Monday, February 9, 2009

How much UPS – Battery “Run Time” is enough?

Written by Ken Baudry

For many companies an electrical design that provides an on-site generator and a UPS will provide a level of availability that meets their needs. The generator serves equipment that is required like lighting and air-conditioning that can be momentarily interrupted and automatically recover on restoration of power.
The UPS serves equipment that is affected by power outages of less then 10 seconds. Examples include IT, Data Processing and Communications.

The generator also provides a continuous feed of power to the UPS. Which means that the UPS only supplies power from the batteries only during the start up and transition from utility to generator. In most cases this is no longer then ten seconds.

When determining how much battery capacity I need, one line of thought is that battery run time only needs to be marginally greater then the time it takes to get the generator started and on line. With this idea in mind, one might conclude that 30 seconds is plenty of time. But there are other considerations when looking at the minimum run time. Larger generators or paralleled generators can take longer, perhaps as much as two minutes to startup, synchronize and come on line. In this case, we might need three or four minutes (minimum).

As batteries age, so does their capacity. Batteries can loose as much as much as 50% of their storage capacity over their life. To have three or four minute’s minimum at the end of life started out as six to eight minutes of initial capacity.
It takes about five times as long to recharge a lead-acid battery to the same level as it does to discharge. If I have a generator and it starts, then I have used a very small amount of the stored capacity. Five times a very small number is still very small.

My eight minutes of battery time, if fully discharged might require as much as forty minutes to then fully re-charge. If we have a generator, then we want to set the generator retransfer time for long enough to compensate for full recharge time. There are a number of assumptions that could be made that would lead to reducing this time but keep in mind that if the batteries don’t have time to recharge, and if the utility experiences multiple failures in rapid succession then there is a chance that they will have the needed capacity.

The other line of thought is that we should buy as much battery capacity as we can afford and why not. More is always better. It’s the safest choice...right?
There are a number of issues with this approach. More run time will mean larger batteries and/or more strings of batteries. Weight and cost are two obvious limitations not to mention that there is a maximum number of batteries a UPS rectifier can charge.

But the biggest limitation is heat. If the air conditioned shuts down, the ambient temperature will climb. ASHRAE TC9.9 recommends and maximum inlet air temperature of 77 Degrees F and a maximum allowable inlet air temperature of 90 Degrees F. If the maximum allowable inlet air temperature is exceeded then critical servers and other IT equipment could be damaged.

A generator may appear to solve this issue, but it’s not always possible to provide generator power for some requirements like small data centers in office towers and VoIP switches in LAN closets. If the generator fails to start, then you are back where you started, without a generator. Air conditioning is typically too large to put on a UPS. So the maximum battery run time capacity, with or without a generator, should be limited to protect against damage to IT equipment from excessive heat.

How fast will my data center or server room heat up? This depends primarily on how much energy (heat) we are putting into the room and how large the space is.

All of the power that the UPS is supplying is turning into heat. The time required to reach maximum allowable inlet temperature can easily be estimated using formula that can be found in ASHRAE or with a quick internet search. An easy way to calculate this can be found at HolisTech Research & Consulting.


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