Post by Deleted on Feb 5, 2019 22:13:18 GMT
Attention is turning back towards the pesky positive earth Freeman
We will be installing a 12v macerating toilet run from an independent 12v 110ah battery charged by 180w of solar along with running a dedicated supply to a 12v coolbox - this will be switched by the mppt solar controller load output - on when it’s shining off when the sun goes to bed.
All fine and dandy when the boat is in use, to take advantage of the solar when the boat is sat for weeks on end on the mooring I have been scheming how to send the spare juice to the current positive earth start and domestic batteries - see here:- thunderboat.boards.net/thread/3405/question-polarity
On the face of it a good old Victron Cyrix 120a vsr is the answer - but! - not so fast... if the independent toilet/cool box battery is fully charged by the solar and the vsr is made - sparking the engine into life will subject the circuit to reverse polarity and make the magic smoke genie 🧞♀️ appear 🤦🏼♂️🧐🔨🔨
So, the electromechanical solution would by to use a DPDT 12v Relay like this little beauty uk.rs-online.com/web/p/non-latching-relays/4927218/ connected NC from battery 2 positive output of the vsr to start battery - and like wise the negative from independent battery to negative of start battery.
The relay coil would be energised by the 12v ignition supply, as soon as the ignition key is turned the relay coil would energise and the contacts would open, disconnecting the vsr and independent battery from the positive earth system.
So far so good, but, any well designed system needs a fail safe - if the relay coil shits itself and the contacts close we again have the potential for magic smoke 🤬🤦🏼♂️
Fisrt thoughts were diodes - namely a Schottky diode - but voltage drop and a marginal output to begin with scotched this idea before it got off the ground.
I then happened across this little YouTube clip.
If I add a PFET to the relay circuit configured thus -
Relay terminal 1 +12v from ignition coil supply to relay coil (energised)
Relay terminal 8 - 12v from ignition coil supply to relay coil (energised)
Relay terminal 2 Pole 1 +12v (from vsr battery 2 terminal) common - PFET pin Drain
Relay terminal 3 Pole 1 +12v NC to start battery - PFET pin Source
Relay terminal 7 Pole 2 -12v from independent battery common - PFET pin Gate
Relay terminal 6 Pole 2 - 12v NC to start battery
Will the PFET protect the vsr and independent battery in the event the 12v DPDT relay coil fails while the engine is running?
Have I got my pin/terminal connections correct?
I have a reasonable grasp of lectrickery - a tenuous grasp of electronics and a tendency to hit electrical/electronic things with a gert big hammer if they don’t do as requested 🔨🔨🔨👍
Help and guidance appreciated as ever 👍🍻
Gazza
We will be installing a 12v macerating toilet run from an independent 12v 110ah battery charged by 180w of solar along with running a dedicated supply to a 12v coolbox - this will be switched by the mppt solar controller load output - on when it’s shining off when the sun goes to bed.
All fine and dandy when the boat is in use, to take advantage of the solar when the boat is sat for weeks on end on the mooring I have been scheming how to send the spare juice to the current positive earth start and domestic batteries - see here:- thunderboat.boards.net/thread/3405/question-polarity
On the face of it a good old Victron Cyrix 120a vsr is the answer - but! - not so fast... if the independent toilet/cool box battery is fully charged by the solar and the vsr is made - sparking the engine into life will subject the circuit to reverse polarity and make the magic smoke genie 🧞♀️ appear 🤦🏼♂️🧐🔨🔨
So, the electromechanical solution would by to use a DPDT 12v Relay like this little beauty uk.rs-online.com/web/p/non-latching-relays/4927218/ connected NC from battery 2 positive output of the vsr to start battery - and like wise the negative from independent battery to negative of start battery.
The relay coil would be energised by the 12v ignition supply, as soon as the ignition key is turned the relay coil would energise and the contacts would open, disconnecting the vsr and independent battery from the positive earth system.
So far so good, but, any well designed system needs a fail safe - if the relay coil shits itself and the contacts close we again have the potential for magic smoke 🤬🤦🏼♂️
Fisrt thoughts were diodes - namely a Schottky diode - but voltage drop and a marginal output to begin with scotched this idea before it got off the ground.
I then happened across this little YouTube clip.
If I add a PFET to the relay circuit configured thus -
Relay terminal 1 +12v from ignition coil supply to relay coil (energised)
Relay terminal 8 - 12v from ignition coil supply to relay coil (energised)
Relay terminal 2 Pole 1 +12v (from vsr battery 2 terminal) common - PFET pin Drain
Relay terminal 3 Pole 1 +12v NC to start battery - PFET pin Source
Relay terminal 7 Pole 2 -12v from independent battery common - PFET pin Gate
Relay terminal 6 Pole 2 - 12v NC to start battery
Will the PFET protect the vsr and independent battery in the event the 12v DPDT relay coil fails while the engine is running?
Have I got my pin/terminal connections correct?
I have a reasonable grasp of lectrickery - a tenuous grasp of electronics and a tendency to hit electrical/electronic things with a gert big hammer if they don’t do as requested 🔨🔨🔨👍
Help and guidance appreciated as ever 👍🍻
Gazza