How a Lead Acid Battery Works

What Actually Happens Inside a Lead-Acid Battery? (Made Simple)

We all know car batteries start your engine, power your lights, and hate the cold, but what’s really going on inside that heavy black box?

Whether you’re a weekend DIYer or just battery curious, let’s crack one open (metaphorically) and break down the chemistry of a lead-acid battery in plain English. No chemistry degree required.

 The Basics: What It’s Made Of

Inside every lead-acid battery, whether it’s flooded, AGM, or gel, you’ll find:

1. Plates made of lead and lead dioxide
2. An electrolyte, which is sulfuric acid mixed with water
3. Separators to keep plates from shorting out
4. A container to hold it all together and (hopefully) keep acid from spilling on your pants
5. These parts combine to make a chemical energy storage system that’s been around for 160+ years and still kicks butt today.

 The Magic Reaction (Charging & Discharging)

Here’s the short version:

1. When your battery is discharging (like when you crank the engine or turn on headlights):
2. The lead dioxide (PbO₂) plate reacts with sulfuric acid (H₂SO₄)
3. The lead (Pb) plate reacts too
4. Both plates turn into lead sulfate (PbSO₄)
5. The acid gets weaker (diluted)
6. Electrons flow out, powering your car
7. When you charge it back up, that process reverses:
8. The lead sulfate turns back into lead and lead dioxide
9. The sulfuric acid gets stronger again
10. Energy is stored up like magic (well… science)

 Why the Voltage Drops When a Battery Is Low

Lead-acid batteries don’t just “run out” like a flashlight bulb going dim, they fade gradually as the chemistry inside changes. This change directly affects the voltage you see at the terminals.

When a battery is discharging (powering something), sulfuric acid inside it is being consumed and replaced with water as part of the chemical reaction. Less acid = lower voltage. That’s why you can measure a battery’s state of charge just by checking its resting voltage with a multimeter.

Let’s break it down:

 For a Standard 12V Lead-Acid Battery

Here’s a general guide to resting voltage (after the battery’s been sitting for at least a couple hours without use):

 Anything below 12.4V for long periods encourages sulfation, especially in AGM batteries.

 For an 8V Battery (Found in Golf Carts, Older Industrial Systems)

These are a bit less common but work the same way chemically:

AGM and gel-type 8V batteries follow the same voltage trend, but they should always be charged with smart chargers that recognize the correct profile. AGM needs a slightly lower finishing voltage to avoid damage.

 For a 6V Battery (Common in Golf Carts, Classics, and Older Equipment)

6V batteries are essentially half of a 12V setup (3 internal cells instead of 6), so the voltages scale down accordingly:

If you’re running 6V batteries in series (common in golf carts or RVs), multiply the values by the number of batteries for a system-wide check.

 Why Resting Voltage Matters

• Immediately after charging, voltage can read high due to surface charge (not accurate). Let it sit 4–6 hours or turn on the headlights for a minute before testing.
• While cranking, voltage will drop—anything below 9.6V under load on a 12V battery may mean it’s weak.
• While charging, voltage is artificially high. Don’t confuse 14.2V (charging voltage) with state-of-charge readings.

What Goes Wrong Over Time?

1. Sulfation: Lead sulfate crystals build up and don’t convert back during charging. This reduces capacity.
2. Corrosion: Plates break down after thousands of cycles.
3. Stratification: Acid and water separate inside, especially in AGM batteries that aren’t charged hard enough.
4. Overcharging: Can boil off electrolyte or warp plates.
5. Undercharging: Encourages sulfation and battery death.
6. That’s why proper charging and occasional testing matter.

 Fun Analogy: A Battery Is Like a Sponge

A full sponge = fully charged battery

A wet but not soaked sponge = partially charged

A bone-dry sponge = flat battery

A crusty sponge with old gunk = sulfated battery that might not come back

You can squeeze water out (use energy), and refill it (recharge), but over time it gets worn out and nasty.

 Final Thoughts

A lead-acid battery is a pretty amazing chunk of chemistry that stores and delivers power through a centuries-old process that’s still super effective today.

Understanding the basics, what it does, how it works, and why it dies, makes you better at maintaining it, testing it, and replacing it before it ruins your morning commute.

Stick around for more nerdy battery breakdowns and maybe grab a multimeter so you can keep tabs on your own!