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Applying a voltage across a chargable battery

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fzn10

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I wana sense the battery voltage as it charges for deisgn purposes but if im appliying a voltage across it, isnt just the votage being applied.
 
Quick answer - yes.

JimB
 
thanks for the reply, but my counter question is that how do they design charging circuitry based on detecting the voltage across the battery terminals if its just the voltage being applied?
 
fzn10 said:
thanks for the reply, but my counter question is that how do they design charging circuitry based on detecting the voltage across the battery terminals if its just the voltage being applied?

It's not, the voltage is dependent on the state of charge of the battery - in effect a battery is similar to a zener diode, in that it clamps the voltage to it's own voltage.
 
Thanks, that makes more sense. so in order to charge does the input voltage needs to be higher than the battery terminal voltage.
 
You do not apply a voltage to a battery to charge it. You apply a current. The battery's voltage rises when it is charging with a current.
If you applied a voltage to charge a battery then the current is completely uncontrolled.
 
audioguru said:
You do not apply a voltage to a battery to charge it. You apply a current. The battery's voltage rises when it is charging with a current.
If you applied a voltage to charge a battery then the current is completely uncontrolled.

Nobody suggested that you should apply an uncontrolled voltage to a battery?.
 
from what i gather from these replies is that if i was using a boost converter i gota control the votlage to make sure that its higher than the battery terminal voltage, and at the same time controll the current. Is that what is happening? how would i know what votlage im applying if the output voltage is just the battery terminal voltage.
 
fzn10 said:
from what i gather from these replies is that if i was using a boost converter i gota control the votlage to make sure that its higher than the battery terminal voltage, and at the same time controll the current. Is that what is happening? how would i know what votlage im applying if the output voltage is just the battery terminal voltage.

Provide a higher voltage from the boost converter, then use a simple series resistor (or a more complicated constant current source) to limit the charging current.

As it's current you're concerned about there's no need to worry about the exact voltage from the converter, as long as it's high enough to provide the current needed (remembering the losses in the current limiter) - a constant current version will compensate for both battery voltage changes (as it charges) and for supply changes (unless it drops too far).

I imagine it would be quite plausible to make the boost converter itself a constant current source? - but that's perhaps getting a bit more complicated?.
 
would this be enough to provide safe charging that would ultimtly lead to the termination of charging when the battery is full?

provide current limiting, this limit is dependent on the maximum charging current the battery can handle. use another comparitor that would gradually decrease the duty cycle of the switch as the battery voltage reaches its maximum safe voltage of 4.2V. is this good?. cause i know how to do that with a boost converter.

Would it also matter if my current is not constant, cause my input is not very stable. just as long as it does not pass the maximum charging current?

thanx
 
fzn10 said:
would this be enough to provide safe charging that would ultimtly lead to the termination of charging when the battery is full?

provide current limiting, this limit is dependent on the maximum charging current the battery can handle. use another comparitor that would gradually decrease the duty cycle of the switch as the battery voltage reaches its maximum safe voltage of 4.2V. is this good?. cause i know how to do that with a boost converter.

Would it also matter if my current is not constant, cause my input is not very stable. just as long as it does not pass the maximum charging current?

thanx

It depends on how you are trying to charge it?, if time isn't a concern then as long as it doesn't exceed the maximum it's OK - it will just take longer.

Are you trying to charge from a solar panel?.

As for determining if the battery is 'full' or not, this is quite complicated, highly dependent on the battery chemistry, but NOT usually as simple as monitoring for a specific voltage.
 
well i want to use it for wind turbine and solar. So i should propably use a chip specificlly designed for charging this type of battery. i didnt want to get into it cause i wanted the least amount of circuitry but that looks like the only option. Thanks for your help.

just out of curiosity, what do u mean by "dependent on battery chemistry", do u mean dependent of different types of batteries like lithuim or Ni, or soething else, casue im gona be using the chargin circuitry for the same battery over and over again.
 
fzn10 said:
well i want to use it for wind turbine and solar. So i should propably use a chip specificlly designed for charging this type of battery. i didnt want to get into it cause i wanted the least amount of circuitry but that looks like the only option. Thanks for your help.

What maximum powers are you expecting from each source?, and what capacity batteries are you planning?.

just out of curiosity, what do u mean by "dependent on battery chemistry", do u mean dependent of different types of batteries like lithuim or Ni, or soething else, casue im gona be using the chargin circuitry for the same battery over and over again.

Yes, Lithium and NiCd are charged (and monitored) completely differently, you can't mix and match battery types - you need to design it specifically for the battery you plan to use.
 
Not exactly sure how much power im getting through it cause i havent been given the environmental data, so just gona give it a blind eye. The total battery capacity that i will be using is around 500mAH, charging time is around 12hrs.

edited this post casue im actually wrong.
 
Last edited:
There is confusion here about whether a battery limits its own charging current. There is also confusion about voltage and current.
 
Replying to the original post by fzn10,

I am working on developing one myself. A few food links for you to study are:
www.mpoweruk.com/chargers.htm
www.batteries.com/*help/glossary.as
www.mpoweruk.com/glossary.htm
www.hardwaresecrets.com/*article/292

To test the charging, you will have to detect the battery voltage. As of what I have experienced, the battery does not clamp any voltage like a zener. For best accuracy, you have to stop charging while you measure. So you can use a "Charge and rest" or a "charge and burp" method and measure battery voltage while the cells are not being charged.

And you ought to use a current source to charge the battery... I believe you are clear about it. Then the charging voltage shall change as the battery charges (small change) so as to keep the current constant...

Charging also depends on the battery chemistry - NiCd / NiMH or LiIon.
LiIon cannot be charged by a constant current source. And NiCd and NIMH have different -ve deltas and different capacities.

Hope this helps.
 
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