Flat battery

Presumably if you're a car manufacturer you install a suitable socket (fuse protected but unswitched) on the wiring loom under the dashboard. It doesn't have to be particularly high current for a small solar charger.

Reply to
Roger Hayter
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Less than a minute. It simply occurred to me to see if it would start.

Reply to
Bert Coules

The OP said that he doesn't have any method of measurement, unfortunately. Might be simplest to charge for 4 or 5 hours, then turn off the charger and turn the headlights on for a minute:

  1. Headlights don't come on - battery buggered. Replace asap.
  2. Headlights come on and stay on - battery ok. Continue charging.
  3. Headlights come on and slowly fade - battery on the way out. The car might start and get you from A to B, but I wouldn't guarantee getting back from B to A! Replace battery asap.
Reply to
Jeff Layman

Thanks for that. Sounds like a good scheme.

Reply to
Bert Coules

If the battery is knackered, won't that impedance increase? As an endpoint, imagine if the battery was completely dry - it's open circuit and so wouldn't accept any current at all, and all the current would be put into the car electronics.

Would sulphation raise the impedance?

Indeed. Although I wouldn't be surprised if some weren't as tolerant as they should be (also that aftermarket radio you got from China...)

Theo

Reply to
Theo

Ah, but is that via the physical steering colum switch or a relay/solid state one buried in a BCU that could be set to "on" in deep leep mode?

Reply to
Dave Liquorice

I have a dashboard push button; but no idea about the deep sleep mode.

Reply to
charles

Car electrical systems need to have a wide voltage tolerance. Thus occurs during "load dumps".

The voltage seen on a car electrical bus *can* spike, but the electronics are also specified to survive this.

This is one reason you should not connect relatively "naive" electronics to the raw electrical system of the car. (A "car stereo" is not the same as a "home stereo" from a "width of supply rating" point of view. The "car stereo" is just a bit armored by comparison.)

You might ask yourself, if the car battery impedance is as low as we think it is, how do *70V spikes* appear on the car electrical bus ? As a Natural Philosopher, you can go find an answer for us. I haven't seen any oscilloscope pictures of car electrical systems which prove these spikes are present.

As an illustration of this, the specsmanship, we can use a TDA2003, an audio amplifier intended for cheap car radios.

UTC TDA2003 LINEAR INTEGRATED CIRCUIT

ABSOLUTE MAXIMUM RATINGS(Ta=25°C)

PARAMETER SYMBOL VALUE UNIT Peak Supply Voltage Vs 40 V DC Supply Voltage Vs 28 V Operating Supply Voltage Vs 18 V

Nominal

Supply Voltage Vs 8 to 18 V

Now, that's an integrated circuit, where the nominal battery voltage might be 12.6V, or maybe 14.4V fully charged or under charge or whatever.

The 18V value is what results, if you jam the field winding to battery, as an alternator test. The alternator can run the battery up to around 18V, by forcing a lot of current into the battery. Your mechanic at the shop might try this. That's why this stereo expects on occasion, to see 18V, and the car radio will continue to play.

Well, what do the other voltages mean ?

The 40V value is a measure of load dump. If there is a transient on the car electrical bus, the 40V value can be achieved. The TDA2003 is designed to disconnect itself, at the 40V level, in an attempt to protect the IC innards. The behavior of some part of the IC changes, when the supply goes over 18V.

BUILT-IN PROTECTION SYSTEMS

LOAD DUMP VOLTAGE SURGE

The UTC TDA2003 has a circuit which enables it to withstand a voltage pulse train, on pin 5, of the type shown in Fig. 23.

If the supply voltage peaks to more than 40V, then an LC filter must be inserted between the supply and pin 5, in order to assure that the pulses at pin 5 will be held within the limits shown in Fig.22.

A suggested LC network is shown in Fig.23. With this network, a train of pulses with amplitude up to 120V and width of 2ms can be applied at point A. This type of protection is ON when the supply voltage (pulsed or DC) exceeds 18V. For this reason the maximum "operating" supply voltage is 18V.

There can be spikes on battery, but you would hope that things like CANBUS or whatever, the spikes would not be seen there.

What is the battery doing when those spikes are present ? You got me there. Looks like the battery impedance at frequency, is insufficient to deal with the spike in any useful way. It suggests an impedance versus frequency curve, of weird shape or behavior. Even bench DC power supplies, have impedance versus frequency curves.

A cheap battery charger, is a half wave or full wave rectified device running off a transformer. A pulsating DC waveform is being applied to battery, but staying at relatively low voltage levels. A good reason for not making sparks around the battery area, is igniting any evolved hydrogen (more of a problem around the battery types where the battery caps are removable, as those have vents). It's for that reason, you could connect the charger while it's off. The electrical spike issue, the car design has taken some of that into account, with load dump specs and the like.

If you're jump starting a car, that's usually done with "hot" cabling. The cabling is connected in a particular sequence, to keep "sparks" away from the battery. You can connect the "hot" to the "hot" first, without sparks. The "ground" alligator jaw, gets fastened to a screw on the metal framework, rather than to the battery terminal, and this keeps the spark away from the very top of the battery. As it's when the second lead is connected (or disconnected), when a small spark flies. When jump starting a car, those "sparks" near the frame, also hint about the state of the two electrical systems on the cars, such as if the second car is a dead short or something. You might see very large sparks or "welding behavior", if the second car had a dead short of some sort. It's better to be making sparks like that, near the frame, than near the top of the battery.

Jump starting a car, is a very good time to be testing the limits of load dump specs.

Paul

Reply to
Paul

Ah - that's why it wouldn't the next time. It's also bad for the engine to start it just for a short time. Ideally, take the car for a drive, or warm it up fully before stopping it. Blip the throttle a few times to while doing this - don't just let it idle.

Reply to
Dave Plowman (News)

Yes to both. But then that is two faults. An incredibly crappy charger and a totally knackered battery.

The old chestnut that only China makes rubbish. ;-)

Reply to
Dave Plowman (News)

BMW have special depot at Southampton where news cars are 'woken up' as part of the PDI. heard this phrase used on a documentary not that long ago.

Reply to
Andrew

I have always done that. Or the boot, depending where it is..

Reply to
The Natural Philosopher

Depending on the motor it takes about 200A to start a car for about 5 seconds.

If a reasonable charge current once started is 5 A that needs 200 seconds to replace the charge.

Or about 3 1/2 minutes.

These figures are well within an order of magnitude. If you start a car you should not routinely stop it within 5 minutes if you can avoid it

Reply to
The Natural Philosopher

This is more commonly referred to after jump starting a car. Not using a battery charger.

And I'd take that with a pinch of salt too.

Reply to
Dave Plowman (News)

Nor have I. Despite playing with car electrics more than most. Except if you were stupid enough to disconnect the battery with the engine running.

As the saying goes, you can make things fool proof. Idiot proof, even. But not c**t proof.

I don't know of any modern alternator where it's remotely possible to bypass the regulator. Certainly not by the average grease monkey. Who knows very little about car electronics, other than fitting new parts.

Love to know where you'd by a cheap battery charger that has a transformer these days. SMPS have been cheaper for many a year.

Reply to
Dave Plowman (News)

I have a very accurate voltmeter fitted to the old Rover. Alternator on that is a slightly more modern unit with 100 amp output. Assuming the battery is reasonably fresh to start with, the voltage, after a cold start will have dropped back to the normal point after a drive to the shops. Say a couple of miles of town driving.

Reply to
Dave Plowman (News)

Bert Coules presented the following explanation :

Better to make the negative connection last and make it first. It is also better practice to make/break the negative connection away from the battery, assuming the battery is connected to the car - connect the negative of the charger, to the car chassis/engine block or etc.. It minimises the chance of a spark igniting gas from a battery.

If you cannot connect away from the battery, always try to waft some air across the battery, to dispel any gas.

Reply to
Harry Bloomfield, Esq.

Dave Plowman (News) formulated the question :

....and if the battery is faulty and open circuit???

Reply to
Harry Bloomfield, Esq.

Bert Coules was thinking very hard :

That is neither a good, nor bad sign. Corrosion just means there has been some acid leakage around the terminal post.

Reply to
Harry Bloomfield, Esq.

Nothing wrong with your figures, but I don't understand the relevance here. All I was trying to point out is a battery which will charge - sort of - but won't hold a charge for very long. It'll lose that charge over a period of several minutes or maybe an hour, and so needs replacing. If the OP stops for less than a minute after driving around with that battery and then tries to restart the car he /might/ get away with it. He might not. The headlight test should show if the battery is dodgy enough that it could fail to restart the car.

Reply to
Jeff Layman

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