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

[ericmark]

That would be FLAT.

The battery in my phone is flat, but the battery in my car is not, it shape does not change, being charged or discharged, there does seem with flooded lead acid a marked decline in available amps as the battery becomes discharged, this is less evident with glass mat or valve regulated lead acid, it also seems odd that discharge a charged battery to 10% and it will fail to start a car, but charge a battery from discharged to 10% and it will start a car.

I have noted with Nickel Iron batteries used on buses and milk tankers they do seem to give full starting amps until the reach fully discharged, which is why they are used with milk tankers to suck the milk in and then still start the prime mover engine after. But alkaline batteries are stored discharged, but lead acid are stored in the charged state.

But when a fork lift is reaching end of shift it goes slow as battery gets need to fully discharged, and electric cars at least a Renault Kango van also goes slow when it reaches end of shift, so it seems there is no different to NiMh or what ever is fitted in electric cars now to lead acid they both with high loads reduce the amps available as they become discharged.

There is clearly even with flooded lead acid a difference between makes and types as to how the battery performs as it becomes discharged, specially when lights left on, and you try to recharge with small battery charger, the threshold between starting and not starting does vary, even when both are claimed to be vehicle batteries. With traction batteries or leisure batteries I have looked at Ah rating and it has claimed 90 Ah and I have put lights on it rated at 5 amp and it has lasted as expected 180 hours, maybe a little more. This is what we are doing with a phone battery, it is being discharged over a long time.

It was claimed the battery was faulty on my wife's car, it would fail to start the car, it was a VRLA in that car, and it failed after just stopping, we would sit there wondering what to do, I would say give it another try and it would spin the engine over as if fully charged, but had acted as if totally discharge 10 minutes earlier, the battery was changed and we retained old one to use in caravan, car has been A1 since change, and old battery seems to be A1 used in the caravan, so some lead acid batteries do seem to have a life of their own.

It just seemed odd that the old Honda Jazz battery recharged so quickly.

 

[JohnW2]

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+?

It sounds as if you might find it helpful to ask Mr Google about "lead acid battery discharge curves", since he has thousands to offer ...

What does 10% charge mean? ... Is it, in the case of Eric's battery, only 3Ah remaining? If so, what is the voltage? ... What values result in that? .... What are the values after that?

some of those curves may help you to understand the answers to some of those questions. It gets complicated because, as has been discussed the actual 'available capacity' of a battery with depend considerably on how rapidly it is discharged (i.e. as has been said, you'll get nothing like 70 Ah out of a fully charged "70 Ah" battery if you draw 100A from it. However, for the purpose of illustration, I'll talk about just one discharge current.

It just comes down to 'areas under curves'. For a particular discharge current, the 'total available capacity' is the entire are under the curve from when it's fully discharged to when it's voltage starts rapidly plunging towards zero .... i.e. the blue area in the following crude sketch (which is notr intended as anything more that a very rough illustration - so don't take too much notice of the numbers or actual shape):

when one gets to the point on the curve that there's only 10% of the (previously all blue) area (available discharge) left (the purple bit here), then that's "10% charged). In practice, one will obviously not go down to anything like zero volts, but the best I can do with some hasty graphics in the middle of the night

That sketch is all wrong, since it suggests that the voltage would not be much over 8V at that '10% point'. In reality I would expect it to be well over 11V. ... but it is about 02:30

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

I think the short answer is that they probably don't need to. In the far off pre-alternator days, there was a need for big batteries, since the dynamo was not man enough to run lights and motors (windscreen wipers, heater fan etc.) etc. However, with alternators, from the energy point of view one only needs the battery to start, and can/could remove it once the engine was running (the battery does serve to help the regulation/stability of the alternator output, but one could theoretically replace it by a very large capacitor).

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

That's not far from the truth since, although much less dramatic than with lithium batteries, the voltage of even lead acid batteries falls pretty rapidly as it approaches the 'flat' condition (no recoverable charge left) - so if you continue to try to draw current when the voltage is down to around 11.8 volts, that voltage will very rapidly fall into its boots.....

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

That ballpark. If you recall, for eric I talked of 250A fior 2 seconds, and ended up with a figure of about 0.14 Ah.

Kind Regards, John
Edit: I perhaps should have added that one probably needs to think about the voltages we're talking about as being 'off load' voltages. Once one connects a load, particularly a large load, the voltage may be appreciably lower because of the battery's internal resistance - which is a totally different matter from 'how charged it is'.

 

[JohnW2]

.... I have looked at Ah rating and it has claimed 90 Ah and I have put lights on it rated at 5 amp and it has lasted as expected 180 hours, maybe a little more.

I think you may want to re-visit that arithmetic and/or typing

Kind Regards, John

 

[ericmark]

I think you may want to re-visit that arithmetic and/or typing

Kind Regards, John

Ups yes 90/5=18 not 180.
As to curves, the curve for a battery that has just been recharged, is different to one which has been sitting for some time, and this is the problem, if sulphur has hardened on the plates it can take 2 or 3 weeks before it is removed again, and the problem for me is how long to leave a battery on maintaining charge when I have one charger and three batteries, if I had spare chargers I would leave on charge for a week at least, but second charger not at home it was left in caravan.

 

[JohnW2]

Ups yes 90/5=18 not 180.

It is, but then I wondered if you had intended to type 0.5A, which would have made your '180' correct

As to curves, the curve for a battery that has just been recharged, is different to one which has been sitting for some time ...

Indeed so - and many other variables, too. A battery which has just had a period of heavy discharge will have a different curve from one that hasn't. A battery which has been charged rapidly may have a different curve from one that has been charged very slowly. ... and 'the way it has been used', in terms of the depth and frequency of charge-discharge )and thermal) cycles will again make a difference. etc. etc.

So many 'differences', in fact, that one really can't talk very sensibly about a 'generic' battery.

... and, as I keep saying, one cannot (should not) forget about internal resistance. I've seen batteries which can be charged (slowly) to their 'full capacity' and which can supply roughly the expected amount of energy to a small load over a long period of time, but which are totally incapable of supplying high currents (e.g. for a car starter motor), even for a very short period of time, because of a high internal resistance.

Kind Regards, John

 

[bernardgreen]

since the dynamo was not man enough to run lights and motors (windscreen wipers, heater fan etc.) etc. However, with alternators, from the energy point of view one only needs the battery to start,

Unless you leave a 6 year old in a Range Rover for a while. He found switches his parents didn't even know about, needed 20 minutes of charging before the engine could be cranked.

 

[ericmark]

I remember the change dynamo to alternator, and how people would try to upgrade, there was in real terms no advantage unless you changed the drive so it could spin faster, because the alternator could spin faster, that allowed it to cut in earlier, its main advantage it was lighter, Lucas dynamos were the odd one out, most dynamos would have the field connected to the live output one end, and other end was earthed by control box, but Lucas had the field earthed and you fed a live supply, may it easier to change polarity.

So yes up to the stop/start system and overrun charging used today, the battery was only needed to start the car and smooth out the supply, alternators were not gradually regulated but were turned on/off with a mark/space ratio as otherwise the control box would over heat, the dynamo had a three stage charging system, no field, resistor supplied field and full field and the control box switched between the three faster than the alternator control box, and the field was also slower building up and clasping so with a dynamo you could remove battery terminal with engine running, with an alternator it would cause a very high peak of voltage and destroy the electronic components.

The old idea was once every two months to use an equalising charge on the battery, since all cells would not be equal it would be over charged once a month or two, which would bring up low cells and gas high cells, this stopped with sealed for life batteries.

Of course early dynamos had no regulator or cut out, there was a free wheel on the drive pulley and on switching on the ignition the dynamo would motor, once the engine was started it would be going faster than dynamo and so start charging, and the field was supplied from a third adjustable brush so you could adjust the charge rate.

I still remember seeing people walking to the radio shop with lead acid batteries to get them charged, single 2 volt cells, used for the heaters on the valves, these were used until completely discharged then recharged, and I was told charged using a diode and resistor from mains so it was nearly constant current, the shop had a bank of batteries on charge and just added or subtracted a battery from the bank as I was lead to under stand, it seemed wrong to allow batteries to completely discharge, but it did not seem to damage them.

 

[JohnW2]

I still remember .... single 2 volt cells, used for the heaters on the valves, these were used until completely discharged then recharged .... it seemed wrong to allow batteries to completely discharge, but it did not seem to damage them.

Even that obviously varies according to the battery chemistry.

I remember my very earliest video cameras (well, the earliest battery-powered ones's - my true 'first', bought in 1980, had a trailing mains lead!) came with a charger which had a button one pressed to completely discharge the battery - they apparently would not charge, much or at all, unless one first 'flattened' them, but I can't recall what chemistry that was (if I ever knew).

Kind Regards, John

 

[ericmark]

Bit more modern than first I used, wound it up, and used super 8 film.

The NiCad battery was known for having a chemical memory so that was discharged first before charging.

 

[JohnW2]

Bit more modern than first I used, wound it up, and used super 8 film.

Oh, I had them (and 'non-super' 8 ones) as well - wind-up, as you say, but I wouldn't call them 'video cameras' (I think we called them 'movie cameras').

The NiCad battery was known for having a chemical memory so that was discharged first before charging.

Indeed - "NiCad" was what I couldn't recall. However, it makes my point that the charging behaviour/characxteristics and charging 'likes/dislikes' vary dramatically with different battery chemistries.

Kind Regards, John

 

[ericmark]

Having recharged the other batteries I put the charger back on the Jazz battery, set to under 20 Ah option as want it to fully switch off once fully charged, it went to 0.8 amp then started to alternate between 0.8 and 0.1 amp and then held on 0.1 amp as shown

I would hope in fullness of time it will start alternating between 0.1 amp and off, but held at 0.8 amp for over an hour, which seems to point to not being fully charged when I charged it last Wednesday around a week ago, so it seems I was right to question speed of recharge. The 20 Ah jump charging battery I charged did in fullness of time stop charging completely, but the 90 Ah ex Jag battery VRLA never actually switched off completely but sat there at 0.1 amp for days, however that battery is suspect, so not surprised. Want the car on Wednesday, if weather OK will walk tomorrow so will see what it does.

 

[ericmark]

Update, today was working in flat and not really good having lead out of cat flap while opening and shutting door all the time, a bit of a problem with energy monitor is it shows only whole numbers, so 2 watt or 3 watt will not show 2.5 watt, current limited to 0.1 amp, so as the volts raise the watts also raise, clearly some loses in the unit, so at around 13.5 volt the monitor starts to flick between 2 and 3 watt.

So after turning off and back on again I get this graph.

Not quite true, on turning back on a couple of minutes at 0.8 amp, but that is not shown, but you can see the wattage raise as the voltage did, this

was what it shows, the monitoring unit is jumping between 2 and 3 watt. Since voltage still raising clearly the battery is still excepting more and more charge. It raises the question how long should you charge a lead acid battery before one considered it fully charged, now at 10 days, and most of that time charging at just 0.1 amp.

 

[JohnW2]

... the monitoring unit is jumping between 2 and 3 watt. Since voltage still raising clearly the battery is still excepting more and more charge. It raises the question how long should you charge a lead acid battery before one considered it fully charged, now at 10 days, and most of that time charging at just 0.1 amp.

As I've said before, the fact that current is flowing into a battery does not necessarily mean that it is 'accepting charge' (or necessarily 'getting any more charged').

Kind Regards, John

 

[ericmark]

As I've said before, the fact that current is flowing into a battery does not necessarily mean that it is 'accepting charge' (or necessarily 'getting any more charged').

Kind Regards, John

If the voltage remains static then I would say any charge is being converted to heat, be it the car radio, or battery losses does not matter, it is not going into the battery, but if the battery voltage is raising then some of that charge must be going into the battery. There is a change taking place.

 

[JohnD]

at around 13.5 volt the monitor starts to flick between 2 and 3 watt.

which, in round terms, is approximately nothing.

My dumb charger has an analogue meter, which drops to (about) zero when fully charged.