only partial charging

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PinkElf
 
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only partial charging

Post by PinkElf »

Hi!


I have read the FAQ beforehands.

I built the Mintyboost according to the tutorial, the measurings were correct.

My problem is:

When I plug in my device (a Motorola Milestone 2) it will charge. It gets very hot, which is normal as far as I understood it. The batteries get hot too, though nowhere as hot as the electronics.
But the device charges only for some 20 minutes, bringing the charge up from say 30% to 50%. Then charging stops (the mobile still has the charging symbole, but the percentage does not rise any higher, the Mintyboost gets cool again).

I tried it with shut off mobile phone and with the running one (with deactivated screen), samye problem.

What can I do and where does the problem lie?

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adafruit_support_bill
 
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Re: only partial charging

Post by adafruit_support_bill »

bringing the charge up from say 30% to 50%
Most modern smart-phones have fairly high-capacity batteries. Much more capacity than can be recharged from a pair of AA alkaline batteries. Most users report that a set of AA alkaline cells will charge their phones by about 20-25%. You can get more charge capacity from C cells. Some report better performance using NiMH rechargeables. Lithium cells also work well.

PinkElf
 
Posts: 6
Joined: Mon May 06, 2013 8:23 pm

Re: only partial charging

Post by PinkElf »

I use Eneloop NiMH rechargables. They have 2000mAh capacity.

The milestone 2 has a Lithium battery with 1300mAh.

The batteries should be able to charge my mobile to full capacity and then some.

According to the (not super-trustworthy) battery tester, my rechargable AA batteries are still good after charging stops.

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adafruit_support_bill
 
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Re: only partial charging

Post by adafruit_support_bill »

I use Eneloop NiMH rechargables. They have 2000mAh capacity.
That is 200mAh at 1.2v (2.4v for the pair). Boosting that to 5v leaves you with 960mA - assuming 100% efficiency. Using a more realistic 80% conversion efficiency leaves you with 768. Then you have to consider the charge efficiency of the phone itself. 80% is a typical figure for that, so we are down to 614. - less than half a charge at best.

Of course, all of that is based on the manufacturer's capacity rating which is based on a 400mA discharge rate. The discharge rate in the MintyBoost to provide the 500mA charge current required by the phone will be more than twice that, so you need to derate that figure even further.

At some point the output voltage will start to droop and the phone will decide that it is not sufficient for charging. This will typically occur somewhat before the cells are completely discharged - especially when discharging at a rate high enough to cause noticeable warming of the cells. It is no unusual for cells to 'rebound' after they cool off a bit.

PinkElf
 
Posts: 6
Joined: Mon May 06, 2013 8:23 pm

Re: only partial charging

Post by PinkElf »

Ah okay I did not think about these Losses.

My battery tester must be broken - I put one of the batteries in my flashlight and it gave nearly no light, so they are truly empty.

If the 2 AA batteries are not sufficient I will see if I can find a bigger case for the Mintyboost to use my 4AA battery holder. I would like to use them because they are the most easy to find batteries and I have a pack of rechargable AA on me anyway. Do you have any suggestion what to do if I wanted to build a device that could charge my device for say 80%?

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adafruit_support_bill
 
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Re: only partial charging

Post by adafruit_support_bill »

With NiMH cells like Eneloops, a 4xAA pack is workable. The output voltage of the freshly charged pack will be slightly over 5v, but it will drop down to about 4.8v fairly quickly under load. Another benefit is that your boost efficiency will improve with the higher input voltage.

I wouldn't recommend a 4x NiMH pack with larger cells, since the voltage of a fully charged pack will not drop so fast. But you could get pretty good performance from a 3xC or 3xD NiMH pack.

The best bet for high capacity charging would be a LiPo pack like this: http://www.adafruit.com/products/353

PinkElf
 
Posts: 6
Joined: Mon May 06, 2013 8:23 pm

Re: only partial charging

Post by PinkElf »

What I wonder now that I read up on the mintyboost "by accident":

You write that:

"Some numbers...

iPhone (all types): 3/4 full recharge"


I am more in the range of 1/5th of a full recharge, the batteries aren't that different. Either this information is incorrect or something does not work right with my kit.

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adafruit_support_bill
 
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Re: only partial charging

Post by adafruit_support_bill »

Where did you read that? That may have been accurate for a 1st generation iPhone, but the 20% range is typical with the last few generations.

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