Alternator does it charge your Battery or only maintain charge.

I would say it does both , re-charge the battery and also keep it fully charged.
Reason being if the battery were reduced to 50% the first time, the next time you use interior lights the rate would be 25% and so on until the time came you wouldn't be going anywhere and you'd eating your bacon and eggs in the dark.

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It depends on your batteries state of charge Roughas.

The alternator tries to put out approx a constant volts.

For sake of this post lets say 14.2 volts.

Now if your battery is at 12v the the alternator will pour in maybe 50 amps and far exceed your little 8 amp charger.

However if your battery was at 14 volts then the alternator has only a difference of (14.2-14v) to push current through the connecting cables and it may only be able to put in a few amps.

This is where your little charger can win because it is designed to raise its voltage so that in can put out a constant 8amps.

For these reasons a normal car alternator will quickly partly charge a battery and replace the starting current but will take a long time to fully charge the same battery.

As robin said above it will take some time but it will recharge the battery. You would have to go on a long drive.

I have been told that if you do not drive for at least 1/2 an hour, you are not replacing any charge.

Be interesting to know how good that advice was :-)

Regards Tony

Good morning Roughas
Looks like you might have to resort to the old Bosch generator from an early Holden ,or some old English car , but steer clear of Lucas generators .

It depends n how flat the battery is.

If you leave the lights on for a couple of weeks, your battery will be dead flat - zero volts. In this state, the internal resistance of the battery is very high and it will accept very little charge. Alternator will be useless for the short period of time you'll run the car. The only way to resurrect a battery in this state is to put it on a charger for a week and see if its still alive. My daughter recently did exactly this and cut her losses and bought a new battery. I've resurrected a couple of batteries like this in the past, but its a long process, and they usually are not as good as they used to be.

If you leave your lights on for a few hours and drop the battery voltage to say 11V (which usually prevents you from starting the car), then the internal resistance of the battery is still reasonable - if it is in good condition, it will take a heap of amps from the alternator and will recharge to a cranking state fairly quickly. But it still takes a long, long time to get back to fully charged, so in this instance, I'd speed up this process by putting the battery on a charger overnight.

Cheers
Phil

Some "interesting" comments here about charging! To answer completely would take an essay, but the quick anser to your original question is yes, an alternator "can" fully charge a battery. The caveat is that it will take some time. Typically an alternator will charge a relatively flat battery to ~80% relatively quickly, but then it takes a considerable time to get to 100%.

For a vehicle starter battery this is rarley an issue as it actually only takes a very small amount of battery power to start a vehicle. The alternator can replace this power within a few minutes of driving, all things be equal. (another caveat!)

As a battery is discharged, its internal resistance gets lower, a flat battery can draw a higher amperage than a fully charged battery during recharge. As the battery is being recharged, the internal resistance increases and down goes the amp draw. If the voltage is now raised (like a multi-stage charger does) you can now head towards 100% recharge, but raise the voltage too high and you start to boil the electrolyte. Or keep the voltage relatively low, but for a long time, and you will also eventually achieve 100% recharge.

Also, a battery sholud be recharged as soon as possible otherwise you can begin to lose capacity. The lead sulphate that is formed during discharge is originally amorphous (jelly like) but turns crystaline over time. Recharging the battery converts the lead sulphate to lead oxide (+ plate) and lead (- plate).

But if the lead sulphate has turned crystaline, this reaction takes some time, if at all, to occur. Its why pulsing charging with a varying voltage can help break down the crystaline lead sulphate and "recover" otherwise dead batteries.

A battery is basically just a chemical reaction and its getting this chemistry right that is critical. There are so many types of batteries (AGM, wet cell, deep cycle etc...) all with their own differences that require supple differences to discharge and recharge regimes to get the best out of them. As they say, "oils ain't oils" applies just as much to batteries!

Cheers

Captain

An alternator will not charge your battery fully.......It pays to look at the original engineering criteria of what the vehicle manufacture wanted to achieve.

FYI... A starting battery has only one goal in life and that's to supply enough energy to start the engine with in a given time frame of around 5 to 15 seconds.... after that it has very little more to do.

When they design a charging system for a vehicle there are many aspects to consider.... what climate is the vehicle going to be operating in.... what conditions is the vehicle going to be operating in.... how long is the vehicle going to be run.... how long is it going to be to start the vehicle.... what load is going to be placed on the battery when the vehicle is not running.... when will the vehicle be run again.... what type or manufacture of the battery is going to be used in the future.... what quality of battery is going to be used in the future.... is the battery going to be maintained correctly.... and what effect on fuel usage and emissions is it going to have.

As you can see there are many criteria the engineers have to deal with, and there are many different charge conditions that have to be addressed.

A vehicle charging system has two goals.... firstly it must supply amply current and voltage to run the vehicle and everything factory fitted under all conditions.... the second thing is to charge the battery enough to enable the vehicle to start the next time.

The second one is the hardest criteria to design with so many unknown variables to consider, there has to be a big variable in charging the battery to not undercharge it or to over charge it, this is where this "a vehicle battery will only charge to 70-80% SOC".... 70-80SOC is reconsigned as a safe limit for charging battery's under most conditions, it enables the vehicle to start quickly, gives OK battery life, has ample reserve and places less load on the charging system.

Hence "lets dump big current and volts into a battery for the shortest period so the battery can start the vehicle next time"

Most charging systems fitted to vehicles prior to 2001 are very dumb in what they can do and how the function.... think " I'm not very smart but I can lift big weights".

Around 2001 more and more vehicles started to appear with a smart charging systems where by an ECU with it's own map would process the information and only charge when really needed, modern day charging systems can control voltage, current and frequency..... the down side of this; seeing it was really designed for better fuel consumption, better emissions, more engine power and making the NVH of the vehicle better is they have lowered the SOC of the battery down to 60-70%.... most new vehicle have smart start technology making them start with in 5 seconds of cranking.

Most new vehicle have time driven auto disconnect of factory fitted electrical gear just in case something was left on like interior lights..... they don't have to headroom anymore to get away with it.

As for secondary battery's....... the engineers who designed the charging system had no interest in even thinking someone may add an aux battery..... with some manufactures you can spec an upgraded charging system designed to charge the starting battery more and also charge a secondary battery fully, GM do it in the USA and some of the commercial vehicle manufactures in Europe (ambulance and rescue vehicles) but to my knowledge this is not available to the general public.

As for your 8 amp charger charging it to 100% SOC..... yes they do because the battery charger manufacture know the fixed non variable conditions the battery will be charged under.

Hi Roughasguts, if you started out "a bit confused" I bet you are totally bewildered by now. Ask a simple question and you get a host of convoluted responses of technical expression which provide no assistance to you.

Doug T provided the simple answer and Phil G qualified it. Then there was a heap of pseudo-science!

When you say "dead flat" I would assume you meant such a low State of Charge that you were unable to start the engine. And when you say "fully charged" I assume you mean to the level that the car manufacturer designed. ( Because there can be other meanings of "dead" and "fully".

The simple answer then is yes, the car alternator can recharge from a discharged condition to a fully charged condition and maintain that charge. An illustration of this is where a car is jump-started from a second battery then proceeds to recharge the original battery to a fully-charged state in due time. A pretty common occurrence.

Using the cranking battery for your caravan lights or radio and discharging it to "50%" may well be below the level needed to crank the engine to restart. Furthermore, repeated discharges to a low level will eventually harm a cranking battery. If you need to run lights from a battery then it should be a suitable type installed in the caravan.

The car alternator is quite suitable to maintain the charge in your battery when that battery is being used as the manufacturer intended, i.e. cranking the engine to start. All the other loads, headlights, fan, radio, etc. are then being supplied directly by the alternator, not by the battery. If this were not so then there would be a lot of cars at the side of the road with their bonnet up!

Thanks Guy's actually that has cleared it up for me!
Don't mind the simple explanation or the technical answer it's all good.

Anyway what I have determined is that my Pajero that only does 20km per week ! and has the new truck battery in it! power everything! and tow vehicle, could do with a battery charge every month along with me Boat battery.

But the Toyota that does some 220 Km a day freeway trip! will not ever need a top up with a charger to keep in good condition.

Why the question is that my lovely 4 year old Son loves to hit interior light switches and has done it to both vehicles the Toyota a 0ne day old battery needed a jump start very next morning! and the Pajero which was left for over a week with the map light on needed a long stint on the charger to get it back to what I hope is a good condition.

Cheers