Mike in DC

Forgive me if this is one of those questions that after hearing the answer you hit yourself in the forehead, Duh!

I've always been curious how chargers measure the voltage of the batteries they are charging. For example, in a previous answer, it was stated that the 977 charges LiPos using constant current up to 4.3 volts. So how does it know when it's reached 4.3 volts?

Obviously, in order to push current into a battery, you've got to apply a voltage higher than what the battery would normally show if there was no charger attached. In other words, if I measured the voltage when the charger was charging, it would have to be something higher than 4.3 volts or there wouldn't be any current going backwards into the battery.

So, how does the charger know what the voltage of the battery would be if it weren't charging? Does it calculate it somehow? Turn off charging periodically? Or doesn't it need to know?

Does the question even make sense or have I completely humiliated myself in front of millions of battery geniuses?

Mike

mehrshad

No, a charger doesn't work this way.
a charger is actually a current source with a voltage limit.
what is a current source?
It is a circuit that sources a constant current (for example 1A) to its load. if you short the output of the current source the current will be 1A !!
when you connect a battery to a charger,the charger will source a constant current to your battery (unless its voltage is higher than maximum of your charger).
when the voltage reaches to its maximum level( lithium types and acid types) the charger begin to reduce the charge current in a manner that the voltage stay constant like a voltage source supply.
this may also help:
http://en.wikipedia.org/wiki/Current_source

(sorry for English)

JackSwagger

Battery charging is a complex process which can fool even the experts at times.
The whole process can be summarized as the steps which occur from the starting to the fully charged.
1). When a charger is atteched to a battery it start the current flow at the maximum rate of the charge source.
2). At the instant battery voltage rises upto maximum limit of allowable voltage to the charge source,the current begins to decline.
3). Charge current will decline to a constant rate current which is an steady state,at that instant the battery is fully charged.
4). When the battery is full charged,attach a low voltage that will remain the battery full charged.

the vibrant

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asterss

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boby12

I like this forum ,because I learn some knowledge before I never know.
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megancheung

A battery charger is a DC power source which is connected to a battery in parallel. (Positive to Positive and Negative to Negative)

A battery charger should only be used with a rechargable battery and with the proper type of battery the charger was designed for.

The charger operates at a voltage slightly higher than the battery voltage. The Higher voltage on the charger pushes current back into the battery in a direction opposite to how it came out. This reversed current flow (Charging Current)causes a reversal of the electro- chemical reaction used in the battery that produced electricity.

The amount of voltage applied to the battery from the charger determines how much charging current flows. Charging current determines how fast the battery recharges.

The speed of recharge effects how much heat is generated during recharge. Too much heat build up(from high charging currents) can damage the battery. (Warp plates, boil off electrolyte, cause pressure build up and rupture the battery)

Recharge rates can also effect longevity of the battery. Trickle charging with low currents is slower but doesn't stress the battery.

countilaw

An example of charging a battery would be place a sponge in a dish of water. The sponge (battery) will soak up the available water. when the sponge is full it won't soak up any more water.

The charger just makes the current available, the battery soaks up the current until it can't absorb anymore.

Frank