Anyone know a source for Z-20 battery replacement?

Does anyone know where a suitable replacement battery for a Z-20 can be purchased? I have one battery that seems considerably weaker than the others (four batteries will be at 11.8 volts immediately after a long ride, and one will be somewhere between 9-10 volts) that I would like to replace before the mismatch causes damage to the others.

I've emailed EVT America about a replacement twice, but haven't gotten any response. I've looked at the B&B's and other brands, but haven't found anything that matches the physical and electrical characteristics of the OEM batteries. I've considered simply installing a BattEQ (if/when they become available) and hoping for the best, but I'd rather take a more proactive approach. When the PHET 12v/36Ah LiFePO4 batteries become available, and if they have a price I can afford, I would consider replacing all five batteries, but LiFePO4 isn't really an option for me at this time.

I'm hoping someone has found a battery that will fit without having to rework the battery box. If anyone has any leads, I would appreciate it.

FYI, for those who don't have the battery size memorized, it's 193mm (l) x 132mm (w) x 170mm (h) and rated for 35Ah at a 20 hour rate.

Thanks,

Jon Stibal

PS: No disrespect to Don Harmon of LifeBatt or other manufacturers of LiFePO4 batteries; PHET is the only manufacturer I've found that has a battery that would physically be a direct drop-in replacement for the Z-20 batteries. I'd be very happy if someone had a 12v LiFePO4 battery in the >28AH range that would fit (and yes, I chose 28 AH specifically because of LifeBatts forthcoming 14AH cells).