Acceptable Current Consumption in Auto

[Newz2000]

What do you consider to be an acceptable amount of current for an on-board computer to use in an automotive environment while the car is not running? For example, the computer (pic based) will be sitting around waiting for something of interest to happen (Marvin, the robot from the book Hitchhiker's Guide to the Galaxy comes to mind when I say this). I'm planning what accessories can be included and I want to ensure not to over-do it.<p>The car will be driven regularly. It will also have a car-stereo installed which will likely consume some current too. The circuit will have much more than just a Pic processor, FYI (maybe a disco ball! Just kidding).<p>Thanks.<p>[ July 24, 2005: Message edited by: Matt Nuzum ]</p>

[sofaspud]

This is one of those "your mileage may vary" answers, but here's what I'd say.
I would target something no greater than the low double-digit milliamp range. But also, I think I would determine what are the "have-to-have" features so that the final product uses as little power as necessary. I don't know what the car stereo configuration is, but generally they only need minimal power for maintaining the clock and presets in memory. My guess is that can be ignored as it's likely in the microamp range.

[Chris Smith]

The safe way to not come out and find a dead battery, when its least convenient, is the second battery under the hood.<p> A simple battery isolator will charge both and stop any discharge from the main battery from going to anything when the car is off, while the wiring on the second battery goes to all your accessories only and you don’t run the risk of discharging your main car battery.<p> This way It doesn’t matter if you screw up on your discharge to your accessories because only the second battery will go dead if you draw too much, or don’t run the car for a while, leaving your main battery safe and charged for the car. <p>If you really want to go fancy, a second alternator can also be installed for fast charges and more banks of batteries with out adding any stress to your main Alternator. <p>Even Acid traces across the top of a battery have been known to discharge a battery over time. And that, is a small draw.

[haklesup]

10s of mA would be OK for a good battery but in winter on a weak battery, it might be enough to kill it overnight.<p>The capacity of your battery will give you the answer. <p>A good way to power the project and make sure there is always enough juice to start the sar is to wire it through a low voltage cutoff switch. I don't have a circuit handy but many DC-AC inverters for cars have them built in. It basically turns you off before the battrery is too dead to start the car. Avoid relays as they use lots of power.<p>Before you make any alterations, measure what the car uses now when off. Simply remove the battery cable and place an ammeter between that wire and the trerminal. Do this with the car off and the key out. This will give you a baseline to compare to. (it is also a useful diagnostic for cars that kill batteries)

[new guy]

A " normal" KEY-OFF battery drain depends on the car and this spec can be found in the proper service manual. I have seen from 12ma to 20ma and this is perfectly normal. AGAIN it depends on the car. I would be concerned if the drain went above 50ma. BUT FIRST I would consult the proper service manual for that particular car! You may be able to go to your neighborhood auto technician to look at his service manuals for the info.

[Robert Reed]

Matt
The rule of thumb for lead acid automotive batterys once fully charged has been a trickle charge rate of 1 milli-amp per amp hour of battery capacity, just to maintain charge and offset internal leakage.This drain occurs at the 13.2 volt float level. At normal terminal voltage (12 .6 VDC) ,this drain would be much reduced, possibly half that amount.This would indicate that a 50 amp hour battery standing alone would discharge it self through internal leakage at a 25 milli-amp /hour rate. If your load were even double this amount, a healthy battery wouln't pay much attention to it, However, I think the guys are giving you good advice about a second battery and isolator. This could be as simple as a pair of suitable diodes (or just one for the second battery) and a small garden tractor style battery hidden in the trunk or under the seat.And #16 wire would certainly be up to the task.<p>[ July 25, 2005: Message edited by: ROBERT REED ]<p>[ July 25, 2005: Message edited by: ROBERT REED ]</p>

[philba]

Seeing numbers like 12mA and up, you should be able to do a LOT better than that using a nanowatt PIC, low power sensor circuitry and a design that lets your PIC stay in sleep mode most of the time (sensor ints wake up the processor). I bet an idle system current consumption could be kept under 1 mA.

[sofaspud]

I'm with philba. Supplemental SLA's and alternators is overkill, unless possibly there is a 1992 battery starting a 1977 Dodge in the dead of winter.

[philba]

by the way, I think a very interesting topic would be low power engineering. I've gathered a few tricks and thoughts but I'm sure I could learn a lot more.