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--- electrical:12v:alternator_charging_hvd [2017/10/11 14:18]
frater_secessus [disconnecting voltage sensing relays]
+++ electrical:12v:alternator_charging_hvd [2022/07/31 00:33] (current)
frater_secessus [solid state isolators]
@@ Line 1: / Line 1: @@
 ====== Disconnecting alternator charging at a voltage setpoint ======
-There are scenarios where one might want to prevent an isolator from combining the house and chassis 12v systems:
+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 needs less voltage than lead-acid (lithium, for example)
+  - 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;  this allows the alternator to charge the house batteries.  These mechanical or electrical means require a small amount power to operate.
+[[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 this power is disconnected the isolator will turn off** and 12v systems will be separate again.((latching relays excepted))  Using a High Voltage Disconnect to kill power to the isolator will effectively stop the alternator from charging the house battery at a given voltage setpoint.
+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 =====
+{{ https://images-na.ssl-images-amazon.com/images/I/51tg8FDT67L._SX466_.jpg?200}}
-===== disconnecting isolators and simple relays =====
+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))
-by 12v trigger.  Example:  12v source that's only active when the key is in the RUN position.  These have 4 lugs:  starting+, house+, trigger+, trigger-; or,
+===== voltage sensing relays =====
+Voltage sensing relays (VSR) typically take power from the chassis battery and have a thin ground to complete the circuit.
-===== disconnecting voltage sensing relays =====
+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
-These usually have 3 lugs:  starting+ in, house+ out, starting-.  The starting- "ground" is only used to complete a small circuit so the relay can run itself.  So it's usually very thin.
+===== 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.

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