I want to build battery backup for my Master Clock so it keeps running when mains fails.
My Master Clock page is here.
Electronic Master Clock

I have the basic charging and backup circit (see diagram) using a 6v sealed lead acid battery.

The circuit works fine but my only concern is that a long power cut will discharge my battery too far and shorten it's life.

Is there a circuit for discharge protection so I can preset a voltage where my battery is taken out of circuit?

I notice solar panel regulators have this function built in but they all work with 12v batteries.

Thanks.
Brett.

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Here is something I posted for just a visual LED indication that a 12V SLA has discharged too low. You can use this basic circuit by tailoring the voltage divider on the Ref input to the TL431, and modifying the output side to switch off your load. Can you do that, or do you want some help?

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Here, I modified it for you: Scale the time axis to match the discharge time of your battery. What do you want to use as the final cutoff voltage? If I didn't guess it, you can play with the resistive divider. I'm taking a wag that the 2n3906 will drive your clock. If not, you could use a bigger PNP or a PFET.

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Thanks MikeMl.

I did measure the current from the PSU of my clock and I think it was 300mA ( I should have written it down). There is also another 250 to 350mA to drive the electro mechanical chime on the hour. This is max 12 x 350mA pulses on the hour into an electromagnet.

The 30 second electromechanical slaves are powered from a seperate supply so I will sort the supply for this later.

In your circuit if the battery was disconnected when it dropped below a set voltage would it try to switch back on when the battery voltage raised again due to no load?

If so I could use the OP transistor to switch off a relay instead of the battery. I could then configure the relay to hold itself off via it's own contacts. Therefore once the voltage threshold had been reached the relay releases shuting down the power to my clock and holding itself off.

A reset button would have to be pressed to reset the relay.

 

Found an article on the TL431 Information On Electronics Articles and News: TL431 Battery Voltage Monitor

 

hi Oliver,
Not cutting across what Mikes doing, but is the battery in question on any sort of float charge.?

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

Hi Eric
The battery is floated at 6.75 volts.

Brett.

 

hi,
Im sure that Mike will modify his circuit so that it has hysteresis from 5.4V to 6.6V.
In that way the auto low cut off will reset to ON when the mains is restored.

OK.

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

The circuit as presented the last time already exhibits some hysteresis, as defined by the 2.2meg feed-back resistor. It creates a nice clean switching threshold, and the circuit will not cut back in until the battery voltage is about 200mV higher than where it cut out.

The attached circuit as modified to increase its load switching capacity by replacing the PNP with a PFET. It will now switch an Amp or three. I also modified it so that it includes the explicit push button to restart it. Note that I lowered the estimated load being switched to 20Ω.

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hi Mike,
My experience with SLA batteries shows that the battery will 'recover' to a voltage much higher voltage than 200mV, as soon as the load comes off.

I would predict the circuit will tend to repeatedly switch on/off, at the battery low point.

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

Good Point! Either modify the circuit to increase the hysteresis or use the version with the push button, which will not start again unless the button is pushed.

 

Thanks you so much for your help on this. The push button idea is great! I will give it a try on breadboard as soon as poss and let you all know how I get on.

Brett.

 

Quick question. The TDL431 is stocked with many codes after the 431. Can you tell me what type in your circuit so I can get the data sheet?

Thanks.
Brett.

 

Here you go. I find these little buggers about as useful as 555s, so I bought 20 from Mouser in the TO-92? plastic transistor package.

 

Thanks again MikeMl.