Battery discharging over night WITH charge controller

Hello, hope someone can shed some light on my problem.

I have a simple solar powered gate opener setup at home. I am having trouble with the battery seemingly discharging over night.

I have 2 x 20w panels wired in series making 24v. These are then wired to a charge controller, which is then wired to 2x 12v 7ah batteries also in series.

Panels : BlueFusion Mono Solar Panels 10W - 100W. For grid-tie commercial system builders and off-grid Caravan, Yacht applications. - newbielink:https://midmarine.com/product/bluefusion-mono-solar-panel/ [nonactive]

Charge Controller : LCD Solar Panel Battery Regulator Charge Controller Dual USB 10A/20A/30A 12V/24V | eBay - newbielink:https://www.ebay.co.uk/itm/LCD-Solar-Panel-Battery-Regulator-Charge-Controller-Dual-USB-10A-20A-30A-12V-24V/123133341839?ssPageName=STRK%3AMEBIDX%3AIT&var=423545070868&_trksid=p2057872.m2749.l2649 [nonactive]

When sun hits the panels the charge controller shows it is charging and the volts go up to 24-26v and the battery level shows full/mostly full. In the morning when I check, the charge controller shows 19-20 something volts and the battery level is empty.

I cant figure out if the charge controller doesn't have a blocking diode or it isn't working, or if the charge controller battery level /volts indicator are just wrong. Or if I am missing something altogether!

At the moment I don't have anything wired to the load of the charge controller as I am just trying to figure out the charge situation.

I thought about putting a schottky/blocking diode on the + side of the wire going to the controller to test if the controller actually does have a function blocking diode or not. But not sure if that is the correct thing to do.

Any help would be greatly appreciated.

I'm sorry I don't understand what you are saying, or asking.

Thanks for your reply. I am a total newbie, so any advice on what to check is welcome.

The batteries are not deep cycle batteries, but as far as i am aware the small batteries i am using are Ok for smaller PV systems such as mine. At least that seems to be the information i am finding online. I am sure you are totally correct about needing Deep cycles batteries for larger systems though.

I am open to being wrong about this, but I am using the batteries used in other similar systems as mine, and are also the batteries recommended by the manufacturer of the gate system.

I have actually purchased a new slightly better charge controller now, and also installed a schottky diode to stop discharge via the PV over night.

One thing you did mention though is something i have had in my mind. If i bring the batteries in to my house and charge them, they will hold their charge when just sat on the bench.

I am still either loosing charge not via load, or the batteries are simply not charging during the day.

The charge controller does show a float charge at least, so im not sure if this is enough to be recharging the batteries after a couple of runs of load.

Thanks again