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How do I test a Battery?

Jeff Mays

Aug 7, 2017
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Aug 7, 2017
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Hi Guys,

I've just had the maintenance man come to service my home alarm system and he told me the batter needs to be replaced.

I've paid for a new battery and have kept the old battery, as this is the second time inn quick succession that I've been told that I need a new battery.

I've used an analogue multimeter and it's giving a reading of 12.5v approx. - does this mean the battery is ok or is there a test I can do to put the battery under load to check whether it really did need replacing?

Grateful for any assistance.



Jeff
 

Bluejets

Oct 5, 2014
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10watt auto lamp as a load. Should maintain around 12v for 5 hours at least. ( assuming a 7Ah battery which most are)
 

dorke

Jun 20, 2015
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W elcome to EP.

1. First thing is knowing the rated battery voltage and ,type.

2. Second the correct way to test a battery is in loaded condition,a multi-meter doesn't do that.
it test in un-loaded condition.The proper load should be selected according to 1.
 

kellys_eye

Jun 25, 2010
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Many alarm systems use simple trickle charge circuitry to keep the battery topped up. This is a very poor way to 'maintain' a battery (given how modern, controller circuitry is cheaply available) but even the best lead-acid rechargeable will have a useful life of around 5 years.

The alarm operating guidance may possibly state that it can operate without power (mains failure) for x-hours and if it can't/doesn't then 'you' might be claiming against the alarm company if you got burgled during a power outage and the battery didn't do as it should.

The maintenance people are, therefore, covering their backs (quite rightly so).

That said, the battery may still have useful life for other purposes if you are keen to 're-purpose' it and discovering just how much life it does have can be convenient.

Connect a car sidelight to it - 12V at 12W (an old indicator bulb is around this level) will drain the battery at a constant 1 amp and measuring how long it takes for the bulb to dim to (say) half brightness will reveal how many ampere-hours there is still left in it.
 

ChosunOne

Jun 20, 2010
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AAGH!! Sorry I didn't look at this thread earlier.

Jeff, first of all, DON'T test the battery by draining it. Draining any rechargeable battery that low will shorten its cycle life drastically. What that test would show is how much storage capacity ( in Ah) the battery had BEFORE you tested it. After the test, it will have significantly less, because deep discharging degrades a battery's capacity for holding a charge, as well as diminishing its cycle life (number of times it can be recharged).

Unless you have a really oddball alarm control panel, it uses a 12V SLA (Sealed Lead Acid) battery, most often either a 4Ah (usually too small, but not necessarily) or a 7Ah. SLA batteries have gotten incrementally better over the decades, so it's possible your 12V battery will be rated at 4.5, 5, 7.5 or even an 8 Ah. I've seen all those ratings.

The measure of whether you need a new battery is how much capacity for storage it has, measured in Ampere-hours (Ah), measured against how much capacity you need to have a viable alarm system, which is a function of how long you need the battery to supply power and still have enough reserve to sound an alarm and notify a monitoring agency at the end of that time.

There will be a rating printed on the battery telling you its Ah capacity when it was fresh from the factory. The capacity diminishes with age and use, and how quickly it diminishes depends on several factors:
(1) How "hard" the battery is used in service, i.e., (a)how much of its maximum capacity gets drained during power outages and (b)rate of discharge.

(1a) Percentage of Capacity: An example is IF you had a 7 Ah battery and a power outage caused it to drain at 500 mA (1/2 A) for 6 hours, that would be a total drain of 3 Ah, = 3/7 or ~43% of your battery's rated capacity, leaving it with (theoretically) 57% reserve. (In practice, you can really only use 80% of the battery's rated capacity--after that, the voltage drops too low to drive electronics designed for nominal 12VDC and most alarm panels in North America shut themselves down.
You can get the maximum use out of your battery if it's never discharged below 20% of its rated capacity.

(1b) Rate of Discharge: Ideally, the lower the discharge rate, the easier the use on the battery and the longer its cycle life. It's recommended not to discharge it at more than the "10-hour discharge rate", i.e., so that no more than 10% of its rated capacity is being discharged each hour. For a 7Ah battery, the maximum recommended 10-hour rate would be 700 mA, and the example I posed above would fall within that parameter.
If the drain in that example were 1A for 3 hours, the total drain would still be 3Ah but it would be a harder use of the battery and would diminish its useful life.

(2) Cycle life: Number of times the battery is discharged and recharged. Even if the discharge is at a gentle rate and doesn't discharge below 20% of capacity, the number of discharge/recharge cycles ages the battery. In the case of a home alarm system, that usually means how often you have line power outages and for how long. (Sometimes it means how often the line power transformer is pulled out of the wall outlet so a vacuum cleaner can be plugged in, and then forgotten).


Those are the main factors that age/diminish your battery's capacity. It's in the nature of alarm systems that each one is customized, so nobody here can tell you how often you need to change it. There isn't any one one-size-fits-all number that can be applied. Generally, most of my customers need to replace their panel battery every 5 years, but I have one customer whose system draws a lot of power and lives in a neighborhood susceptible to power outages, and he has to change his panel battery every year.

North American alarm control panels and systems are different from those used in the UK, but the principles are the same. If you actually need a new battery :"in quick succession", (how much time is that, measured in months?), then you may not have a battery big enough for your system, not unheard of. If you measure the Ampere drain on the battery with the power transformer unplugged from the line power outlet, you can calculate if your battery is big enough.


So after all that information you didn't ask for, here's the answer to your OP question: There is no quick and easy way to test the current capacity of your used battery. Field techs/engineers have to make an educated guess from their local knowledge of factors like age, battery size, current draw from your system on standby, general frequency and duration of power outages in your neighborhood, etc---but they don't know how often you unplug your transformer and forget it, so the best guess is still a guess.

There IS a way to gauge the ball-park capacity remaining in your battery, but it's not quick. If your old battery that you still have lying around isn't dead--i.e. if the system doesn't initiate a "Trouble Condition" when you plug it in--then replace your newer battery with the old one, and let the newer one sit on a shelf for 5 days. This is to allow the chemistry to stabilize. At the end of 5 days, read the OCV--Open Circuit Voltage, with nothing but meter attached to battery.
A brand-new battery fresh from the factory (in other words, with 100% capacity) will read around 13.2 Volts DC.
An old used-up battery with virtually no storage capacity left might read 12.0 VDC.

This is a rough rule-of-thumb that will give you a ball-park figure for the capacity of your 12V SLA battery:
Each SLA cell will read 2.2 VDC when it can store full capacity. When it's shot, it will read 2.0 V or less.
The percentage of capacity is roughly linearly proportional to the difference between 2.0 and 2.2 V per cell, or in a 12V SLA battery, between 12.0 V and 13.2 V.
So a battery that reads 12.6V after being off a charger (and with no other electrical activity) for 5 days, will have about 50% of its rated capacity.

Obviously, most field techs/engineers can't use that test, but a DIY-User can.

For future reference, if you put the word "alarm" in your thread title, it will catch my eye and I'll be more likely to look at it and respond sooner. This forum typically collects or cycles dozens of new threads each day and I don't look at them all, but I try to catch the ones in my field of expertise, alarm systems.
 
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