Power supply with battery backup

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davepusey

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I'm building a power supply (12VDC 1A) that has a SLA battery as a backup.

When the mains power fails, the relay de-energizes, switching the output to the battery, from the power supply circuit.

When mains power returns, the relay switches the output back to the power supply.

As present, I am switching both the +12V and 0V connections via a DPDT relay, but what I want to know is, can I connect all the 0V connections (battery, power supply, and output terminal) together and just switch the +12V with a single-pole relay.

Is this a safe thing to do?

BrownOut

Banned
Connect it properly, and it shouldn't be a problem.

BrownOut

Banned
I don't know exactly what you are proposing, and I don't want to assume. But if you want to connect the ground plane of your power supply to the negaive terminal of your battery, then you should be OK.

davepusey

Member
Hopefully this will make sense of it!

Current circuit...

Proposed circuit...

BrownOut

Banned
I don't see a problem. Make sure the battery is connected to the realy terminal that is connected when de-energized if you're using the actualy power supply to actualte the relay.

davepusey

Member
Yes it is - the idea is when no mains then relay de-energized and battery routed to output, when mains, relay is engaged by power supply and power supply is routed to output.

Think of it like a UPS.

davepusey

Member
Would it be worth adding a diode between battery + and relay for added protection?

Like this...

Code:
                   |\ |
BATT + ]-----------| >|-------------[ Relay Terminal
|/ |

Hero999

Banned
Do you want to charge the battery from the mains?

How long does the battery back up need to last for?

How long will it be connected to the mains for?

You might not need a relay, an LM317 voltage regulator set to 13.8V with the battery connect to its output might do.

BrownOut

Banned
Would it be worth adding a diode between battery + and relay for added protection?

Like this...

Code:
                   |\ |
BATT + ]-----------| >|-------------[ Relay Terminal
|/ |

I don't think so.

davepusey

Member
Just tried connecting as per proposed circuit above and it works fine. The switchover time is shorter too due to only one set of contacts needing to move.

davepusey

Member
Do you want to charge the battery from the mains?

Yes i have a seperate circuit for that.

How long does the battery back up need to last for?

It's a 1.3 amp-hour battery - should last several hours at my loading level of about 100mA.

How long will it be connected to the mains for?

Usually from 8am to 11pm but could be longer and could be shorter.

You might not need a relay, an LM317 voltage regulator set to 13.8V with the battery connect to its output might do.

Can you provide a circuit of your suggestion?

EDIT: Where does the 13.8V figure come from? The battery reads 12.02 on my voltmeter. And the power supply is giving out 11.85 from it's 78S12 regulator.

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Hero999

Banned
How are you charging the battery?

Please post the circuit.

13.8V refers to the maximum float charge for a typical 12V SLA. This means you can leave the SLA connected to 13.8V continuously without any damage. The disadvantage is that it will take ages to charge at 13.8V.

davepusey

Member
How are you charging the battery?

Please post the circuit.

13.8V refers to the maximum float charge for a typical 12V SLA. This means you can leave the SLA connected to 13.8V continuously without any damage. The disadvantage is that it will take ages to charge at 13.8V.

This is the basis for the charger... Sealed Lead-Acid Battery Charger Circuit

New Member
This is the basis for the charger... Sealed Lead-Acid Battery Charger Circuit

What is the supply voltage for the charging circuit?

If you are running it from the 12V reg then unfortunately that wont be a high enough input voltage for the charger, generally a 16 to 18V input is required, so the required charge voltage can be set to 13.8V for float/standby use as already mentioned.

This is covered in the first paragraph under the title "How to use this circuit"

davepusey

Member
Ok let me explain...

I have a 15V 1A transformer (single secondary winding - no center tap) connected to a bridge rectifier (round one with four legs).

The DC output of the bridge rectifier has a 2200uF elec capacitor across it in parrelel.

Reading on meter is 22 volts.

This is then fed into a 78S12 regulator to get a 12V supply.

I was planning to connect the charger before the regulator (at the 22V point).

Last edited:

New Member
Where does the 13.8V figure come from? The battery reads 12.02 on my voltmeter. And the power supply is giving out 11.85 from it's 78S12 regulator.

I think we thought that the 12.02V reading was the actual charge voltage from the charge circuit, it's actually the voltage of the battery with nothing connected?

davepusey

Member
Yes that is correct...

Battery voltage 12.11V
Unregulated supply 22V
Regulated supply 11.45V (this seems low - anyone know why?)

EDIT: My final output for the supply is a LED lighting system that i've built.

At 12.11V when connected direct to battery it will draw 155mA and is quite brightly lit.

On the 11.45V from the regulator it is much dimmer and not lit as brightly as I would light.

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