P4P batteries are compatible with P4P V2 batteries except for every time you switch from a V1 (Phantom 4 Pro V1) to a V2 (Phantom 4 Pro V2) you have to do a firmware update on the battery. A message will appear saying that there is inconsistent firmware found. It takes about 2 minutes to update so it isn't a real problem but now my V1 batteries need to be updated every time I fly with a V2 and vice versa.I can fly on either drone with an inconsistent firmware version but my battery will drain significantly during flight if I haven't made the firmwares consistent prior to take off.
Here are some videos of my testing:
V1->V2: https://photos.app.goo.gl/FjtoZVQgpRNDWCa89
V2->V1: https://photos.app.goo.gl/Mx9NZs9RAvGMwiLQ7

V2 is on the right. V1 is on the left.
I tested this with multiple batteries to make sure it wasn't just a faulty battery.

Is there a way to avoid having to update the firmwares prior to each flight when switching from V1 to V2 P4Ps?

If you're brave you could try the new beta firmware and hope the battery level match with the one in V2.0., look out for the Ultrasonic sensor error thread and reply to Susan.

Edit: See this post.
And please report back if you try and it solve it (or don't).

Hi Luke, thanks for bringing this to our attention. I have reported this to our engineers for review, hope they can consider this.

Hi all,

I'm also flying a P4P and a P4P V2.0... and each time I swap one of the 4 batteries I have between both drones, I still have to do the battery firmware update.
As Luke Skyflyer said, it doesn't take a long time to do this update, however having to do that each time before take-off is pretty annoying...

As this problem is already 3 months old, is there any news from DJI regarding this very strange behavior??
Thanks!

Rolud Posted at 9-10 23:50
Hi all,

I'm also flying a P4P and a P4P V2.0... and each time I swap one of the 4 batteries I have between both drones, I still have to do the battery firmware update.

I don't think anything has changed. We have started seperating out our P4P batteries into a V2 box and a V1 box so we don't mix them up. That's not a good long-term solution though. :/

So if I have several batteries that I was using for my P4P Pro version 1 and I no longer have the version 1 due to upgrading to the version 2. Once I update all of the batteries I won’t have to repeat the process each time as long as they are only being used in my V2?