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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)
Note: this is a basic overview. More details available on the alternator details page.
Alternators can generate massive quantities of electricity when the engine is running, which can be used for charging house batteries. Their ability to generate large amounts of current makes alternators particularly good at charging deeply-discharged batteries (especially lead chemistry batteries)
The vehicle's alternator is designed to turn some of the engine's mechanical power into electrical power in order to
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:
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 by combiners is generally only practical for the bulk charging stage due to relatively low alternator voltage and the long time periods required for absorption; DC-DC chargers can help with the voltage can address the voltage issue. Failure to fully charge lead batts regularly7) will impact battery longevity. For this reason some solar is usually added to the power mix for lead banks. In contrast, Lithium can charge fine from alternator alone, if one drives enough to get sufficent charging.
Combiners are effectively a pass-through:
{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 overheating11) or overloading12) 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. – jimindenver13)
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 fail14) 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:
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.16) 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.
See this sub-article on alternators and heat
If the alternator is contributing 23A and you drive for 20 minutes you can expect to replace 7.67Ah on the drive.19)
23A x 20 minutes / 60 minutes in an hour = 7.666666667Ah
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
Fuel consumption for power generation will be greatest when the vehicle is idled. When charging loads are imposed on a vehicle that is already driving the added cost can be minimal.
Using the 3.6L Promaster with a 40A DC-DC charger as an example, ObvB estimates:
In terms of cost, an average USA price of gas at $3.36, idling for 2 hours to produce 1kWh with a 40A DC-DC charger would burn 2 * (2/3 + 0.074) gal = 1.48 gal = $4.97 / kWh. If you were going to be driving anyway, then the additional cost of turning on a DC-DC charger (using my assumptions) would be about 50c/kWh.20)
It is often said that one cannot parallel batteries of different chemistries. This is not so. The problem would be keeping batteries with different resting voltages connected after charging is removed. If they were left connected the higher-voltage batt would discharge into the lower-voltage batt. Note: an isolator setup is specifically designed to keep this from happening.
As long as the alternator voltage is acceptable to both battery chemistries they can be paralleled during charging.
Example using a flooded starter battery, LiFePO4 house battery, relay, and 14.2v alternator:
Traditional alternators typically try to hold a consistent voltage. The exact voltage being held might vary but it's relatively stable:
most alternators with internal regulators are designed with a voltage turn-down with temperature. Because the battery is often under the hood, it gets hot from the engine and needs a lower charging voltage to remain healthy. Typically a 14.4V regulator target will drop to a 13.8V regulator target by the time the alternator gets to 140F. – MechEngrSGH21)
While this is not intended to be protection from overheating due to high current production, lowering the output voltage will indirectly reduce current when charging by relay/isolator. Example: a setup with accepting 85A @ 14.4v would only accept 65A @ 13.8v.22)
Smart alternators on the other hand 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.23)
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+).
There are several methods of identifying a smart alternator… the most conclusive is the existence of a current sensor on the negative battery cable. It is often a small box with 2 or 3 wires placed right at the battery terminal. In some cases (GM mostly) it may be 6-8“ down the cable from the battery. – MechEngrSGH24)
“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. This short video illustrates how it works.
A constant-duty solenoid is an electromechanical device which uses an electromagnet to complete the charging circuit when the engine is running. The basic idea is the relay uses a low-current circuit26) to activate a higher-current circuit.27)
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 latches28) instead of electromagnets to hold the circuit closed, eliminating that vector of power consumption.29) and therefore heat. SternWake recommends the Blue Sea 901230) 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”31) through the body of the solenoid. The pic above is of a “three post” solenoid – the case is grounded to the chassis through the metal feet.
While they may be externally identical, starter relays and constant duty relays are built differently inside.32)
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. To achieve this the solenoid will pull several amps to run a 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 is used for much longer periods of time and is rated for less current. The lower current means the connection doesn't have to be slammed closed as fast with a powerful electromagnet. As a result this relay type typically draws <1A and the control terminals have resistance of 15-30 ohms.
Note that per Cole-Hersee even a CD relay will get hot:
The coil circuit (control circuit) in a continuous duty solenoid is usually energized for long periods of time. Under these conditions the coil will generate heat and within less than an hour the solenoid housing will become hot to the touch. This is normal. Always make sure that all wiring is properly sized for the load it is carrying, that the terminals are the correct size and have been securely crimped to the wire, that the terminals have the proper torque to the solenoid studs.33)
Cole-Hersee recommends mounting relays with the dimple facing downward:
Our research shows that it might be best to mount the Solenoid dimpled end down. Electromechanical Switches can over time build up deposits due to arcing. By orienting your Solenoid as recommended, deposits will have a tendency to fall to the bottom, clear of the contacts, thus prolonging the life of the Solenoid.
voltage sensing relays (VSR, also called Automatic Charging Relays or ACR) are combiners 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 one34) or both35) 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.36)
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.
Note: voltages-sensing (with or without delay) can be added to plain solenoids.
A “lithium compatible” VSR has a higher voltage setpoint for disconnecting the batteries after charging stops; the charging function is no different.
Reasoning: A traditional VSR might disconnect at ≤12.9v, since that is slightly above resting voltage for a lead-chemistry battery. (When it sees ≤12.9v it knows charging must have stopped and the batts should be disconnected from each other) Fully-charged LiFePO4 rests at higher voltage so the voltage at which the VSR disconnects needs to be higher. If it waited for 12.9v the batteries would get “stuck” together longer than appropriate. In effect the lithium batt would prop up the lead starter batt (and parasitic chassis loads) until the LFP were substantially discharged. So a “lithium compatible” VSR disconnects at something like 13.2v - 13.4v.
Note: a normal VSR can be used with LiFePO4; it may be useful to add a way to disable temporarily it to break the “stuckness”. Also see the Gotchas section for related information.
The Precision Circuits Li-BIM is a lithium-specific dVSR isolator with some differences:
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.50)
An interesting teardown of the Li-BIM can be seen in this video.
The 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.
All late model Essex and King Aire’s use Silverleaf systems with the White Rodgers Solenoid. In 2014, the Mountain Aire will also be using Silverleaf. It is a computerized system that controls all aspects of charging. All late model Mountain Aire and Dutch Star Diesel Pushers use the Battery Isolation Manager (BIM). This is an all in one system that is made by Precision Circuits. Class A models before 2010 use the Bidirectional Isolator Relay Delay (BIRD) with a solenoid. The Bay Star Sport has a manual switch to disconnect power, which is located in the overhead above the entry door. This is similar to the ones in the fifth-wheels. All others have a single lighted switch that is in the front overhead to turn off house voltage.51)
also see DUVAC
Some RVs came with “BIRD”52) controllers that drove the constant duty relay.53) These controllers from Intellictec combine when either side is >13.xv, depending on model/vintage.
on it.”
The functionality of the RV-specific BIRD is largely supplanted by dVSR (see above). In some cases RV manufacturers have moved to the BIM (see above).55)
Intellitec also made a Battery Isolation Controller, which drives the relay based on chassis voltage.56) It also has manual override for self-jumpstarting.
The functionality of the RV-specific BIC is largely supplanted by a VSR (see above).
Silverleaf is a CAN-based system for controlling various functions, including charging relays. Technical reference (pdf)
Note: this type of isolator is no longer common for our uses for several reasons. Nevertheless they might be useful in some setups where bidirectional charging57) is undesirable.
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. The rating is the maximum input from the alternator, and the two “legs” are assumed to each have half the capacity.
These isolators are electronic devices which use diodes to prevent backflow from either battery. Isolators are generally brick-shaped. Silicon diode isolators typically have a 0.7v voltage drop (“forward voltage drop”) between the alternator and house battery when running near rated capacity.58) This may be desirable if the house battery is a wants lower-voltage charging like LiFePO4.59) The slightly-lower voltage will also reduce charging current somewhat.
It is possible to have the alternator voltage-sense the battery voltage through a diode so it sees the voltage as artificially low and so increases its output voltage. The net effect can be almost no voltage loss.
Notes:
These mount between the alternator and the target batteries.
Instead of mounting between alternator and battery it goes between the power source (battery, alternator, etc) and the house battery like a combiner.
When daisy chained with a relay we get the unidirectionality of an isolator without rewiring, alternator-excitation issues, etc, mentioned elsewhere in this article.62)
This type of isolator is similar to the diode-based one above, except that FET components are used instead of diodes, minimizing voltage drop and allowing two-way power if designed to do so. They tend to cost 2x as much as the diode versions.
[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.
All alternators65) need to be able to sense battery voltage to regulate their output. This was traditionally done with a separate voltage sense wire.
“1-wire self-exciting” alternators66) do not have a sense wire and rely on the output wire from the alternator (“B+”) to the starter battery. This is normally not an issue since the battery and alternator are wired to each other.
However when a diode- or FET-based isolator is inserted between them the alternator's B+ can no longer sense the battery voltage; the isolator is one-way by design. The solution is to provide a small amount of current from the starter battery to the isolator's common input terminal where the B+ can sense it. Because this “leak” violates the isolator's one-way principle it should only occur when the vehicle is on.
So a D+ (IGNition) signal is provided to so-equipped isolators on additional “alternator excite” stud67), which triggers the leak from starter battery to common INPUT.
[note from secessus: it's not clear if the leak is momentary or if there is a diode on the leak circuit to prevent charging from flowing down this tiny circuit. Some manufacturers suggesting getting the 12v signal from the starter relay so it is momentary.]
Per Littelfuse (maker of Cole-Hersee isolators):
Most alternators on new vehicles have an integral electronic voltage regulator that requires the use of the 4-stud battery isolator. The small 4th stud is for connection to a circuit switched by the ignition switch… A 4-stud battery isolator can be used with older pattern alternators (in this case the 4th stud will remain unconnected), but a 3-stud battery isolator cannot be used with the Delcotron-type alternator.
Yet another explanation, this time from Victron:
The new Argodiode isolators have a special current limited energize input that will power the B+ when the engine run/stop switch is closed.68)
Because I=V/R we can compare how different “forward voltage drops” affect charging. Assuming 20mR resistance as a baseline, 14.4v alternator output and 12.1v bank voltage:
These are relative numbers, not intended to predict actual charge rates.
.
For many69) 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:
A diode-based isolator may be preferable in niche cases.
See also the effect of alternator charging method on current
See the gotcha section below to see if there are hidden traps in your intended use case.
If an isolator is oversized it will cost more for no benefit, will self-consume somewhat more energy to hold the combining circuit closed,71) 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).72)
Reasonable charging rates can also be easier on the alternator when charging suddenly stops, whether by completion73) 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.74)
Ah = Amps x hours. If your combiner averages 30A and you drive for 1.5 hours it can replace 45Ah (30A x 1.5 hours).
Note: the current produced by direct charging vs DC-DC charging is different.
FLA batteries can accept up to C/5 in Bulk stage.
Example: a 200Ah FLA battery bank will pull up to 40A76) in Bulk charging. An isolator rated for constant duty at 40A77) would be sufficient.78)
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 isolator79) 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.80)
There are mitigating factors that tend to reduce current in real world use:
Note: this is an issue for rigs with secondary alternators dedicated to charging a battery bank. In normal81) setups the vehicle's starter battery acts as a buffer to cushion sudden disconnects.
Sudden disconnection of a large load82) when the alternator is making substatial power can damage the alternator and chassis electronics. Sudden disconnection can occur when:
It's more common in marine setups than vehicles, but external regulators can be used to trick the alternator into outputting specific non-OEM voltages83). 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.
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.
It may be desirable to disable alternator charging on-the-fly when stopped in traffic, on hot days, to avoid charging frozen Li cells,90) stop charging at a given voltage,91) or neutralize gotchas, etc. The method of disabling will vary depending on the gear:
A NC thermal switch affixed to the alternator case might be used to automate this shutoff, or a NO switch to trigger a low-output mode on the Renogy or Leaptrend chargers mentioned above.
A voltage-sensing relay might be used to add voltage-sensing to a relay triggered by D+, or to disconnect at a given setpoint.
In most setups 'dwellers:
Exceptions:
The Blue Sea chart shows wire sizes for both Critical and Non-Critical loads. The difference is in how much voltage sag will be present at high levels of current. Critical in this context refers to electronics with narrow input voltage requirements94), or power transmission where every watt counts95). Both critical and noncritical wiring specs in the chart are safe.
All other things being equal, it is generally preferable to size the wiring to the Critical loads criteria. It makes little sense to buy a 60A DC-DC then choke it to 40A with thin wiring (see below).
There are reasons where one might choose Non-Critical sizing:
We will use alternator voltage of 14.4v and discharged battery voltage of 12.1v below to illustrate how sag affects charging with a relay.96). Current obeys the formula I=V/R, so the greater the voltage sag the lower the current. The actual values here aren't important, only the general pattern:
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
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:
Over time98) 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.
Reminder: it is hard to keep lead batteries healthy by alternator charging alone. In order of decreasing effectiveness:
There are ways to get the alternator to pump out more power:
also see the Alternator Details page
A simple possible approach would be to replace the starter battery with a marine or AGM battery.100)
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.
Given: a 3% maximum voltage drop102) and a 40' round-trip wiring run from alternator to trailer battery we can provide Float voltage to the trailer battery at these rates:
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.
More power (and more appropriate charging voltages) can be passed along separate wiring or down the 7-pin by injecting higher voltages into the harness.103) This section will address 7pin injection.
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.
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).
Examples:104)
This setup would be: starter battery → isolator → inverter → 120vac devices
With a combiner there is no easy way to adjust current with precision; the batteries are effectively paralleled. You can influence the current a bit if you know what's going on.
With relays and isolators charge current is dependent on
….and is governed by the formula ''I=V/R''.
You can't do much about the first two, other than do any elective driving in the morning when bank voltage is likely to be lowest.
The third issue (excess resistance) might be tweaked by the user.
Note that, perhaps counterintuitively, the presence of active solar charging will often decrease alternator charging current because the solar is bumping up the bank voltage (case #1 above).