I've got a roughly 4 yearold Phantom 4 Pro that hasn't been used much in the last 1.5 years. Batteries were stored in a multi-charger in storage mode until about 9 months ago when I moved and the batteries have been unplugged since. Yesterday, I got the drone out and each battery displayed 2-3 out of 4 lights when pressing the button to check power levels. I couldn't locate the multi-charger, so I plugged a battery into the wall charger directly and immediately it switched to a single blinking light. I tried another battery and the same thing happened. After leaving the second battery plugged in overnight, I'm seeing the same thing: a single blinking light closest to the button (LED 4). My third battery has not been connected to the charger and is showing 2 solid and one blinking light when I press the button.

I'm trying to figure out what is going on, and whether or not I need to replace my batteries. Any advice is much appreciated.

Hello there Brack. I am sorry for the trouble this has caused and thank you for reaching out. Can you please try posting a short sample video showing the said issue for us to see and assist you further? In addition the DJI Phantom 4 Pro intelligent flight battery has a Battery protection LED display. Kindly please refer to the post below. Thank you.

Thanks for the help. So I was wrong about it being LED 4, it is LED 1 that is blinking on two of the batteries.

Here are the videos. First is the three batteries currently when pressing the button. The first two only a single LED blinks and the third two are solid and a third is blinking. The first two were like the third before I attached them to power, now they just blink. In the second video you can see the battery just blinking on power. It is this way even after 24 hours being plugged in.

Videos:
https://imgur.com/18KMs9J
https://imgur.com/Tv6FQF5

Hello - the first vid shows the standard display of remaining power in the batteries.  The bottom batteries are very low - 12% or less.  The top battery appears to be okay - a little more than 50% charge.  

There is a risk when allowing your battery to drop down as low as 12% or less because a battery cell can be damaged running that low.  It's something you want to avoid.  If it happens, you want to charge it to at least 50% or so as soon as possible.  

There's also the possibility that you're getting a false reading.  You can turn the batteries on to check the reading.  

When you begin charging a very low battery as shown in the second vid, a blinking lamp of LED 1 is what you would expect to see.   If everything is charging correctly, the blinking lamp will turn solid after a while and the next lamp will begin blinking.  And so on until all four lamps are solid.  

If you leave a low battery on the charger all night and it's charge doesn't improve, it's likely the battery is damaged and should be retired.  There are some things to try, but they are very rarely successful.  

Good luck