Voltage drop when plugged into charge controller? (18-19V -> 12.7V)

Richars65

Hey folks. I'm new to solar and I'm trying to figure out an issue on my camper. 

Basically, I'd been having some trouble with my old SCC and installed a Renogy Adventurer 30A PWM controller.  I have a 100W panel and 2 x sealed lead acid batteries (Interstate HD24-DP).

I initially installed the PV leads first (was an issue in the battery power circuit so wanted to isolate it), but then disconnected everything.  With open circuit, the battery reading is 12.7V, the PV reads 18 or 19V.  I figure great, that'll definitely be enough to charge the batteries, should be set. However, when I connect the battery and PV leads back up, the batteries still read 12.7V but now the PV is down to 12.7 or 12.8V. Not running all kinds of stuff on the circuit.  Any ideas what may be causing the drop?  I have the charge controller set to 12V SLA settings.  Is it possible it's pulling very little because the batteries are fully charged? 

littleharbor2

The battery voltage dictates the charging voltage. If the battery is in a low state of charge it will show on the readout. When you get sufficient sun on the panels the voltage will slowly rise to the absorb setpoint.
12.7 vdc is close to full so you may not see much activity on the controller. Put a substantial load on the batteries and you should see the incoming current or watts, whatever your display shows, rise. SLA batteries can take up to 20% of their amp hour rating so you have 128 amp hours in batteries, you can charge at up to 25 amps. I think one 100 watt panel is about 6 amps at best. Two more panels would give you a good match for keeping those batteries happy.

Photowhit

Richars65 said:

Hey folks. I'm new to solar and I'm trying to figure out an issue on my camper. 

Basically, I'd been having some trouble with my old SCC and installed a Renogy Adventurer 30A PWM controller.  I have a 100W panel and 2 x sealed lead acid batteries (Interstate HD24-DP).

I initially installed the PV leads first (was an issue in the battery power circuit so wanted to isolate it), but then disconnected everything.  With open circuit, the battery reading is 12.7V, the PV reads 18 or 19V.  I figure great, that'll definitely be enough to charge the batteries, should be set. However, when I connect the battery and PV leads back up, the batteries still read 12.7V but now the PV is down to 12.7 or 12.8V. Not running all kinds of stuff on the circuit.  Any ideas what may be causing the drop?  I have the charge controller set to 12V SLA settings.  Is it possible it's pulling very little because the batteries are fully charged? 

When you are testing the solar panel you are reading the open circuit value, or VOC. Once adding to a system, you are looking at the "System Voltage". If in direct sunlight you should see a gradual rise in system voltage.

Understand that 100watt panel isn't very large. 

The batteries you have aren't true deep cycle batteries, but at 61 ah each for a 122 ah (amp hour) total the potential 6 amps from the solar panel represents about a 5% charge rate, without "...running all kinds of stuff on the circuit". Likely the manufacturer would like to see a 10% charge rate.

Richars65

Great thanks both of you for the info. Seems like I need to look for at least another 100w panel or two. 

I can’t recall now where but some post I read in another forum suggested it may be a problem with the batteries themselves. I’d like to try and do some testing to make sure the system is operating as it should.  It seems like I need to put some load on the system (I really only have a fridge and lights, the camper is small). If I understand correctly, I should see the PV vdc go up when the load is put on the system. 

mike_s

What's important is the current, not the voltage. 

A PWM controller basically act like a switch. When bulk charging, the panels are simply connected directly to the battery. The voltage you see will be the battery voltage, which will initially be only slightly higher than when it's not connected to the panel(s). 

As the battery charges, the voltage will rise. When it gets high enough (to the absorption voltage), the PWM controller will begin to switch on/off quickly to maintain that voltage.

littleharbor2

When you put a load on the system the controller will allow current to flow from the panels. The voltage will go up when the charger goes into bulk charging mode and should gradually go up to about 14.4 to 14.6 if you have it set for SLA batteries.

Photowhit

mike_s said:

A PWM controller basically act like a switch. When bulk charging, the panels are simply connected directly to the battery. The voltage you see will be the battery voltage, which will initially be only slightly higher than when it's not connected to the panel(s). 

NO! NO! NO!

It is NOT the battery voltage, but the system voltage. For the current to flow into the battery the potential of higher voltage from the solar panel will keep the system voltage higher than the battery voltage so current will flow into the battery!

In "bulk" the voltage, without a load higher than the potential, will remain higher than the battery voltage until it reaches the absorb set point at which the charge controller will limit the voltage.

If the battery is removed from the system, the voltage will drop.

Photowhit

Richars65 said:

Great thanks both of you for the info. Seems like I need to look for at least another 100w panel or two. 

I can’t recall now where but some post I read in another forum suggested it may be a problem with the batteries themselves. I’d like to try and do some testing to make sure the system is operating as it should.  It seems like I need to put some load on the system (I really only have a fridge and lights, the camper is small). If I understand correctly, I should see the PV vdc go up when the load is put on the system. 

If the system is in 'float' mode you may see an increase in current but not voltage, if the load is less than the available wattage of the solar panel. If larger than the wattage available, the voltage will decrease, but after draining down the battery bank it should move back to a bulk stage of charging.

Here are the stages of charging;

The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

mike_s

Photowhit said:

NO! NO! NO!

Stop being pedantic. This is "Solar Beginners Corner." The difference is only the voltage drop, which wouldn't be much with a 100 W panel. The OP was expecting the panel voltage to stay the same as the open circuit voltage.

Photowhit

mike_s said:

Photowhit said:

NO! NO! NO!

Stop being pedantic. This is "Solar Beginners Corner." The difference is only the voltage drop, which wouldn't be much with a 100 W panel. The OP was expecting the panel voltage to stay the same as the open circuit voltage.

Yep, Beginners corner, don't teach them bad references to begin with and they won't learn them!

System voltage is different than battery voltage.

mike_s

Photowhit said:

System voltage is different than battery voltage.

Not really. Don't confuse things by using the term incorrectly. System voltage is most commonly 12, 24, or 48 volts. It's exactly the same as the nominal battery voltage. It's got nothing to do with any precisely measured voltage while a system is running.

The system voltage refers to the nominal voltage of the connected battery bank such as 12V, 24V, 36V, and 48V. 

https://renogy.force.com/helpcenter/s/article/What-is-the-maximum-PV-input-voltage-of-the-controller-and-the-system-voltage

System voltage is also called rated operational voltage, which refers to the direct current operational voltage (Battery Bank Voltage) of solar power system. Generally, the system voltage value is 12V, 24V or 48V. The medium-scale or large-scale charge controller system voltage value can be 110V and 220V.

- https://zhcsolar.com/solar-charge-controller-settings/

Battery System Voltage
... solar charge controllers below that support 36 volt battery systems in off-grid solar applications. While 36 volt battery systems are not as common as 24 volt or 48 volt systems, ...

- https://www.morningstarcorp.com/voltage/36-volt-solar-charge-controllers/

The charge controller voltage shall be matched with the system voltage. The standard configurations are 12, 24, and 48 volts.

- https://www.opengreenenergy.com/how-to-select-and-size-solar-charge-controller-for-off-grid-pv-system/

we want to look at the nominal system voltage. This will tell us what voltage battery banks the controller is compatible with. In this case, you can use 12V or 24V battery banks.

- https://www.renogy.com/blog/solar-charge-controller-sizing-and-how-to-choose-one-/

Solar charge controllers are rated and sized by the solar module array current and system voltage. Most common are 12, 24, and 48-volt controllers.

- https://www.cedgreentech.com/article/choosing-correct-charge-controller