Small aux LiFePO4 battery, voltage/ignition timer?

[oguruma]

My job necessitates that my car be something of a mobile office, so I want to mount my laptop in the car with a Ram Mounts/Gamber Johnson mount.

I'd like to add a small LiFePO4 auxiliary battery so the laptop doesn't have to pull from the starting battery, and I'd like to add some kind of timer that can sense whether or not the car is running, and if the car the stops running for 30(?) minutes to disconnect the laptop from the aux battery.

The idea being that the laptop will continue to charge for a while after the car stops running. I may add a small mobile LTE router and/or dash cam that runs off the same aux battery.

Basically it would be something like Starting Battery -> 12V Charge Controller -> Auxiliary Battery -> ??? -> Laptop/Electronics.

I know there are Low Voltage Timers that exist in the market but those don't work with LiFePO4 batteries, from what I gather, since it's not adviseable to run a LiFePO4 battery directly in parallel with a Lead Acid/Starting battery.

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[5doorsoffury]

i do this for my job actually lol. life is the best but require specific chargers period. otherwise i would recommend a solid state battery isolator used in audio systems(mechanical ones fail due to carbon fouling from constant arcing of the mechanical connection due to large currents) and a second battery in the trunk if you don't want to fuss around with lithium chargers. maybe a few golf cart lead acid batteries in parallel

there are many systems for timing but they are referred to charge guards. mobile mounts makes a VDPU and ADPU that i recommend. works off alternator ripple/charge or ing input for the timing trigger.

so if it were me id add a 12v honda gel battery in the trunk with that ADPU hooked up with a isolator. the adpu comes with a fuse panel built in so theres no more hardware to buy. this setup allows you to totally kill the back battery that feeds the system and not the starting battery due to the isolator. then the dead battery will be kept off the system with the isolator until it can be properly charged oooooor you turn it on and yolo that thing with all the amps.
EDIT: The oem bms system might hate you fwiw...

 

[oguruma]

i do this for my job actually lol. life is the best but require specific chargers period. otherwise i would recommend a solid state battery isolator used in audio systems(mechanical ones fail due to carbon fouling from constant arcing of the mechanical connection due to large currents) and a second battery in the trunk if you don't want to fuss around with lithium chargers. maybe a few golf cart lead acid batteries in parallel

there are many systems for timing but they are referred to charge guards. mobile mounts makes a VDPU and ADPU that i recommend. works off alternator ripple/charge or ing input for the timing trigger.

so if it were me id add a 12v honda gel battery in the trunk with that ADPU hooked up with a isolator. the adpu comes with a fuse panel built in so theres no more hardware to buy. this setup allows you to totally kill the back battery that feeds the system and not the starting battery due to the isolator. then the dead battery will be kept off the system with the isolator until it can be properly charged oooooor you turn it on and yolo that thing with all the amps.
EDIT: The oem bms system might hate you fwiw...

The problem is that I don't want to kill the aux battery either (and definitely not the starting battery). If I do use an isolator only, it will still allow the electronics to drain the aux battery when the vehicle isn't running.

Does the VDPU/ADPU have the ability to disconnect a circuit that's not directly in parallel with the starting battery?

On my truck, I use a Powerwerx Voltage timer, but in the case, I just have another AGM battery running in parallel with the starting battery. With that, when the voltage is < 13.5ish Volts (no alternator) the timer starts. That only works with another battery directly in parallel with starting battery, though, which you can't do with a LiFePO4 battery.

So, what I need is something that can sense voltage/ignition and then disconnect a circuit that's not actually parallel to the starting battery.

 

[5doorsoffury]

the vdpu is a timed relay basicly connecting and distributeing a feed so take that into account when drawing your schematic so to speak.

as far as killing the battery set the timer appropriately for the amp your pulling vs the battery capacity and when everything turns off you run the car for a bit. ive done mobile offices and the engine needs to be ran no matter what. we kill batteries with 1000cca deepcycle ratings in 2 hours in the work trucks so ymmv.

 

[oguruma]

the vdpu is a timed relay basicly connecting and distributeing a feed so take that into account when drawing your schematic so to speak.

as far as killing the battery set the timer appropriately for the amp your pulling vs the battery capacity and when everything turns off you run the car for a bit. ive done mobile offices and the engine needs to be ran no matter what. we kill batteries with 1000cca deepcycle ratings in 2 hours in the work trucks so ymmv.

The laptop charges at something like 65watts and has a 45watt/hour battery. What's going to kill the battery is if I add a mobile router/dash cam that runs unabated with the engine off for a long period of time.

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