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I'm wondering if I have my car battery hooked up to a power inverter hooked up to a battery maintainer hooked up to the same car battery will it prevent me from needing to jumper cable my car?

I often deplete my car's battery As I live in my car. If on occasion I fall asleep with the car in accessory mode and wake up to a dead battery. So I'm wondering if this setup will assist my alternator in keeping the battery fully charged or is this just a waste of time or even worse is it dangerous?

I have multiple car batteries and I and I'm playing with the ideas of ideas of hooking up a second battery and wiring them in parallel. But I have limited funds and limited knowledge in this field. I have appropriate gaged wires and tools. I know how to splice crimp spider determine positive/negative just to give you an idea of my capabilities.

I currently have a 750 and 450 watt inverter. I drilled through the fire wall to run cables into the car and the inverter run all of my devices. Thank you in advanced for even taking the time to read my question(s).

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  • \$\begingroup\$ No that cannot work since it comes from the same source. A recycled marine (deep cycle) battery in the trunk would work better just for some lights, radio and inverter separate from starter battery then charged by main battery/alternator with a headlight bulb in series for a trickle charge. \$\endgroup\$ Aug 28, 2021 at 5:32
  • \$\begingroup\$ Or replace headlight with jumper cable to avoid surge arc with a switch. they use special power diodes ($) to tandem aux batteries that share charge but not share discharge. \$\endgroup\$ Aug 28, 2021 at 5:42
  • \$\begingroup\$ a new 50 Ah 800 CCA battery at 12V can only supply 600 watt hours or 300 W for 2 hrs more or less. All it takes is one bad cell out of 6 in a 12V battery to make it die quickly. We use to test each cell with battery hose and special bulb to ensure fluid was full and acid strength was matched (Specific gravity). Once a battery is discharged fully it gets worse fast. Battery pulsers can rejuvenate however. \$\endgroup\$ Aug 28, 2021 at 5:49
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    \$\begingroup\$ No, it's just an expensive way to waste battery energy. Think, if putting a few things around a battery could keep it charged, then everybody would be doing it. Get a second battery, dedicate one to running the vehicle, one to powering devices, and use a commercially available switch designed for the purpose (they exist) to keep them charged but separate so you can always start. Keep them topped up with a solar panel if you can. \$\endgroup\$
    – Neil_UK
    Aug 28, 2021 at 5:50
  • \$\begingroup\$ The laws of thermodynamics does not allow this. The energy lost to heat from your battery self-discharge needs to be taken from somewhere else to be replenished. \$\endgroup\$
    – winny
    Aug 28, 2021 at 9:16

2 Answers 2

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As describe it cannot work, it is the perpetual motion machine that would defy the first law of thermodynamics. On the plus side, nothing will go bang, you will end up with reduced battery life.

If you often deplete you cars battery, you best solution is to find a source of power such as solar, or shore power and a battery charger.

A second, but a long way behind, option is to add another battery. Simply wire it in parallel will work, but one battery will discharge more than the other. To wire in parallel properly, you need to balance the wire lengths between each battery and the load.

If you already have a second battery, you would be best to do what most RV users do. Use one as a house battery for running your devices and one for the main car starting. Connect them though a Voltage Sensitive Relay (VSR) , which only connects the batteries when the voltage is above a nominal (usually 13.8V) amount. This way you can always start the car.

Note that charging a lead acid battery takes time - 8 hours or more. Charging only form the car is not good for the battery unless you spend a lot of time driving.

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It seems that you hope to have your battery charge itself.

Using the 750 W inverter for an hour you'll have the following result:

  • Energy out of the inverter = 750 Wh.
  • Energy consumed by inverter = 750 Wh / efficiency = 750 / 0.75 (guess) = 1000 W.

By now you should be able to see where this is headed.

  • Energy into battery maintainer = 750 Wh.
  • Energy out of maintainer = 750 Wh × efficiency = 750 × 0.8 (guess) = 604 Wh.
  • Nett charge = energy in - energy out = 604 - 1000 = -396 Wh.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The self-filling reservoir.

What you are describing is like trying to top up a reservoir using its own water to do it while using a pump that leaks a lot of water away down the river. You'll only end up draining the reservoir.


Note that for the inverter I have started with the output power so I divided by the efficiency to get the input power. For the maintainer I started with the input power so I multiplied by the efficiency to get the output power.

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