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Battery drainage question
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Battery drainage question

by dnalevy on Sat Nov 16, 2013 3:57 pm

Hi all,
Quick question about mintyboost.
If I was to charge an iPhone 4S with 2 fresh duracell (not high drain) alkaline batteries, how much battery perectage increase in the iphone would you realsitically expect I'd get?
Or to put the question another way: If the batteries start out at 3.2V (nominal voltage) how low would you expect them to go before they can no longer supply enough current to enable the mintyboost to output at 0.5A?

Ive tested and it seems to stop charging when the batteries drop to around 2.5V. After that the output current drops to under 100ma.
Is this normal or am I doing something wrong?
I was under the impression the mintyboost would drain the batteries completely, or at least very low.

Thanks!
dnalevy
 
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Joined: Sat Nov 16, 2013 3:39 pm

Re: Battery drainage question

by adafruit_support_bill on Sat Nov 16, 2013 4:15 pm

it seems to stop charging when the batteries drop to around 2.5V.

Standard alkaline batteries are not ideal for the MintyBoost. The discharge curve has a steadily dropping voltage which forces the boost converter to work harder (and less efficiently) to create a 5v output. Alkaline cells also tend to exhibit increased internal resistance as they discharge, resulting in reduced current delivering capability. The Minty will continue to do it's job as long as there is current available from the batteries. It is the phone's charge controller that will stop the charge when it can't get the current it wants.

adafruit_support_bill
 
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Re: Battery drainage question

by dnalevy on Sat Nov 16, 2013 4:28 pm

Thanks for your reply.
So just to clarify, with an iPhone 4S charging at 0.5A with 2xaa alkalies as mentioned above, approximately how far do you expect the battery voltage to drop before the phone stops charging at 0.5A?
dnalevy
 
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Joined: Sat Nov 16, 2013 3:39 pm

Re: Battery drainage question

by adafruit_support_bill on Sat Nov 16, 2013 5:26 pm

That's hard to say. There are wide variations in battery performance and it depends on the initial charge state of the phone. With name-brand high-drain batteries, most people report a 20-25% charge on a set.

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