Renogy DC-to-DC Battery Charger Installation

The Renogy 40-Amp DC-to-DC battery charger had been on my Amazon wishlist for a while, and it was on sale during the holidays, so I bought one. I was planning on getting it anyway for our upcoming Lithium battery upgrade (coming soon). A DC-to-DC charger is required to limit the current draw from the alternator, particularly with Lithium batteries, which can draw a large amount of current from the alternator while charging.

According to Mercedes Body and Equipment Guidelines for 2018 Sprinter and older, it states that the maximum charging current for the auxiliary batteries should not exceed 40-Amps. For now, our current AGM coach batteries will benefit from the DC-to-DC charger as it will receive a proper 3-stage charging (bulk, absorb, float) while driving or idling. 

Disclaimer: I'm not endorsing Renogy or getting paid nor work for Renogy.  I find their DC-DC charger cheaper for the features you get compared to other brands like: Victron, CTEK, KISAE technology, and Sterling. I still have to see its longevity though, only time will tell. 

The DC-to-DC charger can detect the state of charge of the coach batteries and adjust its output accordingly, independent of the engine battery. This means that even if the engine battery is fully charged, the coach batteries can still be in bulk charging mode. The Renogy 40-amp DC-to-DC charger also keeps the alternator's current draw within the MB recommended aux. charge current. Because it isn't completely efficient, I expect it to draw slightly more than 40 amps, which should be fine.

Renogy DC-DC Battery Charger is compatible with multiple 12V batteries: AGM, Flooded, Gel, Sealed, Lithium-iron Phosphate and Lithium-ion. 

Renogy DC-DC Battery Charger Manual  

Parts needed:

• Renogy 40-Amp DC-to-DC charger
• 4-AWG cables (carefully measure needed cable length before ordering)
• 4-AWG 5/16" ring wire lugs (Renogy has M6 (1/4") terminal and MRBF fuse block has M8 (5/16") terminal. A 5/16" ring will fit both or you can order 1/4" and 5/16" wire lugs for each.
• 1/2" Adhesive lined heat shrink tubing
• 1/2" split wire loom
• Firewall rubber boot
• MRBF Terminal Single Fuse Block (optional) You can use an inline fuse holder like ANL, etc.
• MRBF 50-Amp terminal fuse (optional depending on preferred fuse holder)
• 2-Amp ATO blade fuse

One significant challenge is locating a suitable location for the DC-to-DC charger, which cannot be installed in a wet/hot/dusty environment such as under the hood. I decided to make a custom bracket that would fit next to the driver's left footrest. Others have mounted theirs near the front of the driver's seat base; the choice is yours.

I built the mounting bracket out of stock angles and flat bars, then powder coated it flat black. It does involve welding, so you may need to hire a welder to make your own. The mount will be bolted into the firewall with the help of three existing bolts that protrude through the firewall.

Pre-fitting the DC-to-DC charger into the mounting bracket.

I apologize for the blurry photo; it's difficult to take a picture from under the dash. The bracket itself obscures the view of the middle bolt/nut. The alternator input wire, as well as the (+) and ground wires from the charger output are fished through the existing firewall opening (pictured with butyl insulation covering). To protect the wires, a firewall rubber boot is used. The DC-DC charger input ground is connected to an existing chassis ground lug located just to the left of the fuse box/OBD port (not pictured).

The DC-to-DC charger D+ input is directly connected to the Sprinter D+ connection under the driver seat, it's protected by an inline 2-amp ATO fuse.

The EK1 electrical connectors installed under the driver seat base (at the front, on the left-hand side of the vehicle) and has three terminals. Terminal 1(with blue/yellow wire) is the D+ connection.

The DC-to-DC charger output is wired to the coach batteries with 4-AWG wire and a 50-amp MRBF terminal fuse. The charger (+) DC input from alternator is connected to the same post on my Automatic Charging Relay. I decided to keep my ACR in order to have more control over the DC-to-DC charger; I can manually disconnect and reconnect the charger from the alternator if necessary. The ACR also prevents the DC-to-DC charger from immediately connecting to the alternator once the engine is started. I usually wait a few minutes after starting the engine before manually switching on the ACR/charger.

Update: I added a time delay module to the charger D+ input wire to give the vehicle voltage time to stabilize after engine start-up; more details in this post> ACR to DC/DC Charger Transfer Relay.