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Topic: How do NiCad and NiMh chargers detect -deltaV to stop charging? (SOLVED) (Read 2 times) previous topic - next topic

Voidugu

What i am asking is how do the chargers to this. ie how can it be done with a microcontroller. I would like some technical details. I mean, the "loop" that the microcontroller would have to repeat in order to figure out when this -deltaV has been reached and some circuitry that would be required.

@Mr.Retroplayer:
You kind of confused me there. If i understood correctly, you are basically saying that by knowing the voltage you provide to the battery through the series resistor and the voltage drop across the resistor in series with the battery you can calculate the EMF of the battery ?? (ie EMF of the battery = voltage supplied to the resistor and battery - Voltage across the series resistor)
>Wouldn't this equation give me the voltage that is used to charge the battery?
>Would it give me the actual EMF of the battery in order for me to log the data down and be abple to determine the -deltaV?
> Wouldn't i have to stop the charging process, measure the open circuit voltage of the battery and the continue with the charging process in order to measure the EMF of the battery?
>By the way, i know that for lipos, its mostly the cell voltage that changes (drops) while the battery is drained (increase in internal resistance has a smaller effect). Does this not apply for NiCads and NiMhs? Does internal resistance vary greatly while the EMF of the battery remains more or less constant?

@Mr.Lefty:
How do you continuously monitor the battery terminal voltage while charging? What to you mean the change in direction of the voltage?

Again thanks for the help people.

retrolefty

@Mr.Lefty:
How do you continuously monitor the battery terminal voltage while charging?
Continuously monitor = continuously measure = put your DVM across battery while charging and write down voltage every min, see that terminal voltage gradually increases over time as it accepts charge current, reaching a peak value and then falling back slightly, that is end of charge indication.

What to you mean the change in direction of the voltage?

1,2,3,4,5,6,7,6.  The transition from 7 back to 6 is a change of direction relative to prior sequence.

Lefty

Retroplayer


@Mr.Retroplayer:
You kind of confused me there. If i understood correctly, you are basically saying that by knowing the voltage you provide to the battery through the series resistor and the voltage drop across the resistor in series with the battery you can calculate the EMF of the battery ?? (ie EMF of the battery = voltage supplied to the resistor and battery - Voltage across the series resistor)
>Wouldn't this equation give me the voltage that is used to charge the battery?
>Would it give me the actual EMF of the battery in order for me to log the data down and be abple to determine the -deltaV?
> Wouldn't i have to stop the charging process, measure the open circuit voltage of the battery and the continue with the charging process in order to measure the EMF of the battery?
>By the way, i know that for lipos, its mostly the cell voltage that changes (drops) while the battery is drained (increase in internal resistance has a smaller effect). Does this not apply for NiCads and NiMhs? Does internal resistance vary greatly while the EMF of the battery remains more or less constant?


No. Well, yes.... lol.

I am talking about understanding the relationship between voltage, current, and resistance in a circuit series and parallel circuit properties, and Thevenin's thereom. But you don't need to have an engineering understanding of all that. Just know that your charger is providing a voltage. And in a series circuit, all components will drop a portion of that voltage and that will always add up to the total voltage.

So let's say you have 2 resistors and a 9V battery. Measuring the voltage across one resistor reads 3V. Wtihout even measuring the voltage across the other resistor, we already know it is 6V because all the drops must equal the total voltage.

So we put a small resistor in series with the battery and measure the voltage drop across the resistor. We subtract that from the charger voltage and we know the voltage across the battery. We incidently also know the current through the circuit doing this. Ohm's law.

I think you are overcomplicating it and getting hung up on dVt.

To summarize, you are measuring the volage across the added, known-value, resistor. You subtract that voltage from the known charging voltage.

A great resource is The Battery Handbook

retrolefty



@Mr.Retroplayer:
You kind of confused me there. If i understood correctly, you are basically saying that by knowing the voltage you provide to the battery through the series resistor and the voltage drop across the resistor in series with the battery you can calculate the EMF of the battery ?? (ie EMF of the battery = voltage supplied to the resistor and battery - Voltage across the series resistor)
>Wouldn't this equation give me the voltage that is used to charge the battery?
>Would it give me the actual EMF of the battery in order for me to log the data down and be abple to determine the -deltaV?
> Wouldn't i have to stop the charging process, measure the open circuit voltage of the battery and the continue with the charging process in order to measure the EMF of the battery?
>By the way, i know that for lipos, its mostly the cell voltage that changes (drops) while the battery is drained (increase in internal resistance has a smaller effect). Does this not apply for NiCads and NiMhs? Does internal resistance vary greatly while the EMF of the battery remains more or less constant?


No. Well, yes.... lol.

I am talking about understanding the relationship between voltage, current, and resistance in a circuit series and parallel circuit properties, and Thevenin's thereom. But you don't need to have an engineering understanding of all that. Just know that your charger is providing a voltage. And in a series circuit, all components will drop a portion of that voltage and that will always add up to the total voltage.

So let's say you have 2 resistors and a 9V battery. Measuring the voltage across one resistor reads 3V. Wtihout even measuring the voltage across the other resistor, we already know it is 6V because all the drops must equal the total voltage.

So we put a small resistor in series with the battery and measure the voltage drop across the resistor. We subtract that from the charger voltage and we know the voltage across the battery. We incidently also know the current through the circuit doing this. Ohm's law.

I think you are overcomplicating it and getting hung up on dVt.

To summarize, you are measuring the volage across the added, known-value, resistor. You subtract that voltage from the known charging voltage.

A great resource is The Battery Handbook


Well to be fair you also might be making it more complicated then it really is. In context with a nimh peak charger there is no reason for a series resistor to be used to detect when 'peak' voltage has been reached, in fact I'm not sure you could even detect it that way, just measurement directly across the battery terminals will work. A series resistor's voltage drop tells you about the value of current flowing into the battery rather then it's gradually increasing terminal voltage as it accepts a constant current charge from the charger. Now there might have to be a voltage divider wired across the battery terminals to keep the measurement value within say an arduino analogRead() range and scaled back up in software to track when peak voltage has been reached, but that is not effected by the current flow through the battery.

Lefty

Retroplayer


Well to be fair you also might be making it more complicated then it really is. In context with a nimh peak charger there is no reason for a series resistor to be used to detect when 'peak' voltage has been reached, in fact I'm not sure you could even detect it that way, just measurement directly across the battery terminals will work. A series resistor's voltage drop tells you about the value of current flowing into the battery rather then it's gradually increasing terminal voltage as it accepts a constant current charge from the charger. Now there might have to be a voltage divider wired across the battery terminals to keep the measurement value within say an arduino analogRead() range and scaled back up in software to track when peak voltage has been reached, but that is not effected by the current flow through the battery.

Lefty


Perhaps you could explain how you are supplying 7.2V to charge a 7.2V battery and with only the battery in parallel with the 7.2V supply you would expect to read anything different than 7.2V? That makes no sense whatsoever.

Yes, you are measuring current. With Ohm's law, you have three components: voltage, current, and resistance. You need to know two of them to solve for the last unknown. So you use a known resistance, measure the voltage drop (which give you current) and calculate that the voltage across the battery by subtracting that voltage drop from the 7.2V using Thevenin's thereom. This is considered a half bridge circuit, where a wheatstone bridge would be a full bridge.

THAT makes it sound complicated. The OP doesn't really have to understand how all that works to apply it.

But to think that you would expect to ever measure anything different from a 7.2V supply in parallel with one component (the battery) makes no sense at all. And THAT was what the OP was (rightly) confused about.

BTW, I am an electrical engineer in the aerospace and defense business. I would hope that I could understand something as simple as a battery charger. Just saying...

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