# Float Charger

Discussion in 'General Electronics Chat' started by qitara, Mar 19, 2014.

1. ### qitara Thread Starter Member

Jan 18, 2013
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0
Hi

How does a float charger sense the voltage of the battery, does it stop and measure and resume charging if the correct battery voltage is not reached ?

In other terms how does it measure the battery voltage without measuring its own (Charger) voltage ?

2. ### MrChips Moderator

Oct 2, 2009
14,505
4,271
A charger (and every power supply) has internal resistance.

Imagine two batteries connected with a resistor in between the two batteries.
Current will flow from the battery with the higher voltage to the battery with the lower voltage. The amount of current will be determined by the voltage difference and the value of the interconnecting resistor.

If the resistance were zero ohms (and both batteries had zero internal resistance) the current would be infinite and that would not be good for either battery.

Because of series resistances the voltage that the charger measures at the battery terminal is the voltage of the battery, not the voltage of the charger. The voltage of the charger is actually higher than the voltage of the battery otherwise the battery would not receive a charge.

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3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,968
616
It is the diode in the circuit. If the voltage drop across the diode is large enough, the diode is forward biased, it conducts. When the battery is charged, the voltage across the diode becomes much smaller, if it is smaller then voltage needed to forward bias the diode, then diode becomes reverse biased and it stops conducting.

Once battery discharges some. The voltage drop across the diode increases, if the increase is large enough to forward bias the diode, then diode start conducting again. Once battery is charged to certain level, the voltage across the diode decreases, diode becomes reverse biased again and the charging stops, again.

So the whole process keeps floating, charge, discharge, charge again, discharge again.

4. ### qitara Thread Starter Member

Jan 18, 2013
93
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this is clear for me but what i cant get my head around is how the charger measures the voltage, does it do that while charging or does it stop the charging and measure ?, because if the charger does the measurement while charging wouldn't it read its own voltage because its higher then the battery voltage (I am referring to smart chargers)

Last edited: Mar 20, 2014
5. ### crutschow Expert

Mar 14, 2008
16,508
4,441
No. The charger voltage, when connected to the battery, is the same as the battery voltage since the charger is actually regulating the charging current not the voltage. As the battery charges and the voltage rises the smart charger notes this rise and automatically reduces the current until the battery reaches the float voltage, at which point the current is reduced to a small maintenance charge. (This is mainly referring to lead-acid battery charging. Other battery type chargers may use a different protocol.)

6. ### qitara Thread Starter Member

Jan 18, 2013
93
0
How is it possible to charge the battery if you are applying a voltage identical to the battery voltage ? shouldn't the charger voltage be a bit higher in order to pump juice in to the battery ?

Wouldn't both voltages cancel each other out

7. ### #12 Expert

Nov 30, 2010
17,888
9,296
How is it possible to measure the same voltage on both ends of a wire?

The wire that is connected to the battery is also connected to the battery charger.
Measuring the voltage on one end of the wire will show the same voltage that is on the other end of the wire (if the wire is the proper size for the job). By measuring that voltage, you can tell if you have applied enough current for the battery voltage to rise to the proper voltage.

8. ### crutschow Expert

Mar 14, 2008
16,508
4,441
The charge voltage has to be slightly higher than the battery electro-chemical voltage but that's not the only component of the battery voltage. The external charging battery voltage is determined by the cell resistance as well as the electro-chemical cell voltage. Thus the charge voltage is higher that the battery electro-chemical voltage by the amount required to overcome the IR drop of the battery resistance to deliver the desired charge current.

Thus, assuming the wires connecting the battery have negligibly low resistance (which they do for normal charging currents) then the charger output voltage basically equals the battery charging voltage. As the battery electro-chemical voltage rises the charger detects this and reduces the current which reduces the contribution of the battery resistance to the battery voltage. Eventually as the battery voltage increases and the charging current drops to the maintenance value, the battery electro-chemical voltage becomes roughly equal to the external battery and charging voltage. It is then a fully charged battery.

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9. ### qitara Thread Starter Member

Jan 18, 2013
93
0
Is the current reduction achieved by reducing the voltage output of the charger ?

10. ### crutschow Expert

Mar 14, 2008
16,508
4,441
The reduction in current is caused by the rising battery voltage, not a reduction in the charger output voltage. Is it still not clear that the external battery voltage and the charger output voltage are essentially the same (differing only by the small IR drop in the wires)?

Edit: Look at it this way. Suppose the charger was just a voltage equal to the desired battery float voltage with no other circuitry. When first connected to the battery the current would be limited by the difference between the charger voltage and the battery electro-chemical voltage divided by the battery resistance (and any other resistance in series with the battery). As the electro-chemical voltage increases, the voltage difference will also drop and thus so will the current. Eventually as the electro-chemical voltage reaches the float voltage value the current will drop to near zero. Thus you have a decrease in current with no change in the charger voltage. Remember that it's the difference in voltage divided by the resistance that determines the current, not the absolute voltage.

Last edited: Mar 20, 2014
11. ### wayneh Expert

Sep 9, 2010
13,607
4,402
Another way to look at this might help clear the fog.

If no current were flowing, the battery would set the voltage. The voltage seen by the regulator would be "true" (no current) battery voltage. But current IS flowing. The voltage seen is higher and no less "true", but it is under a different set of conditions and has a different meaning. Conversely, if you measure the voltage of a battery that is running a few headlights, it's less than if no current is flowing.

The concept of a battery having internal resistance is a useful way to look at it. If you want to pass current through a battery in either direction, it offers some resistance to that current and therefore the voltage seen across the poles will change with the current.

I hope I haven't made it more confusing.