I have a 140 amp alternator (internal regulator) servicing my house LFP pack that is managed by a REC Active BMS.
The engine start battery (AGM) is isolated from the house pack and is charged by a DCtoDC charger.
I want the REC to turn the alternator on/off via the alternator\'s ignition wire (which effectively is Field).
I only want the alternator\'s field energized when the engine is running and the BMS is OK to Charge HOWEVER the alternator Field needs to be energized from the House LFP and not the engine\'s AGM.
Is there a way to do this with only one relay?
My thought was to have the engine\'s ignition key (the AGM) energize the relay when the key is on and have the BMS pull to ground when it is charge ready. All negative terminals are joined so the current would pass. Not sure if the BMS would be happy pulling 12 volts to ground tho.
Otherwise I need two relays to make an AND gate.
It sounds easy enough. Why not just add a paralleling relay off of your BMS contactor coil and put its N/O contacts in series with the ignition input wire to your alternator.
BMS trips, ignition input to alt breaks, alt turns off.
Engine off, alternator turns off, and both BMS and engine ON to re-enable.
You will need a 10 amp relay and try to find one with a low parasitic draw.
Am I missing something?
You could also use the inverter disconnect opto output with a relay. This would effectively disable the alternator before a hi limit hard trip occurred by setting CHAR to about 3.6 and CHIS to 3.5 (on the SI-Q series BMS)
In my setup, I\'m planning to only have a single solenoid and have it for low voltage disconnect (discharge) as all of my charge sources (less the alternator) are Victron and controlled by DVCC. The MultiPlus inverter/charger prevents me from having a true dual bus setup.
This is my current idea using two relays. The BMS Charge OK signal is gated by the ignition relay and the voltage from the LFP bus is gated by the BMS Charge Ok signal. This means the ignition needs to be on before the BMS can close the second relay and send voltage to the alternator. It also prevents unnecessary current draw by the BMS holding a relay closed all the time vs only when the engine is on.
Thinking about this more.
Could I use the CHARGE NO wire to provide field voltage in leu of a second relay?
I read in this post https://www.offgridsoftwaresolutions.com/ws500-user-forum/topic/rec-active-bms-2-relay-operation-with-dch-and-ch-re1lo-and-re1hi/
\"The customer may connect any signal to the CHARGE F input.\"
What is the current rating of CHARGE F?
cool, its a 5 amp relay, with a 3.6A slow blow fuse as I recall (fuse is internal replaceable PCB socket mount)
Thanks.
3.6A would be pushing it. Probably better to just use a second relay and avoid blowing that little fuse.
The alternator is the only charge source not controlled by the CAN bus so using Charge NO on a relay for Alt Field works great.
Sometimes the answer is right in front of you and you can\'t see it.
From that same forum post:
\"Some of the devices are controlled by applying ground so ground is connected to fused input and NO then connected to a controlling device.\"
So my initial idea of one relay is possible. Ignition switch energizes the relay and the BMS via Charge NO pulls that to ground when it want\'s a charge with Charge F is connected to ground.
Cheers
As I get closer to wiring this up, I wanted to confirm my understanding on charge control is correct.
ABMS is at stock configuration:
- cell over-voltage switch-off per cell 3.85 V
- over-voltage switch-off hysteresis per cell 0.25 V
- cell end of charge voltage 3.58 V
- cell end of charge hysteresis per cell 0.25 V
- Any cell reaches Over Voltage Switch-Off of 3.85v:
- Charge Relay switches OFF
- High cell balances down to 3.60 (over voltage switch off minus hysteresis) and not ALL cells are over 3.58v
- Charge Relay switches ON
- All cells reach or exceed 3.58v:
- Charge Relay switches OFF
- Any cell goes under 3.33v (End of Charge minus hysteresis) but NONE above 3.85v
- Charge Relay switches ON
Is this correct?
