In a dual-battery system²⁾ some of the alternator output is used to charge the house batteries. Since the house battery is electrically isolated from the starter battery when the ignition is off³⁾, these are sometimes called battery isolators.

• when batteries are deeply discharged
• when combined with solar or some other higher-voltage charging source
• when used with battery chemistries like lithium that do not need staged charging.
• when used with a battery-to-battery boosting charger that can produce correct charging voltage

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. It is unlikely to get the house battery completely charged unless one is driving significant distances. This can have an impact on battery longevity. If one drives often a combination of the alternator doing bulk charging and a small solar doing the rest is good match.

Charging the house batteries from the alternator increases the load on the alternator and can be expected to contribute to earlier failure. SternWake reports idling while charging causes a sharp increase in alternator temperature.⁴⁾ So don't idle it!

Also see Is Solar Mandatory? for ideas on how to charge primarly by alternator.

Power from the alternator is shared with the house battery using an isolator of some sort. This allows the house battery to charge but does not allow the house battery to pull power from the starter battery.

1. constant-duty solenoid switch - 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.⁵⁾. SternWake recommends the Blue Sea 9012.⁶⁾
   Solenoids can be used for self-jumpstarting if the chassis battery has enough juice to engage the solenoid.
2. voltage sensing relays - this kind of isolator is installed between the house and starter batteries. It does not get trigger voltage from the fuse panel but rather reads the voltages of one or both batteries to know when to switch on.
   This kind of isolator may have a “combine” override function to enable self-jumpstarting.
   1. single voltage sensing - this type reads the voltage of only one battery. In the case of an RV it would read the voltage of the starting battery. When it is high enough (ie, charged or being charged by alternator) it connects the starting and house batteries. Example: Sure Power 1314. [secessus says: “IMO the practical benefit (if any) to charging the starter battery “first” is keeping the load on the alternator reasonable.”]
   2. dual voltage sensing - this type reads the voltage from both batteries and when either is high enough it connects the batteries. This may or may not be what an RVer wants, as it could consume some solar power to to engergize the solenoid and charge the starter battery. It would also backfeed excessive equalization voltages to the coach system. Example: Sure Power 1315
3. DC-DC isolators (aka b2b isolators) that boost charging voltage to Absorption voltages
4. solid state isolator - an electronic device which uses 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 lower-voltage chemistry like LiFePO4. Some solid state isolaters use FETs and diodes in tandem to reduce voltage drop.
   Note: solid state relays can't combine batteries for self-jumpstarting.
5. manual switch - 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.

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.⁷⁾
If an isolator is undersized (less common) it will not be able to carry enough current, resulting in overheating and/or sudden shutdown.

FLA batteries can accept up to C/5 in Bulk stage.

Example: a 200Ah FLA battery bank will pull up to 40A⁸⁾ in Bulk charging. An isolator rated for constant duty at 40A⁹⁾ would be sufficient.¹⁰⁾

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 isolator¹¹⁾ 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.

• Because lithium can use about 80% of it's capacity instead of 50% for lead-acid, 120Ah of lithium is a common replacement for 200Ah lead-acid banks. So the lithium bank would “only” draw 120A instead of 200A.
• drop-in lithium banks like Battleborn have a battery monitoring system (bms) to shut down charging if current exceeds specs.

Sudden disconnection of a large load¹²⁾ when the alternator is running can damage the alternator and any operating chassis electronics. Sudden disconnection can occur when:

• an isolator shuts off due to current beyond it's rating
• a BMS shuts off lithium charging. This can include overvoltage, overcurrent, temperature extremes, etc.

Alternator charging may bring some battery chemistries (like lithium) to unsuitably high voltages. A high voltage disconnect can restrict alternator charging to lower voltages.

Idling the engine to run the alternator can seriously overheat the alternator. It is usually cooled by wind from the vehicle's forward motion; a fan may help cool the alternator.

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.¹³⁾

Note that you will only have to run the POS+ leg of wire to the house battery as the chassis ground is the other leg.

There are ways to get the alternator to pump out more power:

• a higher-output alternator will put out more power
• a different voltage regulator for older vehicles, as demonstrated by SternWake, increases the voltage available for charging but also increases the coach voltage.
• a Sterling “fake load” regulator will cause the alternator to put out more amps and then will DC-DC convert the voltage up to correct charging range.¹⁴⁾. This is the opposite direction of how MPPT charging works. Also see b2b chargers.

A simple possible approach would be to replace the starter battery with a marine or AGM battery.¹⁵⁾

• Split Charging Guide - a British page. Note the following differences in terminology from American English:
  • “split charging” == alternator charging
  • “leisure battery” == house battery