Car battery - stuffed?

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richms

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#2739619 6-Jul-2021 11:23

Groucho:

Has anyone had any success with those solar panels that plug into what is formerly known as the cigarette lighter to trickle charge?

AU and JP law mandate that the socket is disconnected with the key off because of stupid kids burning themselves when parked up and left alone. You need to go to the fuse box or battery.

They do stuff all since the full sunlight voltage is basically a maintenance charge, so when its not dead flat the current tapers off to nothing well before its actually charged up. Perhaps if you had 2 in series and a MPPT controller to make it work it would actually charge, but IME even with 3 of them in parallel on a car that is so flat it needed jumping to move across the lawn they will not bring it back up.

Richard rich.ms

tripper1000

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#2739630 6-Jul-2021 11:43

jpoc: Are you sure about this? It just doesn't sound at all right to me.

Most mains battery chargers will charge at 2A so, over 8 hours you would put 16 AH into the battery. That amounts to about 20% of the total charge in a typical 80AH battery.

In a modern car, the alternator will supply between 50 and 100A.

Most of that current will be available to charge the battery and so it will take fifteen minutes to put 16AH into the battery as opposed to 8 hours with a charger.

Even a trip to the dairy will do more good than a couple of hours on a charger.

I know that you can pay the big bucks and get a faster charger but even those appear to deliver only about 15% of the current of a typical alternator.

If your car has an ammeter (less common now days) or it you chuck a current clamp on the battery lead, you will see cars (old and new) don't charge batteries as fast as you assume.

First just because your alternator can make 50 - 100amps doesn't mean that the battery can take it that fast.

Second refer my post about ECU controlled alternators. Just because your alternator can make 50 - 100amps doesn't mean the ECU will let it make it when you need it.

Third Alternator current maximum ratings are not "at" 14v they are typically at 10 or 11 or 12v and at the alternator output terminal, not the battery terminal. You will only ever see those currents briefly into a dead flat battery. As the surface charge/voltage of the battery comes up past 13v the alternator is unable to push its theoretical maximum any more. Just the voltage drop in the wiring giving 0.5 volt drop between alternator and battery massively reduces (read 50%) the current coming from the alternator. Also, fun-fact, most alternators won't produce their full output until the engine is at over 2,000 RPM - they're just not spinning fast enough so your little pootle to the dairy won't be doing quite as much good as many think.

Forth, as Timmmay points out, the fuller a battery is, the slower it takes charge. Getting the full charge into a battery isn't a question of big currents, it is a question of time. Reason is that when you charge a battery the chemical changes happen immediately on the plates surface, but it takes hours for the chemical changes to permeate to the full depth of the active material (remember, the more capacity the battery has the thicker the active material there is on the plates). If you jump start a car, take it for a 15 minute drive as you suggest, shut it off and monitor the battery voltage, you will see the voltage go from ~14v when first shut down back down to ~12.2v over the next few hours. The 13-14v you are seeing initially is called surface charge (the charge is only surface deep) and over the next few hours the active material homogenises (the deep active material "charges up" while the surface stuff material "discharges", so that after a few hours the voltage you see is more accurate representation of the true overall state of charge. This is why it take so long to get the last little bit in - it isn't a question of big currents, or grunty chargers, it is just a question of time.

You can liken it to cooking bacon vs cooking a roast. It doesn't matter how powerful your oven, it takes time for the heat to penetrate the full depth of the meat. You can nuke it and cook the surface in 15 minutes, but the insides are still cold and you haven't cooked it properly. It is the same for big batteries - it takes time for the recharging chemical changes to penetrate the full depth of the plates. A 15 minute nuking makes it looked charged, but let it sit for a while and it becomes obvious that you only charged the surface.

Remember that just because a recently flat battery can now start the car doesn't mean it is fully charged - it will be mostly flat and it will but sulphating/aging at an accelerated rate. In the interest of battery longevity, you need to fully recharge a battery as soon as practical after it has been flattened.

tripper1000

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#2739640 6-Jul-2021 11:55

The reverse of the forth point is also true when discharging a battery. You will notice that if you crank an engine until the battery goes flat, then wait a while, you will often get a few more cranks out of it - yes? This is because you have flattened all the active material on the surface, but give it a little time for the chemical changes to homogenise through out the plate, and the surface will be recharged by the deeper active material. But the same recover is not true if you leave the lights on - this is because if you flatten the battery gradually (the opposite of recharging it gradually) the chemical changes can keep up as you go, and you thoroughly flatten all the active material, not just the surface.

tripper1000

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#2739645 6-Jul-2021 12:06

I don't know how far you want to go down a battery rabbit-hole, but the time necessary for chemical changes to homogenise is why car batteries capacity is rated over 20 hours when you read the fine print. If you look at the graphs on the makers web sites, you will see that the faster you discharge a battery, the less power you get out of it. If you discharge a battery in an hour you can get as little as 1/2 the rated capacity out of it - simply because there isn't enough time for all the active material to convert and give up its charge. The same is true for recharging - it will look done after an hour of rapid charging, but you need to give it up to 20 hours to thoroughly recharge - which brings us back to my mains charger can't be beat comment. 20 hours of driving is impractical but 20 hours of mains charging is easy.

1101

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#2739695 6-Jul-2021 13:36

Nothing worse that being stuck somewhere with a dead battery.

So I would replace it . Why take the risk.
But get the car checked out first, to find out why it drained so low.

I actually keep my old batt in the boot, in case I leave the lights on or something & cant start the car .

kotuku4

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#2739708 6-Jul-2021 14:05

The auto electrician I use would typical use a high current charge to desulfate the plates, boiling the electrolytes, on conventional lead acid batteries. Then cool down, top up and load test. And test in vehicle. I bought an autotool bt battery tester for checking my batteries, and charging.

If in doubt test and replace.

Many newer cars have electro magnetic clutch on alternator and smart charge controller. These do crap out.

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