User Tools

Site Tools


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

Note: this is a basic overview. More details available on the alternator details page.


The vehicle's alternator is designed to turn some of the engine's mechanical power into electrical power in order to

  1. recharge the vehicle's starter battery after starting the engine
  2. run electrical accessories

dual-battery systems

We can, within limits,2) use this power to charge our batteries or run our electrical loads. This page is about how to do that with a dual-battery system (i.e. starter battery + house or aux battery).

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 sometimes called battery isolators.

The setup is typically:

  starter battery -> fuse -> wire -> isolator -> wire -> house battery

We might call them “isolators” generically but there are three kinds of devices used to charge from alternator:

  • combiner - which parallels the starter and battery bank together under certain conditions. Inexpensive, historically most common. Includes manual switches.
  • DC-DC charger - includes electronics to adjust voltage and current. More expensive, increasingly common
  • actual isolators - which split alternator power and distributes it separately to starter and house batteries.4) Uncommon these days, but could be useful in some scenarios where some amount of voltage and some current attenuation are desirable.

when alternator charging works well

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.5) mainesail6)


With lead chemistries, alternator charging with isolators is generally only practical for the bulk charging due to relatively low voltage output and the long time periods required for absorption; DC-DC chargers can help with the voltage can address the voltage issue. Without DC-DC it is unlikely to get lead-chemistry batteries fully charged – they require several hours of charging at highter voltages than most alternators put out. Failure to fully charge lead batts regularly7) 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.

Isolators have no way to limit current other than hitting their max rating and failing or tripping.8) This might mean the battery can pull more current than is good for itself or for the alternator. This typically is an issue only with oversized banks or small alternators.

Vehicles with smart (variable voltage) alternators may not be suitable for charging with normal isolators.9) See below.

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

effect on alternator

{WARNING from secessus: idling to charge can cause alternator temperatures to spike, damaging the alternator or its diodes. So don't do that. }

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 overheating10) or overloading11) the alternator; alternator failures from aux battery charging are quite rare.

I have created hundreds of designs and installed around 100 systems, many with isolators or solenoids. In four plus years not one customer has come to me saying that their alternator failed. I do tell them not to sit in a hot parking lot idling their engine to charge their batteries. – jimindenver12)

But see this cautionary tale of using a 60A (!) DC-DC charger to charge 200Ah of AGM from a 145A alternator.

If/when the OEM alternator does fail13) a higher output one can be installed for not much more than it would cost to replace the original.


Be aware of how heat affects the alternator and its health:

  • alternators are about 50% efficient; making 400w of power means it's also making 400w of heat
  • heat is hard on internal components; excess heat can cause derating or failure
  • idling greatly reduces the ability of the alternator shed this heat
    • the alternator's own internal fan14) is running slower
    • ambient temperatures under the hood are higher; there is no forward motion to introduce cooler air through the car's grille

If the bank is slurping a lot of current and you are stuck in traffic on a hot day it might be a good time to disable alternator charging (see below).

SternWake reports idling while charging causes a sharp increase in alternator temperature.15) To avoid this, do your alternator charging while driving so airflow over the hot alternator will help cool it. Other measures included additional alternator cooling or pulley size tuning to alternator RPM at idle.

In a comment on an excellent video about charging lithium from the alternator WorkingOnExploring talks about adding overtemp protection with:

a KSD9700 120C, normally closed thermal switch epoxied to your alternator case. That way, if some abnormal event occurred and the alternator overheated, it could deactivate the [combiner] till the alternator cooled. It would likely cost less than $20 to install this thermal safety.

charging current patterns

  • charging lead chemistries directly from the alternator tends toward
    • high initial inrush currents
    • then a linear taper of current as the bank comes up to alternator voltage16)
  • charging lithium batteries directly from alternator tends toward
    • high initial inrush currents when SoC is low, then
    • middling and gradually-tapering currents across the middle 80%, then
    • very low currents as the bank approaches full charge17)
  • charging lead with DC-DC tends to be
    • rather flat at the charger's rating until
    • Absorption voltage is achieved, at which point it begins a linear taper.
  • charging lithium with DC-DC tends to be
    • rather flat and dictated by the charger's rating.
    • At the tail end of charging will taper off.

alternator current rating

In general, vehicles with higher-rated alternators (150A, for example) will handle a given load better than vehicles with lower-rated alternators (60A, for example). The rating in Amps will be listed on the window sticker, often on the alternator housing itself, or can be looked up using a VIN decoder for your automaker.

see this related article on assessing how much current you can safely take from the alternator

effect on GDI engines

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.”18)

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.”19)

smart alternators

Traditional alternators typically try to hold a consistent voltage.20) Smart alternators talk to the vehicle's ECU (computer) and can vary output voltage wildly moment by moment 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.21)

When the smart alt goes low voltage the normal relationship between the two systems (higher-voltage chassis charging lower-voltage house battery) is disrupted. It can result in rapid ON/OFF cycling of the isolator (voltage sensing types) or the discharge of the house battery into the starter battery (solenoid type triggered by D+).


  • the usual solution is to use a DC-DC charger with an awareness of smart alternators.22) The DC-DC is already upconverting chassis voltage to house battery charging voltage, and does not mind unusual source voltages from the alternator.
  • Some smart alternators can have the “smarts” disabled or tricked into producing higher voltage; this depends on the alternator/vehicle.
  • in extreme situations the OEM alternator might be replaced with a traditional one, or a dual alternator setup (one smart, one traditional) might be engineered

further reading on smart alternators


“Split charge relay (SCR)”, “split charger”, “automatic charge relay (ACR)”, “Voltage sensing relay (VSR)”, solenoid, relay, etc.

Power from the alternator is shared with the house battery by paralleling the two sets of batteries at certain times. This allows the house battery to charge but does not allow the house battery to pull power from the starter battery when not combined.

constant-duty solenoid 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 latches23) instead of electromagnets to hold the circuit closed, eliminating that vector of power consumption.24). SternWake recommends the Blue Sea 901225) 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.

See this video that shows the theory and practice of how these relays work.

Note: some solenoids only have three terminals: 2 big load terminals and 1 small control terminal. This type gets the “ground”26) through the body of the solenoid.

starter relays vs constant duty relays

While they may be externally identical, starter relays and constant duty relays are built differently inside.27)

The starter relay needs to switch huge currents for brief amounts of time. The switching has to be very fast and powerful to minimize arcing. The solenoid will pull several amps to run the powerful electromagnet. The solenoid will not overheat because it is only “on” for a few moments. The control terminals typically have resistance of 3-4 ohms.

The constant duty relay runs constantly and is rated for less current. The electromagnet typically draws <1A and the control terminals have resistance of 15-30 ohms.

voltage-sensing relays

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 one28) or both29) 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.30)

starter battery priority

Some VSR have a feature where they delay the connection a few seconds until the starter battery has recovered a bit from starting the engine. Often misunderstood as “charging house batteries after the starter battery is fully charged”, the typical criterion is chassis voltage of ≥13.4v. Contrary to common belief the starter battery is not fully charged at that point but the current inrush to it has settled down enough that the alternator can do other things.


  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 above resting voltage (ie, being charged by alternator) it connects the starting and house batteries.
    [secessus says: “IMO the practical benefit (if any) to charging the starter battery “first” is keeping the load on the alternator reasonable. In practice, the isolator generally connects the two within a few seconds.”]
  2. dual voltage sensing - this type reads the voltage from both sides and when either is high enough it connects the batteries. This may or may not be what an RVer wants
    • Blue Sea - unless otherwise noted, these ACR combine at 13.0v after 120 seconds or 13.6v after 30 seconds. Disconnect at 12.75v after 30 seconds.
      • SI-ACR - 120A. SI is for starter isolation, a way to prevent unintentional self-jumpstarting. If the SI post is wired to the output of the starter relay37) the ACR will be disabled whenever the starter itself is engine. SI is used mainly in the marine world to minimize the effect of house bank voltage sag during combined starting; this protects marine nav, communications, and similar electronics. If the SI post is not used it acts like a normal dVSR.
      • BlueSea BatteryLink 7611 - 120A. Connect 13.0v38), disconnect 12.75v39). If “auxiliary battery priority” is triggered the cut-out voltage drops to 12.25v. This can be triggered by D+ so it's only active when the engine is running. It can also be always-on from 12v, in effect acting like an LVD. manual
      • BlueSea ML-ACR 7620 - 500A. This is a latching relay which allows the solenoid to de-energize after changing state. This minimizing parasitic losses40) and heat. The ML has Starter Isolation as described on the SI-ACR. Allows self-jumpstarting. Cut in and cut out voltages are slightly different; see the manual.
      • BlueSea ML-ACR w/manual controll 7622 - 500A. Version of the ML-ACR above with a manual control knob41) to override isolation or combination if desired.
    • Precision Circuits BIM. Latching relays in 160A and 225A versions. . Disconnects after 1 hour max to prevent overcharging: “The BIM does not guarantee 100% battery charge, but prevents harmful battery charge levels.43) Sig terminal can use used for self-jumpstarting.
      Also see Li-BIM below.
  3. DC-DC isolators (aka b2b isolators) that boost alternator voltage to more appropriate levels for lead chemistries and can do multistage charging. These have their own page.

Note: voltages-sensing (with or without delay) can be added to plain solenoids.

lithium-specific VSR The Precision Circuits Li-BIM is a lithium-specific dVSR isolator with some differences:

  1. the VSR circuit disconnects at 13.4v when there is no charging occuring on the alternator side. This eliminates the “gotcha” of the circuit staying closed due to high Vbatt.
  2. the isolator opens the circuit (disconnects) regularly to allow alternator cooling. Charge for 15 minutes, disconnect for 20 minutes, repeat. So the BIM is charging ~43% of total runtime.
  3. the isolator disconnects when it detects >= 14.4v on the alternator side, to avoid overcharging

The unit supports self-jumpstarting but a switch must be installed by the user. 160A and 225A models are available. BB suggests the BIM is recommended for lithium banks >= 300Ah.44)

An interesting teardown of the Li-BIM can be seen in this video.

manual switch simplest and least-featured isolator is a 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.

proper isolators

solid state isolator: diode-based

Note: this type of isolator is no longer common for our uses.

These are proper “isolators” and never combine the starter and house batteries. The isolator is a Y connection that receives power from the alternator and distributes it separately to the starter battery and house battery.

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. The slightly-lower voltage will also reduce charging current somewhat.



  • solid state relays typically can't combine batteries for self-jumpstarting
  • the unidirectional nature of the diode isolator may be desirable to prevent “backflow” of higher voltage from the house bank to the chassis.
  • diode-based isolators are typically installed between the alternator and starter battery. This is in contrast to solenoids and VSRs which can be daisy-chained off the starter battery. The batteries are, in effect, always isolated and never electrically combined.
  • It would be possible to insert a two-terminal diode isolator between the starter and house batteries
    • starter batt –> LVD or IGN –> relay –> diode isolator → house battery.
    • Ctek Smartpass (see below) is a commercial packaging of this idea
  • some “1-wire self-exciting” alternators46) require a diode isolator with an Alternator energize feature.47)
  • some diode-based isolators (Victron, see above) have a feature to slightly tweak alternator voltage upwards to compensate for voltage drop across the isolator

solid state isolator: FET-based

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.


  • the Ctek SmartPass.48) is a MOSFET-based isolator with additional features/logic:
    • Connect at >13.1v, disconnect at 12.8v, or 11.8v / 11.4v with smart alternators.49)
    • Battery overtemp protection at 140F.
    • Starter assist when starter batt voltage ~6v.
    • Will trickle charger starter batt when house battery is charging from other means.
    • See this article about DIYing a similar solution.


[note from secessus: “not sure what's inside these solid state isolators”] The Magnum Energy ME-SBC is notable for some unusual features:

  • configurable connect/disconnect setpoints
  • ability to drive a solenoid, which allows for much greater current

Xantrex makes a 15A Digital-Echo Charge isolator.

The Mastervolt Charge Mate Pro 90 is an electronic current-limiting isolator.


how to choose

For many50) use cases a plain constant-duty solenoid triggered by an ignition circuit will augment aux battery charging nicely. It can deliver large amounts of current when battery state of charge is low, and is quite inexpensive. The wiring might cost more than the solenoid.

When access to an ignition circuit is impractical, a voltage sensing relay will do the job, no external trigger required.

In some cases a DC-DC charger is preferable or mandatory:

  • when using alternator as the sole form of charging lead chemistries
  • when the vehicle has a “smart” alternator51)
  • when using an alternator to charge large aux battery banks that may strain the alternator
  • when alternator and aux battery bank voltage are different (12v alternator and 24v bank, for example)

See the gotcha section below to see if there are hidden traps in your intended use case.

sizing an isolator

If an isolator is oversized it will cost more for no benefit, will self-consume somewhat more energy to hold the combining circuit closed,52) and may take more physical space.
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 when deeply discharged (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. In practice, they tend to pull about the same as AGM.

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).53)

Reasonable charging rates can also be easier on the alternator when charging suddenly stops, whether by completion54) 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.55)

flooded lead-acid

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

Example: a 200Ah FLA battery bank will pull up to 40A56) in Bulk charging. An isolator rated for constant duty at 40A57) would be sufficient.58)

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 isolator59) 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 has the potential to accept massive amounts of charging, up to 1.0C. All other things being equal, heaviest current will be pulled when battery bank voltage is the lowest.60)

There are mitigating factors that tend to reduce current in real world use:

  • resistance in the charging path often limits lithium charging current to ~0.5C.
  • Because lithium can use about 80% of it's capacity instead of 50% for lead-acid if one is upgrading to a LiFePO4 bank with the same usable Ah the Li bank will be ~0.6 the rated capacity of the lead it replaces.
  • Li voltage remains in the thirteens for most of its usable capacity, reducing the voltage delta mentioned above.

sudden disconnection

Note: this is an issue for rigs with secondary alternators dedicated to charging a battery bank. In normal61) setups the vehicle's starter battery acts as a buffer to cushion sudden disconnects.

Sudden disconnection of a large load62) when the alternator is making substatial power can damage the alternator and chassis electronics. Sudden disconnection can occur when:

  • an isolator shuts off for whatever reason
  • a BMS shuts off lithium charging. This can include overvoltage, overcurrent, temperature extremes, etc.

externally-regulated alternators

It's more common in marine setups than vehicles, but external regulators can be used to trick the alternator into outputting specific non-OEM voltages63). Balmar appears to be the industry leader in external regulation.

Note that while your battery bank might like higher voltages the vehicle chassis may not.

secondary alternators

In RVs with heavy electrical consumption a secondary alternator may be installed for aux power and charging. It runs off the engine and effectively replaces the generator; some systems will auto-start the engine similar to how gens can auto-start. The secondary alt is typically rated for heavier current and/or externally-regulated (see above). It may be run off a smaller pulley that increases alternator RPM at idle for more power and/or cooling.

Challenges include hefty cost, already-cramped space in van engine bays, mechanic unfamiliarity with non-OEM systems, and potentially-increased time running the engine.


The average user will likely not notice these effects; some of them rather subtle.

  • Solar charging while the isolator circuit is closed (ie, batteries connected) can pass higher-than-normal voltage to the chassis and starter battery. Workaround: see notes on HVD and DC-DC charging below.
  • Voltage-sensing relays can be unintentionally triggered64) or “held closed”65) by voltage from the solar-charged side in some scenarios. Workaround: address with HVD as below if desired, or with a DC-DC charger, or by adding a switch to disable the VSR.66)
  • In early morning or other times when house battery voltage is lowest, a plain solenoid may unintentionally allow depleted batteries to pull down the starter battery. Workarounds: use a VSR, a DC-DC charger, or start the vehicle immediately after inserting the key67).
  • Solar charging while the engine is running may get "stuck" at alternator voltage. Workaround: higher solar wattage, DC-DC charger, or diode/FET-based isolator, or a switch to disconnect isolator after alternator voltage is reached. The Victron Cyrix-ct isolator could be useful here, as it appears to disconnect >13.8v.68)
  • Alternator charging may bring some battery chemistries (like bare lithium cells with no BMS) to unsuitably high voltages. Workarounds: A high voltage disconnect can restrict alternator charging to lower voltages. DC-DC chargers can also regulate voltage provided to the house battery.

disabling alternator charging

It may be desirable to disable alternator charging on-the-fly when stopped in traffic, on hot days, etc. The method of disabling will vary depending on the gear:

  • bluetooth-enabled DC-DC can often be disabled or derated from the app
  • relays triggered by D+ can be disabled by a switch on the D+ wire
  • VSR triggered by voltage can often be disabled by a switch on the thin ground wire on the VSR itself.69)

wiring for the isolator

To get the most current size the wiring to the critical loads spec in Blue Seas' chart70). One might also connect at the alternator rather than the battery to further reduce resistance when the starter battery is recharging.71) Note that you will usually only have to run the POS+ leg of wire to the house battery as the chassis ground72) can be the other leg.73)

Note: critical in this context refers to electronics with narrow input voltage requirements74). Both critical and noncritical wiring specs in the chart are safe.

Maximum current sizing would be important for DC-DC chargers, as it would be a waste to bottleneck power to the spendy chargers. With plain isolators less-than-maximum current can be tolerable or even preferable. This would allow for wiring that takes up less space and costs less.

We will use alternator voltage of 14.4v and discharged battery voltage of 12.1v below to illustrate.75). Current obeys the forumula I=V/R, so the greater the voltage sag the lower the current. The actual values here aren't important, only the general pattern:

  • theoretical max with oversized wiring: 115A
  • critical load wiring (3% sag): 93.5A
  • noncritical load wiring (10% sag): 45A


Let's assume a 200A AGM bank that sits 10ft (20ft of wire for the complete circuit) from the starter battery or other connection point. 200Ah of AGM will pull ~60A (0.3C) at 50% depth of discharge.

If we want to make full use of DC-DC charging we would

  • plan on a 60A DC-DC charger. Note that due to voltage boosting, 60A of charging to the battery may require ~20% more current (72A) from the battery.
  • fuse for 72A + 20% = 86.4A between chassis and charger
  • select 2awg from the chart (20', critical, 70A-80A)

A plain isolator can't boost voltage while meeting a charging current target so we only have to worry about 60A. Run critical spec wiring if you want “full blast” and noncritical for reduced current:

  • use any isolator rated at least 60A
  • fuse for 60A + 20% = 72A between chassis and charger
  • select wiring from the chart
    • 4awg (20', critical, 60A), yielding ~60A when first connected. Maximum charging current for 200Ah of consumer-grade AGM.76)
    • 6awg (20', noncritical (10% voltage sag), 60A), yielding ~30A when first connected. Minimum charging current for 200Ah of AGM.

Over time77) isolator charging will deliver the same Ah back to the battery bank with either critical or noncritical wiring. The critical wiring will deliver high current that falls off sharply and linearly. The slope is quite steep. Noncritical wiring will deliver moderate current that also falls off linearly although more gradually. The slope is shallower.

A side effect is that if one drives on very short trips the critical wiring may deliver more Ah to the bank in the limited time available. The noncriticial wiring would be gentler on the alternator.

alternator hacks

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

  • a higher-output alternator can handle higher continuous output in a given set of conditions
  • 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.78). 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.79)

charging trailer batteries

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:

  • battery hot lead, typically black.
  • ground80) wire, typically white.

The minimum size for these wires is 12ga and some heavier models use 8ga.

current on different sized wires

Given: a 3% maximum voltage drop81) and a 40' round-trip wiring run from alternator to trailer battery we can provide Float voltage to the trailer battery at these rates:

  • 12ga ~5A
  • 10ga ~8A
  • 8ga ~10A

workaround: voltage boosting

DC-DC charging

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.

high-voltage boosting

More power (and more appropriate charging voltages) can be passed along the 7-pin by injecting higher voltages into the harness.82)

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.

workaround: heavier wiring

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.

isolator without a house battery

Q. An isolator is typically used to charge a house battery, so why install an isolator if you have no house battery?

A. because an isolator can bring Big Current into the cabin for other uses, and do so only when the engine is running. Ciggy ports are typically limited to 10A (120-150w).


  • 120vac 300w rice cooker running on 500w MSW inverter, (~330w total after inefficiencies).
  • 120vac 300w wall charger for your solar generator running on a 400w PSW inverter.
  • if you have a stout enough alternator, a 120vac 700w instant pot running off an inverter.

This setup would be: starter battery → isolator → inverter → 120vac devices

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
and understanding the alternator has it's own stuff to run – wipers, fans, headlights, ECU, etc
starter battery and house battery
sits between alternator and batteries rather than between starter and house batteries
ie, an isolator
daily is ideal
diode-based isolators will reduce current somewhat due to forward-voltage losses
as when idling
as with giant AGM or lithium banks
all alternators have a non-infinite lifespan, not just ones used to charge aux banks
or fans
this happens because lead chemistry voltage rises (and acceptance drops) linearly while charging
this happens because lithium has a flat middle in the voltage curve with steep angles at either end
the exact voltage might vary, but it's relatively stable
triggered by D+, and DIP or other setting might be required
return circuit
dVSR - d is for dual
apparently time-based, see documentation
34) , 38)
time-based, see documentation
not to D+/IGN
typically 0.5A
yellow knob visible on top of housing
bonus points to the Jaycorp for printing the connect/disconnect setpoints right on the isolator!
variable voltage
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.
same reason headlights are turned on at the donor car when jumpstarting
likely with peak tolerance of 60A or so
assuming you aren't applying heavy loads like a microwave while driving
100A peak
It's actually the delta (difference) between alternator and battery voltage but we assume that alternator voltage is stable
single alternator
including a charging battery
even “smart” staged charging
both VSR and dVSR
a momentary-off switch would kill the connection, although an ON/OFF switch might be useful for other purposes
ie, do not leave in the Accessory position which would drain the starter battery
the VSR requires a ground to make a complete circuit to run internal electronics. Breaking this circuit turns off the VSR
distance includes the entire circuit, even if NEG is bonded to the chassis
“negative return”
setups with separate grounds like tow vehicles and travel trailers will likely need to run both wires
not an issue with most van gear), or power transmission where every watt counts((not applicable to alternator charging
We will also assume a resting resistance of 20m Ohm to complete the formula
high end AGM like Lifeline requires 0.5C, or 100A for 200Ah
half hour? hour?
negative return
14v at the alternator and 13.6v at the battery
disregard the relay complication; you can use a toggle switch if desired
while driving, not while idling
electrical/12v/alternator.txt · Last modified: 2023/03/20 10:56 by frater_secessus