, battery conditions, and load conditions are going to all affect the "max" current). With a calibrated/accurate DVM, you can measure the battery voltage and the controller output voltage (on its output terminals) and make sure that everything is working to specifications (proper voltage, not much voltage drop, bulk/absorb/float stages, etc.).

-Bill

Re: concern

Yes, just disconnect the panels from the controller (or at least one lead), and put your Fluke set to 10 amp scale (and leads in appropriate holes in meter) and measure the current in full sun (panels pointing within 10 degrees of sun).

Solar panels will not be damaged by the "short" of the current meter... The current will be Isc (Current, short circuit) rating of panels...

In your case, it should be Isc of the two panels in parallel--roughly equal to or slightly greater than Imp.

Yes, you could put a 2.4+ amp load on the battery--but now you have more "parts" to figure out what is happening... Is the controller limiting current, or are the panels? Is the controller limiting current because it is working properly or broken. It is even possible that the amp meter on your controller is not accurate.

One quick and dirty test to see what the controller is doing--in full sun, measure the solar panel voltage at the controller, and the battery voltage at the controller with your DVM... Assuming it is a PWM (not MPPT) controller, if the controller is trying to pass full panel current, the voltage at the controller input should be roughly equal too, or +1 volt higher than the battery bus voltage... If the solar panel voltage is more than +2 volts higher than the battery bus voltage, it is probably trying to throttle back the charging current to the battery (controller "switch" is not 100% closed).

By the way, check the Deka battery specs... It appears to be a Gel Cell type battery... And you should not be charging it much more than C/20 (5%) of the 31AH--or ~1.55 amps... (sorry, I have to go right now--so I cannot hunt up the mfg. specs. at the moment).

-Bill

Re: concern

The charger is probably controlling charging current by operating at a specific voltage set point... Basically, if you set your charging point at xx.x volts, the battery chemistry will probably limit the current to an "acceptable" value...

From the MK Battery website, PDF Spec. sheet:

Charge Voltage @ 68F (20C): Cycle 2.30 to 2.35; Float 2.25 to 2.30 v.p.c (Volts per cell?)

Important Charging instructions: Warranty void if opened or improperly charged. Constant under or overcharging will damage any battery and shorten its life! Use a good constant potential, voltage-regulated charger. For 12-volt batteries, charge to at least 13.8 volts but no more than 14.1 volts at 68F (20C)...

And found another PDF document on charging:

Max. [Bulk Charge State] Current (Monoblock Battery Design) Gel: 30% of 20hr Rate

I assume that this is for a multi-cell battery (12 volt) and the Bulk charge rate of 30% of 20hr rate (hmm, this is not clear to me--but it appears that this would be):

(31 AH / 20 Hours) * 0.30 = 0.465 amps

But that would mean that it would take 20/0.3=67 hours to charge from "dead"... Is that what they really mean? Or 1/67=1.5% charge rate...

Or, do they intend:

31 AH * 0.3 = 9.3 amps...

But that is way to much for what Wind-Sun's FAQ says for a Gel Cell battery.

Then we have another GEL Charging FAQ (read whole thing--not everything has been excerpted here):

Bulk Stage I1
Maintain Current <= 30 A per 100 Ah C20
Typically, Constant Current, but Constant
Power, or Taper Charge Permitted
End when voltage = 2.30 to 2.35 V/cell (20°C)

Absorption Stage V1
Maintain Constant Terminal Voltage
(Adjusting only for changing battery
temperature)
Voltage = 2.30 to 2.35 V/cell (20°C)

Optional Accelerated Finishing Stage I2
Maintain Constant Current:
1 to 1.5 A per 100 Ah C20
Charge for 1 to 4 hours based on Ah accumulated
in first two stages:
<25% of C20 – 1 hour
25% to 50% of C20 – 2 hours
>50% of C20 – 4 hours

There is a bunch of technical information (including the above links) here.

It appears that MK Batteries has similar charging profiles between their AGM and GEL batteries (other than exact voltage settings)... And that a charge rate of 25-30% of the 20Hr capacity of the battery is correct (~9 amps in your case)... The low 1-1.5% charge rates is the "trickle charge" limit for long term storage (if the battery is stored more than 3 months without load/charging).

-Bill

Re: concern

Because, the wording in the first battery spec. was unclear--they talked about 30% of the C@20Hr rate... Which would have been 0.3*C/20 (and given a 67 hour charge time)...

But, they appeared to really mean that you can charge at 0.3*C ("C"apacity measured at the 20Hr rate)--which appears to be the correct interpreation because the mfg. estimates a 4 hour charging cycle for a battery taken down to <25% state of charge (I think that is what they are saying)... Which is really quite good--AGM's are the only other "typical" lead acid battery that can charge at those higher rates.

The 1.55 Amp rate appears to be the maximum "trickle" charge rate.

At least, according to MK Batteries, they do not have the low charge rate restriction that Wind-Sun found/saw that Gel Batteries typically needed for long life...

In fact, if you go through the online specs. for MK Batteries, you find that the AGM and Gel batteries have the same charging profile--only slightly differing voltage steps (because of slightly differing construction/chemistry?).

-Bill

Re: concern

Yes, when "we" talk about charge and discharge rates, it is usually C=xxAH (when measured at the 20Hr rate--X amps such that 20 hours will discharge 100% of the battery's energy)--that C/10 or C/20 is XXAH/10 or XXAH/20...

1.55 amps would be the maximum long term charge rate (it appears)... But, there is really no reason to charge at that high of current for a battery that is in storage (there are charts of storage time vs temperature where it can be several months or longer before a Gel battery needs to be recharged in storage).

Remember that the company is talking about 1.55 amps 24 hours per day--you are able charge at that rate for 3-5 hours per day... So it is much less overall.

If the battery where in storage (no load), the amount of trickle charge needed is probably on the order of a few 0.1'ths of amps (low self discharge).

But, the recommendation of "...at least 13.8 volts but no more than 14.1 volts at 68F (20C)..." for "charging" and 0.30 volts less for float charging will probably take care of your needs. If your charger does not have a "float" mode--you can probably just crank the charging voltage towards 13.8 volts (if you don't draw much power from the batteries) or towards 14.1 volts if you take significant amounts of power every day (~10%+ or so).

In the end, you need to make sure your "loads" are less than the ability of your solar panels to put energy into the system (allow to recharge batteries plus losses)... And given the variations in seasons, weather, etc... You either end up with a system that is way over-sized, can only be used 6-9 months of the year, and/or you have a backup power source (generator, wind turbine, AC mains, etc.).

I don't think there is anything "wrong" that you can fix... It is the limitations of the panels (and probably controller) you have (and the system overall).

-Bill