When Does a DC-DC Battery Charger Start or Stop Charging?
What Is a DC-DC Battery Charger?
A DC-DC battery charger, also known as a battery-to-battery charger allows you to charge your auxiliary battery from your alternator while driving.
Renogy 12V 30A Dual Input DC-DC On-Board Battery Charger with MPPT (RBC30D1S) and 12V 50A Dual Input DC-DC On-Board Battery Charger with MPPT (RBC50D1S) do more than that. They allow you to charge the auxiliary battery with a starter battery connected to an alternator or with solar panels connected directly to the battery charger.
Charging Sources
The RBC30D1S or RBC50D1S DC-DC battery charger with MPPT model has two charging sources:
- Starter battery connected to a vehicle's alternator
- Solar panels
The charger can be connected simultaneously to the solar panel and the automobile starter battery, or separately to either the solar panel or the automobile starter battery to charge the auxiliary battery. The charging logic varies depending on the chosen connection method.
Solar Panel as the Charging Source
When solar panels are the only charging source, the charging logic is as follows:
| Solar Output Voltage | Battery Charger Status |
|---|---|
| > 15V for 10s | Starts charging* |
| > 25.5V | Stops charging |
*Note:
The RBC30D1S model has a maximum charging current of 30A, while the RBC50D1S model has a maximum charging current of 50A.
In this mode, the battery charger prioritizes charging the auxiliary battery. When the output voltage of the solar panels exceeds 15V, the charger begins charging the auxiliary battery after a 10-second delay. However, if the output voltage of the solar panel exceeds 25.5V, the charger stops the charging process. Charging resumes when the solar output voltage drops below 24.5V.
The battery charger charges the starter battery at a limited voltage of 13.8V and current of 15A (for RBC30D1S) or 30A (for RBC50D1S) after the auxiliary battery is fully charged. During the charging process, the battery charger charges the starter battery for one minute and pauses for 30s to check the battery voltage:
- If the starter battery voltage is less than 12.7V, the charging resumes.
- If the starter battery voltage is higher than 13.2V, the charging stops.
Starter Battery as the Charging Source
When the starter battery works as the only charging source, the charging logic varies specific to the connected alternator:
| Alternator Type | Starter Battery Voltage | Battery Charger Status |
|---|---|---|
| Traditional alternator | > 13.2V for 15s | Starts charging**/td> |
| < 12.7V | Stops charging | |
| Smart alternator | >12V for 15s | Starts charging** |
| < 11.5V | Stops charging |
**Note: The RBC30D1S model has a maximum charging current of 30A, while the RBC50D1S model has a maximum charging current of 50A.
- For traditional alternators, the battery charger charges the auxiliary battery if the starter battery voltage remains above 13.2V for 15 seconds. The charging process stops when the starter battery voltage falls below 12.7V.
- For smart alternators, the battery charger charges the auxiliary battery if the starter battery voltage remains above 12V for 15 seconds. The charging process stops when the starter battery voltage falls below 11.5V.
For both traditional and smart alternators, the battery charger ceases charging the auxiliary battery when the solar battery voltage exceeds 16.5V. Charging will resume once the solar battery voltage drops below 15.5V.
Both Solar and Starter Battery as the Charging Source
When both the starter battery and solar power are used as charging sources, the battery charger prioritizes taking power from the connected solar panels as long as the solar power is sufficient.
If the solar panel cannot keep the auxiliary battery at constant charging, the battery charger will automatically adjust to allow both the solar panel and the starter battery to charge the auxiliary battery simultaneously, with each source providing no more than 50% of the charging power.
How to Determine When the Auxiliary Battery is Fully Charged?
The battery charger features an automatic detection system that identifies when the auxiliary battery is fully charged based on the charging stage:
- For lithium batteries, the battery is considered fully charged once it maintains boost charging at lower than 3A for 30s.
- For non-lithium batteries, the battery is considered fully charged once it enters float charging mode.
