2017 Yamaha bad charging issue?

[ParCarWoes]

I'm trying to help the owner out. He bought a Yamaha and said it's about 3 years old. Already on its second set of batteries and these are getting weak. He said it is losing power by the time he's on the 9th hole. It is being charged by a Delta-Q model 913-4800-09 charger.
After being on the charger for a few days it shows a green light next to the full battery symbol and a yellow light next to the symbol for what I guess is line current.
I forgot to see what brand batteries are in the cart.
I put a multi-meter and measured each battery voltage. Four of them were at 8.40 volts and two were at 8.00 volts.
Is this probably a charger issue, or something onboard the golf cart?
Thanks!

[INTHEWINDTOO]

Have you measured each battery voltage while driving? And after they begin to weaken?

[ParCarWoes]

I have not driven it.
I checked today and it has US8VGC batteries which are in the 10 pre-programmed types listed for the Delta-Q charger.
The 2 weaker batteries are now showing 7.87V dc as it sits on the charger.
Obviously the charger is not always supplying current to the batteries.