Regarding Vehicle Battery Charger

Once I went to a mechanic to get my bike's battery recharged. I saw him increasing the current using a knob. But the voltage was constant. My question is how come the current is increased with a constant load. As per my knowledge the amount of cuurent drawn depends on the load. Can anyone clear my doubt please. I have googled it alot but unfortunately did not find any helpful information.

Thanks in advance,
Ajish

Interesting observation. I found this article on charging Lead-Acid batteries. See if this helps...

Some how it was little helpful but not completely. Please do help if anyone knows the answer to the question posted. Anyways the link is useful.

I suspect the battery was dead or fully discharged when the technician put it on the charger. To see how it works, connect a 12 volt bulb across the charger and start increasing the current from 0 to full capacity. The bulb will glow with full intensity when the current reaches the bulbs maximum consuming limit. It will not increase beyond that limit coz the voltage is fixed. Same works with the battery. He would have checked what current the battery consumes in that condition ( may be dead or deep discharged) or it is shorted.
Cheers

load of the battery is constant ......

what he did he tried to increase the charging rate but this is not good thing (this will decrease the battery life)........

mrarslanahmed said:

load of the battery is constant ...... what he did he tried to increase the charging rate but this is not good thing (this will decrease the battery life)........

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Yes its not good but as i assumed if the battery is deeply dicharged or amost dead, this technique will let know wether it rejenuates or not.
Cheers

Yes you are right ........ may be he tried to check if the battery is dead or deeply discharged.....

If he measured the output current at the initial stage....wont this would have helped the technician?

I am trying to ask is there any other way to check if the battery is working or dead other then feeding it with maximum current level.....?

First, battery chargers are not all alike. Some use electronic circuits to control the charging current, detect the state of the battery, sense when it's full and switch to trickle charging, etc etc. The more basic ones are fully manually controlled. Let's assume that what you saw was the manual type.

Imagine two batteries of unequal voltages and their negative terminals are connected together. If you place something, say a resistor, between the two positive terminals, current will flow from the higher voltage to the lower voltage. Like this:



The strength of current flow is given by I = (V₁-V₂)/R. For the same voltage difference, the lower the resistance, the higher the current flow. If the resistance remains the same, the greater the voltage difference, the higher the current flow.

In a battery charger, the lower voltage represents the battery being charged. The higher voltage represents the charger output. At the heart of a simple charger is a transformer with multiple taps which give out different AC voltages. This voltage is rectified with diodes to give a DC output. This voltage has to be a little higher than the voltage of the battery to be charged.

In the simplest type of charger, the resistor may simply be the combination of transformer internal resistance, rectifier resistance, connecting wires, contact resistances, etc. There may also be an external physical resistor of variable resistance that can be switched to give fine current control while the transformer taps provide a coarse setting.



What you saw was probably the mechanic switching the tap points to get the desired charging current. If the charger was a more sophisticated type, he was changing the charging current with some electronic circuitry.

@pjdd...
nice info. BTW, i didn understand how the high rate of charging damages the battery. All vehicle batteries are 12V, then why do they require center taps? it's 12V need always.

Thanks Pjdd your post was really helpful.

---------- Post added at 20:02 ---------- Previous post was at 20:01 ----------

@pjdd...
nice info. BTW, i didn understand how the high rate of charging damages the battery. All vehicle batteries are 12V, then why do they require center taps? it's 12V need always.

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But the Ampere-Hour ratings are different...

Embedded_Geek said:

Thanks Pjdd your post was really helpful.
But the Ampere-Hour ratings are different...

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IMO, Amp-Hour rating has nothing to do with center taps/no of centre taps.
BTW, do yo know ans for: How the high rate of charging damages the battery

Assuming we are talking about flooded lead acid battery's, high charge rate can damage the battery by boiling the electrolyte away.
Deeply discharged battery's (in case of car battery's which are thinplated) would not accept a high charge current at first due to sulfation.
A good charger will limit the charge current upto 10,5 to 12 Volt's (the pre-charge fase) then it would go to the bulk fase of charging upto ~12,8-13,2 Volts (the voltage of the charger can go higher then this depending on the battery type, temperature etc.) after the battery will be ~85-90% full it should go to float charge (which uses a lower voltage/current) some also use a equalizing charge.
It really depends on the battery type, state of health, temperature, amount of AH's etc. etc. what current and what voltage You should use to keep your battery in a healthy state.

Battery acts like a zener diode. That is it will try to maintain its voltage. So 12V battery, although stays at 12V but it loads the charger more if it tries to increase its voltages. If the charger has less voltage than battery, there will be no load on charger. If charger has more voltage than battery, battery will pass more current until charger voltage drops to near battery voltage. Now voltage is same but current is increased. This current can be monitored with a series ampere meter.

walkura said:

Assuming we are talking about flooded lead acid battery's, high charge rate can damage the battery by boiling the electrolyte away.
Deeply discharged battery's (in case of car battery's which are thinplated) would not accept a high charge current at first due to sulfation.
A good charger will limit the charge current upto 10,5 to 12 Volt's (the pre-charge fase) then it would go to the bulk fase of charging upto ~12,8-13,2 Volts (the voltage of the charger can go higher then this depending on the battery type, temperature etc.) after the battery will be ~85-90% full it should go to float charge (which uses a lower voltage/current) some also use a equalizing charge.
It really depends on the battery type, state of health, temperature, amount of AH's etc. etc. what current and what voltage You should use to keep your battery in a healthy state.

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Sulfation? i didn understand how that impacts. BTW, the charge current is high at the beginning, that contradicts your statement(i'm not sure about this, but the link in above post says that).

ALERTLINKS said:

Battery acts like a zener diode. That is it will try to maintain its voltage. So 12V battery, although stays at 12V but it loads the charger more if it tries to increase its voltages. If the charger has less voltage than battery, there will be no load on charger. If charger has more voltage than battery, battery will pass more current until charger voltage drops to near battery voltage. Now voltage is same but current is increased. This current can be monitored with a series ampere meter.

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Battery => Zener Diode?? I've heard they use special diode circuits for the purpose while charging. Any sources for this?

Thanks

Alertlinks is quite correct, a good Pb-acid battery acts very much like a large zener until it is ~80% charged.

Orson Cart said:

Alertlinks is quite correct, a good Pb-acid battery acts very much like a large zener until it is ~80% charged.

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Ya, but how?? So, doesn't it require any diode circuitry do design a charger?

I was trying to describe electrical behavior of battery during charging and comparing it with a diode called zener diode, not to implement any changes to charger.
Zener diode - Wikipedia, the free encyclopedia
Zener diode - Wikipedia, the free encyclopedia
Zener effect and Zener diodes
It is zener effect which resembles. (Same voltage but different current). Load is not constant as you suspect.

But the voltage was constant. My question is how come the current is increased with a constant load. As per my knowledge the amount of cuurent drawn depends on the load.

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Load = power = voltage x current ( ohm's law).

When a battery discharges sulfates are formed on the plates (especialy in deeply discharged battery's), when You charge properly You would recover most of the plate area.
Yes ofcourse You can try to charge straight away full current, but You will shorten the lifespan of your battery (the sulfates won't go away from all the surfacearea of the plates and the amount of Ah,s decreases with every charge cycle).
Just google battery sulfation and You will get 1000's of hits about this proces.