The major downside to DC-to-DC chargers is cost, especially when compared to how many amps they can pass when compared to plain isolators might pass as much current as the alternator, battery, and wiring could handle. This isn't as big a limitation as it might seem since lead-acid current acceptance drops substantially throughout Absorption, and lithium owners often limit charging to C/5 for maximal longevity.
However, for those with absurdly gigantic (like 10kWh+) battery banks, alternator setups, and wallets, DC-DC chargers up to 200amps are available. [Untested theory: b2b chargers might work well beside normal isolators; this is essentially the $$$ CTEK SmartPass setup described below.]
In addition, even DC-DC chargers can fail to charge lead batteries fully. Lead-acid batteries require two things to get fully charged:
DC-DC chargers are often triggered by the IGN circuit, but voltage sensing can be added to that circuit. Others are triggered by chassis-side voltage alone.
DC-DC chargers typically do not support self-jumpstarting, although the need for jumpstarting may be reduced with combo units (below) that maintain the starter battery from solar power.
Dual-input (alternator + solar) units make charging source decisions that are sometimes quirky. See below.
DC-DC chargers come in different ratings (10A, 40A, 60A, etc) which increase significantly in price as the rating increases. In general, the stated rating is on the output (battery charging) side.3) Since DC-DC chargers are usually boosting (increasing) voltage, a DC-DC charger will typically pull somewhat more current from the chassis than it provides to the house battery.
Example:4) - Remembering that W = V x A, 40A @ 14.4v (576w) to house battery would be
The DC-DC should be big enough to meet your charging targets given the amount of driving you do and the contributions of other forms of charging. It should be small enough that the alternator can provide the power continuously.5) An oversized DC-DC charger cannot protect the alternator.
Note that bigger is not always better; different battery chemistries and capacities accept current at different rates. Buying more charging capacity than the battery wants6) is a waste of money. Buying more charging capacity than your alternator can deliver constantly can overheat/damage the alternator.
Assuming a 100Ah bank:
Isolated DC-DC chargers have negative terminals output side (battery bank) “galvanically isolated” from the input side (alternator). It is not needed for most use cases:
Generally speaking, one only needs to isolate the negatives when the installation does not share a common negative return path, such as may be the case on many fiberglass boats that don't have a chassis and may not have a common grounding bus to which all their battery banks connect. – Justin Cook9)
But note that a trailer does not always have common grounding:
Galvanically isolated is a nice thing to have, especially if you are charging house batteries in a trailer from the tow vehicle engine. The issue relates to how the house battery is grounded to chassis ground when the tow vehicle is disconnected. All in one RVs and vans don't have this issue (probably) so I would not expect them to need galvanic isolation. (emphasis added) – HaldorEE10)
Combining the solar charge controller and DC-DC charger in one unit can have benefits in cleaner/simpler installation, starter battery maintenance from solar, and perhaps cost over separate components. The downsides are less configurabilty11), single points of failure, and limited options on solar input.
Combo units typically have much lower maximum panel input voltages (as low as 23v) than a standalone charge controller would (60v - 150v). This limits the solar configuration in a few ways:
With the bluetooth dongle and app installed13), the profiles can be adjusted and power output reduced in 10A increments.
This device's behavior regarding solar+alternator charging is widely misunderstood.
The 50A dual-input charger prefers to charge by solar if possible, up to 50A. If it cannot meet charging demands alone and the engine is running it will will charge up to 25A by solar and 25A by alternator:
Using the above information and a theoretical scenario where we have 15A14) of solar charging available during the daytime, an alternator capable of delivering 50A, and a bank able to accept 50A of charging:
Behavior is the same for the 30A model (pdf) except 15A + 15A vs 25A + 25A above.
UPDATE: a Renogy comment on this video suggests the DCC series switches to alternator-only “when the PV is below around 15V, it charges at full capacity from the [alternator]”.
Note: some have reported better alternator charging performance with the D+ (smart alternator) wire connected, even with plain alternators. Oversizing cables from the alternator may also help. 15)
Because of the behavior described above, people who want maximum alternator charging sometimes add a way to disconnect solar panels. This is typically done by adding some kind of disconnect on the panel POS wire going to the solar charge controller:
If the service battery is in float charge stage, the starting battery will be charged at the same time. The charging voltage is limited at 13.8V. The charging amperage is limited at 25A.16)
Note: since the Renogy Lithium profile doesn't have a float stage starter battery maint may not be available in that profile.17) At least one report claims the starter battery is maintained in the Li profile.18)
Further reading: a post describing functionality of the charger
Note: some manual versions brand this unit as part of the “Rego” line.
The RBC series 50A charger has a 50V PV input limit and built-in bluetooth. It can also boost 12v panel voltage to charge 24v banks. A RBC30 is reported to exist.
The existing manual is minimal and specs/behavior are largely undocumented as of Jan 2024.
The lithium charging logic is charge-and-stop:
Customer support is generally regarded as excellent.
When the alternator input terminals detect at least 13.2v the charger will be switched to alternator charging.26) This appears to be hardcoded:
With two inputs available, the house battery will be charged from either the engine while underway, or via the solar panels when stationary. The process to choose either engine or solar is fully automatic… – source (pdf)
The Kisae does not maintain the starter battery. However, the manual gives instructions on feeding power to the starter battery when that battery's voltage is too low:
For DMT1230, 1250 using CH 2 (Solar Array/Panel): Turn unit off. Use a proper jumper to temporary connected CH2 “+” to Ch 1 “+”. Remove the jumper when CH 1 is above the unit start up voltage (8Vdc) and restart the unit.
This unit also seems to be sold under the Ring and Merlin brands.
SRNE makes 30A and 50A combo unit similar to the Ring and Kisae. Since SRNE makes things that are rebadged by other sellers it is possible they are the original manufacturers. The BT dongle appears to be the same as the EpEver and Renogy models.
The manual claims the charger can charge from both sources at once (“hybrid” or “Mode 3”); this may require setting the N13 value to
Like the Renogy DCC30 and 50, when charging from both sources each can only output 1/2 the rated current (25A solar, 25A alternator, for example).28)
The brochure claims the charger will maintain the starter battery from solar (“Mode 4”), apparently at 13v. Starter batt charging current ≤15A with the 30A model and ≤25A with the 50A.
Rich makes a 50A DC-DC w/MPPT. It appears to be very similar to the SRNE listed above.
Configurable PV or alternator priority. The manual mentions charging from both at the same time and the firmware has an N13 setting like the SRNE above.
When both sources are active it will likely be 25A+25A29); see notes on SRNE above.
13.8v to the starter battery, at up to 1/2 the rated current.
Redarc “dual input” DC-DC chargers have integrated MPPT controllers. They appear to do both buck and boost conversion as solar input voltage is 9v-32v.30)
These units do not appear to maintain the starter battery from solar. Redarc does document how to insert a switched relay in between the starter and aux batts to self-jumpstart.
Note: the Ring appears very similar to the Kisae (above).
The manual shows “boost” (lower) and “charging” voltages (higher) and “bulk” voltages. The charging diagram elsewhere in the manual does not use these terms.
Unclear; the manual does say:
The charger will shift to solar charge mode when the input terminal is connected to the solar panel
…which suggests the unit goes to solar-only mode when panel voltage is detected.
The Sterling BBS1230 Pro Batt Ultra is a 30A boost/buck 30A charger. The solar input max is 31V and 16A with a 350w output maximum. Alternator input voltage is 11v-20v and 30A.
The solar and b2b charging can be turned off separately.
The charger charges only from alternator when engine is running:
When your engine is running, the BB aspect of the BBS charges. When the engine / ignition is off, the solar charger works. They do not work simultaneously.
the manual for the pre-2022 version incorrectly claims the unit maintains the starter battery.
The Votronics Triple is DC-DC, MPPT solar, and shore power charging from 110v-230v, 45Hz-65Hz.
The naming convention is shore power amps33) / DC-DC amps / solar wattage
Max solar input voltage for all models is 36Voc. Minimum panel wattage is 50w-60w depending on model.
Solar can be prioritized over shore power charging. The Solar Operation section of the manual suggests that alternator charging has priority over solar. The default priority appears to be:
The starter battery will be maintained at ≤5A by solar or mains charging. Max input voltage is 32v.
Note: the LiFePO4 profiles charges to 14.5v and floats at 13.6v. The STD and GEL profiles have slightly lower voltages and may be worth considering.
The default behavior appears to be:
if solar input power is strong enough. When solar input cannot provide enough energy to charge the battery, the Battery Charger will draw power from the alternator.
There is a solar priority mode:
Press the Solar Priority button to prioritize input from the solar panel over charging from the alternator… As long as the solar panels produce more than 25W (for 7410) or 50W (for 7411) of power, the unit will select charging by solar. If the auxiliary battery decreases below a certain level, the unit will charge by alternator regardless of solar input.
This mode appears to be:
Solar or alternator, with alternator priority:
Our charger utilizes the DC source before tapping into solar power36)
Reverse Charging Ready: Don't let a flat starter battery hold you back! Our charger swiftly breathes life back into your battery to get you back on the road in a flash.37)
[Note from secessus: output is reported to be low and it's unconfigurable other than for battery type, but it's 1/2 the cost of similar combo units. Pick your poison]
At 0:10 of this video it claims the unit charges from one source at a time:
at a time, leisure battery can only be charged by one of them
at 0:30 in the video it displays the text ALTERNATOR CHARGING PRIORITY and the voiceover seems to confirm this.
There appears to be no starter battery maintenance.
AK makes a 25A combo unit with a
no information available
no information available
House and starter batteries are combined when the starter battery holds >13.1v for 5 seconds.40) The batteries are isolated when the starter battery is “<12.8V, for 10 sec… or service battery voltage > starter battery voltage”.
The MPPT charger will charge the house battery at up to 20A. CTEK specifies 50-300W of panel, though the 23v Vmax limits the unit to nominal 12v panels. Note: polycrystalline panels may be more useful here, as their Voc should be a bit lower than mono.
The 250-series will charge simultaneously from solar and alternator up to the 20A output limit. No details are given in the manual.
The SmartPass accessory adds up to 80A-120A (see below) of alternator charging to the D250's output under certain conditions for a theoretical charging max of 100A-140A, depending on version:
The smartpass also adds a form of self-jumpstarting.
Related: see this article on DIY “smartpass” functionality
The D250SA trickle charges the starter battery from a solar panel at intervals of 3 seconds if the service battery is fully charged.48)
The Sterling Pro Batt Ultra series can be configured to exact setpoints. The BB1260-12 is a 30A example (manual) and costs about $50 more than the D250S above. Note that Sterling rates DC-DC chargers on input, not on output.
The BB1240 has a starter battery maintenance (reverse charging) feature. This might be true for all the 2022 (“red case”) redesigns.
DC-DC chargers in 20A, 40A, and 60A output. Switched on by ignition circuit on the D+ port, low output mode (12.5% of rated current) triggered by 12v on a dedicated terminal. 49) DIP switches for battery type, Vabs and Vfloat. manual (pdf) teardown Note: some versions of the manual may have a typo.
These chargers appear to have a 3 hour absorption duration. This is usually insufficient for lead chemistries, but still longer than the 2 hour duration of the DCC series above. From the manual:
By default, absorption will not exceed 3 hours to prevent overcharging.
60A DC-DC with different form factor and a configurable USER profile. Important differneces from teh 1212 line above:
They have modes for “AGM, gel, calcium content, VRLA and standard lead acid batteries”.
Victron makes several DC-DC chargers, some of which are configurable by bluetooth. 12v examples:
See information on isolated v. non-isolated chargers above.
Orions tend to run hot50) so mount in an area with sufficient ventilation.
This YT video covers Orion configuration in considerable detail.
Bogart's SC-2030 can work as a DC-DC charger as well as a PWM charge controller.
Votronics makes a range of standalone DC-DC chargers
When sensing voltage it connects at >13.3v, reduces charging at <13.0v, and disconnects at <12.9v after 30 seconds.53)
The starter battery will be maintained with ≤1A current when lead banks are >13.1v, and when lithium banks are >13.5v.54)
The Sterling A2B uses DC-DC conversion to boost voltage but has no ability to limit current.
Charging a 24v bank from 12v alternator is possible but gear selection is relatively slim.
Victron makes 12/2455) DC-DC chargers with inputs 8-17v and outputs of 20-30v.
Also see the Votronics section above for their DCC 25A 1224 charger.
Sometimes we want to replace an existing relay charging setup with DC-DC. There are some things to consider
For these reasons it is sometimes preferable to leave the relay in place and install the DC-DC downstream from it:56)
starter battery -> relay -> POS wire to the living area -> DC-DC charger -> battery bank
Caveat: if the DC-DC has a starter battery maintenance function and you want to use that then the relay should be removed. Otherwise the circuit will be broken by the relay with the key off so power cannot flow back to the starter battery.
DC-DC chargers are typically rated on their output power, which means they can pull more than their rating from your alternator. This could be a consideration if you are running a small alternator and relatively big DC-DC.
Example: Relatively low chassis voltage, 13.4v. Relatively high charging voltage, 14.8v.57). 50A output @ 14.8V requires 57.98A from the alternator after DC-DC boosting and losses.
Carefully observe voltage input maximums on units with built-in solar charge controllers. If you will be in freezing conditions allow headroom to account for the effects of temperature on solar cell voltage.
Voltage-sensing units58) can misbehave when an undersized starter battery charger/maintainer is attached. For this example we assume a 10A maintainer and a 20A DC-DC charger.
The workaround is to either charge the house battery first or to buy a big enough starter battery maintainer to support both the starter battery and DC-DC charger loads.
DC-DC charger (B2B) options and general discussion on DIY Solar Power Forum.