UPS that contain internal batteries are usually designed to provide only a few minutes of runtime at full load. Long runtimes (long runtimes are defined in EN62040 as runtimes over 30 minutes)can be obtained by operating the UPS at lower powers. This of course isn’t always feasible so additional battery packs, or a large external battery bank can be added to some UPS systems. The problem here is that the internal charger within the UPS is seldom large enough to recharge the battery in a timely fashion (or at all). This is where extended run UPS Systems come in.
Typically a UPS internal charger has a 1Amp rating. This means it provides at best case an amp of current back into a discharged battery. If that battery has a 7 Amp Hour rating then you can see that it should take 7 hours to charge a fully depleted 7Ah battery using a 1A charger. In the real world it doesn’t work like this of course. Battery recharge is quite efficient when the battery is flat, but as it charges it becomes more difficult to put charge into it and therefore the charging time is longer.
So imagine taking a unit with a 1A charger with an internal 9Ah battery string and adding several additional battery strings to it. So if we do some rough maths, the UPS will charge its own internal string in 9 hours. The next additional string added will increase this to 18hours. The next to 27 hours. You can see how the timescales mount up, meaning that if you have several attached battery packs you need to factor that recharge may take several days to get the battery up to capacity.
An extended run UPS tends not to have internal batteries but the space taken up with a high capacity charger, typically up to 12A. This allows the connection of multiple battery packs or a high capacity battery with the knowledge that it can be recharged.
Chargers can also be connected in parallel. So, for some UPS such as the VFI-B, we can add additional battery packs which contain their own internal charger. This way charge is supplemented and recharge time dramatically reduced. However this method can only be used for basic constant float charge applications. In more advanced charging applications that follow 3 stage charging we have no option but to use higher capacity. Products such as the TX and VFI-BL offer high capacity chargers that can also be down graded. It is possible to overcharge batteries by not current limiting the charge current. For example, maximum charge current is C/4, so for a 9Ah battery this is a little over 2A. Charging with a 12A charger would overcharge and damage the battery, giving the option of using the UPS with its internal batteries, with the option of adding a battery cabinet to extend runtime.