How to know whether a battery is fully charged?

Hi everyone, I'm having problems in knowing when are the rechargeable batteries fully charged.
I had a battery charger system. Can anyone tell me what can i do to make the charger stop charging when its full.

Usually when voltage on battery terminals reaches a certain voltage level battery is full or empty.

You didnt specify what battery tech, Li-Ion, Li-Po, Lead Acid, NiMh, NiCd,...

For specific battery tech see charging graph and specific way to sense when battery is near full charge state, such as case with Li-Ion/Li-Po, NiMh batteries.

Different Battery chemistries have very different charge termination methods.

Only with lead-acid batteries can the state of charge be reliably determined by a simple measurement of the battery voltage. Other battery chemistries require special circuits to monitor the rate-of-change of the voltage and/or the battery temperature to determine when it is charged. These monitor conditions also vary with the chemistry so what works for a NiMH battery will not work for a Li ion battery.

This complexity is why cheap NiMH battery chargers charge at a slow rate (which will not damage the battery for a short overcharge) for a long time period, whereas a smart charger can charge the battery much faster using a higher charge rate, since it monitors when to safely terminate the charge.

Several IC manufacturers make specialty chips designed to monitor the charge of specific battery types. That's generally the easiest way to build a smart charger.

A cunning way would be to measure the internal resistance of the battery. i.e. pulse a load on it of say, 1 A, and measure the amount the output voltage falls. A good battery should change by less then 20 m|V/cell.
Frank

Charging Mechanism is different for different batteries.......Whereas to check the charging state....measurement of battery voltage is straight forward method...
Threshold for battery Full charge and Low depends on battery you can refer to battery specification...accordingly circuit has to be designed..using comparators...zeners..transistors etc...Once charge full is detected you can cutdown the supply by switching off the series MOSFET....

As,crutschow explains it is usually easy to know for lead-acid batteries if it is fully charged or not.
A fully charged 12 V lead-acid car-battery will sit idle at 12.6 volts;Using this voltage you can use current-sensing resistors to determine state of battery.

If your charging current is not high & your battery casing becomes too hot to touch,it may be because of overcharging the battery.

rahdirs said:

As,crutschow explains it is usually easy to know for lead-acid batteries if it is fully charged or not.
A fully charged 12 V lead-acid car-battery will sit idle at 12.6 volts;Using this voltage you can use current-sensing resistors to determine state of battery.

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Lead Acid have voltage 12,72V-12,73V (new battery with 6 cells) after 24h of resting when charging process is over.

I didnt catch how to use shunt resistor to determine state of battery can you explain more ?


Overheating of battery is not only linked with overcharging, this situation can be due internal cells damage, what cause short short circuit or similar problems (Lead Acid, NiMh, NiCd,...). Heat can develop even when battery is empty and on start of charging process.

tpetar said:

Lead Acid have voltage 12,72V-12,73V (new battery with 6 cells) after 24h of resting when charging process is over.
I didnt catch how to use shunt resistor to determine state of battery can you explain more ?

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I was talking about using sense resistors.There is a slight difference between sense & shunt resistors.A shunt resistor is used to bypass the unwanted current.A current sense resistor is used to convert current into voltage drop. This voltage drop is further used to control other device.

Current-sense resistors can be used to determine how much current is being applied to a battery pack during charging in order to prevent overcharging or overheating in the batteries.
They can also be used to monitor the voltage drop across a sense resistor, connected in a series between the negative battery terminal and ground, and to determine the charge and discharge activity of the battery.

I wasn't trying to say that over-heating is the because of overcharging,i was just trying to say it may be one of the reasons behind overheating.

rahdirs said:

I was talking about using sense resistors.There is a slight difference between sense & shunt resistors.A shunt resistor is used to bypass the unwanted current.A current sense resistor is used to convert current into voltage drop. This voltage drop is further used to control other device.

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I must admit this is something completely new for me. I've never heard for this before, this give new definition to shunt resistor.

Can you explain me difference between shunt resistor and current sense resistor please. What resistance value have that resistors and how they looks like.

rahdirs said:

Current-sense resistors can be used to determine how much current is being applied to a battery pack during charging in order to prevent overcharging or overheating in the batteries.

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Yes, shunt resistor can be used for that, but there is little difference between new and used batteries and delivered Ah to them for full charge state.



;-)

tpetar said:

Can you explain me difference between shunt resistor and current sense resistor please. What resistance value have that resistors and how they looks like.

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They are just an ordinary resistor.There is only a difference in how they are used.
Measuring a current means converting it to a voltage, which may then be compared with a threshold, digitised or otherwise processed by a current sense circuit. There are several solutions for doing this, including current transformers, hall-effect sensors and magnetoresistive sensors. However, the simplest and, in many cases, lowest cost method is to employ Ohm’s law in the form of a current sense resistor.

The rating would depend on the application where they are used,high-current,low-current etc.. so that power dissipated by this resistor is negligible.
There are many applications for these as given in this manual from TT Electronics "Current Sense Resistors-Application Note".
The attached image is from that manual

rahdirs said:

They are just an ordinary resistor.There is only a difference in how they are used.
Measuring a current means converting it to a voltage, which may then be compared with a threshold, digitised or otherwise processed by a current sense circuit. There are several solutions for doing this, including current transformers, hall-effect sensors and magnetoresistive sensors. However, the simplest and, in many cases, lowest cost method is to employ Ohm’s law in the form of a current sense resistor.

The rating would depend on the application where they are used,high-current,low-current etc.. so that power dissipated by this resistor is negligible.
There are many applications for these as given in this manual from TT Electronics "Current Sense Resistors-Application Note".
The attached image is from that manual
View attachment 91972

Click to expand...

Why you then earlier make difference between current sense resistor and current shunt?

Power dissipated by shunt resistor (current sense resistor) is not negligible specially at higher current (Higher current even more then few amps) and higher resistor values (we speak about mR values up to 0,1mR-0,22mR if we have more this is normal resistor or heater ).

Today Hall sensor ACS712-20 (for 20A) or similar in shop is around 1eur, in comparison with standard power resistor which is 0,2-0,5eur + you need opamp. Plus impact on circuit what shunt resistor make in comparison with Hall sensors.