One question of great interest to people when discussing UPS (Uninterruptible Power Supply) Systems is how much runtime will they get. Sure, it’s the main purpose of a UPS to provide backup power, although it is not the only function. A lot of misconception arises from this, in that a UPS that provides longer runtime is perceived as better than one that provides shorter runtimes. This may be true is some instances, but in the main: Runtime=Batteries. The more batteries you have (or higher capacity batteries) – the more runtime you will get.
There are many factors that also impact your runtime, such as:
- Ambient Temperature The warmer it is the better your batteries will perform, however this comes at a cost. As temperature increases the battery life expectancy decreases rapidly.
- Battery Health Batteries have a useful working life. The more they are used, the less able they are to be recharged fully, resulting in reduced runtime.
- Load Variance A effect known as Peukert’s Law extends runtime if the load is not constant but is low for a period allowing the battery to “catch up”
- Load Power Factor The higher the load power factor the more power you draw from the batteries. This is also why you may see different runtimes quoted for different UPS systems that use the same batteries due to specifying runtime at different load power factors.
- Inverter Efficiency The more efficient the inverter the less power is wasted and the more battery power can go into supplying the load.
- End Of Discharge Point The UPS is programmed to switch off when a certain battery voltage is reached. In many cases this is 1.75Volts per cell, or 10.5V for a 12V battery, however this may be as low as 1.65Vpc. The lower the EOD voltage the more runtime you will achieve, however this is at the detriment of reduced battery life and increased recharge time.
Most UPS Systems use Valve Regulated Sealed Lead Acid Batteries or VRSLA for short. Some may not include the “Sealed” as technically it is not correct – as they have valves – and so you may see VRLA used instead but they are talking about the same technology. The workhorse of the UPS industry for systems from 500VA to up to 10KVA is a battery about two thirds the size of a house-brick and is rated at 12V and 7.2Ampere-Hours or Ah. The Ah rating is a measure of the batteries capacity. A battery rated at 12V 9Ah will give more runtime than one rated at 12V 7.2Ah for example. Except for small UPS systems there is usually several batteries connected in series to give a higher terminal voltage and hence lower current for design reasons. For example, 1KVA UPS usually have 3 batteries, giving a 36V voltage, 2KVA 6 batteries – 72V and 3KVA 8 – 96Volts. 6 & 10KVA systems usually have around 20 giving 240Vdc nominal input, although these figures will vary from product to product. For example, lets say a 2KVA UPS contains 8x7Ah/12V batteries. Upgrading to a 3KVA UPS that uses the same battery will not give you any more runtime, as, like I said above – runtime=batteries, and so a common misconception is that going for a larger UPS will automatically give you more runtime. This need not necessarily be the case.
Some UPS Systems allow the connection of external battery packs in order to prolong runtime. This is a great way to provide guaranteed availability of power, however a point very often overlooked on extended run UPS Systems is the recharge time. Many UPS systems are fitted with a charger rated at no more than 1Amp, which is fine for a battery string rated at anywhere up-to 10Ah. For example, the recommended charge current for VRSLA batteries is between C/10 to C/4 where C is the Ah rating. At currents lower than this the batteries will charge but will take significantly longer to do so.
For example, let us assume we’ve discharged a UPS battery rated at 10Ah. It will take over 4 hours to recharge this battery to 90% of capacity and a further several hours to fully recharge. If we double the battery bank by adding in another battery pack, the charge time will now be 8 hours to get to 90%. Add in another and now it’s 12 hours. So before you go buying a multitude of battery packs, ensure that the charger is up for the job of recharging the batteries after a heavy discharge.
Our long runtime UPS Systems for applications such as emergency lighting are fitted with a 5A charger for exactly this reason.
One other note of caution comes with the use of line interactive UPS Systems for extended run applications. As line interactive units are not “on” all the time, many are not designed with long run times in mind. The best technology for long runtime applications is online double conversion technology. Since this technology is “on” all the time, it does not matter whether power is being taken from the mains or from the battery, it will continue regardless.
