Minty Boost voltage dropping and causing charging to stop?

[kriston]

Hi!

I just made my MBv3 kit yesterday, and have been having weird problems with it. Finally got my hands on a digital multimeter, and just figured out why the problem might be. Not to say that I know how to fix it... That's why I'm posting here!

Problem: My iPhone4 is plugged in with fresh AA batteries. It starts charging with that "tee-clink" sound, has the pretty charging symbol and green bottle of powah. After anywhere from 1-10 minutes, my iPhone4 makes that same "tee-clink" sound, and when I take a look at it, it has stopped charging.


So what I did today was to test the voltages. With those fresh batteries plugged in, and no iPhone, I was getting 4.96V across the outer pins. Plugged the phone it, and left it to charge. After a few minutes, it made a 2nd "tee-clink" sound and stopped charging. I checked the voltage across the outer pins, and it was pulling about 4.3V. I unplugged the phone and checked the voltage across the outer pins again and it had gone back up to 4.96V.

Tried it with a pair of freshly charged 2450mA NiMH rechargables as well, and same sort of a thing.

Does anybody know why it might be happening? I'm going away for a 4 day hiking trip on Monday and I would like to be able to bring this with me!!!



On a separate note: Somebody mentioned that a pair of fresh AAs gives me about 50% of my iPhone4's battery back. What should I be using if I want something closer to a full charge? A pair of Cs? Ds?
(I'm trying to guesstimate... If I were to use a full load of charge of my iphone a day, how many batteries would I need to keep it usable throughout the trip? )

[adafruit_support_bill]

Most likely the current demand is causing the voltage to sag which causes the phone to go out of charge mode. When the voltage comes back up it starts charging again. I get the same behavior on my iPhone from one of my 3rd party wall-wart usb chargers - especially if the phone battery is very low to start with.

This page describes how much capacity you can expect from various battery options: http://www.ladyada.net/make/mintyboost/power.html

[kriston]

Most likely the current demand is causing the voltage to sag which causes the phone to go out of charge mode. When the voltage comes back up it starts charging again. I get the same behavior on my iPhone from one of my 3rd party wall-wart usb chargers - especially if the phone battery is very low to start with.

Is there anything that I can do about that then? Because if fully charged batteries are unable to maintain a voltage, this isn't much use to me.
Is it possible that I've done something wrong? Or that one of my parts are faulty? Or that something is happening that is causing the sag?

[adafruit_support_bill]

You might have better luck with C's or D's. According to LadyAda:

Larger capacity batteries also have a positive side effect that they dont droop or sag as much under high loads and have lower internal resistance!

[kriston]

You might have better luck with C's or D's. According to LadyAda:

Larger capacity batteries also have a positive side effect that they dont droop or sag as much under high loads and have lower internal resistance!

I'll check tomorrow if I can get a C or D sized holder. And maybe rechargeables as well. Thanks!

[tinsmith]

Many C and D sized rechargeables are effectively repacked AA rechargeables, for what that's worth.

[kriston]

Many C and D sized rechargeables are effectively repacked AA rechargeables, for what that's worth.

ya, I saw that comment about getting *good* C/D sized rechargeables, and not the re-packaged ones.

I'll see what I can find.

[yeoj4151]

Or instead of upping the available current you could just up the supply voltage so that when it does sag it can stay above the charging threshold. To do this go out to radioshack or other place you can grab hobby electronics and get a AA battery holder that takes 3 AA's is series making it 4.5Volts input (closer to 3.9 if you are using recharagable), instead of the standard 3Volts(2.4-2.6 recharagable). This means that when it pulls all the current it can from the batteries and their supply voltage drops to handle the load, it has farther to fall in order to stop providing the 5Volts out. Its also more efficient for the Ni-MH, and Ni-Cd rechargeable batteries.