To safely and efficiently charge a 12V lead-acid battery with a 48V lithium battery, it is necessary to solve the problems of voltage matching and charging control. The following are the specific plans and precautions:

I. Core Issues

1. Voltage mismatch

A 48V lithium battery (about 54.6V when fully charged) is much higher than the charging voltage of lead-acid batteries (13.8 to 14.4V).

Direct connection can burn out lead-acid batteries or cause danger.

2. Different charging characteristics:

Lead-acid batteries need to be charged with constant voltage and limited current (such as 14.4V constant voltage + 10% to 20% current of capacity).

The direct output of lithium batteries cannot be automatically adjusted.

II. Feasible Solutions

Option 1: Use the DC-DC step-down module + charging control

Step:

1. Select the voltage reduction module:

The input range covers 48V

The output can be adjusted to 14.4V (the charging voltage of lead-acid batteries).

The current should match the capacity of the lead-acid battery (for example, a 2A module is selected for a 10Ah battery).

2. Connect the circuit:

48V lithium battery → [DC-DC module (output 14.4V)] → [12V lead-acid battery]

3. Precautions:

The module should have constant voltage (CV) and constant current (CC) functions, or an additional current-limiting circuit should be installed.

Monitor the charging voltage to prevent overcharging (disconnect when the lead-acid battery voltage is ≥14.4V).

Solution 2: Dedicated lead-acid battery charging controller

Applicable scenario: The charging process needs to be managed automatically.

1. Select the controller:

Supports 48V input and 12V lead-acid battery output.

Built-in charging algorithms (float charging/equalization charging/temperature compensation).

2. Connection method:

48V lithium battery → [48V-12V charging controller] → 12V lead-acid battery

Advantages:

Automatically switch charging stages (constant current → constant voltage → float charging).

Anti-reverse connection and overcurrent protection.

Option 3: Through inverter + standard charger

Step:

Use a 48V to 220V inverter and connect it to a mains lead-acid battery charger.

The output of the charger is connected to a lead-acid battery.

Disadvantages: Low efficiency (loss from two conversions), only suitable for temporary use.

III. Key precautions

Voltage accuracy: The charging voltage error of lead-acid batteries should be ≤±0.2V. If it is too high, it will cause gas evolution; if it is too low, the battery will not be fully charged.

Current limit: The charging current is ≤ 20% of the lead-acid battery capacity (for example, a 20Ah battery is charged with 4A).

Protection functions: It must have overcharge protection (automatically cut off power when fully charged) and reverse connection protection.

Heat dissipation: High-current DC-DC modules need to be equipped with heat sinks or fans.