How many amps to battery bank

andyrud · July 2020

I want to add an additional 24 volt panel to my existing 1200 watt array. The 24 volt array is controlled by a midnight solar 150 controller that coverts the power so it can be input into a 12 volt 600 amp battery. My question is this:

The 24 volt panel says it will produce approx. 8 amps. How many additional amps will go into my battery, 8 amps or 16 amps?

Thanks in advance
Andy R

Photowhit · July 2020

andyrud said:
The 24 volt panel says it will produce approx. 8 amps. How many additional amps will go into my battery, 8 amps or 16 amps?

It depends on that type and wattage your panel is...
You want the new panel to be like the panels you currently have AND wired in the same manner.

So if you have a 1200 watt array made up of 4 - 300 watt - 72 cell panels all in parallel, you could just add your new panel in parallel with the other 4. PROVIDED it has 72 cells.

If you have 6 - 200watt - 60 cell panels, and they are wired in 3 strings of 2 panels, you would want to add a pair of panels, adding 1 won't help much.

Assuming you are adding panels in a manner that they would be useful, then you can expect about 75% of the wattage of the panel, divided by whatever the charging voltage is; So if you add a 200 watt panel and charging at 14.5 volts, you should expect about (150/14.5=) 10.3 additional amps when you battery bank can use it.

Photowhit · July 2020

Once you battery reaches Absorb voltage, you battery won't need the extra current and naturally take less over time until you reach Float.

Here's a bit of info on battery charging.

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.

Marc Kurth · July 2020

I wish more people actually understood your explanation above! People often argue with me and insist that their charging system modulates the charging current downward in order to avoid overcharging during the absorb phase.

Photowhit · July 2020

High praise coming from someone with your background.

It's not an easy thing to describe.

How many amps to battery bank

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