Battery charger

[Magnum]

I can use some electronics help.

The 200 m.a. AC/DC adaptor did not supply enough current to keep the
12 V battery charged all the way.

It's a lead acid sealed battery hooked up to a 12 Vdc answering machine.
It functions as a battery backup.

Battery specs

Standby use = .195 amps
Cyclic   use  = .39 amps

The answering machine uses a 200 Ma power supply.

Would 500 m.a. be sufficient ?

[dedndave]

well - if the answering machine is on all the time, then the 200 mA was pretty well spent just keeping that going
a lead-acid battery cell has a voltage of ~2.1 V at full charge, no load
you may want to put some kind of regulator in there to insure the battery voltage does not exceed ~13.4 V when the charger is on
but - 500 mA provides you with 195 mA to run the machine, leaving up to 305 mA to charge
if the charger is on and the machine is off - 500 mA
these batteries are pretty stout and can take a little misuse - just not constant

[Magnum]

well - if the answering machine is on all the time, then the 200 mA was pretty well spent just keeping that going
a lead-acid battery cell has a voltage of ~2.1 V at full charge, no load
you may want to put some kind of regulator in there to insure the battery voltage does not exceed ~13.4 V when the charger is on
but - 500 mA provides you with 195 mA to run the machine, leaving up to 305 mA to charge
if the charger is on and the machine is off - 500 mA
these batteries are pretty stout and can take a little misuse - just not constant

Thanks.

[hutch--]

Andy,

A 9 volt AC mains transformer, a cheap bridge rectifier and a capacitor over about 24 volts and you will have a very reliable and clean 12 or so volts. The capacitor is better bigger than smaller, 4000 microfarads or bigger but it should make a very clean power supply for either the gadget or the battery charger. Note though that you would need additional circuitry if you want it to reduce its charge as the battery approaches full charge.

[dedndave]

a capacitor is nice if you intend to run on AC power with no battery connected
but, batteries make pretty good capacitors - they will keep the ripple voltage low for you

with no battery connected - full wave bridge rectifier
C = I / 2 f Vpp
where f = 60 Hz, I is in amps, and C is in farads
if you want to keep the ripple below 0.5 Vpp with a 200 mA load, 3000 uF or more is required

[Magnum]

Andy,

A 9 volt AC mains transformer, a cheap bridge rectifier and a capacitor over about 24 volts and you will have a very reliable and clean 12 or so volts. The capacitor is better bigger than smaller, 4000 microfarads or bigger but it should make a very clean power supply for either the gadget or the battery charger. Note though that you would need additional circuitry if you want it to reduce its charge as the battery approaches full charge.

After being gone for a while, I found the answering machine was dead.

The battery was almost completely discharged and the adaptor was putting out 1.9 Volts of what used to be
13.9 volts.

[dedndave]

see what it's putting out with no battery or answering machine connected

[Magnum]

1.9 volts

[dedndave]

dead
i have a box full of adapters - lol
you can probably get one at radio shack

[Magnum]

I found the real problem.

I am using a power strip with 4 outlets.

Two of them were bad.

Other 2 showing around 122.6 Volts A.C.

So, my answering machine and my power adaptors work fine.