OT Car batteries going flat.

Yup - on it, thanks. The key turns, but it's rubbery and doesn't seem to be catching on anything. Tempted to squirt some WD40/lithium grease/solvent in the hole - what's the worst that could happen?! I had a quick look online, but doesn't seem to be a common problem (Audi 2007 A3 8P)

My car hasn't noticed.

It originally had a lead-acid that lasted 10 years or so.

The replacement is a calcium one; I was a bit dubious about this, but needed one in a hurry.

It is getting a bit tired now. 10 years or so later...

Andy

Is your Lidl charger one of the earlier ones- still a Smart C-Tech clone but the earlier Lidl batch?

If so, check YouTube. I?m pretty sure there is a mod to get around the ?really flat battery? problem. The latter clones either have a similar mod or are different design.

And I don't understand why you choose to argue about something you obviously know nothing about.

Ah - right. You've changed your tune. Now cost is more important than performance. Do make up your mind.

I've heard this said. Have had a calcium type in an older car and can't say it was any different. Or rather it said calcium on it. Think it was Halfords own brand.

The Lidl unit is reverse polarity protected by a relay on the output, coil fed via a diode from battery volts. If the battery volts are very low, it won't switch on.

The old linear charger would likely blow its fuse if reverse connected.

Trick with the Lidl one - if the battery is very flat, but you want to try recovering it, is to do a jump start with the charger connected. Switch it on, then remove the jump pack.

But far better not to let the battery get that flat. ;-)

Just trying to save the planet without blowing the car up.

Cheers

A better way in this case then might be to sell the laptop PSU on eBay and use that money to buy a dedicated charger. ;-)

Cheers, T i m

I just slap an old, dumb charger on to get the smart one started.

Agreed. Unfortunately, guessing how long it will be before my wife's unused car (she can't drive until 21st of this month, having suffered unexplained blackouts last year) goes flat is a bit of guesswork - and I keep getting it wrong.

We could have taken it off the road and disconnected the battery, but it also functions as reservce vehicle for me, so I can still get to work if there is a problem with mine - in the past I'd have lost a full week's pay on a couple of ocassions without that option, as I was working away and needed the car for that, but couldn't get a part immediately.

SteveW

The wall adapter on your laptop has special properties. It has a "current limiter". The wall adapter shuts off, if the output is short circuited. The wall adapter has a current rating, like 19V @ 3amps, and it is the amps number which the wall adapter "enforces". When the current flow goes over the nameplate value, the 3amps, the wall adapter shuts off.

We can draw a circuit diagram.

------------ = 3 amps , R = 2.13 ohms R

Checking the math, and moving from right to left in the diagram:

By placing that resistor in the path, we are ensuring the adapter limit of 3 amps is not exceeded. If we used a single 1 ohm resistor, way too much current flows from the adapter, and the wall adapter shuts off.

Another way to do it, is "automate it". By placing a constant current source device in the path. When you use a constant current device, you no longer have to sit there adjusting any resistors. With a CC device in place, you can disconnect the 12.6V battery, and put a screwdriver across (+) and (-) and short out the right hand side of the diagram. And the wall adapter won't shut off, because the CC device never allows more than 3 amps to flow.

I could show you a simple CC device, consisting of an LM350, heatsink and resistor, but that's not necessary for this project.

Note that, when you use a Resistor, such as our 2.13 ohm device above, it gets hot. How hot ? Well, we work out the power by calculation.

P = V * I = (R*I) * I = I^2 * R

P = I^2 * R = (3 amps * 3 amps) * 2.13 ohms = 19.17W

Since resistors tend to come in standard values, I could use three one ohm 10W resistors in series

---- 1 ---- 1 ---- 1 ------ (makes a 3 ohm 30W resistor) 10W 10W 10W

And the 19W dissipated in 30W worth of resistors, would make the resistors "damn hot". With power resistors, you don't rest your fingers on them.

By erring on the "safe" side, and using 3 ohms total, instead of 2 ohms total, I'm staying on the "safe" side of the 2.13 ohm calculation. The higher the resistance, the less current that flows.

The above is an illustration of safely connecting two "hard" sources to one another. The wall adapter is a voltage regulator (a voltage source). The car battery is a voltage regulator (at the moment, wants to make 12.6V in the example). If I drop a screwdriver shaft across the battery terminals, the battery tries its hardest to maintain the 12.6V. It can put 9V @ 150A through the screwdriver shaft or 1350W. Enough to make a smallish screwdriver "glow red". Don't do that!!! I had a friend get burned one day, when his metallic watch strap contact the (+) on the battery. His watch strap heated up and burned him. It burned the pattern of the watch strap into his wrist. No joke! You could tell the whole time he was healing, it was painful. He no longer wears "jewelery" while working on the car (at the time, an Austin Mini).

In the above example, I made an "assumption" that the battery is at 12.6V. When battery chargers (the cheap "iron" kind) are made, the design has to assume the user will click the two clamps together, drawing sparks. This is *very hard* on the charger, and causes the selenium rectifier inside to get very hot. You might even smell the poor thing cooking. Well, your wall adapter circuit with the 2.13 ohm resistor, is not immune either. We assumed the battery was

We're using the resistor in the above diagram, so that we can coax the wall adapter to keep running for the hours it will take to charge the battery. You should *not* leave that circuit hooked up forever. It's to be disconnected before the battery is charged all the way to 19V. Unless a battery is sulphated, or electrolyte stratified in the battery (dense electrolyte on bottom, almost water on top), it should remain stiff and stop at 14.4V or so. When a battery is sick, it'll charge higher than that. Which for most practical purposes, is useless charging.

Batteries also give off gases. By construction, some batteries are able to recombine the reduced amount of gases inside, and don't have vents. Batteries with vents, can lose gases as well as water, and may need to be topped up with *distilled* water, not tap water. Distilled water is available at the hardware store for this purpose. Maintenance-free batteries lack such a filling requirement.

The reason these work, is the selenium rectifier and the transformer characteristics, serve to "soften" the output of the charger. It's like the circuit has a bit of resistance in it. The people in the sci.electronics newsgroup, there are a couple individuals over there, that design "minimal component" hardware like this, intended to shave every nickel of cost out of goods. And those are the kinds of people who contribute to designs that are simple looking like this. (For example, one of those people did the plugin "Glade air freshener" that heats up and spreads its stink through the air :-) )

--- || ------ selenium rectifier --------- (+) \||/ /||\ [AFAIK, half-wave rectified] \||/ --- || ----------------------------------- (-)

This one by comparison, looks to be full wave. And the rectifiers are packaged different than mine. This one is fancy, with a meter on the front, and a 6/12 selector switch.

Unfortunately (for our car battery) our distance from the (nearest) shop and the difficulty parking close by means that it would probably lose more charge in the trip than it gained. ;-)

Cheers, T i m

If it'll keep going in current limiting mode, yes, if not, no. If you do use it do keep an eye on the charging progress and stop when near full, the brick will overcharge it otherwise, which is not ideal.

NT

Wrong again. No idea why you make a habit of it. If instead of pratting you'd followed what I said you'd not be arguing now. For some reason you've become a troll.

NT

No I haven't

You're becoming pathetic.

Think you should go into politics. Only answers you give are to the question or point you'd like to have been asked or made.

Or perhaps you're simply thick.

bye troll

Looks like I spoke too soon about my battery. I knew it was getting a bit old - but today I went to take the car out.

It's had an old model railway controller trickling into it all day, and it's back to 12.5. Tomorrow I'll see what it's fallen back to.

Andy

Hard to be sure which way it will go. I?ve seen them recover and be fine for several years- apparently as normal- others just peg out the next time you leave it over night.

I assume you?ve not got a proper charger. When you get a chance, it is worth investing in one of the modern ?smart? chargers. Lidl or ALDI do then for about 15 pound maybe every 3 months. They are copies of a more expensive one but excellent value. I have the ALDI one but bought the Lidl one for my son- in- law, it has the same functions.

One of those things you rarely need but ....