The only problem I see with the timer on the car is that it won't keep time when you're not plugged in. So when you're out driving around, the timer isn't turning, and you'll have to get in there and reset it every day.

They make heavy duty weatherproof timers that you could put on the garage wall or outside wall of your house and just wire up a GFCI outlet to it. Set it to come on a couple hours before you normally leave for work, and all you have to do is plug in the car every evening.

http://www.homedepot.com/cmc_upload/...75007406_4.jpg

I agree that it's only needed for a few hours, but my point was that the energy cost is negligible. Plugging it in probably saves more than $0.50 of fuel when you start it cold. I only plug mine in a few nights per year (when it's going to be below about 10 F), I've never bothered to buy a timer.

Craig,
can you describe the trickle charger you have mounted? I really like the idea of having one on the car but I'm a little scared due to what J Davis posted about one ruining a battery even though it wasn't being used.

heck
 

1995 e300d 178,000.
Even in Northern Virginia, inside my attached garage, I plug my car's block heater in overnight. I drive my wife and daughter to work and school and its worth $.50 to hear them ooh-ing and ahh-ing when the heat blasts out before we hit the end of the driveway. I kind of like it too. My 85 300td stays on tender always as it is only driven during hunting season. Wish that one also had a block heater. I get in the 85 some mornings at 3:00 a.m. and its coooold.

The one I have is designed to be left installed, it's a "tender" type charger that has a bracket that fits under the battery. I don't remember which brand I have, but it's something like this:

http://www.pacificbattery.com/ob15.html

I bought it few years ago at an Interstate Battery store, as I recall it was pretty cheap, maybe $30 or $40. I have it connected so it works whenever I plug in the block heater. I've never had any problem with my battery draining.

As a side benefit you are slightly heating your garage with the 400 watt heater in your block. The heat is there doing what it was meant for, heating you block. The heat that escapes is not totally wasted, it heats the air in your garage.
-Jim

All,
As an added bonus, the trickle charger also adds a little heat to your battery. Not much, but the battery looses performance as it gets colder so anything helps.
-Jim

2.5Turbo,
The problem of the timer not keeping time while unplugged was what made me add the line about "Maybe I'll just not worry about it!"
-Jim

The risk is only if you keep the charger attached, unpowered, for months on end without driving the car. Overnight will do no harm. Sitting for a week will do no harm. J Davis's problem came when he was gone a long time and had to leave the car sitting. In fact, he was on the cusp of the next problem...if you have to leave a car sitting for a long time (~6 months), you should remove the battery and store it somewhere safe.

Except that mine was a boat battery. And it was removed from the boat. And it was somewhere safe. I just forgot to unplug and unhook it. :)

-Jim

I went back and re-read your original post on this subject. Interesting. It would be worth testing some trickle chargers to see if they draw current when unpowered, and how much. I wonder if a component in your charger failed? Even new diodes leak a tiny bit (usually only a few microamps), of course. Hmmm, an interesting problem. It certainly bears remembering.

Pretty much any charger will draw some current. Diodes have a very high resistance in the reverse direction, probably talking microamps here not milliamps, a dashboard clock will draw many times the current of this reverse-resistance load.

If you want to see if it is significant, and don't have a DC ammeter, try the smallest light bulb in series with the charger. Oh, another thing. If there is any kind of sensing circuit for reverse polarity hookups etc., such as a red light, it too could draw some current.

Typically when I store batteries long-term I use a small "ice-cube" type of 12v converter and a timer, have it hooked up and charging for an hour or less per day. The internal resistance of the battery isn't infinite either, it will self-discharge over time.

A blocking diode installed on the positive wire going from the charger to the battery would totally prevent this from happening. (By this I mean the problem of destroying a battery with a charger attached but not plugged in.)

http://www.the12volt.com/diodes/diodes.asp

I've done some poking around. The trickle charger I have that I might install is a 1A trickle charger. This diode, http://www.radioshack.com/search/index.jsp?kwCatId=&kw=1n4001&origkw=1N4001, is rated at 1A continuous and 30A surge. It can withstand 50V applied to it of the wrong polarity without allowing reverse current. It is 59 cents from Radio Shack. For less than a dollar, plus some soldering I can make sure that my charger will not kill the battery.

-Jim

My charger was an old Sears 2/10 amp charger. It died a few years ago. Remember that I had mine attached to the battery for a LONG time, 6 months, with it not plugged into the wall.

-Jim

Use a relay
 

I know I have heard that leaving a charger connected to a battery shouldn't drain it, but I don't want to take any chances, (and I needed something to tinker with at the time) so I mounted relay in the engine compartment (inside a box)....House current clicks the relay "on", and connects BOTH the neg. and pos. leads from the charger to the battery. When I unplug it, both pos. and neg. are disconnected so that the charger is COMPLETELY isolated. (I didn't use a standard Bosch 12V single pole relay....I used a "double pole" 120V relay with blade connectors that plugs into its base...I mounted it onto a piece of oak and covered it with an inverted deep grey plastic wall outlet box, and cut slots for wires. I attached the whole unit to the wheel hump)

(Nice idea with the Radio Shack diodes....more compact than my system!)

I figure that if the battery is good, it really does not need to be on the charger all night....I have the system set up so that the charger and the block heater go on at the same time at about 3 am.

What I am looking for now is a nice small low profile ceramic heater, about 750 watts, that I can mount out of the way in the passenger footwell, and hook it up to the same system so that in addition to heating the engine and charging the battery, the cabin will be nice and toasty in the morning. This does make me a little nervous....I don't want to melt the interior, or worse, set the vehicle on fire. So I am not looking for a cheap $10 piece of crap.

Regards,

Mark

That is a sweet idea with the relay! I've never really used them before so it did not come to mind.

On the diode idea, is 1 amp enough? I know that is what the charger is rated for so it should be.

750W + 400W + maybe 50W for the charger = 1200W. That is a good bit of power. That is 10 amps of current for 3 hours. Is it really that cold where you live that you need 750W of heater in the car? Heck, an electric blanket running for 3 hours would put a good bit of heat in the car. Or maybe this:

http://www.jcwhitney.com/autoparts/Product/tf-Browse/s-10101/Pr-p_Product.CATENTRY_ID:2011034/p-2011034/N-111+10201+600002307/c-10101

It is 900W, but is like what you are looking for.

-Jim

Thanks guys for clarifying things. I was in Wal-mart today and saw they had a Black and Decker charger for only $26. I may go back and purchase it.

If I have this straight, so long as the charger is either plugged in and used every few days, or even if it's not, so long as the car is started and run every few days, there is no chance of the charger doing any harm to the battery or anything else even without installing a diode. Do I have it right?:stupid:

And it's fine to leave it plugged in not only overnight but even for a couple of days?