On 3/3/2016 6:27 PM, firstname.lastname@example.org wrote:
Remember, the internal resistance of a 9V alkaline battery is 1.5-2
ohms. The short-circuit current is about 3-3.25A because the voltage
falls to about 5V when short-circuited. So you'd need a high-wattage
resistor to protect against a short circuit, but only about a 5A diode.
But those cigarette lighter plug battery-keepers are not going to short
out. Even a "dead" car battery is not a short to ground. The diode is
only to stop the car battery from sending current into the 9V battery.
I suspect that they just use a diode. A 20 watt resistor would be
expensive (and large), but even if they wanted to protect against a
short circuit a 5A diode is just a few cents. If they really want to get
fancy they could use a resettable polyfuse but that costs about 10x the
cost of a diode.
On Friday, March 4, 2016 at 4:11:10 AM UTC-5, sms wrote:
You don't need a high wattage resistor. You've completely neglected
that the resistance of the resistor gets added to the circuit. The
backup battery only has to supply a small current to the car when it's
turned off, ~30 ma max for my car. You could put in a 100 ohm resistor.
I was going to check current drain on the battery for our car to
get a feel for how long a 9V "transistor" battery would last.
(The car battery is equipped with a shunt which I figured would
be a convenient way to get at this measurement!)
But, simply turning the ignition to "accessory" (to provide power
to the "cigarette" outlet) made it clear that a 9V battery wouldn't
last long enough for me to raise the hood!
[First, I'd have to check the workshop manual to determine HOW power
is gated to the cigarette lighter outlet; i.e., if there is a
semiconductor switch between the lighter and the battery, then
the car might not take kindly to trying to *inject* power through
For our vehicle, ACC mode illuminates three graphic displays (LCD's),
the instrument panel plus lots of little lights (probably all LED's)
scattered around the dash and console. This would be in addition to
the keyless entry electronics (that are powered regardless of ignition
state) as well as any "memory keeping" circuitry throughout the
vehicle (auto-power window memories, seats/mirrors, etc.), "HomeLink"
remotes (for the garage door opener), cell phone (BlueTooth) interface,
No idea if the concierge-service radio link (and/or "emergency 911")
circuits are powered ("standby").
Of course, you'd have to plug the device into the lighter, exit
the vehicle (which turns on the courtesy light) and close the door
BEFORE disconnecting the battery cables and keep things in that state
until AFTER you'd replaced the battery.
Note that "battery" is available at the OBD-II connector. But, no
idea whether this is directly connected to "Battery" (which means
supplying any power through this connector would power EVERYTHING
that hangs off the battery) *or* if it was designed specifically
to support just the ECU functions (and, as such, could be used as
a "backup battery" source)
Note to self: when it comes time to change battery, drag out a lab
supply to power the *entire* vehicle during that transition (don't
rely on some little battery!)
On Friday, March 4, 2016 at 5:47:56 PM UTC-5, Don Y wrote:
Are any of the small battery memory holders intended to be used
with the car switched on to accessory? I can't imagine they are.
In accessory position a lot of stuff is going to be powered up.
I would think you could either connect them to a cig outlet that
is always hot or hook them up directly to a jumper terminal,
the cables, etc.
On Wed, 02 Mar 2016 18:07:26 -0500, email@example.com wrote:
And when the new 12V car battery is connected, that 9v battery better be
removed really fast, or there's a good chance it will explode.
Sure, the voltage is only 3v more, but you're dumping hundreds of amps
into that tiny battery.
When 9v batteries are used as backup power in devices such as your home
clock radio, they are NOT wired directly into the power supply. They
supply power to the clock chip and are isolated from the power suppply
with special circuitry. If you post this to one of the electronics
newsgroups, I imagine someone could give you an actual schematic.
But be my guest, and blow it up. It's only a one dollar battery and I'm
sure no one will die when it blows, but you wont catch me doing it.
Having hot battery acid all over the interior of my car is not something
I want to clean up. And there is a slight chance it could get hot enoigh
to start a fire, if it's touching flammables.
On Wed, 02 Mar 2016 23:15:41 -0500, firstname.lastname@example.org wrote:
Like i said, be my guest and blow the 9v battery up. I'm sure you'll get
a thrill out of it. You're right, there is no ACID in it. My bad choice
of words, but there are chemicals that will make a big mess. Sure, you
can add a diode and will likely prevent blowing up the 9v battery, but
that diode will also lower the voltage, which may or may not keep the
Then again, this whole thread is stupid. How many minutes (or hours)
will be spent wiring up the 9v battery with a diode and doing all of
this, when it only takes a minute to reset the clock after the car
battery is changed. The whole topic is pointless! Change the battery and
reset the frikkin clock. DONE!
And on Sunday March 13, 2016 @ 2am, that clock will need to be reset
anyhow, when we switch to daylight savings time. Change the battery on
Sat March 12, or Sun March 13, and set the clock one hour ahead at that
If the OP dont know how to set his car clock, that info is easily
avialable online. There's probably even a youtube video about it...
On Thu, 03 Mar 2016 17:56:20 -0600, email@example.com wrote:
It's not the clock. It's the security code on the radio, and possibly
the keyless entry too. It's not hard to rig up a memory saver that is
perfectly safe. With something as simple as a light bulb to limit the
current it is 100% safe. and 9 volts is almost double what is required
to hold the memory..
That said, I just changed the battery on my truck and car last week
with no memory saver - but the car, the newest of the 2, is 14 years
On Thu, 03 Mar 2016 20:18:02 -0500, firstname.lastname@example.org wrote in
Just curious about something. Would connecting a small 12V battery
charger onto the vehicle battery _cables_ before they are disconnected
from the battery work? Something like this
(Amazon.com product link shortened)57098480&sr=1-6&keywordsv+battery+charger
seems like it would be able to retain memory (although not as
convenient as something plugged into the cigarette lighter
Web based forums are like subscribing to 10 different newspapers
and having to visit 10 different news stands to pickup each one.
I don't think folks truly understand just how much data is "stored"
in modern vehicles (granted, OP was referencing a 2010 unit).
Our vehicle has some 200 "customizations" -- PER DRIVER. And, this
does NOT include:
- AM/FM/SXM presets
- "Favorites" (audio/GPS/phone)
- Address Book (Name/Address/Phone/GPS coordinates/icon)
- "paired" bluetooth devices (phones, etc)
- speed dial
- speech recognition "tweaks"
- seat/mirror positions
- GPS "avoid areas"
Many of these settings are time-consuming to make (do you even KNOW what
your radio presets *were*? your GPS destinations?). And, virtually
impossible to "remember" -- even if the name of the setting conjures
up some *vague* recollection as to its intent!
Typing much text on a touchscreen is an exercise in frustration!
And, I have no idea how you'd reload a radio preset for a station
that can't be *tuned* locally! (i.e., do you then have to drive to
that market just to tune in those station(s)?)
One thing sorely missing (at least in our vehicle) is the ability
to download current settings to a thumb drive (ideally, in XML
format so you can edit them off-line) -- as well as the ability to
UPLOAD settings from said drive!
I made a set of cheat sheets (two, double-sided 8.5x11 laminated
sheets that I keep in a seat back pocket) enumerating all of these
settings -- along with their defaults and our "preferences". So,
*some* of these can be restored with just a lot of patience...
[Early on, I learned that the smart way to address GPS destinations
is to create *those* offline and upload them to the car. Anything
created ON the car is stuck there! So, not to be used unless necessary!]
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