# Spec'ing an always-on alarm/tracker for a car battery

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?

• 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. – kenny Jan 23 '13 at 19:37
• 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. – Gustavo Litovsky Jan 23 '13 at 19:56
• 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. – cnst Jan 23 '13 at 20:10
• @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. – cnst Jan 23 '13 at 20:49
• 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. – cnst Jan 25 '13 at 3:27

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