The original Rayovac 840 was used on many older computers as either the main or replacement to on board battery and is Alkaline not rechargeable. Most of the external battery packs were not rechargeable. You should use 3 regular Alkaline batteries, not any Nicad, Lithium, etc and it should work just fine. My Laser 486 has an ext battery (from factory) and it uses standard Alkalines. The onboard barrel batteries were rechargeable Nicad or (later) Lithiums and are the cause of many bad board due to leakage. Consider your self lucky yours had an ext. battery pack (like my Laser 486), if not it could be a corroded mess.

Thanks for the information Horun, I'll chuck in some regular batteries in holder. I was dreading a barrel battery having leaked all over. But for $50 it was worth the risk.

I recommend the TADIRAN TL-5242, it's lithium and it has the right connector for Compaqs of this era like the Prolinea 4/25s and Deskpro XE 466.