DIY PowerBank

[bobdabiulder]

Hi!  I have two VERY cheap Li-ion 18650s from Ebay for <$5.  I understand that they suck.  I am trying to use them in parallel to charge my phone, with the charging circuitry from an older powerbank.  I got it to give around 1A for a while, but now it only gives a very steady 140mA out +/- 5mA.  How can I fix this?  THANKS.  

 

P.S. tell me if you need more info.

[RGProductions]

3 minutes ago, bobdabiulder said:

Hi!  I have two VERY cheap Li-ion 18650s from Ebay for <$5.  I understand that they suck.  I am trying to use them in parallel to charge my phone, with the charging circuitry from an older powerbank.  I got it to give around 1A for a while, but now it only gives a very steady 140mA out +/- 5mA.  How can I fix this?  THANKS.  

 

P.S. tell me if you need more info.

That probably means the cells are not providing enough voltage,so the circuitry is having to output less current to get to 5 volts.

[bobdabiulder]

The cells are at 4.01v, and judging by the inductor and some tests, it appears that the charging circuitry im reusing has a buck/boost converter to get +5v out.

[RGProductions]

2 minutes ago, bobdabiulder said:

The cells are at 4.01v, and judging by the inductor and some tests, it appears that the charging circuitry im reusing has a buck/boost converter to get +5v out.

Huh. I can't really help beyond here.

[dany_boy]

I can. Could you provide a sketch of your schematic?

[dany_boy]

Is it something like this?

Sorry for the awful sketch, but really not feeling like firing up eagle

[bobdabiulder]

Yes, except I have a battery protection board in between.  However, I have tried bypassing the first protection board since the booster is one itself.  That was unsuccessful.  

[dany_boy]

Can you measure the input and output voltages at the same time when the current begins to drop?

[bobdabiulder]

Sure, soon.  Output should be 5v, my phone didn't like 4.2 directly from the batteries.

[dany_boy]

If you could post some data regarding the components (specially the boost converter) that would also be very helpful. Keep in mind most boost converters don't like working below their rated minimum voltage. Some pictures would do nicely for additional reference.

[bobdabiulder]

The boost converter worked fine on the original power bank, and it works fine with other test voltages down to 3.2v.  It seems that no matter what it defaults to 140mA.

[dany_boy]

Make sure your batteries don't go bellow those 3.2 volts, and also consider the constant voltage charging stage from Li-Ion batteries consumes much less current.