Can run alarm on batterycharger?

For a simple burglar alarm I need 12 V and 500 mA max. and a batterybackup. I have found a 12 V lead flees battery 2,3 Ah and a battery charger (600 mA at 13,8 V). Now I would like to ask if some one knows if it is possible to run the alarm direct over the charger and battery, and then save a 12 V power supply?

I am new to this

Morten

Depends on the charger. Two potential problems:

The charger is not well filtered, and has a lot of ripple. The battery might be able to smooth it out to the point where the ripple won't effect the alarm.

The charger is not properly voltage-limited, and will cook the battery if left on 24/7.

If you connect a smoothing capacitor accross the o/p of the charger it'll give you a good supply, but wil increase the volatge, which on a charger with no load will be higher than 12v anyway, whats the max voltage your system will run at?

Hi Mike

Thanks for your answer. The charger should be designed for this lead flees battery and have protection against overcharging. The voltage are decreased when the battery are fully charged.
Do you think it was reasonable to put in a timer so that the charger only runs eg. an hour a day?

Morten

Hi dr pepper

Thanks for your answer. The alarm runs at 10 - 15 V/DC or 9- 12 V/AC I have learned today that a series diode should keep neg. spike away. Now I think a zener diode to ground (15 V) should keep overvoltage away?

Morten

morten said:

The charger should be designed for this lead flees battery and have protection against overcharging. The voltage are decreased when the battery are fully charged.
Do you think it was reasonable to put in a timer so that the charger only runs eg. an hour a day?

Click to expand...

Here is how I would find out:

First connect the charger to the battery with no other load connected. Use your DMM to monitor the battery terminal voltage for the first few hours. If the charger is a modern u-processor controlled charger, the battery voltage will slowly increase up to ~14.6V during the first few hours (how long depends on how much the battery was discharged initially), and then the voltage will drop to ~13.2V and remain there until the charger is turned off (or disconnected from the battery). A momentary interruption of input power, or disconnection from the battery, will restart the cycle over again.

Now try it with the load connected across the battery. One possibility is that the charger tries to get the battery voltage up to 14.6V, but due to the load current, it cannot, and it keeps trying forever. If that happens, you will damage the battery if the final resting voltage (after several hours) is higher than 13.8V. This would be because the charger is not designed to drive a load in parallel with the battery.
Another possibility is that the charger switches to the lower voltage, and now the load current causes the battery voltage to sag below 12.9V, which over time will chronically undercharge the battery, causing it to sulphate, ruining it.

If, after several hours with the load connected to the battery, the battery voltage rests at >12.9V but <13.6V, then you are ok to leave the charger connected all the time.

The timer idea should be used only as a last resort, if the charging cycle hangs up.

I wouldnt run the charger every day for one reason, it sounds like this is a multi stage charger, multi stage chargers deliberately overcharge the battery to reduce sulphation, doing this every day might dry out the cells.
Once charged the charger will drop the volts to 13.8 and then maintain that voltage (maintenance charge), if this is a multi stage charger just leave it on.

Hi dr pepper

Thanks for your answer. The charger describes it self as a battery keeper, to mc batteries to leave the charger on all winter for batterystorage.

Morten

Hi mike

Thanks for your comprehensive answer. I will make the test you describe.

Morten