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What is the specification of a standard car battery (e.g. of a 2008 Jetta)?

Say, I want to connect an always-on alarm or a tracker to the electrical system of the car.

Is it likely to deplete the charge of the battery such that I won't be able to start the car? Are OEM batteries or the always-on circuitry in modern cars protected from this kind of thing?

Also, suppose the device I want to install has its own backup battery, and specs of such battery are provided, as well as the time that it's supposed to power the unit for. Considering that car batteries should supposedly never be fully depleted in order to remain capable of holding the full charge and of powering the starter, how would I do the math to calculate how many days I can leave the car undriven for? Once driven, how long would it take for the battery to recharge?

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  • \$\begingroup\$ If you plan on using a lot of current, more than a trickle, I would consider having your own battery and hooking it up to the ignition on wire to charge your battery when the vehicle is running. \$\endgroup\$ – kenny Jan 23 '13 at 19:37
  • \$\begingroup\$ Make sure to protect any circuit you make for the car from the EMI and noise of the car's electrical system. It's very hard on electronics and they need to be properly protected. \$\endgroup\$ – Gustavo Litovsky Jan 23 '13 at 19:56
  • \$\begingroup\$ No, not a lot of current at all: obviously, an always-on specialised alarm/tracker that can sustain a day or two on its own cheapo "mid-size" battery is likely to run quite some time on a much-bigger car battery. However, the question of "how long" is still there. \$\endgroup\$ – cnst Jan 23 '13 at 20:10
  • \$\begingroup\$ @GustavoLitovsky, I plan on buying a pre-made unbranded device like TK103B or similar. Some of them are offered with some backup batteries that are supposed to power the device from 8 hours to a couple of days in the fully-tracked mode. Said Li-ion batteries usually have the spec of something between 1 and 3 mA·h at 3.7V. \$\endgroup\$ – cnst Jan 23 '13 at 20:49
  • \$\begingroup\$ I've looked at my VW A5 / 2008 Jetta 2.5L SE, and its battery has the following markings: 1J0 915 105 AD, 12V 61 Ah 300A DIN, 540A EN/SAE. So, seems like a 60 A·h battery, which is 720 W·h. \$\endgroup\$ – cnst Jan 25 '13 at 3:27
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Battery capacities are usually provided in amp-hours or milliamp-hours. A quick search shows that typical car batteries have a capacity of around 45 amp-hours. That means with a one ampere load, it has enough energy to run for 45 hours. This isn't the most accurate way to think of capacity, since the total energy available depends on the manner in which it's used (quickly? slowly?) but it should suffice for some back-of-envelope calculations.

So to estimate the runtime, divide the capacity in amp-hours by the average current requirements of your device. If your device requires 50 mA on average, then:

\$ 45Ah / 50mA = 900 h \approx \text{37 days} \$

Most car batteries are not designed to be deeply discharged as part of normal operation, so avoid that. It's hard to say if your car has any sort of protection against running the battery down enough that it won't have enough to start. It would depend not only on the car, but also on which circuit the device is connected; some may have protection while others may not. I know my car does not offer any protection (even the dome light will run the battery down), but as the car in question gets newer and more expensive it becomes more likely there will be some circuitry that will do something intelligent.

If your device has batteries of its own that will power it should the main power be lost, then you can simply add that runtime to the estimate runtime of the main battery.

It's hard to say how long it will take to recharge the battery. It will depend on the charging method (fast charger? slow charger? the car's alternator?) as well as the type of battery, but something an hour and 2 days is likely. See this page from Interstate Batteries for more information on that.

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