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Words of Wisdom: “With a isolator you would run the truck early to get a fair amount of the bulk charging done and let the solar finish it off the rest of the day.” – jimindenver1)

Alternator charging

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 alternator charging works well


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.4) 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.

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

effect on alternator

Charging the house batteries from the alternator increases the load on the alternator and can be expected to contribute to somewhat earlier failure. The shortened life may be trivial if alternator charging is done thoughtfully, and may be sudden if done without considering the effects of higher alternator loading.

SternWake reports idling while charging causes a sharp increase in alternator temperature.5) To avoid this, do your alternator charging while driving. This will ensure airflow over the hot alternator.

Lithium in particular has low internal resistance and can pull 1C (100A for an 100Ah bank) or more. 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). Victron posted 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 charging. Some Li bank owners use DC-DC isolators which limit themselves to a particular output (20A, 60A, 100A, etc).6)

Reasonable charging rates can also be easier on the alternator when charging suddenly stops, whether by completion7) 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.8)


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.9). SternWake recommends the Blue Sea 9012.10)
    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.

sizing an 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.11)
If an isolator is undersized (less common) it will not be able to carry enough current, resulting in overheating and/or sudden shutdown.

flooded lead-acid

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

Example: a 200Ah FLA battery bank will pull up to 40A12) in Bulk charging. An isolator rated for constant duty at 40A13) would be sufficient.14)

AGM lead-acid

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 isolator15) 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

Sudden disconnection of a large load16) 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.17)

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.

alternator hacks

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.18). This is the opposite direction of how MPPT charging works. Also see b2b chargers.

using the coach battery only

A simple possible approach would be to replace the starter battery with a marine or AGM battery.19)

further reading

  • Split Charging Guide - a British page. Note the following differences in terminology from American English:
    • “split charging” == alternator charging
    • “leisure battery” == house battery
2) starter battery and house battery
3) or house battery voltage is below a given setpoint
4) DC-DC chargers can help with the voltage
7) same reason headlights are turned on at the donor car when jumpstarting
11) an electromagnet holds the parts of the active circuit together. When power to the inolator input is cut the electromagnet can no longer hold the circuit closed. The circuit is open and the batteries are isolated from each other.
12) 200Ah/5
13) likely with peak tolerance of 60A or so
14) assuming you aren't applying heavy loads like a microwave while driving
15) 100A peak
16) including a charging battery
electrical/12v/alternator.txt · Last modified: 2019/10/17 10:54 by frater_secessus