Hello Philip,   I don't know of any third party sources. Maybe the community can assist on that.   If you contact Sales and ask for the Spare Parts department (these are service part numbers) they should be able to quote you for battery.   Spare Parts (800-357-3355)   PS4210 Part Number Part Description 4VKD1 KIT,ACC,BTRY,BBU,6.6V,FRU K4PPV KIT,ACC,BTRY,BBU,7.3V,2,LI,NMC     PS6210 Part Number Part Description 24P87 KIT,ACC,BTRY,BBU,6.6V,FRU 4VKD1 KIT,ACC,BTRY,BBU,6.6V,FRU CX9PN KIT,ACC,BTRY,BBU,6.6V,FRU K4PPV KIT,ACC,BTRY,BBU,7.3V,2,LI,NMC       PS6610 Part Number Part Description 24P87 KIT,ACC,BTRY,BBU,6.6V,FRU K4PPV KIT,ACC,BTRY,BBU,7.3V,2,LI,NMC    
I was wondering if anyone had a suggestion for a reputable source for EqualLogic cache batteries I had a bad (and expensive) experience with a vendor who sold me faulty batteries and I don't think they are directly available from Dell (if they are available from Dell, please let me know)      
Cache battery replacement for PS4210/6210/6610