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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 off, these are sometimes called battery isolators.
when alternator charging works well
limitations
isolators
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.
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.
4). SternWake recommends the
Blue Sea 9012.
5)
Solenoids can be used for
self-jumpstarting if the chassis battery has enough juice to engage the solenoid.
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.
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.”]
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
DC-DC isolators (aka
b2b isolators) - this kind of controller boosts the chassis voltage to voltage appropriate for proper Absorption. The most common example is the
CTEK D250SA, a 20A solar/alt controller. The
SmartPass accessory bypasses the D250SA when the house battery wants > 20A of current. This will generally happen in the first half of Bulk mode. When the house battery demand drops below 20A control is handed back to the D250SA for boosting to Absorption voltage.
6)
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.
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.
gotchas
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.
wiring
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.7)
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:
using the coach battery only
A simple possible approach would be to replace the starter battery with a marine or AGM battery.9)
further reading
Split Charging Guide - a British page. Note the following differences in terminology from American English: