battery again

[rhinosaw]

further to my battery query about 10 days ago when I said that my new battery was reading 12.4 v

Now after a couple of days idle the battery read just under 12.2 v, went for a longish drive , then left the car for 4 hours and the battery read over 12.5v. alternator is reading OK.

just wondered if the 12.2 on a new battery that was in a car not used for approx 72 hours was Ok. EFB stop/start battery by the way
thanks

 

[Old Salt]

A modern car with ecu, radio memory, alar, etc will flatten a battery in 2 to 3 weeks, if left standing, due to the constant drain although it is only milli amps. Friend of mine in the 80’s had an ameter fitted and drove 20 miles to work each way. By the time he got to work the alternator had just replaced the charge from starting the car for his journey. To top it up he had to drive further. At this time of year with the headlights and wipers on, heated screens etc it will take even longer.

I would get a battery charger, compatible with your battery type, and charge it for a minimum of 24 hours. Modern chargers will alter the charge as required. Aldi and Lidl both sell a reasonably priced smart charger (when available).

 

[EFLImpudence]

Friend of mine in the 80’s had an ameter fitted and drove 20 miles to work each way. By the time he got to work the alternator had just replaced the charge from starting the car for his journey. To top it up he had to drive further.

It might have taken that time to replace the total drain since last used but, assuming an instant start, it should only take a few seconds to replace the starting load.

So if there is excessive drain between starts and it is never completely replaced then the battery will become lower and lower.

At this time of year with the headlights and wipers on, heated screens etc it will take even longer.

Not much longer.

The system needs checking to ensure that there is no drain which should not be there.

When I had such a problem, I found that a USB charger left in the cigarette lighter constantly drew 60mA and a faulty relay constantly drew 100mA.
160mA does not sound much but added to the expected permanent drain the battery will be flat in a week.

I have had no trouble since but the battery after resting is never over 12.4V

 

[Vicario]

My car had a 'parasitic drain' which left me with a dead battery if left unused for more than a few days. The AA man asked if I relied on opening the door to switch off the radio rather than using the on/off button. When I started using the button the problem disappeared. The switch at the door was the problem.

 

[Harry Bloomfield]

Friend of mine in the 80’s had an ameter fitted and drove 20 miles to work each way. By the time he got to work the alternator

Ammeters were never fitted with alternators, because the amount of current involved from an alternator, meant far to thick cables were involved. Alternator fitted cars had voltmeters instead, to keep an eye on the charge/battery state.

 

[Harry Bloomfield]

It might have taken that time to replace the total drain since last used but, assuming an instant start, it should only take a few seconds to replace the starting load.

Batteries behave rather strangely, in as much as their voltage can rise very rapidly to match the charge voltage, which can inhibit them from taking up a charge rapidly. Cranking an engine over can take a 'great lump' out of a battery, even if the engine starts easily. I agree with it taking just a few seconds for the battery voltage to recover, but I would dispute your few seconds to put the actual charge back in. Putting the charge back in to a fully charged state, could take as long as 30 minutes, because the charge rate declines rapidly, once the battery voltage matches the charge voltage.

Think in terms of a 110amp alternator, on a car with no extra loads needing to be supplied. Call it a 55amp hour battery, which is almost flat. You would expect on the face of it for the 110amp alternator, to be able to recharge the 55amp hour in around 30 minutes of driving. Not so, because of the above effect - it can take 24 hours of constant running to restore that battery to a full charge. Not much different to the time it takes a modern charger to restore a battery to a full charge. You cannot rush the charging of a battery, except by dramatically increasing the charge input voltage, with the inherent risk of over charging and wrecking the battery.

 

[Harry Bloomfield]

A modern car with ecu, radio memory, alar, etc will flatten a battery in 2 to 3 weeks, if left standing, due to the constant drain although it is only milli amps. Friend of mine in the 80’s had an ameter fitted and drove 20 miles to work each way. By the time he got to work the alternator had just replaced the charge from starting the car for his journey. To top it up he had to drive further. At this time of year with the headlights and wipers on, heated screens etc it will take even longer.

Yes, modern cars, with modern electronics, rely upon being used regularly to keep the battery topped up against the constant drain of maintaining the elecBatronics. Two or three weeks parked is probably the safe limit for being stood without being used, because of the drain. If their is an issue with excessive drain, it can be awfully difficult to actually measure and even more difficult to track down - because there is such a wide difference between the maximum current and the possible drain when measured at the battery - a few hundred amps, versus drains of a few milliamps.

I spent a week on and off, working on my car, trying to track down a drain of 100mA, which pulsed rapidly and regularly on and off. I was lucky that my car has a removable bus-bar, which only fed everything but the starter motor's heavy current, so I was able to connect a meter across the bus-bar and remove it, to get a discharge measurement. Every time a fuse was pulled, it would wake up the car's electronics, so wait several minutes for it to 'sleep' again and maybe a hundred circuits to be checked by pulling fuses. Eventually, I pulled the one fuse which was the 'always on' circuit for the towing system and the drain dropped to a much more slowly pulsing 20mA. It was the voltage sensing relay, for fridge and leisure battery charging, just set to engage at just a touch too low a voltage. It engaged, which pulled the battery voltage down a touch, so it disengaged, battery voltage rose again, it re-engaged - rinse and repeat, until it had flattened the battery.

Back to the OP's problem....

11.7v = 10%; 11.8v = 20%; 12.2v = 30%; 12.3v = 40%; 12.3v = 40%; 12.35v = 50%; 12.4v = 60%; 12.5v = 70%; 12.55v = 80%; 12.57v = 90%; 12.6v = 100%

These are the battery at rest voltages, battery not under charge for at least 1 hour and with no loads applied to a standard lead acid battery, rather than an EFB battery.

 

[JohnD]

Cranking an engine over can take a 'great lump' out of a battery, even if the engine starts easily. I agree with it taking just a few seconds for the battery voltage to recover, but I would dispute your few seconds to put the actual charge back in.

maybe

but let's suppose the starter draws 400amps

For five seconds

that's 2,000 amp-seconds

And the alternator charges at 20Amps

For 5 minutes

that's 6,000 amp-seconds

my figures are made up, I don't know the real ones.

 

[Harry Bloomfield]

my figures are made up, I don't know the real ones.

That is all any of us can do, because the numbers vary so rapidly in the process of charge and discharge. What you cannot assume, is that the 20amp charge immediately after starting, will continue, because it will not it rapidly declines to a couple or so amps as the battery voltage rises to meet the alternators voltage. The rapid drop in charge current after an engine start, was much more obvious back in the days of dynamos, when ammeters were fitted. Voltmeters, even if they are fitted, tell you nothing about the state of charge when the engine is running - apart from the fact that the charge system is working.

 

[cdbe]

Batteries, I hate them. Probably because I drive older cars on short frequent journeys (school run). Current (new to me earlier this year) car is a 14 year old Golf diesel, I think the current battery is the original from the factory. It has never failed to start in the current weather, but it feels weak - that initial struggle before the starter spins into life. I've metered it (after 12 hours) and it's showing 12.1v. I've bought a new battery and shoved it in the boot along with my jump leads till it's needed, but with the weather warming up the old battery seems a lot happier again so I'll eke it out a bit longer.

I have read that on new stop start cars they don't fully charge the battery with the alternator - apparently the ECU knows how much is needed to start it. Even the old Golf worries and turns off the radio when I'm sat waiting for the wife to finish shopping. The old Berlingo used to go into eco-mode after 20 minutes and you had to start the engine for a moment (probably used loads of juice) to get the radio back.

 

[Harry Bloomfield]

Batteries, I hate them.

You are not alone with that.

I have read that on new stop start cars they don't fully charge the battery with the alternator - apparently the ECU knows how much is needed to start it.

Yes the theory goes that keeping a battery fully charged constantly, reduces its useful life - so they call 80% near enough full.

 

[Old Salt]

Ammeters were never fitted with alternators, because the amount of current involved from an alternator, meant far to thick cables were involved. Alternator fitted cars had voltmeters instead, to keep an eye on the charge/battery state.

He had an ameter, a voltmeter only tells you whats going into the battery. An ameter will tell you the state of the battery.

 

[Harry Bloomfield]

He had an ameter, a voltmeter only tells you whats going into the battery.

T'other way around and two m's. Centered pointer ammeter showed you what current was going into or out of a battery, but they were not fitted with alternator because the cables needed would be too big - plus there was much greater electrical current demand on cars, than in the 40amp generator days. So instead, they began fitting volt meters, which were generally more useful with an alternator.

 

[Old Salt]

Whatever

 

[EFLImpudence]

Eventually, I pulled the one fuse which was the 'always on' circuit for the towing system and the drain dropped to a much more slowly pulsing 20mA. It was the voltage sensing relay, for fridge and leisure battery charging, just set to engage at just a touch too low a voltage. It engaged, which pulled the battery voltage down a touch, so it disengaged, battery voltage rose again, it re-engaged - rinse and repeat, until it had flattened the battery.

That was the relay I mentioned which was drawing 100mA all the time.

I don't have a caravan so just disconnected it.

I assumed its coil and Voltage sensor was mistakenly connected to the BATT+ instead of IGN+ as, as I understand it, it should only switch on the leisure battery and fridge circuits when the alternator was supplying >14V.