Best to not let your car battery get below 12.4V or you will lose some of it's service life. But that won't give you lots of parking recording time, so you have to trade some battery life if you want more time. I personally think 12.2V is a good compromise, and 12.0V can be acceptable knowing that you may be replacing your battery 20% sooner than if you ran no cams. Also not that many folks have measured variances of + or - 0.2V with their kits, so best to actually measure the voltage after selecting your setting
I've got a large but cheap battery and I push the limits pretty hard. I've done the math based on what my previous battery did and those results came to be a cost of about $0.25 per day per cam ($0.50 per day for a 2-channel cam). I'm OK with that but if I had a car which used a small expensive model-specific battery them I might
not be OK with the cost numbers it produced
The average single-channel cam draws in the neighborhood of under 1AH in recording. To make the math easier, and to make sure any errors are on the safe side that's the figure I used with my calculations though most actually draw around .75AH
I have an 80AH battery which costs about $140 today (it was $110 when I did my figures). You begin shortening a lead-acid car starting batteries service life when you draw more than 10% of it's capacity, so I'm safe with 8 hours of single cam recording. I use double that figure for a 2-channel cam though it's probably more like 1.5AH draw, again just to play safe. And I record about 12 hours every night, so I'm using somewhere around 20% of the battery capacity, which is going to shorten the battery life about 10%- the excess amount over the safe level. I know I get around 5 years service with the battery and no cams because I've driven this style of van for over 20 years now, so you can figure my costs from there (it's too early in the morning for me and math today
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I never checked voltages so I can't say about them any more than to note that those numbers I recommend seem to align closely with my actual experiences. And since I'm not using any type of low-battery-cutoff, there have been a few instances where I forget to turn my cams off and killed the battery along with a few other times where my headlights did similar
I can use one of my tool batteries to get myself going since it's my workvan, but the average driver would need to carry a jump-pack for similar results. Because of the excessive abuse my 2-year old battery is getting quite weak and will need replacing in no more than a couple months time. Ouch. And a good lesson for others about why you should have and use a HWK with a sensible cut-off setting, especially if your vehicle has a small battery the way most now do
Phil