Quick and Nasty Battery backup circuit. Need input.

Other Hardware General Electronics
1

Hi Guys

Hope someone can shed light on this, I need to make a quick circuit to act as a battery backup circuit.
The image attached shows a load connected to two power sources, one the main supply from an isolated ac to dc converter that provides 5v and the second power source is a battery pack fitted with a buck boost to maintain 5v. The battery pack is charged through a different circuit from the main supply however the charger does not connect the battery to the load once main power source is removed.

Will the attached circuit work, what is the pitfalls ? any input would be appreciated.

Thank you

quick and nasty .png

2

Diodes work better on the upstream side of a voltage regulator. With your circuit, the 0.7V diode drop will change the 5V to 4.3V.

3

Hi Aarg

Thank you for feedback, I toke this into consideration, and the booster used on the battery is set to 5.66v and the trim on the primary line set to 5.7v, I will be using schottky diodes with a voltage drop of .4 so that should not be a problem.

4

Rustie0125:
Hi Aarg

Thank you for feedback, I toke this into consideration, and the booster used on the battery is set to 5.66v and the trim on the primary line set to 5.7v, I will be using schottky diodes with a voltage drop of .4 so that should not be a problem.

Then why didn't you mention it? :frowning:

5

Does the battery get charged while main power is connected?

With the simple diodes and adjustable power converters, you can set the battery's power converter to a fraction of a volt lower, so that the battery isn't consumed in normal operation. But 3A is a lot of draw. The main power converter will drop its voltage a little under that load. That allows the battery to supply some of the power, draining your battery even though you're connected to main power.

So, you would want to keep topping up the battery while main power is connected.

But if you're doing that, then why not charge the battery from main power and only have one power converter going from the battery to the circuit? Saving one diode drop may be significant for the battery life.

If the battery doesn't get charged (like a 3V lithium coin-cell battery) then you never want it able to draw power while main power is connected. Then the simple diode circuit is not appropriate.