# Flooded lead acid cell impedance

Discussion in 'Electronic Design' started by [email protected], Dec 30, 2012.

1. ### Guest

Could some electronics guru please shed some light
on this ? What is an approximate impedance value
for a flooded lead acid cell battery, e;g., a 12V
32 AN cell. I have gone through a number of online
sites and material, e.g., 'Battery Univerdity but
that has not been of much help. Thanks in advance,

2. ### Sjouke BurryGuest

wrote in :
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.

3. ### Spehro PefhanyGuest

On Sat, 29 Dec 2012 23:41:53 -0800 (PST), the renowned
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
will be worse cold.

Best regards,
Spehro Pefhany

4. ### mikeGuest

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
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.

Let it 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.

5. ### mikeGuest

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 !

7. ### Guest

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.

8. ### Jasen BettsGuest

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

9. ### Tauno VoipioGuest

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.