Electrical – Approval before using built portable charger on a smartphone


I've finally managed to set up a more efficient portable charger that uses 8 AA alkaline batteries(actually not so efficient).


simulate this circuit – Schematic created using CircuitLab

There are two switches in the schematic:CH1 and CH2.When all the alkaline batteries are fully charged,only CH1 is closed.That will make the regulator dissipate less power.After the batteries are discharged enough(until each one of the batteries reaches ~1,15V) so that togheter they output <7 volts,CH1 is opened and CH2 is closed to continue charging(until the top 2 batteries each reach 1,2V and the rest reach 0,8V(per battery),shutting down the voltage regulator).

I'm not really sure in what package is the regulator,but it has a heatsink attached and I attached another bigger improvised heatsink.It's not heating up too much.

I am sadly using batteries from different brands and I don't think they have the same age(I used some more than the others),but I am constantly keeping an eye out on them.

According to the datasheet,it should output 5V and 1A.

The role of this pack is to charge a dead phone battery for a bit so you can solve an emergency(call someone).

I already tested it on another phone(and it worked great) and on another device which showed some unexpected behaviour(the battery charge level flipping back and forth between fully discharged and a bit charged,but it had some problems before testing the circuit on it).

The original charger of the smartphone I wish to connect it to has an output of 5 V and 1,3 A

To conclude:
Can I freely use the charger with my smartphone without causing damage(Would you use it on your phone?)

EDIT:I understand that the two-switches-activated-simultaneously problem and the differences between the batteries caught your eye,but I would like an answer more focused on the current and voltage supplied to the smartphone,as well as the way the regulator outputs power(is it ok if it's continous?should it be pulsed so it won't cause any damage?)

Best Answer

Can I freely use the charger with my smartphone without causing damage(Would you use it on your phone?)

Yes and yes.

The 7805 does all the work for you by setting the output voltage at 5v. You don't have to worry about the current output because the phone will take care of that by limiting its current draw such that output remains at 5v. Worst case (ie the phone doesn't control current draw), the 7805 will protect itself with its short-circuit protection.

Now just because it's safe to use and I would be fine using it, doesn't mean you should use this design. As other have pointed out, it's going to be very inefficient and you're going to have a bunch of batteries at differing states of charge. You want to try to keep all batteries at a similar charge so that you have an output you can count on. If you have some dead cells mixed with charged ones you might not realize it at all. Then when one starts leaking acid and corroding, your backup battery pack is no good anymore.

What you really want here is a switching boost regulator. Not only will that allow you to use all your batteries consistently and maintain equal charge, but it will reduce the number of batteries (smaller pack) needed for the same purpose. It will also have the benefit of being able to function on a lower total battery charge, giving you a longer charge from the same batteries.

Depending on your budget (shoe-string or super shoe-string), switching regulators might be too pricey for what you want. A neat little DIY circuit you should check into is the Joule Thief. It's not very efficient, but is extremely simple and very affordable to build. Better still, it can be used in conjunction with your 7805 to give you a stable and phone-safe output. There are literally hundreds of sources for circuits like this on Google if it's a route you're considering.

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