Hello!

I am just getting into the world of components and programmable logic but have been tinkering with things for decades, so please pardon my simplicity.

Nutshell version:
What is the simplest way to accomplish the following:?
-If 12V battery drops below a certain value (say 10.5V) for a certain period of time (say 5 seconds), a contact is closed for 1 second.
(the contact would then be a pulse to the remote starter module to start the vehicle [the remote starter itself times out at a configurable interval] then the cycle repeats).

I camp in remote places quite a bit and need to run a DC-to-AC inverter to power a few basic things with a typical constant load around 200W. I have a 2005 Ford Escape Hybrid that does a good job of generating electricity and storing it, so the gas engine doesn't need to stay running. It will periodically restart for about 1 minute every 5 minutes to maintain a 40-43% SOC on the HV battery (which in turn, charges the 12V battery that I am drawing from). I also have an aftermarket remote start system that is able to manage starting and stopping of the ignition upon a signal, so the hard part of "hotwiring" it to run is done. I just need to send a signal to the remote start module to do its thing.

(I can provide much more detail on the logic programmed into the vehicle and how it does certain things)

In a nutshell, rather than constantly starting and stopping the engine every few minutes, I'd like it to only run when the 12V battery reaches a low level (something the vehicle logic doesn't include). It seems if I run it for about 10 minutes every 2 hours, that's sufficient to keep things running so I've considered doing a simple timer (rather than low voltage detection) as well. I realize the 12V battery isn't going to like this cycling, I eventually plan to incorporate a deep cycle battery for this (the 12V battery only runs accessories and contactors, cranking is handled by the 300V battery).

The inverter itself has a piezo beeper that activates at low-voltage (<11.0V), so I thought somehow I might be able to tie into the buzzer as a "trigger". However, this also briefly activates during the startup surge of some devices (hence the desire for a sort of delay in confirming the low voltage condition is steady).

As I previously stated, a timer function that accomplishes this every X hours is also an option. Part of me wants to limit the thermal cycling of the vehicle, so running for 10 minutes per hour (regardless of SOC) may accomplish that.

Perhaps I'm overthinking this, and I assume I'd be able to build something with an Arduino once I fully understand them. But is there a way I can accomplish this with simpler individual components like relays and timers?

Thanks much for your advice and I look forward to paying it forward once I know a bit more about all of this.

Pete

 

I also have an aftermarket remote start system that is able to manage starting and stopping of the ignition upon a signal, so the hard part of "hotwiring" it to run is done. I just need to send a signal to the remote start module to do its thing.

So you could do everything you want manually, you just need an automated button pusher? Do you know anything yet about how the current button works? It should be easy to replace that button with an automated switch but the details matter, such as whether it connects to V+ or ground of the remote. You don't need an Arduino but of course you can always throw one in if you like.

 

If 12V battery drops below a certain value (say 10.5V) for a certain period of time (say 5 seconds), a contact is closed for 1 second.

That can be done with a TL431 programmable reference voltage (to accurately detect the battery voltage), triggering a 555 one-shot timer, which momentarily turns on the relay.
I can supply a schematic for that, if you like.

But 10.5V is generally considered to be a totally dead battery.
I wouldn't go any lower than about 12.4V, where a lead-acid battery is about 75% charged.
That minimizes the stress on the battery, especially if it's not a deep-discharge type.

 

In general, your car's alternator is not designed to bring a dead battery to full charge. You should research this aspect, you might want to take a different approach, or as @crutschow mentioned at least set the threshold much higher.

One other thing is safety, an engine that might suddenly start itself is dangerous. An interlock that prevents operation if the hood is open would be a good thing. There may be other conditions to consider, and other sorts of failsafes.

 

Below is the LTspice simulation of a circuit that should do what you want.
It uses a TL431 programmable reference to detect the trip voltage, with an RC input to the output P-MOSFET relay driver to generate the 1 second output relay closure (a 555 is not needed since the pulse time accuracy is not critical).

The TL431 conducts when the Ref voltage is above 2.5V, which turns on Q1.
When the Ref voltage drops below 2.5V, Q1 turns off, which generates a negative going pulse through C1 at M1's gate.
This turns on M1, and consequently the relay.
When the gate capacitor voltage charges back to near the battery voltage through R3, Q1 and the relay turn off. This takes about 1 second for the R3C1 value shown.

R2, pot U2, and R4 form a voltage divider to generate the Ref voltage that determines the battery voltage V(bat) where the TL431 changes state.
Pot U2 adjusts the trigger point to the exact battery voltage you want.
R8 provides some positive feedback hysteresis at the trip point so the circuit doesn't oscillate around that point.

For the given component nominal values, the simulation shows the relay pulse occurring when the battery voltage drops to 12.38V.
(Rtst is just for the simulation).