Also not to be rude, but understanding a device's data sheet is fundamental to using the device in a design.
Learning how to read a data sheet is basic to designing an electronic circuit.
I know many attempt a design without understanding a data sheet, but the results are often not pretty.

Agreed. All the more reason not to try a complex circuit design when an already purpose-built circuit exists. For people learning, it's best to play it safe and not do risky stuff. I try to learn and understand these things before I utilize them in my designs. Because if I don't understand how it works, then I won't understand when something goes sideways how to fix it, or how to improve the design, etc.

 

REally, if the goal is simply to stop working when the battery voltage drops below some higher value, the simple scheme will be to add a series Zener diode that will stop conducting when the total applied voltage drops below the sum of the USB adapter input plus the zener voltage. NO, it is not efficient, but it would be simple and effective.

 

REally, if the goal is simply to stop working when the battery voltage drops below some higher value, the simple scheme will be to add a series Zener diode that will stop conducting when the total applied voltage drops below the sum of the USB adapter input plus the zener voltage. NO, it is not efficient, but it would be simple and effective.

Yes that would be simpler, but as you said less efficient, and this will likely be a fairly high power application, probably into the 10s of watts, could easily be looking at over 1A, maybe 3A+ if these are high wattage modules. That's a lot of wattage to lose over the diode, and heat to dissipatee. Efficiency itself isn't even necessarily the biggest point, but rather the heat to dissipate.

 

WHAT does the TS intend to power with the battery?? and why use a USB scheme supply ???

 

WHAT does the TS intend to power with the battery?? and why use a USB scheme supply ???

If you go back to the first post, it looks like he is intending to power 2 separate USB boards for charging devices like a cell phone or tablet. Each of those can be 30-45W depending on a few factors (I have used all the way up to 65W circuits).

 

REally, if the goal is simply to stop working when the battery voltage drops below some higher value, the simple scheme will be to add a series Zener diode that will stop conducting when the total applied voltage drops below the sum of the USB adapter input plus the zener voltage. NO, it is not efficient, but it would be simple and effective.

The limited voltage selection and large voltage tolerance of a typical Zener makes that a problematic solution.

 

Any Zener diode rated between five and ten volts, and enough current, should work.

 

Any Zener diode rated between five and ten volts, and enough current, should work.

Seems that would give a rather inaccurate cutoff.

 

I qualified the suggestion initially by stating it was not efficient, and certainly it is not efficient. BUT it would be simple and effective and easy to apply. Certainly that is the trade-off.

 

The limited voltage selection and large voltage tolerance of a typical Zener makes that a problematic solution.

The tolerance doesn't need to be *that* tight for this application as long as the user goes on the safe side (a higher cutoff). The much bigger issue is really efficiency and heat dissipation.