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vehicle battery voltage

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No lower than 12.4 volts if you want to avoid replacing your battery annually or even twice annually.

I've experimented with this for several years. Auto starting batteries aren't intended to lose more than 10% of the charge. Damage begins below 90% charge remaining and accumulates with each occurrence.

This is the open circuit voltage, when there's no load on it. If there's a load on it, you can't determine charge state from the voltage.

If you replace your battery with a combo marine/start battery, you'll get better performance.

Bottom line is...don't subject your vehicle battery to cab loads. Battery and alternator are engineered just for the engine's needs.

If you still want to do it, buy an Optima Spiracell Yellow-Top battery. Read the manufacturer info on it for clues as to how low you can let your open-circuit voltage go without damaging it. Leave a healthy margin in there to start the engine and budget for a new alternator every once in a while.


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John, look into installing a second battery with a cross over switch. Camper truck item. Then check out adding a solar panel charger.

A high compression engine will obviously need more battery power than that little 4-banger you got, [:)] which might start back up with 11 volts from the battery. Get a crank for it. [:)]

The International 6 cylinder, about 230 CI, can be started with a crank. Mine was a '52.

I restarted my mom's VW bug by rolling it off the road into a wide ditch and popping the clutch in 3rd, neat trick. That was also a 6 volt. A long song on the radio could kill that battery.

It's true,Tube radio, probably drawing 8 amps.

Marc is correct as usual, that the car battery is not the best power source for your office, unless you are running the engine.

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Gasoline auto engines weren't designed for idling. A new engine block I put in my Toyota lasted one year before it started burning oil again, from idling.

My current vehicle is a diesel but I won't idle that either, not since they reduced the sulfur content of diesel. It makes the fuel too dry and idling will ruin the block.


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John, it doesn't hurt your battery to have light parasitic draws. Most vehicles these days have continuous draws for the various keep alive systems on the vehicle. Put a marker bulb in series with a battery cable with all systems off and some vehicles (Fords mostly)will smoke the filament, others make it glow brightly and almost all of the rest will make it glow dimly.

The ECU, BCC, Radio and ICC are all always on. Plugging in your phone or flashlight won't make a bit of difference as long as voltages don't drop below 11.25 volts or so.

Marc is correct saying gas engines don't like to idle- they're most efficient at wide open throttle. That said, today's computer controlled engines will idle for years at a time with no carbon build up and no ring fouling. If you monitor the data streams from the ECU you'll see that it quickly adjusts pulse width to accommodate the less than ideal conditions that idling presents and within 4 or five minutes you'll see LT fuel trims leaning out in response to ST trends.

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