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)
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
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.
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
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
) that boost charging voltage to Absorption
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.
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.6)
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:
using the coach battery only
A simple possible approach would be to replace the starter battery with a marine or AGM battery.8)
Split Charging Guide
- a British page. Note the following differences in terminology from American English: