I have two APC backup power supplies from the mid to late 90's. One is a Back-UPS 450, the other is a similar APC unit from the same model line but a slightly smaller unit (forget exact model) Both batteries have long since died. Replacement batteries go from $20-30 each from what I see. These would be used for devices that only consume small amounts of power. One would be used just for a Arris Cablemodem, the other would be used just for a Netgear WiFi access point. Any issues using a 20 year old UPS with a new battery?