Lead Acid Battery "Internal Resistance" and Temperature


by thender
Tags: acid, battery, charging strategy, h2so4, internal resistance, lead, lead acid battery, temperature
thender
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#1
Nov29-13, 09:37 PM
P: 20
Hello,

I've reached my wit's end trying to understand the theory behind charging lead-acid batteries. I believe I have read over twenty different articles, maybe six hours worth of studying.

According to what I've read:

Charging strategy for a lead acid battery (like the one in your average automobile) is a delicate matter. If the battery is not maintained approximately 100% charged it will suffer. Discharged batteries will have lead sulphate deposits harden and crystallize on the plates, overcharged batteries will boil the electrolyte, losing water and possibly sulfur as well.

That much I mostly understand. The part that I don't understand is why a battery's capacity is considered to be temperature dependent, such that at 40 degrees fahrenheit it may only have 70%. Automotive batteries have "Cold Cranking Amps" ratings afterall.

Beyond that, the battery temperature needs to be taken into account when charging, it seems that cold batteries need higher charging voltages, and warm batteries need lower voltages.

Then there are some people who reference the "internal resistance" of the battery. I believe the internal resistance includes both the resistance of the electrolyte and the metal straps and lead plates. At low currents the metal parts of the battery may not affect the current much, but I don't know about high currents.

A good automotive battery drops down to maybe 10 volts during cranking, but rises back up. I am not sure why the voltage decreases that much.

I think the temperature affects the electrolyte somehow. Maybe it effects the concentration? Maybe it effects the ionic conductivity?

I pulled my last hair out today and decided to ask for help.

What I am really trying to understand is how a strategy for charging an automotive lead acid battery works with regard to the battery voltage, state of charge, temperature, and charge rate.

I had wanted to understand if there were any capacitive or inductive components to it as well, ie, impedance. But for now I am just trying to grasp these other more basic aspects.

Thanks for any help,
-Andrew
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Borek
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#2
Nov30-13, 03:34 AM
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It is a matter of changes in ionic conductivity. See for example

http://en.wikipedia.org/wiki/Electri...ure_dependence

(not an explanation per se, but it shows how large the changes are as a function of temperature). Lead plates can be safely ignored.

During cranking it is an ohmic drop, AKA ohmic loss, AKA IR drop. To quote wikipedia:

The magnitude of the emf for the battery (or other source) is the value of this 'open circuit' voltage. When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source. It can, however, be inferred from a measurement of the current I and voltage difference V, provided that the internal resistance r already has been measured: ℰ = V + Ir.
emf stands for electromotive force (http://en.wikipedia.org/wiki/Electromotive_force).
AlephZero
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#3
Nov30-13, 01:39 PM
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In practice, you don't need much "strategy" for charging lead acid batteries. So long as the charging current is fairly low (say less than 5 amps) you can overcharge a typical car battery for ever, without harming it.

So all you need to do in practice is to make sure the charging voltage is low enough so the charging current is low when the battery is fully charged. That's what the voltage regulator on the car alternator does. The usual voltage is about 14V. The charge taken from the battery to crank the engine is replaced fairly quickly (e.g. in 10 to 20 minutes) and the rest of the time the battery is being harmlessly overcharged.

Note that while the engine is running, the power for the car electrics like the lights, aircon, sound system, etc is coming from the alternator, not from the battery.

MalcolmB
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#4
Nov30-13, 01:55 PM
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Lead Acid Battery "Internal Resistance" and Temperature


Hi Thender...

This may help: - http://batteryuniversity.com/learn/a...d_acid_battery

Cheers

Malc
dlgoff
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Nov30-13, 02:32 PM
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Quote Quote by MalcolmB View Post
Hi Thender...

This may help: - http://batteryuniversity.com/learn/a...d_acid_battery

Cheers

Malc


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