We are in the process of choosing the battery for ac and dc ups. Our client is a Eroupen company which dictates the battery types. They object to SRA or VRLA type of battery. They only accept vented Lead acid or Ni-cad battery. This plant will be built in North America. NiCad battery manufacturer is not available locally. No recycling facility for Large Ni-CAD Battery in North America. I am trying to get some expert opinions about Vented Lead acid and VRLA (SLA). Any inputs would be appreiated.   

For serious applications, there seems to be one choice, vented lead acid. I've never seen anything else in a substation. UPS manufacturers will try to pass off all sorts of other things.

How does the cost compare, Hamid?

Bill
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"Why not the best?"
Jimmy Carter

The costs for VRLA is about 30% of Ni-Cad of the same capacity. They don't seem to care about the costs. They accept Vented lead acid but not VRLA. Not sure why. There may be some mindsets in IEC world, Ni-Cad is better and maintenance free and has longer life.

VRLA batteries are not maintenance free batteries, just maintenance impossible batteries. They still need maintenance, but you can't perform the maintenance. Ni-Cad batteries don't like being kept close to full charge at all times.

I suspect they accept vented over VRLA because they're aware that the failure rate of a vented is far lower than a VRLA.

As for the Ni-Cad, beware of not just the upfront cost but what it will cost to dispose of these batteries at end of life. I'm in Australia, and the last time I needed to get rid of a bank of these they needed to be shipped to New Zealand for recycling, and the cost was exhorbitant.

Thanks guys for the excellent inputs. It looks like the best way to go is to go with the vented Lead acid, which is more reliable than VRLA. We will drop out the Ni-Cad for sure.

Vented Ni-Cad has big advantages if your installation is not in a temperature-controlled environment because Ni-Cad chemistry is very temperature tolerant. The operational life is longer than for vented lead-acid, and the weight is significantly lower which can be an advantage in, say, an offshore installation. We see both in UK substations, although lead-acid is certainly more common.

Disposal costs in the UK are such that Ni-Cads can be disposed of for a nominal cost, if you shop around, and lead-acid cells are collected free-of-charge.

For small substations, the dc power requirements are often small. Ni-cad batteries are available in smaller ratings than lead-acid and can be economical.

Here on the west coast, some utility companies require NiCad plants for their station battery source. (Pacific Gas and Electric in northern California). We have specified many types, but the NiCad, for their cost and disposal costs still seem to be popular with utility and large industrial users. We have had much success with these systems, both 24VDC and 125VDC for station battery plants.
Regards,
EEJaime