cell internal resistance.

[lord loh.]

My past few threads have been related to making a battery charger. I have finally managed to make a constant current source. But I am still to decide on it's compliance.

I have to charge 6 NiCd cells in a series.

For this test purpose, I have connected two cells in a series (NiCd 700mAh).

I am going to use the -ve delta technique to terminate charging.

So I am now monitering the battery voltage while the battery is charging and by switching off the supply.

Now I would like to calulate the internal resistance of the cell. How do I do it?

R=V/I

My I is 740mA. But what do I take my V as? The cell volatage while there is no charging, the cell voltage while charging or the sum of the two voltages?

I am right now doing (Vcharging+Vcell)/740mA

The idea is that the cell voltage shall be opposing the charging voltage. So the actual voltage put up by the constant current source is higher than the Vcharging by Vcell.

Or should I do R=Vcharging/740mA

I thank everyone for their help so far...

 

[Nigel Goodwin]

Now I would like to calulate the internal resistance of the cell. How do I do it?

There seems no reason to want to?, but it's VERY low for NiCd's and NiMh.

You can measure it (but I still don't see any reason, apart from it's educational value).

1) Charge the battery fully.

2) Place your voltmeter across it.

3) Get a low value resistor, with an ammeter in series with it (on a suitable range).

4) Read the voltage off the voltmeter.

5) Apply the resistor/ammeter across the battery.

6) Read both meters.

7) Subtract second volt reading from the first, giving the voltage drop.

8) Using voltage drop and current, apply ohms law to find internal resistance AT THAT CURRENT.

 

[lord loh.]

There seems no reason to want to?, but it's VERY low for NiCd's and NiMh.

I ought to decide my VCC and circuit to set the compliance range (The range of resistances for which my circuit shall act as the current source) of my current source.

8) Using voltage drop and current, apply ohms law to find internal resistance AT THAT CURRENT.

So does the internal resistance vary? If I used a different value resistor, would the internal resistance be different? Is internal resistance a function of output current?

 

[Nigel Goodwin]

There seems no reason to want to?, but it's VERY low for NiCd's and NiMh.

I ought to decide my VCC and circuit to set the compliance range (The range of resistances for which my circuit shall act as the current source) of my current source.

8) Using voltage drop and current, apply ohms law to find internal resistance AT THAT CURRENT.

So does the internal resistance vary? If I used a different value resistor, would the internal resistance be different? Is internal resistance a function of output current?

Yes, to some extent, but it's so low you can ignore it in calculations, it's swamped by external factors - UNLESS you place a short across it, when the battery destroys itself - due to it's low internal resistance!

 

[eng1]

It would be nice to sense both the voltage and the temperature of the battery. It is possible if you use a PIC with ADCs. Then it could control the current with PWM (you could start with a soft charge...)

 

[audioguru]

Why not look at a datasheet for a battery charger IC. The engineers have already designed it very well, describe all the functions and protections and you won't need to re-design a wheel and not have a fire or an explosion.

Maybe you should be using their wheel, after looking at datasheets of rechargable batteries and and tutorials of recommended ways to charge them from a battery manufacturer.

 

[eng1]

Check the datasheet of MAX713.

 

[audioguru]

I can't remember which type of battery or what size, but you should look on a battery's datasheet for spec's about it:

 

[lord loh.]

I looked into the datasheet of LM3647 (obsolete) and have also been to the maxim site.

Most maxim ICs are for high end work and require computer bus connections. lm3647 is a SMD so are maxim's IC.

SO now I am planing to implement the lm3747's charging algorithm with soft charge / fast charge / Topping charge / Maintaince charge and PWM using a PIC.

But for all this I do need a current source. And do need to know the internal resistance to design a current source with the desired compliance range.

The source is to be designed to be versetile to charge 2200mAh cells as well. So 700mAh shall be achieved by PWM.

Can anyone suggest me a transistor that is good for switching and can handle ~3A? Nigel suggested BD131 (general purpose power amplifier) and TIP41 which has a rating of 6A(too much and may be expensive)

 

[Nigel Goodwin]

I looked into the datasheet of LM3647 (obsolete) and have also been to the maxim site.

Most maxim ICs are for high end work and require computer bus connections. lm3647 is a SMD so are maxim's IC.

SO now I am planing to implement the lm3747's charging algorithm with soft charge / fast charge / Topping charge / Maintaince charge and PWM using a PIC.

But for all this I do need a current source. And do need to know the internal resistance to design a current source with the desired compliance range.

The source is to be designed to be versetile to charge 2200mAh cells as well. So 700mAh shall be achieved by PWM.

Can anyone suggest me a transistor that is good for switching and can handle ~3A? Nigel suggested BD131 (general purpose power amplifier) and TIP41 which has a rating of 6A(too much and may be expensive)

Have you ever considered trying google?.

This website https://www.angelfire.com/electronic/hayles/charge1.html has every thing you need, except you have to write your own PIC software!.

And you DON'T need to know the internal resistance to design a current source - for a start, a CONSTANT current source is constant regardless of the internal resistance, secondly it's so low it could (and SHOULD) be ignored in any calculations.

 

[lord loh.]

Thanks Nigel, The circuit given did give me a few leads, but I might considering using the charge and burp method of charging that might mean a bit of modifications.

Thanks a lot Nigel

 

[eng1]

lm3647 is a SMD so are maxim's IC.

MAX713 comes in a 16-DIP package, I've got one (6$). It basically needs resistors, caps, a diode and a BJT as external components.


Then, did you think of the benefits from using a MOSFET instead of the BJT?

 

[Thunderchild]

the only perpose to measure the internal resistance is to determine the short circuit current ans the voltage out put under a cetain load but these are little practical use

 

[audioguru]

When you know a battery's internal resistance then you can calculate how much its voltage drops when loaded, with any current, not just a short.

 

[Nigel Goodwin]

When you know a battery's internal resistance then you can calculate how much its voltage drops when loaded, with any current, not just a short.

Except the internal resistance isn't a constant, and in this case we're talking about CHARGING the battery, and not discharging.

 

[Thunderchild]

just pump the battery with the current you want to charge it full stop who cares about the internal resistance