The core reason why lead-acid and lithium batteries are "incompatible" is not that they can both discharge, but rather that their three major systems—voltage, chemistry, and protection—are completely different. Mixing them equals instantaneous damage and safety risks.

I. Different Voltage Systems

Item
Lead-acid (12V example)

Lithium-ion (12V example)

Single Cell Full Voltage

14.4 V

16.8 V

Open Voltage

12.8 V

12.6 V

Cutoff Voltage

10.5 V

8.4 V
Results of Mixing:

Using lead-acid at 14.4V to charge lithium battery → will never fully charge, capacity decreases by 30%;
Using lithium-ion at 16.8V to charge lead-acid → overcharge, water loss, bulging, lifespan halved.

II. Different Chemical Systems

Lead-acid: Three stages (constant current-constant voltage-trickle flow), trickle flow 2-4 hours, desulfurization;

Lithium-ion: Two stages (constant current-constant voltage), no trickle flow, continuous low voltage = lithium plating + bulging.

Mixed Use Results: Lead-acid charger for lithium batteries → Continuous low-voltage trickle charging, internal lithium plating, cycle life drops from 800 cycles to 400 cycles.

III. Different Protection Systems
Lead-acid: No individual cell BMS, relies on charger cutoff voltage;
Lithium battery: Must have BMS, single cell cutoff is 4.2V, mixed use without BMS = overcharge and explosion.

Mixed Use Results: Lead-acid charger for lithium batteries → BMS repeatedly disconnects, temperature rises by 10℃, risk of thermal runaway increases.

IV. Real-world Test Case
2023 Hangzhou Test: 48V 20Ah ternary lithium lead-acid charger (58.8V) 72h:
Capacity decreased from 85% to 47%;
Internal temperature 78℃, 2 bulges, recovery success rate %.

V. Summary in one sentence: "Different voltages will result in incomplete charging, different chemistry will cause bulging, and different protection systems will lead to explosions"—Lead-acid and lithium batteries have completely different voltage, chemical, and protection systems, requiring dedicated chargers. Using the same charger will result in instant damage and safety risks!