Flooded lead acid cell impedance

snipped-for-privacy@gmail.com wrote in news:7ed07fe6-3028-4811-8862- snipped-for-privacy@googlegroups.com:

Crancking amps mostly suggest battery impedance, Just devide the voltage drop(2-4V) by the cranking amps gives the internal resistance. It is a gamble what the voltage drop is, some define 10 volt out at CA, others define 10V at Ca.

Okay, I'll give this a shot.. dunno your specs for that battery, but here's a (presumably) similar battery:-

Page 68 has the discharge curves (voltage as a function of charge state for various output currents).

The internal resistance is a function of output current, time, temperature as well as charge state.

From the curves you can read off the "impedance" as delta-V/delta-I for various values of charge state and current.

As you can see, the impedance increases as the battery is depleted-- a change in current from C/3 to C/100 (almost open circuit) results in about a 1.9V drop at 10% charge. At 100%, about 0.9V, so you could say the internal resistance more than doubles. If 'C' is, say, 300Ah then the C/3 = 100A and C/100 is 3A, so delta-I is 97A, and the effective internal resistance changes from about 9mOhms to about 20mOhms. It will be worse cold.

Best regards, Spehro Pefhany

That's a very vague question. What are you trying to accomplish?

Do you really care about the impedance? Over what frequency range, state of charge, temperature, age???

Maybe you're interested in the equivalent series resistance? But what is that really?

Maybe you want to know how much the voltage will droop when you load it? That's easy...well...no it isn't.

A battery that's been sitting will have some relatively high voltage. That voltage will drop rapidly at first, then settle down to some lower rate.

The only way I've found to reliably gauge actual battery performance in an application is to emulate the application.

Typically, I start with a fully charged battery. Let it sit for a while. Add the anticipated load and sit for a while. Add or subtract come current and measure the change in voltage. Use deltaV/deltaR as the dynamic resistance.

The definition of "while" in each case depends... Graph the data and pick a time that works to get you the info you need.

Realize that sulfation will dramatically increase the dynamic resistance as the battery ages.

Another interesting experiment is to use a pulse load. There'll be an initial step in voltage that's sorta related to resistance and a more gentle slope that's sorta related to chemical reactions.

When people ask that sort of question, they're often contemplating HIGH charge/discharge currents. That's a different bucket of worms.

Situation gets even more complicated when you charge the battery. If you assume a 'full' charge is at 13.8 volts, then you might expect that if you apply 14.8 volts ( ie a 1 volt difference ), then at say 10 mohm impedance, as measured rom the discharge current capability, you might expect 100 amps to flow into the battery. Not so ! The battery voltage will 'float' up to 14.8 or even up as high as 15.5 volts or so, but the current will fall to only a few ma. Electro-chemistry is not anything like a conventional ohmic resistance !

Thanks to each of you for your comments. I am aware of the complicated nature of battery chemistry. I am creating some first-cut SPICE simulations of a battery system, and was looking for am approximate value to get started with, before adding a full- scale battery model.

close enough to 0 that you don't want to short circuit it ever.

A good 12 V car battery has a short circuit current in excess of 2500 A, if you have a good enough short.

Please be careful with rings and metal watch bands.