Ibex Manufacturing, Inc.
Application Note 7
So How Do I Build a UPS (Uninterruptable Power Source)?
This note discusses various methods of providing power to an application that won't be interrupted by a mains power failure.

The simplest method is shown in the following figure. A battery charger is permanently connected to a battery and to an application's circuit. When the mains power is active, the charger powers the circuit and simultaneously recharges and maintains the battery. In the event of a mains power failure, the battery (which is already connected) keeps power applied to the circuit. When the mains power failure is over, the charger again recharges the battery while simultaneously powering the circuit.

A battery that is discharged during a mains power failure does not "steal" all of the charger's output current when the mains power is restored. The voltage that the battery discharges to, during a power failure, acts as a floor when the power is restored. The battery voltage can't drop further when charging commences. In practice, when the battery is being recharged, the battery voltage rises immediately unless the battery is quite large. The application's circuit draws whatever current it requires, with the battery getting the rest.

Obviously, the current draw by the application's circuit must be less than the maximum current output of the charger. Otherwise, there won't be any current left over to recharge the battery. Also, the application's circuit must be able to accept the full voltage range of the charger. For instance, at room temperature the battery voltage can reach 15V (for a 12V system) during charging. At very cold temperatures, the voltage can be higher. Consult the individual charger's electrical data sheet for more details.

A very important aspect to consider is how deep the battery discharge is likely to be during a power outage. A 12V battery should not be allowed to discharge below 10.8V (open circuit voltage). Much below this voltage and the battery starts to deteriorate. If the battery is allowed to go totally "flat", it's Ah rating will be greatly reduced or the battery may fail completely.

Using a charger that automatically disconnects the battery at the 10.8V level costs a bit more. However, it may pay for its extra cost the first time it disconnects a battery that has discharged to a too-low voltage.

The recharge time for the battery will be a function of the battery's Ah rating and the amount of charging current left over by the application's circuit. Another application note discusses battery charging time.

The next figure shows a possible way to connect both a charger and a power supply to a load. This allows the use of a small charger whose only responsibility is to maintain the charge of the battery. The power supply is used to power the load while the main AC power is on. This method can be less expensive when powering a large load.

There is, however, one caveat with this method. The power supply voltage must be as high, or higher than, the highest battery charger output voltage. At room temperatures and above, the charger's voltage will be 15V or less (12V system). At -40C the voltge may be as high as 16V. If the power supply's voltage is too low, the load may clamp the charger's output at too low a voltage to sufficiently charge the battery.

The rectifier in series with the power supply may be omitted if the reverse current drawn by the supply (from the battery) is at an acceptable level. It's recommended that schottky rectifiers be used, as they have a much lower voltage drop than standard rectifiers.

Multiple voltages can be obtained from a battery system by cascading chargers and batteries. Application Note 5 discusses this option. For loads that draw a large current, this may be the best solution. However, for lower current loads, a single charger and battey with a DC/DC converter, may be a less expensive option. Stepping down the battery's voltage is straightforward and isn't discussed here.

Refer to the following figure for a useful scheme to step up the battery voltage. Stepping up the battery voltage by simply using the DC/DC converter's output alone requires that the converter handle the full power requirement of the higher voltage load. Instead, if the converter's output is cascaded with the battery's voltage, the converter needs to handle only half the power.

In this example, suppose the 24V load draws 0.5A = 12 watts. If the DC/DC converter's output is cascaded with the battery, it must supply 12V x 0.5A = 6 watts.

The total current that the battery/charger must then supply is: 12V load current plus DC/DC converter input current plus 24V load current.

Important: For this scheme to work, the DC/DC converter must have an output that is electrically isolated from its input. Also, the considerable variation in voltage across the battery will appear at the 24V output. If tight regulation is needed for the 24V supply, then there may be no option other than using a 24V DC/DC converter.