More that 99.9 per cent of the time batteries are just big, heavy lumps that fill the empty recesses of alarm panels and smaller access control panels. But without the guarantee they provide of operation during power loss, security systems would be in serious trouble.
Most modern security systems use rechargeable batteries – lead-acid (L-A) – is by far the most common, thanks to a life expectancy over 5 years, the fact they are widely available and are very cheap to manufacture.
Maintenance issues
Modern batteries are reliable and long lived if they are maintained properly. Neglect a battery or subject it to extreme conditions and abuse, and it will fail. There are a number of key factors that will effect the life expectancy of a battery including temperature cycling, operating temperature, thermal runaway, shelf life, charge-rest-discharge cycling, rate of charging and rate of discharging. Other factors include loss of electrolyte, deep discharging, physical damage, overcharging, memory effect and the cutoff voltage of the equipment supported.
Because such factors impact on each other in the field it’s extremely hard to predict battery life expectancy with real accuracy and this means it’s equally tough to predict exactly when batteries should be replaced. This is partly due to the fact test methods are not perfect and partly because a weak battery that’s capable of supporting a 2-zone alarm panel for 12 months won’t be capable of supporting a 16-zone panel with additional wireless and access control expansion for 6 months.
Testing Batteries
There are a number of issues that surround the testing of rechargeable batteries. Old hands can ignore this advice, but technicians just starting out should try to get familiar with battery states and how these effect life expectancy in a given system. Use the same testing tool each time bearing in mind that most batteries will give slightly different readings depending on which instrument is used to test them.
As a new installer test new batteries, middle-aged batteries and elderly ones. When a dead battery comes into the workshop in another installer’s vehicle for recycling or to be thrown out, test it. Get a feel for battery readings. When testing, check the voltage for a few seconds and keep an eye on it. A good battery will have a steady reading, while an older one’s reading will fall fast.
Never use an ammeter to test a rechargeable battery – use them on dry cells only. Rechargeables have got high amperages that are capable of burning out an ammeter, damaging the battery itself or even scorching your fingers – and watch leads, too. Testing L-A batteries with ammeters makes leads seriously hot very fast.
Use a voltmeter and a load technique when testing and be sure to disconnect the charger and don’t check the battery when it’s been depleted by power failure as it won’t give you a clear idea of its state. Using a load technique for battery testing achieves an accurate measurement of the battery’s condition while carry a load – 1amp is sufficient. Under these circumstances you’ll see a slight voltage drop and then a levelling of the voltage in batteries with life left in them.
Replacing Batteries
If a battery is suspected of being about to fail then no chances can be taken. There are 2 ways to handle such a situation. The first is a timed discharge test – a constant current discharge. You measure the time it takes for terminal voltage to get to an endpoint voltage you’ve decided on already. Discharge current in amps is set equal to battery capacity in ampere hours.
The problem with this technique is that it takes time you might not have and demands you use equipment that’s probably back at the workshop, not with you in your vehicle or tool bag. In such circumstances just change the battery. If you need to change the battery, be careful to replace it with the same type so charging requirements are identical. In the event of undercharging a battery the system may fall over when power fails, while overcharging will damage plate grids and lead to gassing.
And be careful to get the voltages right. This is basic stuff to experienced technicians, but a lack of concentration and care can lead to errors. Most modern alarm and access systems use 12V DC, but fire alarms and integrated access solutions may need 24V DC. Manufacturers of batteries and security equipment make their product so that battery voltages are nominal, allowing the system to manage lesser or greater voltages if required. Alarm and small access systems typically function in the 9-16V DC range.
Factors reducing the life expectancy of batteries include:
* Cleanliness of terminals
* Shelf life
* Rate of charging/discharging
* Overcharging
* Deep discharging
* Memory effect
* Thermal runaway
* Operating temperature
* Temperature cycling
* Charge-rest-discharge cycling
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