It depends on the type of lead-acid battery and how long you want it to last, but the short answer is:

Only about 30–50% of a lead-acid battery’s rated capacity is realistically usable.

Here’s the breakdown.

Why you can’t use 100%

Lead-acid batteries are very sensitive to depth of discharge (DoD). The deeper you discharge them, the fewer cycles they survive.

Using too much capacity causes:

• Sulphation

• Plate shedding

• Rapid loss of capacity

• Early failure

Usable capacity by battery type

🔋 Starter (car) batteries

• Designed for short, high-current bursts

• Usable capacity: ~10–20%

• Regular deep discharge will destroy them quickly

Rule: Never use these for energy storage.

🔋 Leisure / Marine / Semi-deep cycle

• Compromise design

• Recommended usable capacity: ~30–40%

• Occasional deeper discharge is OK, but not daily

Example:

• 100 Ah battery → 30–40 Ah usable

🔋 True deep-cycle lead-acid (flooded, AGM, gel)

• Designed for repeated discharge

• Usable capacity: ~50% (60% max if you accept reduced life)

Example:

• 100 Ah battery → 50 Ah usable

Cycle life typical:

• 50% DoD → ~500–800 cycles

• 80% DoD → ~200–300 cycles

AGM vs Gel vs Flooded

AGM tolerates slightly deeper discharge but still degrades fast beyond ~60%.

Important gotchas

🔹 Peukert effect

At higher discharge rates, usable capacity is less than the rated Ah.

Example:

• 100 Ah battery at 20-hour rate

• At high load → may behave like 70–80 Ah

🔹 Temperature

• Cold → less available capacity

• Heat → more capacity now, shorter lifespan later

Simple rule of thumb

For lead-acid systems, size your battery bank so your normal usage never exceeds 50% state of charge.

If you need to regularly use more than half the rated capacity:

• You need more lead-acid batteries

• Or switch to LiFePO₄, where 80–90% is usable

If you like, tell me:

• Battery type

• Ah rating

• Application (solar, caravan, glider ground power, etc.)

and I’ll give you a precise usable energy figure in Ah and Wh.

Short answer: yes — roughly, and in practice often better.

Let’s put proper numbers on it so it’s clear.

Lead-acid vs lithium (LiFePO₄ assumed)

🔋 7 Ah lead-acid

• Sensible usable DoD: ~50%

• Usable capacity:
  7 Ah × 0.5 = 3.5 Ah

If you go deeper than this regularly, lifespan drops fast.

🔋 5 Ah lithium (LiFePO₄)

• Sensible usable DoD: 80–90%

• Usable capacity:
  5 Ah × 0.8–0.9 = 4.0–4.5 Ah

So in real use

✅ A 5 Ah lithium comfortably replaces the usable capacity of a 7 Ah lead-acid
✅ Often gives more usable energy, not less

Extra advantages lithium has (often overlooked)

• Higher voltage under load
  Lead-acid sags badly; lithium stays near nominal voltage

• No Peukert penalty
  Capacity stays close to rated even at higher currents

• Much lighter
  Big deal for portable / aviation / field equipment

• Much longer cycle life
  ~2000+ cycles vs ~300–500

One important caveat ⚠️

This assumes:

• LiFePO₄ chemistry, not cheap lithium-ion without protection

• A proper BMS (over-current, over/under-voltage, temp)

• Compatible charger (14.4–14.6 V for 12 V LiFePO₄, no float)

If this is a drop-in replacement for SLA (alarms, UPS, glider equipment, etc.), check:

• Low-temperature charging limits (LiFePO₄ must not be charged below 0 °C)

• Charger float behaviour

Bottom line

Yes — a 5 Ah LiFePO₄ is a direct practical replacement for a 7 Ah lead-acid, and usually an upgrade.