When a lead-acid battery’s display is used with a lithium‑ion battery, the state-of-charge reading is inaccurate. The root cause lies in the fundamental differences between the two battery types in terms of voltage characteristics, state-of-charge measurement principles, and system design. A detailed analysis follows:

I. Voltage characteristics differ from the discharge curve.

Voltage range differences

Lead-acid battery ( 48V System) Full-charge voltage is approximately 55.8V , the undervoltage point is approximately 41V ; and 14 The full-charge voltage of a ternary lithium battery reaches 58.8V , the undervoltage point is 45V5。

The voltage of a lead-acid battery decreases nonlinearly as the state of charge drops (with a stable voltage at full charge and a sharp voltage drop in the latter half), whereas the discharge curve of a lithium battery is much more gradual. 17。

Result: The lead‑acid display divides the battery’s state of charge into segments based on the lead‑acid voltage range, but the lithium‑ion battery’s actual voltage falls into an incorrect segment, causing the displayed level to jump erratically—for example, showing insufficient charge when fully charged, or indicating a high charge level even at low battery levels. 57。

Virtual Power Phenomenon and Load Fluctuations

Lead-acid batteries exhibit a pronounced “false‑charge” phenomenon: when subjected to high inrush currents, the voltage drops sharply and the state‑of‑charge indicator plunges, only to rebound once the current stabilizes. By contrast, lithium‑ion batteries are less sensitive to current variations; however, lead‑acid‑type displays continue to interpret voltage according to the lead‑acid load‑voltage curve, thereby exacerbating display inaccuracies. 178。

II. Incompatible Principles of Electric Charge Measurement

Lead-acid batteries rely on the voltage method, while lithium-ion batteries require the coulomb‑counting method.

Due to cost constraints, lead-acid batteries typically rely on the open-circuit voltage method or the load‑voltage method to estimate state of charge, without accounting for factors such as actual capacity degradation and temperature. 167。

Lithium batteries are typically equipped with BMS (Battery Management System), combined with the voltage method + The ampere-hour integration method (which statistically analyzes discharge current over time) offers higher accuracy. 13。

Issue: The lead-acid display cannot read the lithium battery. BMS The ampere-hour integration data, which relies solely on voltage signals, is inevitably inaccurate. 36。

Differences in Battery Pack Complexity

Lithium‑ion battery packs consist of multiple cells connected in series, requiring cell‑level balancing to address individual cell‑to‑cell variations. In contrast, lead‑acid battery management systems lack built-in calibration logic and cannot compensate for state‑of‑charge discrepancies arising from inconsistent capacities among the cells in a lithium‑ion pack. 38。

III. System Design Mismatch

Lack of hardware and algorithms

Lead-acid battery meters are typically designed to be universal, with their voltage range matched to lead-acid systems solely through a voltage‑divider resistor; they do not include a dedicated lithium‑battery calibration interface or algorithm. 58 . Some users, when modifying their setups, attempt to connect resistors or Zener diodes in series (such as 4.3V Zener diode) compresses the voltage signal, but it can only improve performance for ternary lithium batteries; due to differences in their voltage profiles, it remains unsuitable for lithium‑iron‑phosphate batteries. 56。

No capacity fade compensation

Lead-acid displays have a fixed, default capacity, whereas lithium‑battery capacity varies dynamically with temperature and cycle count. After modification, the display can no longer accurately reflect the actual capacity, and the error becomes even greater at low temperatures and as the battery ages. 67。

IV. Differences in Temperature and Environmental Adaptability

Lithium batteries experience more pronounced capacity fade at low temperatures than lead-acid batteries (e.g., -10 Capacity decreases at ℃. 30% ), but lead-acid displays lack a temperature compensation mechanism, so at low temperatures they may indicate a falsely high state of charge. 378。
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