Best place to tap 12 V that's only active when the engine is running?

[Discoverado]

I don't know if this engine has an oil pressure sender, but if it does that would be a source for a signal when the engine is running, but maybe not the alternator.

Interesting idea. I'll have to look into this more.

My house battery and charger are installed under the backseat. I thought maybe I could tap into the rear seatbelts (the wires that are used to alert the driver on the dash whether or not seatbelts are connected), but they were only registering less than 1 volt and not the 12 volts I need.

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[colinl]

Fuse 3 and Fuse 6 are the only 12V switched that I found in the body control module panel found in the front passenger footwell area.

I used a piggyback fuse holder I got from Amazon. Because of the height of the fuse holder and the 90 degree angle, I found fuse #3 much easier to use.

 

[RCollins]

Interesting idea. I'll have to look into this more.

My house battery and charger are installed under the backseat. I thought maybe I could tap into the rear seatbelts (the wires that are used to alert the driver on the dash whether or not seatbelts are connected), but they were only registering less than 1 volt and not the 12 volts I need.

Keep in mind, most sensors are one wire, which supplies ground when activated or deactivated. In other words. the oil pressure sender switch will supply ground when there is no pressure present. That would illuminate the warning light. So there would be power in the wire all the time, just an open circuit when there is pressure. You would need to tee into the plumbing and get a sender that was normally open, and close on pressure rise, for that to work for your purposes,

 

[TheWizziard]

Can you expand on this? I guess I don't understand how I'd use this.



Yeah sounds like we're looking for the same thing. You bring up an interesting point, though. I have the factory 400W inverter, and I think it's only powered when the engine is running. Maybe there's an existing remote line for that we could take advantage of?

I believe the factory inverter is controlled by a signal from the BCM over the LIN (local Interconnect Network). That is how thew F150 400w invertor works.

You may be able to pick a signal from 100v output using a relay.
Are there any LED's showing that the inverter is on? You may be able to get a signal from the LED circuit.

 

[Darnon]

Can you expand on this? I guess I don't understand how I'd use this.

A bit more research and it looks like this might not be entirely the most ideal solution, but I'm just a dumb mechanic and not an electrical enginerd.

A zener diode is like a normal diode in that it normally allows current mostly in one direction, but differs in that at a certain rated voltage it allows current bi-directionally. So an appropriately sized zener diode (it seems in practice it might not be entirely as simple as just selecting a 13v) could theoretically be selected that when the alternator is outputting a charging level of voltage it closes a relay that connects your auxiliary charger to power.

 

[GPSMan]

You may have missed it.
Way back in 2005 I did something like this but tapped the fuel pump or fuel injector wire. Think to yourself: "What else is only on while the engine is running?"

Or, as some eluded to:
Use a voltage cut-off circuit.

So the circuit is "normally closed" and opens any time the voltage drops below x volts ( 13v ).

 

[TheWizziard]

Fuse 3 and Fuse 6 are the only 12V switched that I found in the body control module panel found in the front passenger footwell area.

I used a piggyback fuse holder I got from Amazon. Because of the height of the fuse holder and the 90 degree angle, I found fuse #3 much easier to use.

Another member is using use spots 1 & 2 with a Micro3 fuse tap. They are listed as unused. He claims that they are switched. I haven't tried it yet.

This doesn't help the OP as he wants the start circuit.

 

[Discoverado]

Or, as some eluded to:
Use a voltage cut-off circuit.
So the circuit is "normally closed" and opens any time the voltage drops below x volts ( 13v ).

So the charger I'm using essentially has this functionality built-in. However, the problem I'm having is that when the starter battery is fully charged the voltage output by the alternator is lower and is triggering the charger to turn off. I've noticed that right when I start the vehicle the alternator voltage is higher to recharge the starter battery, but it soon drops back down. When I get on the highway and am cruising, the voltage coming off the alternator is low enough that the charger cuts out. That's why I'm trying to use the remote signal into the charger (it's designed to be triggered that way for "smart" alternators) so that the charger knows when the engine is or is not running.

 

[MakinDoForNow]

Can you expand on this? I guess I don't understand how I'd use this.



Yeah sounds like we're looking for the same thing. You bring up an interesting point, though. I have the factory 400W inverter, and I think it's only powered when the engine is running. Maybe there's an existing remote line for that we could take advantage of?

Any time the 12v buss has over 13 volts on it the dcdc converter or the alternator is on and you can switch on your device, whatever it might be. When voltage is less than that charging current is not available!

 

[MakinDoForNow]

So the charger I'm using essentially has this functionality built-in. However, the problem I'm having is that when the starter battery is fully charged the voltage output by the alternator is lower and is triggering the charger to turn off. I've noticed that right when I start the vehicle the alternator voltage is higher to recharge the starter battery, but it soon drops back down. When I get on the highway and am cruising, the voltage coming off the alternator is low enough that the charger cuts out. That's why I'm trying to use the remote signal into the charger (it's designed to be triggered that way for "smart" alternators) so that the charger knows when the engine is or is not running.

So your charger is automatically shutting off when voltage on 12v buss drops below it's minimum input voltage. Would it be possible to eliminate your charger and place a dedicated bms on your non vehicle battery. This would place a drain controlled by your bms on the vehicle battery and that would keep alternator or dcdc converter active (just like any other accessory device) until your bms stops charging. ????

 

[MakinDoForNow]

FYI. I had a 2012 Honda crv that in order to keep 14.4v on 12v buss in order keep 12v battery charged I kept running lights and cabin fan speed on 2. So maybe just turn some accessory on. Need voltage meter in power point to see if enough drain to keep voltage on buss high.

 

[Discoverado]

When voltage is less than that charging current is not available!

With a standard alternator that may be the case. But with the Maverick the alternator is more responsive. AKA if I pull current from the battery the alternator can accommodate.

So your charger is automatically shutting off when voltage on 12v buss drops below it's minimum input voltage. Would it be possible to eliminate your charger and place a dedicated bms on your non vehicle battery. This would place a drain controlled by your bms on the vehicle battery and that would keep alternator or dcdc converter active (just like any other accessory device) until your bms stops charging. ????

So I think the functionality you're describing is essentially what the charger should be doing. However, I don't want it to keep charging until it's full because this puts the starter battery at risk of being run down too far (for example, if the house battery charges off the starter battery while the engine isn't running).

FYI. I had a 2012 Honda crv that in order to keep 14.4v on 12v buss in order keep 12v battery charged I kept running lights and cabin fan speed on 2. So maybe just turn some accessory on. Need voltage meter in power point to see if enough drain to keep voltage on buss high.

Yeah I see what you're saying and that could be a workaround. Right now I have the charger signal line tied to a manual switch so that I can control when it does or doesn't charge - essentially serving the same purpose as keeping an accessory on. But I know one of these days I'm gonna forget to turn it off when I park and will soon have a dead battery. In fact, I've already forgotten a few times but luckily it wasn't too long and the starter battery was still able to start the engine!


Thanks for all the ideas so far! I have some more detective work to do tonight, that's for sure.

 

[MakinDoForNow]

Could you just put a sensor switch in pos input line to your charger that would open when voltage dropped to whatever setting you desired say 13.8-14v??? Of course that may delay topping off your battery but would not drain vehicle battery even when vehicle is topping of it's battery?
Such as
https://www.amazon.com/Voltage-Prot...voltage+sensing+switch&qid=1691446152&sr=8-14

 

[Darnon]

Way back in 2005 I did something like this but tapped the fuel pump or fuel injector wire. Think to yourself: "What else is only on while the engine is running?"

It's actually not uncommon for modern Fords to power the fuel pump circuit as soon as the door is opened or the key is inserted. This primes the pump in preparation to start.

 

[GPSMan]

It's actually not uncommon for modern Fords to power the fuel pump circuit as soon as the door is opened or the key is inserted. This primes the pump in preparation to start.

Well not in 2005! 🫢

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