Battery charging, times don't seem to add up.

Yesterday car old Honda Jazz would not start, took other car, and on return put battery on charge, I would guess around 30 Ah, two hours at 3A and six hours at 0.8A = around 12 Ah, OK left it on charge but 0.1A is neither here or their, so it would seem either car fails to start when battery still at 50% and since all lights etc worked it is clearly not completely discharged, or battery has degraded to 50% of original, or not charging fully, however it now spins the engine over without a problem.

However in the past I have not had the option of looking on the PC and seeing how much energy has been supplied by the battery charger, so going back 50 years batteries could have failed to start cars when 50% discharged, but I simply had no way to measure it back then.

The Ah is measured as a 10 or 20 hour rate, so does not really apply to cranking, the car does 2 miles a week on average, and was not used last week, last filled tank well before Christmas, so failing to start is no surprise, but how long I should recharge for is unknown, likely some sulfation has taken place, voltage is 12.9 every time I look, but likely to remain at that as at 12.8 the charger will return to 0.8A voltage will quickly raise, then at 14.4 volts it drops again to 0.1A so 98% of time will show 12.9 volts.

So all my years of training mean nothing with this new technology, as I was taught to use a hydrometer, even if I still had one it would not help with a sealed battery. So question is at what point of its capacity will a car battery fail to start a car? Should it start at 10% or does it need 50%?

A battery with only 10% charge would be able to start a car if the internal resistance of the battery was low enough to allow the cranking current to pass through the battery without creating a large voltage in opposition to the battery polarity.

But with most battery technology ( chemistry ) the full surface area of the plates is not available ( sulphated or similar coating ) until the battery is almost fully charged and the charging current can start to break down the coating.

What is the definition - or values - of a car battery with only 10% charge and how do you tell? With a loading meter?

Does it mean Eric's 30Ah battery will be only 3Ah?

Do car batteries have a C rating?

ericmark said:

Yesterday car old Honda Jazz would not start, took other car, and on return put battery on charge, I would guess around 30 Ah, two hours at 3A and six hours at 0.8A = around 12 Ah, OK left it on charge but 0.1A is neither here or their, so it would seem either car fails to start when battery still at 50%

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The fact that a charger has supplied a certain amount of energy does not necessarily mean that anything like that amount of energy will have been 'stored' in the battery, since some (increasingly more as the battery ages) will just be turned into heat.

Kind Regards, John

2 miles a week is never going to replace any charge back into the battery. If you think of the current at start up, from a fully charged battery, takes a big chunk of the charge you then need to run for a few miles, (5-10 I've been told), just to maintain it near the original charge. Even when your ignition is turned off your car is still using minute amounts of the charge to keep the clock up to date, the mileometer reading etc.
I would leave to fully charge and if it starts the car make a note of the date. Then take daily readings and see if they drop even when the car is not being used.

My bottom dollar says your battery is knackered/dead/not worth a light.
But; you haven't fully charged it yet. I take it that it is a modern lead acid, possible calcium type. I'd want that to be sitting at 13.8v at the battery terminals for =>12 hrs before I thought it was charged and then it would do it good to see 14.2-14.4v for a couple of hours.
A near flat car battery should take more than 3A initially - either your charger isn't up to it or that battery isn't taking charge properly. If it is still on the car try the charger with your lights on and see what it can muster.

One battery charger is in the caravan so only have one in the house, I have five batteries to maintain, so the charger rotates between batteries not regularly charged by the car, I as @conny says expect the battery to discharge, and normally will be charged around once every couple of months, but due to Christmas and illness the old Jazz has not had regular runs, so I expected it to be discharged in a way, but also expected it to take longer to recharge, First hour as expected around ¼ hour at max of 3.8 amp dropping to 3 amp for around 2 hours then a long time at 0.8 amp, and finally dropping to 0.1 amp, area under the graph is the energy put into the battery, rough calculation shows that is about half the total declared capacity of the battery.

Problem for me is I have never been able to measure the energy used by a battery in the past, it would be charged using constant voltage rather than constant current, and the current would gradually taper off, because the charger has 4 steps, it is reasonably easy to see each step so 2 hours at 3 amp is clearly 6 Ah and 6 hours at 0.8 amp is around 10 Ah so only around 16 Ah of power was put into the battery before the final stage at 0.1 amp I will normally leave for days at 0.1 amp if not weeks, and it is likely still turning a little sulphur back into acid, but very little, it was in real terms fully charged after putting about 10 Ah back into the battery.

But I have never bothered before to note at what point a battery fails to start a car, and for a car in regular use, it likely takes less than 10% of battery capacity to start the car, likely more like 2% or cars which stop engine when sitting at traffic lights would not really do very well, had I not been monitoring the charge I would have assumed it would charge at max of 3.8 amp for a good length of time, but that seems rare, in the main batteries removed from caravan for recharging even at 110 Ah only charge flat out for 2 hours, then are at 3 amp for a good length of time, and at 0.8 amp for a few days, and it seems the smart charger does squeeze more back into the batteries than the float charger built into caravan.

ericmark said:

But I have never bothered before to note at what point a battery fails to start a car, and for a car in regular use, it likely takes less than 10% of battery capacity to start the car, likely more like 2%

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I doubt whether 'how well charged' the battery is represents a major part of the equation. A relatively 'easy to start' engine should only require a second or two, and I think that a typical starter motor will draw maybe 250A when turning a cold engine - but 2 seconds at 250A is only about 0.14 Ah. I suspect that internal resistance (an increasing issue as the battery ages) is probably an important part of the equation.

ericmark said:

... likely more like 2% or cars which stop engine when sitting at traffic lights would not really do very well ...

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Maybe that's why we're being told that we need electric cars

Kind Regards, John

John Double u twa said:

That would be FLAT.

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Perhaps you should tell my iPhone that it shouldn't work when there is only "10% charge" in its battery.

... and the conceptual difference between a car battery and a phone battery is?

More to the point, one is lead acid, the other is lithium.

JohnW2 said:

... and the conceptual difference between a car battery and a phone battery is?

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One is for very high current, short duration and the other the reverse.

EFLImpudence said:

One is for very high current, short duration and the other the reverse.

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True, but I still don't think that a car battery with "10% charge" in any way qualifies as 'flat' ...

... if (as has been known to happen!), the not-very-young battery in my car goes totally flat (no attempt to turn starter motor, and even dashboards lights etc. don't come on), then less than half an hour's charging at 3-4A with my crude cheap-and-nasty charger is usually enough for it to be able to start the car with no problem. Even if the charging process is 100% efficient (i.e. all energy 'charges the battery', rather than heating it) that's only 1.5h Ah - 2 Ah, much less than 10% of the battery's full capacity ... and I wouldn't personally use the word 'flat' to describe a battery that will start my car, would you?

... don't forget what I wrote yesterday to eric - that, energy-wise, it probably takes less than 0.2 Ah to start a car - the internal resistance being the main potential problem.

Kind Regards, John

winston1 said:

More to the point, one is lead acid, the other is lithium.

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True, but see my post #21, and bear in mind that my car battery is lead-acid.

JohnW2 said:

True, but I still don't think that a car battery with "10% charge" in any way qualifies as 'flat' ...

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This is what I am trying to determine.
What does 10% charge mean?
Is it, in the case of Eric's battery, only 3Ah remaining? If so, what is the voltage? Still 12V+?

... if (as has been known to happen!), the not-very-young battery in my car goes totally flat (no attempt to turn starter motor, and even dashboards lights etc. don't come on),

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What values result in that?

then less than half an hour's charging at 3-4A with my crude cheap-and-nasty charger is usually enough for it to be able to start the car with no problem.

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What are the values after that?

Even if the charging process is 100% efficient (i.e. all energy 'charges the battery', rather than heating it) that's only 1.5h Ah - 2 Ah, much less than 10% of the battery's full capacity ...

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So, why do they make them so big if that will do?

and I wouldn't personally use the word 'flat' to describe a battery that will start my car, would you?

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I suppose not because you wouldn't think it was flat but I am not sure. 11.8V is classed as totally discharged.


My van's nearly new battery used to go 'flat' (unable to start - clicking starter motor) after three or four days not used until I found 160mA (1.92W @ 12V) of continuous parasitic drain but, according to the 70Ah, it should take eighteen days.
Starting batteries do not like continuous drain.

... don't forget what I wrote yesterday to eric - that, energy-wise, it probably takes less than 0.2 Ah to start a car - the internal resistance being the main potential problem.

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I can't argue with your figure. Is that about 750 Coulombs - 250A for 3 seconds?