hardcore misery

i've tried to cut my cellphone charger's output jack and plug it to my cellphone to measure its voltage, but the multimeter doesn't display its voltage, but it displays battery's resistance.

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Nigel Goodwin

It won't connect to the battery, it probably connects to the input of the charging circuitry.

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hardcore misery

...and the charging circuitry is not connected to the battery?

this is one of the main problems on our project (auto-shut off cellphone charger)

the comparator should sense a voltage across the cellphone, but unfortunately we've not expected this problem.

Nigel Goodwin

Of course it is, but that's no help to you.

You don't seem to have researched the project very well?, the phone will most probably already have a properly designed one internally.

Sorry, but you should have!, it's an obvious occurance.

A comparator also isn't much use for what you want anyway, presumably you also haven't researched charging techniques for the battery chemistry involved?.

I hope this 'project' is a school one?, if it's a university project it's not looking good for you!.

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hardcore misery

this project is for our electronics design course...

we've not researched about charging techniques for cellphone batteries..

one question sir, if a battery is overcharged for a period of time, does it mean that the battery voltage exceeds on its rated full battery voltage?(ex. full battery voltage is 4V, if overcharged, does battery voltage exceeds 4V?)

audioguru

Do you know which chemical type is the cell-phone battery? Do you have the datasheet from the battery manufacturer?

If it is Lithium then it is about (some are a little different) 4.2V when fully charged. If it is overcharged then it gets very hot and might catch on fire. The lithium battery has a built-in temperature sensor that connects to the 3rd pin. It is a backup and shuts off the charger if the voltage comparator fails to stop overcharging.

Maxim-IC make battery charger ICs. Look at their datasheets to see everything involved with charging batteries.

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Uncle $crooge

Nigel Goodwin

Certainly with NiCd and NiMh the battery voltage FALLS when it's over charged, you have to detect this drop, so a comparator isn't any help.

You need to research specific charging requirements for your particular battery chemistry - Lithium Ion is common in modern phones, and has difficult and dangerous requirements, which is why it's usually built-in.

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hardcore misery

so is there any hope in our project?

based on the schematic, we can't put the -V of comparator to the +V terminal of cellphone battery because at 'no load condition' (charging voltage is greater than or equal to the Vreference of the comparator, the comparator is OFF) and the comparator will sense a 0V across cellphone which turns ON the comparator and eventualy it will turn OFF again because the comparator will sense the charging voltage which is greater than the Vreference, hence the comparator will ON/OFF at a rapid motion...

if we will tap the -V of comparator to the +V of the charger...
at 'no load condition' again the comparator is OFF, and we can't charge the cellphone because the there is no connection between the +V of the battery and the +V of the charger.

If we set the circuit to be in charging condition at no load... the battery voltage should exceed the Vreference of the comparator in able to turn it off...

Gayan Soyza

Its good if you can design a charger for Li-ION but its really hard.

This is the most important before building any battery charger first.

I will strongly suggest you to look into this special Li-ION charger IC's introduced by MAXIM.

MAX1551
MAX1736

For me Comparator idea is not a good for battery charges.Because its a very basic level technic.Battery Manufactures they won't suddenly stop their charging when it reaches to its cutoff mark.They will look (sense) over & over again the voltage drop happening during charging period.

Thats why I still on research on building my own Li-ION battery charger.

I have an idea to detect the upper voltage limit of the battery periods of time in regular intervals by use of a micro controller.This is the only thing I can do at home.

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My Website
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JimB

I have to ask, what is the aim of the project?

Is it to improve the charging of cellphone batteries?
If so, I think that the phone manufacturers would have that well sorted by now. One of their selling points would be good battery life between charges.

Is it to save the planet from global warming from all that horrible C02 produced by cellphone left on charge unnecessarily?
If so, you need to be in at the design stage for the whole battery/phone/charger system.

As an afterthought, you could try investigating the current drawn by the phone from the charger during the charge cycle, and use that as the basis for switching the charger off.

JimB

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audioguru

A cell phone already has a charger circuit in it.
Take the battery out of the cell phone then charge it with your own charger circuit.

If the battery is a Lithium cell:
1) Limit the charging current to what the battery manufacturer recommends. 2) Use your comparator to stop the charging when the battery voltage reaches 4.2V or whatever the manufacturer recommends.
3) Use the temperature sensor in the battery as a backup to stop the charging if the battery gets too hot because your comparator might have failed.

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Uncle $crooge

hardcore misery

this came through my mind, and i've consulted this on my teammates, and they disagree with this idea because our project is designed to charge the cellphone with the battery inside and shut it off when it is in fully charged condition, and not to take out the cellphone battery and charge it..(and we have to improvise a battery pack for li-ons cellphone battery, if we will pursue this idea)

hardcore misery

this project is to be designed to avoid overcharging of cellphone batteries. this is just a design project for out electronics design course, in fact, tommorrow is our initial defense of our prototype( all we can present is the multiple charging system of the cellphone charger)

hardcore misery

As an afterthought, you could try investigating the current drawn by the phone from the charger during the charge cycle, and use that as the basis for switching the charger off.

is this the same as the voltage comparator? at no load condition, the charger must be off.. if there's a load, the charger is on, and if the battery is fully charged, the charger is off, basically, no load and full battery condition is the same ( but how can we achieve this condition?)

Nigel Goodwin

Doesn't the phone already do this?, the internal charger circuitry presumably is there for that very reason?.

I suspect your initial defence is doomed to failure, as it's blatently obvious your team has'nt done any research whatsoever for your project.

It's rather like trying to defend a project for fitting cars with rubber typres!.

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