I’m looking at battery options to upgrade the box on my Astir, but noticed something we barely think about – the battery is plugged into the aircraft with a latching DIN type plug which is lovely, but the battery itself is about 6 inches away in an open area just connected with Lucas spades with no stress relief on the cable or secondary cover, and nothing other than cable colour to indicate polarity if were to need reconnecting. Pretty much standard in gliders, but surely it’s actually rather sketchy? Lead acid batteries don’t even have built in short protection. (Obviously it works in practice, but still.)
How much of a lead acid battery capacity is available to use?
ChatGPT said:
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:
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Sulphation
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Plate shedding
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Rapid loss of capacity
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Early failure
Usable capacity by battery type
🔋 Starter (car) batteries
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Designed for short, high-current bursts
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Usable capacity: ~10–20%
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Regular deep discharge will destroy them quickly
Rule: Never use these for energy storage.
🔋 Leisure / Marine / Semi-deep cycle
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Compromise design
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Recommended usable capacity: ~30–40%
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Occasional deeper discharge is OK, but not daily
Example:
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100 Ah battery → 30–40 Ah usable
🔋 True deep-cycle lead-acid (flooded, AGM, gel)
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Designed for repeated discharge
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Usable capacity: ~50% (60% max if you accept reduced life)
Example:
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100 Ah battery → 50 Ah usable
Cycle life typical:
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50% DoD → ~500–800 cycles
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80% DoD → ~200–300 cycles
AGM vs Gel vs Flooded
| Type | Practical usable capacity |
|---|---|
| Flooded | ~50% |
| AGM | ~50–60% |
| Gel | ~50% |
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:
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100 Ah battery at 20-hour rate
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At high load → may behave like 70–80 Ah
🔹 Temperature
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Cold → less available capacity
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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:
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You need more lead-acid batteries
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Or switch to LiFePO₄, where 80–90% is usable
If you like, tell me:
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Battery type
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Ah rating
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Application (solar, caravan, glider ground power, etc.)
and I’ll give you a precise usable energy figure in Ah and Wh.
You said: So a 5ah lithium would replace the usable capacity of a 7ah lead acid?
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
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Sensible usable DoD: ~50%
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Usable capacity:
7 Ah × 0.5 = 3.5 Ah
If you go deeper than this regularly, lifespan drops fast.
🔋 5 Ah lithium (LiFePO₄)
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Sensible usable DoD: 80–90%
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Usable capacity:
5 Ah × 0.8–0.9 = 4.0–4.5 Ah
So in real use
| Battery | Rated | Usable |
|---|---|---|
| Lead-acid | 7 Ah | ~3.5 Ah |
| Lithium | 5 Ah | ~4–4.5 Ah |
✅ 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)
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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:
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LiFePO₄ chemistry, not cheap lithium-ion without protection
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A proper BMS (over-current, over/under-voltage, temp)
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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:
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Low-temperature charging limits (LiFePO₄ must not be charged below 0 °C)
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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.
