video survaillance

Hello All, I have a 2006 BMW 530i, My vehicle has been vandalized 2 times at my workplaces' parking lot. The lot does not have surveillance so I want to install one in my car. I"m looking at several of them that record to an SD mem card, HD and even have IR for night recording. My problem is power. I haven't found any that can record for more than 4 hours on its built-in rechargeable battery, I need it to record for at least 8 hours. The camera(s) come with a cigaret lighter charging cable, however none of my 12v cig outlets are "hot" while the car is off.   The cams I've been looking at have an input voltage of 5 to 7 VDC, (the provided charging cable must be stepping down the voltage). Having said that, can anyone shine some light on what my power option are so I can have this cam work for 8 hours?

It's essentially a case of your battery reserve. You could use a second battery (via an isolator), or add a battery protector aka low voltage cut-out or LVCO (in either case - whether to leave enough cranking capacity or protect your 2nd battery), or determine what you have to play with. The latter might be based on allowing (say) 10% discharge. Hence if it's a 60AH battery, you have a max of 6AH = 6AH/8hours = 0.75A (or 12V x .75A = 9W) available. (Mind you, as the battery ages and loses capacity....) The 10% is based on the ROT (rule of thumb) that a cranking battery should not be discharged more than 20%, but that's for a reasonable or rated battery life, and it does not mean that the car will still crank and start with a 10% or 20% discharged battery, though they usually will. (The ROT used to be 30%. That might have been with thicker battery plates before higher cranking & load currents became the norm.)

Go to your local stereo shop and see how much they will charge you to wire one of your cigarette lighters to constant instead of switched power.

oldspark wrote: It's essentially a case of your battery reserve. You could use a second battery (via an isolator), or add a battery protector aka low voltage cut-out or LVCO (in either case - whether to leave enough cranking capacity or protect your 2nd battery), or determine what you have to play with. The latter might be based on allowing (say) 10% discharge. Hence if it's a 60AH battery, you have a max of 6AH = 6AH/8hours = 0.75A (or 12V x .75A = 9W) available. (Mind you, as the battery ages and loses capacity....) The 10% is based on the ROT (rule of thumb) that a cranking battery should not be discharged more than 20%, but that's for a reasonable or rated battery life, and it does not mean that the car will still crank and start with a 10% or 20% discharged battery, though they usually will. (The ROT used to be 30%. That might have been with thicker battery plates before higher cranking & load currents became the norm.)

OK, I'm a bit unclear,based on the formula, I'm understanding that in an 8 hour period the usage is .75amp is that correct?

Yes. Though that is an approximation because battery capacity depends on its discharge rate. Car etc batteries use a 20-hour discharge rate, aka C20. A 60AH C20 battery might be rated as 40AH @ C5 (5 hour discharge period). The AH is the product of discharge current x discharge time. That means a 60AH C20 battery should supply 60AH for 20 hours which equates to 60Ah/20h = 3Amps for 20 hours. (Hence ~12V x 3A = 36Watts for 20 hours.) Assume you limit discharge to 10% (ie, 90% capacity remaining for cranking etc), then that allows 60AH x 10% = 6AH. 6AH over 8 hours is 6/8 = .75A for 8 hours. That means ~12V x .75A = 9W.... ... so the camera should be no more than 9W. (You can size a 2nd battery from the above... If the camera is 1A, then that is 1AH per hour. 1AH x 8h = 8AH. So if a deep cycle that you will take down to 50% discharged, that means 8AH x 2 = at least a 16AH battery. Or a cranker to 20% discharge means a 8AH/.2 = 40AH battery.) That's assumes a full capacity battery - ie, not old. You'll have to adapt for your battery size and desired discharge limit. I chose 10% as "half of the allowable 20% ROT discharge for cranking batteries. You might decide 5% etc. The less the better. The more discharge a battery is, the quicker sulfation builds up which eventually destroys the battery (if not removed whilst still "soft" by fully recharging and (occasional) equalisation charges.

oldspark wrote: Yes. Though that is an approximation because battery capacity depends on its discharge rate. Car etc batteries use a 20-hour discharge rate, aka C20. A 60AH C20 battery might be rated as 40AH @ C5 (5 hour discharge period). The AH is the product of discharge current x discharge time. That means a 60AH C20 battery should supply 60AH for 20 hours which equates to 60Ah/20h = 3Amps for 20 hours. (Hence ~12V x 3A = 36Watts for 20 hours.) Assume you limit discharge to 10% (ie, 90% capacity remaining for cranking etc), then that allows 60AH x 10% = 6AH. 6AH over 8 hours is 6/8 = .75A for 8 hours. That means ~12V x .75A = 9W.... ... so the camera should be no more than 9W. (You can size a 2nd battery from the above... If the camera is 1A, then that is 1AH per hour. 1AH x 8h = 8AH. So if a deep cycle that you will take down to 50% discharged, that means 8AH x 2 = at least a 16AH battery. Or a cranker to 20% discharge means a 8AH/.2 = 40AH battery.) That's assumes a full capacity battery - ie, not old. You'll have to adapt for your battery size and desired discharge limit. I chose 10% as "half of the allowable 20% ROT discharge for cranking batteries. You might decide 5% etc. The less the better. The more discharge a battery is, the quicker sulfation builds up which eventually destroys the battery (if not removed whilst still "soft" by fully recharging and (occasional) equalisation charges.

WOW! oldspark, I'm going to have to read that over a few times to try to understand it better. Seems like this is going to be a trial and error type of thing. I'm going to buy the cam and try it out, I'll wire up one of my 12v socket so its constant and go from there. Thanks for your help your a knowledgeable person.

No need to quote my replies (IMO!) - it is taken that a reply refers to the last. Otherwise I clarify what post I am replying to if not obvious nor the OP's, and when quoting, I will only quote the relevant part(s). In my world, that's standard "etiquette". Sorry - that's a bit of an aside. but having recently whinged to someone (another forum) about wasted bytes, screen space, and paper...    And do not read the above as an attack etc - I merely meant it as a suggestion or tip! Now, back to business... THANKS for your compliment. (But) Though knowledgeable, it doesn't mean I'm correct! But maybe find the specs of the camera(s) you seek and post it if you want to size a 2nd battery, or calculate the drain on yours. Obviously as low a powered camera as possible bu with the required features. (Warning - many camera systems suffer from an inability to identify cultprits - cheap low-res or poor CMOS quality etc.)    I recall many cameras being 9W or about .75A = 750mA draw (@12V, though conventionally 9W/13.8V = 650mA @ 13.8V), but near-enough is good enough. Battery capacity is not an exact science (those beasts live and breath and age!!). And I/we understand that your "camera load" means the lot - ie, camera and recorder or transmitter etc, and maybe other bits like a low-voltage cut-out circuit and relay (ie - 10mA + 100mA)... Many vehicles have other drains anyhow (alarms, remotes, etc) that mean many tens of mA drain, hence requiring a drive at least once a week... There are lots of ways to tackle this... Assuming cranking/starting is crucial, then many might chose a second battery. But for daily starting, why bother? That is where info from others hereon is VERY useful. (I see myself as an "experienced theoretician", these others are "knowledgeable practitioners".) FYI - IMO the ideal is either a low-voltage cutout to disconnect alarms & remotes etc to leave sufficient starting capacity, else a 2nd battery. The 2nd battery can be smaller to only cater for alarms etc... Or why not a 2nd cranking-capable battery so if your main fails, you simply swap, or jumper? There are qualifications with 2nd batteries - ie, automated battery isolators, AGM if not in the engine bay, etc. But I use the UIBI isolator which is essentially just a relay sized to my choice ($3 for 30A etc). (Just Google " uibi isolator".) [There are some very recent posts re relay connecting issues (with batteries) that are quite amusing... And not that I mentioned quoting of FULL lengthy replies for no reason... LOL!]    My 2nd battery is whatever spare battery I have (typically ~40AH AGM and wet cells, interchanged with the main battery) after all, one day my main battery will fail to crank.