====== Disconnecting alternator charging at a voltage setpoint ======

There are scenarios where one might want to prevent a [[electrical:12v:alternator|battery isolator]] from combining the house and chassis 12v systems:

  - the house battery might be a chemistry that could get overcharged by the alternator (lithium, for example)
  - the house battery might have a resting voltage high enough to trigger the house side of a dual-sensing relay (LiFePO4)
  - the combined circuit can feed higher-than-optimal voltages back from solar charge controller to the chassis' electronics.
  - a [[electrical:12v:b2b|b2b charger]] in parallel with an [[electrical:12v:alternator|isolator]] could have the same backfeeding result


===== how it works =====

[[electrical:12v:alternator|Isolator/relays/solenoids]], etc, use mechanical or electrical means to combine the chassis and house 12v systems.   These mechanical or electrical means will require a small amount power to operate.((latching relays excepted))  

If the device's own power source is disconnected it will turn off and 12v systems will be separate again.((again, latching relays excepted))  Using a [[electrical:12v:hvd|High Voltage Disconnect]] to kill power to the device will stop the alternator from charging the house battery at a given voltage setpoint.

Shutting down the device will require different approaches, depending on where it gets the power to operate itself.
===== solenoids and relays =====
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Purely Mechanical devices like solenoids and relays use 12v //trigger// or //exciter// voltage((usually from the accessory circuit)) to operate.  They will either have 4 lugs or 3 lugs;  in the latter case grounding is done through the base of the device.

Power to these devices is disrupted by interrupting power to the trigger+, which is one of the small lugs.((or the only small lug in the case of 3 lug relays))


===== voltage sensing relays =====

Voltage sensing relays (VSR) typically take power from the chassis battery and have a thin ground to complete the circuit.  

Power to VSRs is disrupted by cutting the ground wire. One way to reestablish the ground (thereby turning the device on) is HVD --> relay NO contacts --> ground wire in/out

===== solid state isolators =====

It is unlikely you will need an HVD with a diode-based isolators because:

   - the isolator can't backfeed voltage back to the starter battery, and
   - voltage drops up to 1v in the diodes when passing alternator --> isolator --> house bank

But it is theoretically possible alternator voltage is so high that even with the drop you might want an HVD.  Some solid state isolators have a trigger or exciter input;  control with HVD as with solenoids and relays.  

Three lug isolators without exciter input might not be usable with HVD.