The bottom line is that current simply flows where it is needed, batteries will take what they need when batteries are combined, and the voltage becomes equal among the new combined bank. Unless your charger, alternator or solar/wind system is pumping out an incorrect voltage for you bank you will not over charge using an ACR. mainesail2)
aka Split Charging for our European friends
In a dual-battery system3) some of the alternator output is used to charge the house batteries when the engine is running. When the engine is not running the house battery is electrically isolated from the starter battery to keep from draining it and leaving you stranded. So the devices that handle the isolating/combining duties are called battery isolators. There are several different types.
Normally only the POS4) wire is used to route power with an isolator; the NEG side is the chassis ground. If no chassis ground is available a NEG wire will need to be run also.
Alternator charging is generally only practical for the bulk charge stage due to relatively low voltage output and the long time periods required for absorption.5) It is unlikely to get lead-chemistry batteries fully charged unless one is driving for several hours. Failure to fully charge lead batts regularly6) will impact battery longevity. If one drives often a combination of the alternator doing bulk charging and a small solar doing the rest is good match.
See Is Solar Mandatory? for ideas on how to charge primarily by alternator.
Charging the house batteries from the alternator increases the load on the alternator and can be expected to contribute to somewhat earlier failure. In practice it's usually a non-issue if one avoids overheating8) or overloading9) the alternator.
If/when the OEM alternator does fail10) a higher output one can be installed for not much more than it would cost to replace the original.
SternWake reports idling while charging causes a sharp increase in alternator temperature.11) To avoid this, do your alternator charging while driving so airflow over the hot alternator will help cool it.
High-performing engines with Gasoline Direct Injection can experience carbon fouling on the backside of intake valves, which can cause “drivability woes… misfires” and dislodged carbon flakes can cause “turbocharger issues… catalytic converter damage.”12)
Anecdotally the problem appears to be worsened with excessive idling, although this may be a subset of situations where “the engine doesn’t run long enough or hard enough to get hot enough to burn any of the carbon off.”13)
Traditional alternators typically try to hold a set voltage. Smart alternators talk to the vehicle's ECU (compuuter) and can vary output voltage wildly depending on present conditions. It might unload the alternator during heavy acceleration to reduce parasitic losses, or run the alternator at high voltage just after starting to speed up the recovery of used energy.14)
The use of plain isolators with smart alternators may result in the rapid cycling of the isolator's circuit (with voltage sensing types) or the discharge of the house battery into the starter battery (solenoid type).
DC-DC chargers and/or isolators with an awareness of smart alternators may be required.
Furher reading: Redarc - standard vs smart alternators
Power from the alternator is shared with the house battery using an isolator (“Split charge relay” / SCR) of some sort. This allows the house battery to charge but does not allow the house battery to pull power from the starter battery.
A constant-duty solenoid is an electromechanical device which uses an electromagnet to complete the charging circuit when the engine is running. Solenoids are generally cylindrical. Energizing the solenoid will cause a 0.5A - 1A current drop between the alternator and house battery. Exception: Latching isolators use latches15) instead of electromagnets to hold the circuit closed, eliminating that vector of power consumption.
16). SternWake recommends the Blue Sea 901217) although non-marine units in the $20-$50 range are more common in vans.
Solenoids can be used for self-jumpstarting if the chassis battery has enough juice to engage the solenoid.
voltage sensing relays (VSR, also called Automatic Charging Relays or ACR) are solenoids with a bit of extra logic to know when to connect/disconnect. The VSR does not get trigger voltage from the fuse panel but rather reads the voltages of one18) or both19) batteries to know when to switch on.
This kind of isolator may have a “combine” override function to enable self-jumpstarting.
in its simplest form, all an ACR really does is parallel batteries when charging is present and un-parallel batteries when there is no charging present. It does this automatically with no human forgetfulness.20)
Note: voltages-sensing (with or without delay) can be added to plain solenoids.
Note: this type of isolator is no longer common for our uses.
These isolators are electronic devices which use diodes to prevent depletion of the starter battery. Isolators are generally brick-shaped. Diode-based isolators have a 0.5v - 1v drop between the alternator and house battery. This may be desirable if the house battery is a wants lower-voltage charging like LiFePO4.
This type of isolator is similar to the diode-based one above, except that FET components are used instead of diodes, minimizing voltage drop. They tend to cost 2x as much as the diode versions.
A manual battery switch normally has 4 positions: A, B, A+B, and Off. A would be for the starter battery and used during starting. B would be used for house use when one is not driving. A+B could be used to combine both sets for starting or for charging while driving. This kind of setup is prone to user error. A manual switch has no current or voltage losses.
The Precision Circuits Li-BIM is a lithium-specific isolator with some differences:
[note from secessus: “not sure what's inside these solid state isolators”]
The Magnum Energy ME-SBC is notable for some unusual features:
Xantrex makes a 15A Digital-Echo Charge isolator.
The Mastervolt Charge Mate Pro 90 is an electronic current-limiting isolator.
If an isolator is oversized it will cost more for no benefit and will self-consume somewhat more energy to hold the combining circuit closed.25)
If an isolator is undersized (less common) it will not be able to carry enough current, resulting in overheating and/or sudden shutdown.
Most AGM will pull about C/3 (33A for a 100Ah bank) but premium brands may do more. Flooded lead-acid batteries tend to pull less current (C/5, 20A per 100Ah of bank). If your flooded back will only pull ~40A, or your AGM bank 70A then there is little reason to spend more money on a 150-200A isolator.
Lithium in particular has low internal resistance and can pull 1C (100A for an 100Ah bank) or more. Victron posted a video showing it is possible to “smoke” an alternator while charging lithium. Since lithium does not care much about state of charge, there is little reason to go for maximum force lithium charging. Some Li bank owners use DC-DC isolators which limit themselves to a particular output (20A, 60A, 100A, etc).26)
Reasonable charging rates can also be easier on the alternator when charging suddenly stops, whether by completion27) or BMS intervention. Blue Sea makes an alternator field disconnect which shuts down alternator power just before disconnecting the load, but this may be chiefly applicable to marine alternators. Others have discussed installing a small lead-acid battery parallel to the Li bank; in theory this could soften the blow from Li leaving the circuit. Other sources suggest the presence of the starter battery would be sufficient.28)
Consumer-grade AGM batteries typically will accept C/5 - C/3.
Example: a 200Ah AGM bank will pull up to 67A in Bulk. A 75A isolator32) would be sufficient.
Note: high-end AGM like Lifeline, Odyssey, Rolls, etc, can pull massive current when charging. 200A+ would be possible for the example bank and could shorten the life of a stock alternator.
Lithium also has the ability to accept massive amounts of charging. It will do so across the entire charging range, as lithium does not have an Absorption phase the way lead-acid does.
Example: 200Ah of lithium could easily accept 200Ah+. There are mitigating factors, however.
Sudden disconnection of a large load33) when the alternator is running can damage the alternator and any operating chassis electronics. Sudden disconnection can occur when:
The existence of the starter battery should attenuate the problem but keep the issue in mind.
These side effects are usually unimportant and the average user will likely not notice them. They are listed here for completeness.
2 gauge copper wire connecting the coach and house is recommended for most alternator charging installs. SternWake recommends attaching at the alternator rather than the battery.38)
Note that you will only have to run the POS+ leg of wire to the house battery as the chassis ground is usually the other leg.
There are ways to get the alternator to pump out more power:
A simple possible approach would be to replace the starter battery with a marine or AGM battery.40)
Some amount of power can be passed along the 7pin harness, usually enough to maintain the trailer battery's voltage and run small loads. For the purposes of this discussion the important wires in the 7-pin are:
The minimum size for these wires is 12ga and some heavier models use 8ga.
A configurable DC-DC charger might be able to pass high enough voltage to overcome sag. If the 7-pin is the conduit then we are still limited to the currents listed above.
The basic idea is alternator → DC boost to 36v or something → run down the 7pin charging wire to the trailer → MPPT charge controller → battery
Using 12ga wire as an example, 5A @ 13.6v = 68w. After the same 3% voltage drop and MPPT conversion losses the boosted setup would deliver 166w, and be able to “smart charge” the trailer battery at appropriate voltage.
It's also possible to run a separate and heavier cable from the TV to the trailer; this would minimize voltage sag. If a plain isolator is used with heavier wiring the voltage will still be insufficient to fully charge lead batteries.
Heavier cabling + a DC-DC charger could provide correct voltage to the trailer battery.