Renogy 12V 20A DC to DC On-Board Battery Charger: how it works

The Renogy 12V 20A DC to DC charger is an on-board unit that charges an auxiliary battery, often called a house battery, from a vehicle's starter battery or alternator. It is aimed at RVs, commercial vehicles, boats and yachts, where lights, fridges and other equipment run off a second battery bank. It measures 8.4 x 6.9 x 2.7 inches (about 21 x 17.5 x 7 cm) and weighs 3.1 lb (about 1.4 kg).

Why use a DC to DC charger

Connecting a house battery straight to the alternator can undercharge it, because alternators are designed for starter batteries, not for the different charging profiles of deep-cycle batteries. A DC to DC charger sits between the two. It takes the vehicle's power and delivers a controlled charge suited to the house battery. This matters especially with lithium batteries, which need a different charging curve from lead-acid.

Battery types supported

Renogy says the charger works with a range of alternator types and charges AGM, flooded, gel and lithium deep-cycle batteries correctly. Check that your battery chemistry and voltage match, and select the right setting as described in the manual.

Three-stage charging

The charger uses three stages: bulk, boost and float. In general terms, bulk delivers the maximum current while the battery is low; boost holds a higher voltage to finish filling it; float then holds a lower voltage to keep it topped up without overcharging. Renogy says this automatically charges the house battery to 100%. The 20A figure is the maximum output current, so larger battery banks will take longer to fill.

Protections and isolation

Renogy lists over-voltage, over-temperature and reverse-polarity protection. It also isolates the input from the output, which it says prevents incorrect voltage transmission and stops the two sides from interfering with each other. Isolation is useful in vehicles because the starter battery and the house battery stay electrically separate.

Protecting the starter battery

The charger can detect whether the engine or alternator is running. Renogy says this prevents the starting battery from being drained and protects the alternator. In practice that means the charger only draws power when the vehicle is generating it, so your engine battery isn't depleted while parked.

Size and installation

The unit is compact and sturdy, intended to fit where space in a vehicle is limited. It is rated for use in a range of conditions. Installation involves correctly sized cables, fuses and secure mounting, so follow the manual and local codes, or have a qualified installer do the work. Wiring is on the battery side of a vehicle, where mistakes can cause damage.

What to check before you buy

• Battery type and voltage: confirm that your house battery is a supported 12V type.
• Capacity: compare 20A with the size of your battery bank and how quickly you need to charge it.
• Alternator type: check whether your vehicle has a standard or smart alternator and what the manual advises.
• Cables and fuses: plan for the cable gauge and fuse sizes the manual specifies.
• Mounting space: confirm there is a suitable location with enough airflow.
• Wiring skills: decide whether to install it yourself or get a professional.

The short version

• A 12V 20A DC to DC on-board charger that charges a house battery from the starter battery or alternator.
• Supports AGM, flooded, gel and lithium batteries.
• Three-stage charging: bulk, boost and float.
• Over-voltage, over-temperature and reverse-polarity protection, with input-output isolation.
• Compact at 8.4 x 6.9 x 2.7 inches and 3.1 lb.

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