Battery Grip Question

I have 2 cameras, the 7D2 & 5D3. Both have grips. The 5d3 has a Canon Grip, and the 7D2 has a Vello Grip.
I notice something that I think is strange when charging the batteries.
One of the batteries charges in less than a minute while the other takes 30 min to 1 hour to charge.

Question: Is there some way that the Grip is only using one of the batteries at a time, and when that first battery gets low in charge, it switches to the other battery?

No matter which Grip (Canon or Vello) that I use the charging happens the way I describe.

Anyone know why this is happening?

What batteries are you using?

I have the Canon OEM grip for my 7D2 and Canon, DotPhoto and ExPro batteries all seem to discharge at fairly similar rates. Note the DotPhoto and ExPro batteries are used in pairs (in the grip), the Canon battery is on it's own - too stupidly expensive to buy another and the others work very nicely!

If you have the batteries registered and study the battery information as you shoot, you will see that when starting with two batteries of current equal charge (such as 100% right after recharge) the camera alternates between the batteries shot-for-shot.

If the batteries are not equally charged, the camera draws from the higher-charged battery until they are equal, then goes to alternating mode.

At least, that's what it does when working properly.

What is the difference between the batteries (Brand? Age?) and how are you charging them?