Simple 12V "UPS" Circuit

Attached, I've included a simple circuit that I'm thinking will work for a simple 12V "bumpless" UPS-type of a circuit. Normally, I power my radios from my 12V lighter, but there are times when I shut that down or the power drops temporarilly - like when you start the car - and I'm looking for a small 12V SLA battery to "carry" the load for a short time until the car's voltage returns. Will the attached circuit do the trick? Or, does the circuit need to be modified in any way to keep current from draining back to the car's circuit? Add another diode?

Looking for any feedback. All I'm looking for the battery to do is to carry the load temporarilly and that it does not get charged by the car nor does it feed-back into the car's circuits.

Can you recommend diodes for the circuit? It has been years since I've built stuff like this, so please bear with me!

Thanks.

Well, you'd need another diode to "OR" the batteries together. Otherwise, the SLA battery would try to power your electrical system during engine cranking.

You'll really need a charger/maintainer circuit, too - otherwise your SLA battery will have a short life indeed. Charging requirements for SLA batteries are different than automotive batteries.

I'm assuming that the attached updated circuit is what you mean, right? Also, as the battery will only be used for short times, I'll maintain its charge separately, so no need for a charging circuit.

Also, is there a simple addition to this circuit that could indicate via an LED when there is power flow from the battery?

Thanks.

danf100 said:

I'm assuming that the attached updated circuit is what you mean, right?

Click to expand...

Yes. You'll need to select diodes that can handle the total power required by your audio system.

For example, if you had a 15W x 15W system (stereo, 15W per channel), that's 30 Watts.
I = P/E (Current = Power / Voltage), so I = 30/14, roughly 2.15A.

If you don't know the power rating of your audio system, find the fuse for it in your vehicle's manual. Use a diode with an average current rating higher than the fuse current rating.

Note that you must also use a fuse in the wiring between the positive terminal of your battery and the diode. Otherwise, if there is a fault in the wiring or radio, you will likely wind up with a fire.

Also, as the battery will only be used for short times, I'll maintain its charge separately, so no need for a charging circuit.

Click to expand...

OK. If it stays partially discharged for any length of time, it's life will be shortened considerably.

Also, is there a simple addition to this circuit that could indicate via an LED when there is power flow from the battery?

Click to expand...

Not that I can think of offhand.

There is a commercially made version of something close to what you are looking for, check here: http://www.westmountainradio.com/SuperPWRgate.htm

SgtWookie said:

For example, if you had a 15W x 15W system (stereo, 15W per channel), that's 30 Watts.
I = P/E (Current = Power / Voltage), so I = 30/14, roughly 2.15A.

Click to expand...

No.
The amplifier is probably not 100% efficient. If it is class-AB then 30W goes to the speakers and another 30W heats the amplifier. So the current is 60W/13.8V= 4.4A.

i need 1000va ups circuit.
please help me with full description.
thank you

Can't say for sure if this could be a working solution, but it might help the thought process. If the sensing circuitry detects the current draw by the ignition switching, then by detecting this would allow you to make a connection to the backup battery, and when that connection is made, an LED could indicate the battery is in use. This would also enable total isolation of the backup battery and reduce its discharging.

You also might want to think of a using some sort of constant trickle charger
for the backup battery by using the car battery as the source....but only ON when the car is on... ignition controlled.

Now we just have to wait for the SMART GUYS to respond...!

I've been looking or something similar to this. My situation is that I have an iPod interface from Harmon Kardon that keeps turning off when I trigger my snow plow blade on my truck. The voltage drops and causes the HK to attempt to shutdown or at the least hiccup. I need a voltage conditioner to bridge the dips. I was wondering if anyone had any additions to these diagrams that they could think of, or would these work as depicted? It's been high school since I've done any tinkering with electronics projects but sounds like a fun way to spend an afternoon.