However it is not just a process of continually adding battery packs until the desired runtime is achieved. This is because the UPS needs to recharge the batteries in a reasonable timeframe, and is why there is a recommended maximum to the number of additional cabs that can be fitted. For example, let us take a system with a built in 1A charger, and this is connected to it’s internal battery string with a 9Ah (~PAGE-Ampere Hour-~) capacity. This means in an ideal world with no losses, the charger can put 1Ah back into the battery each hour, thereby taking 9 hours to recharge. If we add an 18Ah battery pack to this, then the recharge time is now 27hours. Another 18Ah battery pack now extends this to 45 hours and so on.
Some Extended Run UPS Systems do not contain any internal batteries but a high capacity battery charger. These are designed for high capacity battery banks or the connection of many battery packs. For example a unit with a 5A charger can charge the 45Ah battery pack above in 9 hours.
Note that these recharge times are ideal. In reality charging lead acid batteries has inefficiencies that increase the closer to capacity the battery is charged. This is why battery recharge times are normally quoted to a value – for example 90% of charge.