When Down Time is not an Option

Improving reliability of a DC-UPS System

For industrial applications uninterruptible power supplies (UPS) have become more and more important. For example, very cost effective DC-UPS systems are used for a safety shutdown, finishing processes or providing the energy for a communication system.  

Current market solution

Today the common market practice to provide a 24VDC back-up system utilizes two 12V batteries connected in series (Fig. 1). There are inherent problems in utilizing batteries if the unique factors affecting a battery’s performance are not taken into consideration. Applying two batteries in series adds to the problems and makes a totally reliable system basically impossible to build.

Fig. 1  Functional diagram (standard)        two batteries in series

Problems with batteries

Typically, maintenance-free rechargeable lead acid batteries are used because of their worldwide availability and low cost. What makes a battery application so critical is that the lifetime of this type of battery is limited and should only be used in certain conditions. Ignoring these limitations shortens the lifetime and reduces reliability of the battery back-up tremendously.

	Temperature: The lifetime is reduced dramatically at higher temperatures (Fig. 2). An increase of 10°C halves the lifetime, so the design engineer must attempt to place the battery in the coolest location in the control panel or make other provisions for keeping the battery away from excess heat.
Fig. 2   Temperature / Life characteristics

Deep Discharge: The number of charging cycles that the battery is subjected to greatly affects the lifetime and performance of the battery. The amount of energy drawn from the battery with each use has a dramatic impact. For example, if you are taking 30% of the energy out of the battery, up to 1200 charging cycles are possible. If 100% of the capacity according to the battery manufacturers specifications are used, about 180 cycles are possible and a deep discharge can limit the life of the battery to only a few cycles.

Problems with UPS systems using two batteries in series

As mentioned before in typical UPS systems two 12V batteries are connected in series. Here only the sum of the voltage can be measured and therefore, special conditions are required to ensure a properly working battery management system. To make the UPS work without failures both batteries have to be absolutely identical. This can only be guaranteed if you are using paired batteries out of the same production lot with the primitive state “clean”. This may not be a problem when installing a new system. When the batteries have to be replaced out in the field after a period of time, local distributors that provide the batteries and may not  be able to supply the batteries in that way.                                                                                                            

If paired batteries are not being utilized or if one of them is not working correctly, the performance and reliability of the system will be greatly compromised. Most problems occur because of mismatched voltages and show up in various situations.

Fig.4  Not paired batteries

Charging: During charging one of the batteries will see a higher voltage than the other (Fig. 4). The effect is that one battery will charge faster than the other, 
resulting in overcharging and the unit will get warm. This causes an aging effect that damages the battery. The result is a loss of capacity and an even greater mismatched voltage.

At higher ambient temperatures the high voltage can also cause dangerous outgassing. H2 gas can emit from the battery forming an explosive environment.

Discharging: The other battery in the system that is seeing the lower voltage will be undercharged (Fig 4). When using the UPS in buffer (battery) mode the voltage will dip to a very low level which could result in a deep discharge, possibly damaging the battery as explained before.

Battery test: During the battery test a UPS tries to measure the condition of the batteries to ensure the reliability of the system. Unfortunately, a true reading can not be given because the controller of the UPS can only measure the sum of the voltage of the two batteries. The measurement might not reflect the true condition of the batteries.

Taking all these limitations into account makes it difficult to build a reliable system with a UPS system that uses two batteries in series. Each discharge cycle makes the system more and more unreliable resulting in a system break down without warning.

PULS solution – “One Battery Concept”

For that reason PULS has developed the DIMENSION U-Series DC-UPS supplying 24VDC with only one single 12V battery. The PULS UPS eliminates all the problems encountered with two batteries in series. This concept enables a secure and reliable system with a focus on a long battery lifetime by providing superior battery management.

Fig. 5   Functional diagram (PULS)

The principle of the “One-Battery-Concept” is pretty simple. A step-up converter amplifies the 12V from the single battery to a stabilized 24V (Fig. 5). By eliminating the second battery a very precise charging management is possible. All critical parameters can be controlled precisely to ensure a reliable system and enable a long battery lifetime.

This unique technology does not only solve the problems of the two batteries in series, but it offers several other advantages for the application.

Other advantages are:

• Easy procurement of battery because matched units are not required..
• Small size & lower weight of the battery modules that can be integrated on the DIN-Rail.
• Designed for an easy usage.
• Diagnostic functions like “Check Wiring” or “Replace Battery” with signal outputs.
• Buffer time limitation for extending the battery lifetime.

Array of PULS UPS products

UB10	UB10	Accessories
Fig. 7   UB10.241	Fig. 8   UBC10.241	Fig. 9    UZK12.071

Offers to “Waste Water Treatment” applications with the UB10.245

A typical application in “Waste Water Treatment” is a small panel controlling lift stations that are spread out over a remote area. The DC-UPS provides the 24VDC to guarantee continued operation of the whole control system in case of a loss of power. The control system is not the only important part, but the communication device also needs power for remote telemetry because the lift station is far away from the plant where everything is monitored and controlled.

These two functions are very important because if the lift station is flooded, waste water will pollute the environment and high fines will have to be paid by the utility.

So engineers look for a very reliable system that fits all their needs. Consider the ambient temperatures in a panel standing in Florida with sunshine all day long. No exception to quality and reliability can be made here.

	The PULS UB10.245 (Fig. 10) is a very reliable unit. It has a very high lifespan and MTBF specified in the datasheet. Also the superior battery management described above is included to build a reliable system.
Fig. 10   UB10.245

One of the most important features of this unit is that it has a dual output that can feed your controls with 24VDC and also feed your communication device that is working with 12VDC (Fig. 11). Other advantages include less panel space, easy assembly and less cost because only one unit is needed.

Fig. 11    Functional diagram UB10.245

Summary

In conclusion, using a DC-UPS solution from PULS gives you a high technology solution that enables you to build a reliable system by overcoming the problems with batteries. At the same time PULS is providing a GREEN solution with the DC-UPS efficiency of 97.8% combined with a PULS power supply with efficiencies up to 95.4%.  Protecting the environment by saving energy while providing the best reliable solution is important to PULS.

For more information or technical support please contact PULS L.P. or visit our website.

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