Battery drain issues with Phonak Cross

A friend is deaf in one ear and now needing a hearing aid for the better ear. The first recommendation was a recharable Phonak with Cross. However, she could never get the battery to stay charged for more than 6 hours using the cross. The provider ordered a second unit with the same result, so they are now going to try Signia. She also felt like she had to turn the volume setting UP when using the cross.
I am a LONG time hearing aid customer. My husband started wearing aids in the late 1970’s and I have worn them for over 20 years, so I am a big ADVOCATE and I feel badly that she has had this experience.
We use the same Audi - - and I love her and feel she works hard, with all the major brands - - to fit to the INDIVIDUAL.
My brother wears a Signia with a Cross and has never had this issue.
Any thoughts/experiences with this issue would be greatly appreciated.