low battery warning

[philba]

I've got a security system that is giving me a "low battery" alert. I measured the gel-cell battery (unloaded) at 13.64V. The battery leads from alarm are showing 13.75V (110 mV difference). I assume this is sufficient difference to kick off the low-bat warning since the gel-cell hasn't charged up to that. I don't really understand lead-acid battery charging in great detail, though.

I know I'd need to cycle test the battery but is it reasonable to assume that the battery is going bad? It is only 5 years old. The environment is a heated basement and it's done maybe 5 full cycles in its life (though none recently). It seems like it should give me longer life than that.

should I be looking elsewhere for a problem? I'll replace it since that's the cheapest option but it does seem premature.

the security system is a gemini

 

[Nigel Goodwin]

It sounds a little overly sensitive?, that doesn't sound very low to me?.

However, you say it's 'only' 5 years old - how frequently are you supposed to replace it as part of the annual maintenance?.

 

[Hero999]

What's the normal battery voltage?

 

[philba]

It sounds a little overly sensitive?, that doesn't sound very low to me?.

However, you say it's 'only' 5 years old - how frequently are you supposed to replace it as part of the annual maintenance?.

yeah. as I was looking at new batteries, it turns out they all say 3-5 yr replacement so maybe I'm doing the right thing replacing it. I don't know if 3 to 5 is real or the equivalent of "repeat" on shampoo instructions.

 

[RODALCO]

Low Voltage alarm

Most security alarms have a 12 Volts back up battery in it in case the power is off for a period of time.
If the mains is present the voltage will be around 13.6 to 14 Volts.
The low batt alarm should not come on when the battery is being charged anyway, which means the voltage is over 12.6 Volts for a lead acid battery at rest e.g no load applied.

In your case if the battery is over 5 years old i would consider replacing it.
Sometimes these older batteries get a weak cell which draws the voltage down permantly or intermittendly.
For the sake of a 40 $ NZ 25 $ US battery (12volts 7 Amp hrs)or godd security that money is well spend to replace it.

If you check the batt with a load 12 volts lamp 21 Watts, and the voltage at the terminals drops well below 12 volts rapidly, replace it.

On your new battery write the date on it, before putting it in.

 

[zevon8]

Just throwing this out there........

Many alarm systems have wireless sensors, such as a remote fire/heat sensor, or a sensor located in the attic, etc or sometimes just where is would be a pain to hard wire. Even some "wired" systems can have one remote wireless sensor.

Anyways, the point of my rambling is that some systems will report "low battery" when its the wireless sensor that has a low battery, not the main unit.

The gel cell should, once settled, sit at about 12.8 volts when fully charged.

 

[philba]

Most security alarms have a 12 Volts back up battery in it in case the power is off for a period of time.
If the mains is present the voltage will be around 13.6 to 14 Volts.
The low batt alarm should not come on when the battery is being charged anyway, which means the voltage is over 12.6 Volts for a lead acid battery at rest e.g no load applied.

In your case if the battery is over 5 years old i would consider replacing it.
Sometimes these older batteries get a weak cell which draws the voltage down permantly or intermittendly.
For the sake of a 40 $ NZ 25 $ US battery (12volts 7 Amp hrs)or godd security that money is well spend to replace it.

If you check the batt with a load 12 volts lamp 21 Watts, and the voltage at the terminals drops well below 12 volts rapidly, replace it.

On your new battery write the date on it, before putting it in.

thanks, by the time I read your response, I already got the new bat (8 AH, btw) and installed it. I've played around a bit with the old one. Put an 80 mA load (.01C) on it. Initially it read 12.75 but after 20 minutes it was down to 8V. Definitely bad.

by the way, good call on the price - 24.99 $us.

 

[Hero999]

Are you really sure it's a 12V and not a 15V battery?

 

[philba]

Are you really sure it's a 12V and not a 15V battery?

????

wierdest question I've gotten in a while...

 

[Hero999]

Why is that so strange?

If it's a 15V battery then of course it'll give a low battery signal at 13.75V.

 

[philba]

where might one find a 15V gel-cell? especially for a standard device like a security system. sure it's possible but so are lots of other voltages that they don't make.

by the way, I may not know a lot about lead-acid cells but i do know that a 12V lead acid battery is usually charged to around 13.75V or higher. a 15V gel-cell (if they exist) would probably get charged to 17 to 18V.

also, the issue isn't unloaded voltage but rather the battery's behavior under load as was mentioned above. once i put a small load on it, the truth came out.

 

[Nigel Goodwin]

Why is that so strange?

If it's a 15V battery then of course it'll give a low battery signal at 13.75V.

Lead acid cells are nominally 2V each, and you have six of them for a 12V battery - you can't make 15V.

 

[HiTech]

My maintenance dept. has just changed out around fifteen 12v - 7ah gel cell batteries used for backups to the control electronics for the HVAC systems. They pretty much all measured 13 to 13.7 volts but many of them could barely illuminate a small 12v lamp used as a test load. Even after charging them, and rejuvenation, they still didn't hold a charge very long. So the voltage reading (unloaded) is somewhat meaningless. I did experiment with a couple of them by prying off their sealed top covers to expose the individual cells. I removed the rubber caps, injected a small amount of distilled water into each of the six cells and replaced the covers. Throw it on the charger at a slow rate and voila! The battery has sprung back to life but still not like a new one. I have kept a few of them to use around my home for running two-way radios portable-like during a power failure.

 

[RODALCO]

Never heard of 15 Volts security alarm systems or batteries.

As Nigel sais 7½ cell batteries don't exist.

Most standard domestic alarm systems are 12 Volts dc.
The batteries under charge can go up to about 13.8 Volts.

 

[Hero999]

Lead acid cells are nominally 2V each, and you have six of them for a 12V battery - you can't make 15V.

A 12V battery is normally 13.8V when fully charged, which gives 2.3V per cell so seven cells would give 16V which isn't that far off 15V. I agree though it's highly unlikely, I've never seen a 15V gell cell before but I wouldn't completely rule it out.

 

[Nigel Goodwin]

A 12V battery is normally 13.8V when fully charged, which gives 2.3V per cell so seven cells would give 16V which isn't that far off 15V. I agree though it's highly unlikely, I've never seen a 15V gell cell before but I wouldn't completely rule it out.

I would!, they are multiples of 2V, so can't be 15V - admittedly under certain conditions of charge it would certainly be possible to read 15V (for a short time), but that doesn't mean it's a 15V battery.

 

[zevon8]

Actually, a fully charged flooded cell battery, like a car battery, will read 12.65 Volts. Each cell will produce between 2 and 2.1 Volts, depending on age, plate wear sediment and electrolyte balance. A "hot" battery, one that has just come off charging will sit at very close to the charge voltage applied, say 13.8Volts in a car. 13.8 Volts is halfway between minimum charging voltage 13.2 and 14.4, where gassing will occur, and you boil the battery. This is why 13.8 was chosen for cars. Once the battery has settled, and cooled down, it will read 12.65 again. Cool down and normalization depends on the mass of the battery and its ability to cool.

Gel batteries tend to run a little higher, but not much. They should be charged differently also, constant current to get them up to nominal voltage, then constant voltage float for maintaining them. Mostly in alarm sytems a fixed voltage regulator keeps them alive, since tehy go in new, and rarely get flattened.

 

[oldtimer]

A lead acid cell fully charged will have an open cicuit voltage of 2.2 Volts. In a 12Volt battery there are 6 cells in series giving 13.2 Volts. Do not allow more than 14.1 volts across it or you will puncture holes in the plates the debris will then fall to the bottom causing shorts. Check the elecrolyte with a hydrometer the reading depends on temprature but should be about 1280.