Lead-acid batteries have been around for more than 150 years, and they're still the most common battery in cars, 4WDs, boats and caravans. But "lead-acid" covers a few different types. This guide explains the differences, how to charge them, how to test them and how long they last.
What is a lead-acid battery?
A lead-acid battery stores energy using lead plates sitting in sulphuric acid, called the electrolyte. It's rechargeable, reliable and relatively cheap, which is why it's still so widely used.
There are three main types of lead-acid battery. The difference is how the electrolyte is held inside.
| Type | How the acid is held | Sealed? | Maintenance |
|---|---|---|---|
| Flooded (wet) | Liquid, free to move | No | Needs water top-ups |
| AGM | Soaked into glass mats | Yes | None |
| Gel | Thickened into a gel | Yes | None |
EFB batteries, used in many stop-start cars, are an upgraded version of the flooded battery.
Is AGM a lead-acid battery?
Yes. AGM stands for Absorbent Glass Mat. It's a lead-acid battery where the acid is soaked into fine glass mats instead of sloshing around as a liquid. That makes it:
- spill-proof, so it can be mounted on its side
- maintenance-free
- better at handling vibration
- better at handling deeper discharges than a flooded battery.
So every AGM battery is a lead-acid battery, but not every lead-acid battery is AGM.
Is a lead-acid battery a wet battery?
Only the flooded type. Flooded batteries are often called wet batteries because the acid is a free-flowing liquid. AGM and gel batteries are sealed and aren't considered wet.
AGM vs flooded at a glance
| AGM | Flooded | |
|---|---|---|
| Maintenance | None | Water top-ups |
| Spill risk | Spill-proof | Can spill if tipped |
| Vibration | Handles it well | Less resistant |
| Deep cycle life | About 500 to 1,000 cycles | About 300 to 500 cycles |
| Price | Higher | Lower |
Cycle life depends on how deeply the battery is drained each time. The figures above are for deep cycle use.
How to test a lead-acid battery
The simplest test is a resting voltage check with a multimeter. Let the battery sit for a few hours without charging or use, then measure across the terminals.
| Resting voltage (12V battery) | State of charge |
|---|---|
| 12.6V or higher | Fully charged |
| 12.4V | About 75% |
| 12.2V | About 50% |
| 12.0V | About 25% |
| Below 12.0V | Flat |
Voltage only tells you how charged the battery is, not how healthy it is. To check health, the battery needs a load test, which shows how it performs when it's actually working. A battery that reads 12.6V but drops away quickly under load is worn out.
For a full step-by-step guide, including the starting and charging tests, see our guide to testing a car battery with a multimeter.
How to charge a lead-acid battery
Lead-acid batteries need the right voltage to charge fully without damage. A smart charger handles this for you, as long as it's set to the right battery type.
The three charging stages
- Bulk: the charger pushes in as much current as it can until the battery is about 80% full.
- Absorption: the charger holds a steady voltage while the current tapers off, bringing the battery to 100%.
- Float: the charger drops to a lower voltage to keep the battery topped up without overcharging.
Typical charging voltages for a 12V battery
| Type | Absorption voltage | Float voltage |
|---|---|---|
| Flooded | 14.4 to 14.8V | 13.2 to 13.5V |
| AGM | 14.4 to 14.7V | 13.5 to 13.8V |
| Gel | 14.1 to 14.4V | 13.5 to 13.8V |
These are general figures. Always check the battery manufacturer's specifications, as they can vary.
How long does charging take?
As a rough guide, a flat 100Ah battery takes around 6 to 12 hours on a 10 to 20 amp charger. A partly charged battery will take less time.
Use the right charger
- Choose a smart charger with a setting for your battery type.
- Don't use a lithium-only charger on a lead-acid battery, or the other way around.
- Avoid old-style chargers that don't switch off, as they can overcharge.
Can I connect lead-acid and lithium batteries together?
Not directly. Lithium and lead-acid batteries work at different voltages and need different charging. Connecting them straight together can stop the lithium battery charging properly and may trip or damage its built-in battery management system (BMS).
If you're running both, for example a lead-acid starting battery and a lithium auxiliary battery, use a DC-DC charger between them.
How long does a lead-acid battery last?
| Type | Typical life |
|---|---|
| Flooded | 2 to 5 years |
| AGM | 3 to 7 years |
What affects how long it lasts:
- How deeply it's drained. Keeping deep cycle batteries above 50% charge greatly extends their life.
- Heat. High temperatures speed up wear.
- Charging. Undercharging and overcharging both shorten life.
- Being left flat. A flat lead-acid battery starts to sulphate, which permanently reduces its capacity.
- Maintenance. Flooded batteries need their water level checked.
Looking after a lead-acid battery
- Recharge it as soon as possible after use. Don't leave it flat.
- Keep the terminals clean and tight.
- Check for cracks, leaks or swelling.
- For flooded batteries, check the water level regularly and top up with distilled water only. Never use tap water.
- When storing, charge it fully first, keep it somewhere cool and dry, disconnect it and recharge it every few months, or leave it on a maintainer.
Lead-acid batteries can release gas while charging, especially flooded types. Charge them in a well-ventilated area, away from sparks and flames.
Quick answers
What are lead-acid batteries called?
Depending on the type, they're called flooded or wet batteries, AGM batteries or gel batteries. EFB is an enhanced version of the flooded battery.
Is an AGM battery the same as a lead-acid battery?
An AGM battery is a type of lead-acid battery. It's sealed and maintenance-free, unlike a flooded lead-acid battery.
What's the maximum charging voltage for a 12V lead-acid battery?
Usually around 14.4 to 14.8V for flooded and 14.4 to 14.7V for AGM. Check the manufacturer's specifications for your battery.
