Understanding your UPS battery’s amp-hour rating

If you’re specifying a UPS system, then it’s absolutely essential that you truly understand its battery autonomy; For how long will it support the load if the AC supplies fail?

What is a battery’s amp-hour rating?

In simple terms, a battery’s autonomy depends on the load it must support and its amp-hour (Ah) rating. For example, a battery system rated at 30 Ah could in theory support a load of 30 A for one hour.

But it’s a little more complicated than that. In reality, a VRLA lead-acid battery’s Ah rating isn’t fixed but depends partially on the rate at which it’s discharged. This is managed by labelling batteries with ‘C’ ratings; these relate their Ah capacity to their rate of discharge. For example, if our 30 Ah system was C20 rated, this would mean that it could provide its 30 Ah capacity while being completely discharged over a 20-hour period. ‘Completely discharged’ means that every cell’s output voltage is reduced to 1.75 V. Discharging cells beyond this will adversely affect their capacity and useful working life.

Battery testing is performed by subjecting the battery to a load size that has been calculated to completely discharge it over the 20-hour period. The total Ah output over this period is recorded and used to specify the battery’s C20 rating.

However, you will find that if you attempt to discharge our battery at a significantly faster rate – say 10 hours instead of 20 hours – its Ah rating will be reduced.  This could amount to something like a 10% reduction, to 27 Ah for example. This loss of capacity arising from faster discharge is caused by heat. The faster a battery is discharged, the heavier the current and the greater the heat produced across the battery’s internal resistance.  The battery energy consumed by this heating effect is denied to the load, so its useful or available capacity is reduced accordingly.

UPS battery amp-hour considerations

Although the above examples have used time periods measured in hours for simplicity of explanation, the reality is that a typical UPS battery system, because of its large load size, will have an autonomy time measured in minutes rather than hours. Nevertheless, the argument informs at least one key question you should be asking when purchasing a battery, or possibly a UPS system if battery capacity in Ah is being quoted: What discharge rate applies to the Ah specification being provided?

We should also remember that this is just one aspect of a battery’s capacity to support the load. Firstly, the battery’s DC output must be fed through an inverter to generate the AC power required by the load, and inverter inefficiency will lead to some energy loss.

The battery’s condition, age, and the way it has been treated and maintained will also affect its capacity as well as its service life. For example, VRLA batteries are typically specified for use at 20°C; running them at 0°C will have little effect on their service life, but could reduce their capacity by 30% or more. Over-temperature operation will reduce their service life, and may cause permanent damage.

Over or deep discharge of the battery will cause sulphation of its internal plates, leading to increased internal resistance and, as mentioned, loss of capacity and service life.

So, after first establishing the battery’s true capacity and fitness to support its load, this capacity cannot be taken for granted; it must be maintained by paying attention to operating and environmental conditions, monitoring, and regular preventative servicing.

Take a look at our knowledge base for more articles concerning UPS battery monitoring, installation and care.

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