[SOLVED] Another question about simulating battery resistance/impedance.

[d123]

Hi,

I'm trying to estimate real battery resistance in a real circuit using 'DC load method' to use in a simulation (to compare reality to simulation's interpretation of a circuit I know works with measured values). Method taken from here:



Formula is:
Battery internal resistance = (Vopen-circuit - Vload) / I load.

Already have read a bit about and see shortcomings of this method, thanks.

Measured real circuit:
Battery Voc = 8.52V

(Ri = internal resistance)

- At POR, Vbatt = 8.49V to 8.5V, I load = 147uA > so Ri = 136 Ohms
- At full load, Vbatt = 8.46V to 8.47V, I load = 2.31mA >so Ri = 25.97 Ohms
- At latched off, Vbatt = 8.47V to 8.48V,I load =1.93mA >so Ri = 25.90 Ohms.

...
Simulation tool has user-settable battery parameter for 'internal resistance' but it is static/fixed AFAIK. I have no idea if the tool considers impedance/varying internal resistance based on varying load values.

1) Do sim tools usually adjust for this or is e.g. 1 Ohm internal resistance fixed once assigned, no matter the load?

2) There is no point attempting to compare real circuit to simmed version for that reason as results will be 'meaningless'?

3) Any workarounds to make the battery model provided a bit more real?

 

[KlausST]

Hi,

Simulation .... a big word.
But every simulation is an approximation to a real circuit. You may have a rough approximation or a very detailed one. Detailed in which regard?

So what exactly do you want to simulate?
The voltage of a battery depends on many parameters. Health, age, temperature, charging state, production....

An almost realistic simulation for a battery (internal resistance) according time and current is about impossible because it does not care about charging cycles, temperature, time of applied current...

This is the reason why "voltage" of a battery is not a very good information about it´s state/health....

Some example:
Charge a lead acid battery with C/10 with CICV method up to 14.4V. (see the given voltages just as examples)
Now disconnect the charger.
You will see an immediate drop in voltage. Maybe down to 14.0V. (this may be explained by a series reisitance)
But within the next minutes it still may drop down to 13.7V..without current, without significant change in temperature, ....
Can´t be explained with resistance or varied charging state ... Hard to explain, hard to simulate....

Klaus

 

[d123]

Hi Klaus,

Hope you're fine and well. Thanks for insight, especially third paragraph of your reply.

While I'm here, attached is a Min Chen & Gabriel Rincón Mora pdf called: 'An Accurate Electrical Battery Model Capable of Predicting Runtime and I-V Performance', in case it's of use to somebody.

Now to answer the question of your second paragraph: My main interest is that I want to compare real results with simulation results as sims are quite hit-and-miss regarding 'trustworthiness' to further know/learn why and when they are reliable and when nonsense. Non-functional theoretical circuits can sim well and functional real circuits not so - it helps to differentiate so as not to waste time (or hope). It has taken a couple of years or more of gaining real practical experience for comparing real to simulated to begin to know better when the sim tool I use is wrong/'lying' and when it is accurately reflecting reality.

I see it's time wasted simming a real, functional circuit due to what you say unless I were capable of making a meaningful battery model.

Next question: Is 'DC Load Method' [Battery internal resistance = (Vopen-circuit - Vload) / I load] useful for anything in a practical way or just number crunching Ohm's Law and not very meaningful for circuit design?

The more I thought about it this afternoon, the less I understood the utility of it, especially as 9V batteries are generally described as 1 to 2 Ohms and my calculations go from 26 to 136 Ohms. Somewhat confused.

 

[BradtheRad]

To deduce battery internal impedance, I think of attaching a load which pulls battery voltage down to 50 percent. Automatically the load resistance is the same as battery resistance.

9V batteries are generally described as 1 to 2 Ohms and my calculations go from 26 to 136 Ohms.

By raw math such a 9V battery drives a load of 1-2 ohms at 4.5 volts and 2-4.5 Amperes. (The total loop is 2-4 ohms.) If you ask me it greatly overestimates the amount of current available from those little cells smaller than AAA.

I don't do such tests often since it depletes a perfectly good battery. Internet blurbs report 9v alkaline capacity around 500 mA. So your figure of 26 ohms is more reasonable for internal resistance.

 

[KlausST]

Hi,

I guess - (means I don't know)
That resistance measurement give different values:
* with different current: example: @10uA / 1mA / 100mA
* with different timing: example: @10us / 1ms / 100ms
* with different temperatures
* with different states: after long time storage down to 50% charge / after high current discharge to 50% charge, after charging from low cgarge to 50% charge

So maybe the value of 1-2 Ohms measurement is done wiith "optimal conditions". The manufacturers will know at which conditions the battery shows the best performance.

Klaus

 

[Easy peasy]

a very simple method for DC impedance is to quickly and accurately measure the terminal volts, for 20% load and 50% load, on the load increase and for the load decrease Z = delta V / delta I, average the two measurements ( up & down ) to get around the inherent hysteresis, at higher frequencies the inherent C of the battery can and does give misleading results ... about 0.5 seconds needed on each load.

 

[d123]

Hi,

Thanks, Brad. Your reply is very helpful, and I certainly agree about the 1 - 2 Ohms/500mA. Wouldn't be eager to drain a 9V to test real impedance and then have no charge left in it, as you say in your third paragraph.

Thanks, Klaus. It seems to me, in my admitted big ignorance, that the 'DC Load Method' is a 'What came first, the chicken or the egg?' - i.e., unless you want to know the max. load possible the baddery could handle, it's just fun with maths as up to that point the load directly influences the internal resistance value. I don't see that as useful for 'realistic' simming objectives.
'optimal conditions' and 'best performance' - good reminder, thanks.

Hi, EasyPeasy. Right, thanks. Your answer helps to see even more that for simming purposes I just have to accept approximations and it's far more productive to have real, hands-on experience in order to evaluate the results as valid or doubtful.

Many thanks.