A DC-DC charger charges your house battery from the vehicle's alternator while you drive. Renogy makes them from 20A to 60A, with and without a solar input. The right size depends on two numbers more than anything else: how big your alternator is, and how fast you need to refill your house battery. This guide walks through both, then matches the result to the Renogy chargers we stock.
The short answer
- 20A: a 100Ah house battery, or short daily drives where a full refill isn't the goal.
- 30A to 40A: the most common van and truck-camper size, for 100Ah to 200Ah of lithium, if your alternator passes the check in Step 1.
- 50A to 60A: 200Ah to 400Ah house banks, a large alternator, and owners who want a full refill on a two- or three-hour drive.
- Want solar on the same box? Pick one of the MPPT models below, and check its solar input limit against your panels.
Step 1: check your alternator first
A DC-DC charger pulls its power from the starter battery and alternator, so the alternator sets the ceiling. In the manual for its 30A and 50A dual-input chargers, Renogy says the alternator should be rated 1.5 to 2 times the charger. That means a 45A to 60A alternator for the 30A model and a 75A to 100A alternator for the 50A model. It also limits alternator input to 400W on the 30A model and 660W on the 50A.
Renogy's alternator guidance differs by model, so use the figure for the charger you are looking at:
| Charger | What Renogy says about the alternator |
|---|---|
| DCC 20A / 40A (Gen 2) | Its quick guide says to check with the vehicle maker before installing that the alternator power does not exceed 720W, with output current in the 75A to 100A range |
| DCC30S (30A) | 45A to 60A alternator, 400W maximum alternator input |
| DCC50S (50A) | 75A to 100A alternator, 660W maximum alternator input |
| DCC 60A | No alternator figure in the manual we read. Check your vehicle's rating, and note the 30A current-limit option below |
If your alternator is small or already busy, ask us before you buy, or pick a charger you can turn down (see the FAQ).
The input side also draws more current than the charger puts out. Renogy's manual for its 20A, 40A and 60A DCC chargers warns that the input can draw up to 50% more than the rated output while charging at full rate. So a 40A charger can pull up to about 60A from the starter battery, and the input cable and fuse have to be sized for that.
Smart alternators: many newer vehicles vary the alternator's output with operating conditions, so its voltage can sit lower than a traditional alternator's. Renogy's chargers handle this, but you have to run the ignition (IGN or D+) signal wire so the charger knows the engine is running. Renogy's quick guide for the 20A/40A charger gives the thresholds: with a traditional alternator it starts charging at 13.2V and stops at 12.7V. In smart-alternator mode it starts at 12V and stops at 11.5V.
Step 2: decide how fast you need to refill
A rough refill time is the amp-hours you need to put back divided by the charger's amps. Charging slows near full, so treat these as best-case numbers:
| House battery (lithium) | To replace (20% to 100%) | 20A | 40A | 50A | 60A |
|---|---|---|---|---|---|
| 100Ah | 80Ah | 4.0 h | 2.0 h | 1.6 h | 1.3 h |
| 200Ah | 160Ah | 8.0 h | 4.0 h | 3.2 h | 2.7 h |
| 300Ah | 240Ah | 12.0 h | 6.0 h | 4.8 h | 4.0 h |
If you drive an hour a day and use about 50Ah overnight, a 40A or 50A charger covers it. If you mostly drive to a site and then park for a week, size the charger for the battery and let solar do the daily work.
Also check that the charger doesn't exceed what your battery will accept. Most Renogy 12V house batteries we sell accept 100A or more of continuous charge. The Core Series 50Ah is rated for 50A, so a 60A charger is too much for it.
Step 3: alternator only, or alternator plus solar?
Some Renogy chargers also take solar input through a built-in MPPT controller, so one box covers both charging sources. That saves a separate controller and a second set of cable runs. Each model has its own solar limit, and these come from Renogy's manuals and product pages:
| Renogy charger | Charge current | Solar input | Good to know |
|---|---|---|---|
| DCC 20A / 40A (Gen 2) | 20A or 40A | None | 300W / 600W max input; charge current adjustable in the Renogy app |
| DCC30S / DCC50S | 30A or 50A | Up to 400W (30A) or 660W (50A) | Charges from solar first, then adds alternator current when needed |
| 40A DC-DC with MPPT | 40A | 13V to 30V, up to 670W (manual) | 616W nominal output, IP31, button and LED setup, no screen |
| IP67 50A dual input | 50A at 12V (solar charging 25A max on 24V) | Up to 720W, 15V to 50V | Waterproof; works with 12V or 24V on either side |
| DCC 60A | 60A | None | 750W rated; a current-limit wire halves output to 30A |
For the DCC30S/DCC50S, Renogy's manual lists the maximum solar input voltage as 30V (30A model) and 50V (50A model). The 50A product page on renogy.com shows 25V, so check your panel's open-circuit voltage (Voc) against the lower figure, or ask us before wiring panels in series.
Step 4: budget for the wiring and fuses
A DC-DC charger needs a fuse on both the starter side and the house side, plus cable heavy enough for the run. A few examples from Renogy's manuals:
- DCC30S / DCC50S: Renogy's manual lists a 40A (30A model) or 60A (50A model) ANL output fuse in the box. On the starter-battery side it recommends a 45A to 60A fuse for the 30A model and 75A to 100A for the 50A. The 50A model wants 8 AWG for runs up to 10 ft, 6 AWG for 11 to 20 ft and 4 AWG for 21 to 30 ft.
- DCC 20A / 40A (Gen 2): 10 AWG (20A) or 8 AWG (40A) to the house battery, and 8 AWG (20A) or 6 AWG (40A) from the starter battery. Renogy bases those figures on runs of about 3 m (10 ft) or less.
- IP67 50A: a 70A ANL fuse on both battery connections, 8 AWG battery cables up to 26 ft (total of both cables), and 10 AWG from the panels.
Mount the charger close to the house battery. In a trailer that means a long run back from the tow vehicle. Thin 7-pin charge wiring can cap what any DC-DC charger delivers, so check that circuit before you buy a big unit.
Which one should you buy?
- Van or truck camper, 100Ah to 200Ah, separate solar controller already fitted: the DCC 40A, or the 20A if your alternator is small.
- New build that needs both alternator and solar charging: the DCC50S (up to 660W of solar), or the DCC30S for arrays up to 400W.
- Engine bay or exposed locker: the IP67 50A is the waterproof option and also suits 24V systems.
- Big lithium bank and a big alternator: the DCC 60A. Its current-limit (LC) wire drops it to 30A, so you can protect the alternator on hot days or at idle.
Not sure your system adds up? Our RV solar and battery calculator sizes the battery, solar and charging together. You can also browse the full Renogy range. Adding a battery too? See Renogy lithium batteries compared. Found it cheaper? We'll beat it.
FAQ
Can I buy a bigger charger and turn it down?
Yes, on some models. The Gen 2 DCC 20A/40A lets you set the charge current in the Renogy app, and the DCC 60A halves its output to 30A when you connect its LC terminal to 12V. That is a good approach if you plan to upgrade the alternator or battery later.
Do I still need a DC-DC charger if I have solar?
Often, yes, if you drive most days, camp in shade or travel in winter. See Do I need a Renogy DC-DC charger if I already have solar?
Renogy or Victron Orion?
If you want one box for alternator and solar, Renogy's MPPT models are the simpler install. If the rest of your system is Victron, see Renogy DC-DC charger or Victron Orion-Tr Smart.
Will a 20A charger hurt a small alternator?
Renogy doesn't give a small-alternator figure for the 20A. Its Gen 2 quick guide asks you to confirm with the vehicle maker that the alternator is within the 720W, 75A to 100A range before installing. If yours is older or already powers a lot of accessories, check its rating first, or ask us.