Because of their electronic nature they are limited to a rated amount of current1) which can protect your alternator from damage. This can be especially important with Lithium and AGM batteries that can demand huge loads from the alternator when discharged.2)
Charger throughput is generally limited to 20-30A3) since higher rates would dictate $$$ internal components, whereas plain isolators might pass as much current as the alternator, battery, and wiring could handle. This isn't as big a limitation as it might seem since lead-acid current acceptance drops substantially throughout Absorption, and lithium owners often limit charging to C/5 for maximal longevity.
Untested theory: b2b chargers might work well beside normal isolators; this is essentially the $$$ 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. Some exceptions do exist; Sterling units are rated on the input (power taken from the alternator).6)
Bigger is not always better, since battery types accept current at different rates and buying more charging capacity than the battery wants7) or alternator can provide is a waste. Assuming a 100Ah battery:
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 Cook8)
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) – HaldorEE9)
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 configurabilty10), 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:
House and starter batteries are combined when the starter battery holds >13.1v for 5 seconds.11) The batteries are isolated when the starter battery is “<12.8V, for 10 sec… or service battery voltage > starter battery voltage”.
The SmartPass accessory and 80A-120A (see below) of alternator-voltage charging to the D250's output when Vbatt is low enough that the alternator can still charge it directly. This combined charging12) will happen in the first half of Bulk mode. As house battery voltage rises to normal alternator voltage the smartpass can no longer contribute. At that point the 20A D250SA continues voltage-boosted charging to Absorption voltage.13)
The smartpass also adds self-jumpstarting.
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 D250-series trickle charges the starter battery from a solar panel at intervals of 3 seconds if the service battery is fully charged.18)
The 250-series will charge simultaneously from solar and alternator up to the 20A output limit. No details are given in the manual.
The D250SA trickle charges the starter battery from a solar panel at intervals of 3 seconds if the service battery is fully charged.19)
Note: a 30A combo model is also available
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 15A21) 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.
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.22)
Kisae makes a DMT1230 DC-DC charger 30A charger with 50v MPPT input and 16v alternator input.23) They also make a 50A model (DMT1250), which charges up to 50A by alternator. Some sources give 50v as max input; check the current manual for your device. Both models list specifiy 500w of panel with up to 20% overpaneling (600w). The Kisae has a minimum solar input voltage of 14.5v.
Customer support is generally regarded as excellent.
By default, when the alternator input terminals detect at least 13.2v the charger will be switched to alternator charging.24) The unit can be set to prefer solar charging instead.
The Kisae does not appear to maintain the starter battery.
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.25)
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 Votronics Triple is DC-DC, MPPT solar, and shore power charging from 110v-230v, 45Hz-65Hz.
The naming convention is shore power amps28) / 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.
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.
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.33)
The starter battery will be maintained with ≤1A current when lead banks are >13.1v, and when lithium banks are >13.5v.34)
Charging a 24v bank from 12v alternator is possible but gear selection is relatively slim.
Victron makes 12/2435) DC-DC chargers with inputs 8-17v and outputs of 20-30v.
Also see the Votronics section above for their DCC 25A 1224 charger.
Observe voltage input maximums on units with built-in solar charge controllers.
Voltage-sensing units36) 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.