audioguru

Connect a low value resistor in series with the negative battery terminal to ground. Then use a comparator that works with its inputs near 0V like an LM393 or LM339 to compare when the current drops to about 3% of the battery cell's rated current.

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Uncle $crooge

hardcore misery

this was suggested by a friend of mine, but during that time i can't understand the principle of it, referring to this schematic, a 10 ohms/5W is added on the negative side of the battery charger(4.2V, on the actual circuit) so can we use this resistor for the comaparator?(btw, are we going to use another comparator?) so therefore we will disconnect the transistor and relay circuit on the given schematic, and can we use logic gates to compare two conditions? a condition for the OFF state when the comparator reaches 4.2V(but battery voltage is not fully charge) and another condition for the measurement of current drop until the battery is fully charged?

what can you advice sir? hoping that we can finish these all before thursday for to be ready with mock defense and finalize our documentations. thank you very much.

audioguru

You don't want to measure the charger's current which includes the relay's current, you want to measure the battery's current.

10 ohms causes a voltage drop of 3V when the current is 0.3A. That is a lot of wasted voltage.

You need a voltage regulator with its output adjusted to 4.2V plus or minus only 1%.

You also need safety features like temperature and pressure sensors and maybe a timer. you don't want the battery to catch on fire during your charging demonstration.

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Uncle $crooge

hardcore misery

we are using a voltage regulator for the charger of battery (set to 4.2V)
we don't have temperature sensor.. so what if the voltage across battery in charging condition reaches 4.2V( as we all know, if we remove the battery, its voltage is not equal to 4.2V) what is the voltage drop across 10ohms?, where can i measure the current drop?

audioguru

If your teacher knows about the hazzards of a Lithium-Ion battery catching on fire when it is overcharged because the charging circuit has failed then your extremely simple circuit will be shot down (in flames).

Most Lithium-ion battery cells have a temperature sensor inside. Use it as a shutdown backup in case your circuit fails.

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Uncle $crooge

hardcore misery

so there will be another circuit for the temperature sensor? is it a voltage comparator again?

hardcore misery

can we know the time for fully charging a li-ion battery through computations?

Leftyretro

As far as I know most li-ion chargers measure and use a battery end of charge voltage to detemine when to stop charging the battery. The temp sensor is an override shutdown of the charging regardless of state of charge of the battery.

Lefty

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Measurement changes behavior

ericgibbs

hi Lefty,
From the graph you will see that the 4.2V state has to be held until the Icharge <= 3% of the 1C rate.

This is where the problem lies with hardcores charger circuit, it now cuts off at 4.2V when it reaches 4.2v!.

Attached Images

screenhunter11.jpg (42.9 KB, 12 views)

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

audioguru

It is too bad that the school kids can't use a lithium battery charger IC that does everything that should be done to charge a dangerous lithium battery.

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Uncle $crooge

hardcore misery

is there a way to measure the Icharge? this is very confusing to me because if you connect a 10ohms resistor between the negative terminal of the charger and battery,

the charge voltage is 4.2V, while for example the battery voltage is 3.7V

at noload, the load current is 420maH, when the 3.7V is loaded to the charger, there will be a voltage drop on the 10ohms resistor (4.2V - 3.7V)
which is equal to 0.5V and has a current of 50maH, so if the battery voltage reaches 4.2V, theres no voltage drop on the 10 ohms resistor and the current drop on the 10 ohms is ZERO?

where can i measure the current drop illustrated on the graph? is it possible to have two conditions on the charger?

condition1: when the battery voltage reaches 4.2V while charging the transistor will turn OFF.
condition2: when current drops at < 3% (which means that the battery is said to be fully charged) another transistor turns OFF

so a RELAY will turn OFF when these to conditions are met.

is this possible? if yes, what way can we do this? through logic gates?

audioguru

Sense and regulate the voltage across the battery to be 4.2V. Then sense the current in the 10 ohm resistor in series with the battery.

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Uncle $crooge

hardcore misery

how can we regulate the voltage across the battery? in charging condition, at this point, there is sudden fluctuation on the voltage at 4.2V. sir if you don't mind, i would like to see some illustrations on what you are trying to explain sir, or just equations or formulas, or examples.. thank you sir..

hardcore misery

how can we regulate the voltage across the battery? in charging condition, at this point, there is sudden fluctuation on the voltage at 4.2V. sir if you don't mind, i would like to see some illustrations on what you are trying to explain sir, or just equations or formulas, or examples.. thank you sir

ericgibbs

hi,
A block diagram of the circuit you require.

Attached Images

ChrgBlk1.gif (3.6 KB, 13 views)

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/