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MG MGB Technical - battery charging time


first,assume my alternator is doing its job
properly and that my battery is relatively new.
further, my battery is completely discharged for
whatever reason.
i get the car started. a jump.

now, how long,approximately, minutes,hours,
must i drive to completely recharge
the battery ?

forgive me, i know i took too long to pose
my question


In practice it won't fully recharge John. Alternators are good at balancing a big load but once the battery voltage comes up, which happens at quite low capacity, they stop charging and may never get the battery back up past half capacity. Batteries generally don't like to be boost charged or deep cycled. Trickle charge it at 1-2 amps for 24-48 hours and if it is a good battery you will be amazed at how much capacity it regains. Otherwise it will seem to recover after a couple of hours running but will let you down again easily. Rich.

An alternator applies a constant (the battery smoothing the peaks and troughs) voltage of about 14v to the battery, and a charger about the same so, out of interest, how does the 'half charge' result? An alternator is also 3-phase at a significantly higher frequency so in fact I'd expect a 'higher' average charge for a given charge voltage from an alternator over a charger. In which case if you find and remove the original cause of the flat battery and drive it regularly and for more than half a dozen miles at a time it *should* get back to full charge in normal use over a period of time. Flattening a battery does slightly reduce its capacity, and regular even slight discharges will significantly affect its capacity until eventually it requires replacement, but that is irrespective of alternator or charger, I have found. If your charger applies more than 14v to the battery it may charge quicker, but be detremental.
Paul Hunt

Hi John.

I can't answer your question, but I can point you in the right direction.

A battery has a 'capacity', which is measured in Ampere-Hours (AH).
There are complications (eg temperature), but (for example) a 60AH battery will theoretically provide one amp for 60 hours.

Charging a battery is an inefficient business, so fully charging a 'flat' 60AH battery would take rather more than 60 hours at a charge rate of 1A.

Alternators have various maximum output currents, but ours are usually about 30A. In my experience the actual long-term charging current is significantly less than this, at something more like 6A, though the current will be high at the start of charging and reduce as the battery 'fills up'.

So, assuming a 60AH battery and an average charging current of 6A, full charge will only be reached sometime after about 10 hours of driving.

An old battery may have reduced capacity, and will reach full charge more quickly - if a battery appears to reach full charge after a short while, this suggests a poor battery rather than a good one, as some ppl assume.



Paul, Don,
Basically, in the 70's, low maintenance batteries appeared (when did you last have to top up your water?). These are lead/calcium and do not fully charge with old 14v regulators. Newer cars charge at a higher voltage.
I've found a link here which explains it well. Rich.

Hi Rich.

Thanks, well worth a read.


Interesting, if not terribly helpful. FWIW the MGB Workshop Manual quotes from 14.3v to 15.5v (dependant on temperature) for dynamo/control box equipped cars, the higher voltage being obtained at lower ambients (14.9v to 15.5v at 10 degrees C). Alternators are quoted at 14.3v to 14.7v with no temperature variation given. The article quotes anything from 13.8v to 15.6v as being suitable charging voltages in some places, although in others it states a *minimum* of 14.8v and elsewhere that 14.5v will overheat the battery! The author's personal experience is that 14.1v kept a battery fully charged on a car used infrequently and for short journeys, and my experience with voltages around 14.5v to 14.8v is that a flattened battery will charge in use with no apparent loss of capacity. However if one drives off after a bump or jump start with headlights, wipers, heater, rear screen etc. all on then the charging voltage will be reduced significantly. It's true I don't top up my batteries often, but certainly the roadster and I'm pretty sure the V8 is the same in that they both have removable plugs *for* topping up. Other batteries I have seen *are* sealed.
Paul Hunt

I once had a dynamo equipped car which a PO had fitted an ammeter to . After a cold start it would charge at 5 amps for about 1 mile , then drop back to almost nothing the charge time would include sitting on the drive at fast idle on choke while I shut the garage up .I would expect an alternator equipped car to be much the same , except it would charge if you just left it ticking over , where the dynamo would continue to discharge .The capacity of a battery is most affected by the rate at which it is discharged , so you will see 60 Ampere hours, at the 1 amp rate for instance . Draw 10 amps and you will get about 30 ampere hours .It is true that once a lead acid comes up to voltage , it will stop charging .
If you assume 100A cranking 5 seconds to start 50% efficincy you will see that these numbers stack up.
S Best

thanks a heap for all the great information.
i really learned a few things !!


As S Best states, the charging rate is based on the voltage difference between the battery and the generator or alternator. The greater the voltage difference, the lower the Ohms (less resistance) in the battery and the heavier the load on the charger. As the battery approaches full charge it presents a very small load to the charger and the amperage draw drops. With an amp meter (all the modern cars only have volt meters) you can judge how discharged or charged your battery is by the amps it is drawing from the alternator or generator. With a generator you can have a bad regulator which will allow the generator to produce a hi voltage and overload the generator and overheat/charge the battery and soon ruin both the generator and battery. Turning on lights, heater fan, etc will of course also cause the amp output from the generator to increase.

Many manufacturers are now saying you should not use the alternator to charge the battery as it will damage the alternator. The truth is that the alternators at max output for a period of time will overheat. Hi output for a few minutes to recharge the starting amps in the battery, the heater on full power,etc. will drop quickly once the battery is charged. With a discharged battery it may take hi output for a half hour or more. This will overheat the hi output compact modern alternator.

The big bene of the alternator is they are lighter, more compact, put out charging voltage at low rpm and are more reliable than generators. The pulley diameters are different as the alternator spins at much higher rpms than the generator.

On my '67 I'm using the early small diameter crank pulley with an alternator using the stock '68 (and later) alternator pulley. I find no problem charging the system with the lower rpms. Of course I'm not running air conditioning, major audio boom power, or hydraulic bounce pumps, just one little 12 volt battery.
Barry Parkinson

Hi Paul, All,

Looking at this now with 30 years hindsight. Around '75 I was selling batteries of the then new "low maintenance" type, Dagenite Demon etc. (These are not sealed, you can still top them up but they don't usually require it in temperate climates.) There were quite a few problems with flat batteries and many of the old school were sceptical of the new technology. This is referred to in the link I posted with reference to older cars.

Fast forward to '95 and we had a new Subaru. First winter left the lights on and flattened the battery. Used my 160A starter/charger, maybe an hour in all to get going again and forgot about it, assuming it would recharge. Next winter same again. Quick restart, blamed myself for leaving the lights on and assumed it would be OK. Then within a couple of weeks it was flat of it's own accord on a cold morning. So, new battery I thought, this one's never been much good. Then I decided to give it one last chance and trickle charged it for nearly 2 days. I remember the charge rate dropped off within minutes but instead of leaving it on the low setting I flipped it through to high (20A) to keep around 2A going through. Even this dropped off to barely readable on the meter, an amp maybe, but it wasn't gassing so I left it charging slowly.

It was like a brand new battery. Cranked faster than it ever had and never hinted at weakness for 3 more years till we sold it. I think it never really had been fully charged by the car in ordinary use. The car drops charging to a very small level, probably 1A, to stop boiling on long trips. It will replace starter charge but I remain convinced it is no good for recharging a flat battery.

Those MGB figures are interesting. From what I can glean, using todays low maintenance type batteries, dynamo setups should be fine and alternators may be OK or not (actual charge voltage, battery type and type of use could conspire one way or the other).

Perhaps this explains why many people find those float chargers useful on classics. My B is dynamo and has date stamped 11 year old Fulmen low maintenance types on which have had a trickle charge every few months (the PO and me). I've not had it long so don't know of any problems but they still have plenty of capacity. Rich.

I have learned loads from reading the replys to this question (as usual) but can anyone tell me why i need to disconect the battery before i can put it on trickle charge. Seems such a waste of time, i was thinking of put a convienient socket in the car so i can use on of these little top up chargers.
John Darby

"The greater the voltage difference, the lower the Ohms (less resistance) in the battery "

For a given state of charge the internal resistance of the battery is constant, not dependant on charging voltage. The higher the charging voltage the higher the current will be, just like if the charger were connected to a fixed resistor. The difference is that the voltage of a battery increases as it charges, and as the voltage difference *reduces* the charging current reduces. The internal resistance changes with the chemical changes inside the battery and is what changes the capacity and discharge rates.

You *don't* need to disconnect the battery before trickle-charging it, and if you have a cigar lighter in the car you can put the appropriate plug on your charger leads and make charging a doddle. The factory lighter is on the purple circuit and always live (can be disconnected with the key out of the ignition on more modern cars) and fused, so if the charger is fused as well all is perfectly safe. Some cigar lighter plugs are rated lower than others, and those with an internal fuse can get warm in use as the current is passing through the thin coil spring that is holding the fuse under pressure. Without a cigar lighter it is much easier to connect the charger to the purple end of the bottom fuse in the engine compartment fusebox and a convenient ground rather than connecting it direct to the batteries, even if you don't disconnect them.

With the battery in an open cradle and a 'normal' garage i.e. not hermetically sealed there should be enough ventilation. However *not* so if the battery is in one of those plastic bins, which are actually storage bins for the spare hole if a CB car has been converted to a single 12v, not for batteries.
Paul Hunt

Hi John.

I have used a trickle charger (via the cig lighter) as you describe for many years without problems.

The reason for disconnecting the battery is, I believe, to protect the alternator from voltage spikes that might damage the alternator's regulator circuit, particularly when connecting / disconnecting the charger.

I can imagine an old, crude battery charger with selenium rectifiers damaging an early generation alternator.

The modern mains powered battery charger / starter units (with silicon rectifiers) would appear to be far more 'brutal' than a trickle charger but are widely used without disconnecting the battery, presumably without problems.



Thanks Don, that makes sence and the tip of using the cigarette socket is great, i would never have thought of that.

Yes no problem usually charging on the vehicle but do be very careful connecting and disconecting any battery, anytime. I always waft a big rag around to disperse any gas first. A battery gives off very flammable hydrogen so no naked lights or sparks from connectors, switches or even lead lights. Switch at the charger or wallsocket well away from the battery and never clip/unclip clips from the terminals whilst powered up.

I've actually been there when a large truck battery exploded as the mechanic disconnected it and no-one was laughing, we all needed a cup of tea. Fortunately the top blew out, the case didn't explode outwards, and he didn't have his face over it. That was an old rubber type but I could imagine a welded plastic case splitting. Rich.

Connecting another battery for a jump start doesn't hurt the alternator (as long as it is connected the right way round) and neither should a charger which has much less energy to deliver. The battery will suppress any spikes from a charger or jump battery, it is very low impedance.
Paul Hunt

Hi all
FWIW I have the 6 volt dual batteries and charge them in the winter about once a month using a trickle charger. I plug the charger into a small plug I fitted in the engine compartment, connected to the starter solenoid and a good earth, I never disconnect the battery.
One other consideration if you are not using the car, is to discharge the battery very slowly over a period of time, then re-charge it fully, then discharge again and so on. This keeps the battery active and helps prolong its life, leaving a battery unused does it no good. A small discharge can be achieved by an alarm system left switched on.
Based on the above, the Lucas 6v batteries have been on our car since we rebuilt it in May 1996, nearly 9 years old and still going strong.
Graham Cherry

"is to discharge the battery very slowly over a period of time, then re-charge it fully"

I have found the opposite. My V8 with an alarm and not used on a daily basis slowly discharged the battery, sometimes to the point where I would have to charge it before I could use it. The batteries were lasting less than 18 months as they were losing the ability to start the car in shorter and shorter times, even though I would sometimes leave them on a charger for an extended period. Eventually I fitted a battery cut-off switch to eliminate all discharge, and now the battery always turns over the engine extremely well even after a month or more. My roadster has no alarm so there is only the microscopic drain of the alternator diodes, but even this is enough to give slightly slower cranking after a month or so. But I always try and run the roadster at a fast-idle for 20 mins or so once a month if I haven't been able to drive it. I replaced the single 12v battery shortly after buying it 15 years ago, then the twin 6v's once since then, and they are nowhere near needing replacement again. I have only ever heard that allowing a battery to discharge was bad for it. BT, who have a much higher investment in batteries than all of us put together probably, changed from charge/discharge system to parallel battery float as their extended 'testing' at lots of sites showed the batteries lasted longer that way.
Paul Hunt

Depends on the battery technology and how far, "deep", you discharge. Old tech, pre 70, didn't mind deep cycling too much but needed trickle charging after. Newer low maintenance types (most cars now) don't like deep cycling at all and will loose capacity as Paul says. RV "recreational vehicle" or "leisure batts" are designed for deep cycling so no problem. Gel batts also actually like it, or rather, they are protected against going too far, but they are a funny breed and have special charger requirements otherwise they stay flat. There is another spill-proof type too as well as Gel but that doesn't like deep cycling. All in all, most car batts at the moment don't like it. Rich.

Just one other thing which may have a large practical bearing on all this. Poor alternator connections, earths or battery terminals can have a large effect on the charging voltage the battery receives due to the large currents and small voltages involved.

To put ohms in context, a piece of ordinary auto wire the length of the car may be .3 ohm. Run, say, 3A horns from it and the nominal 12V from the battery will be less by V = 3A x .3R which is .9V so the horns get 11.1V.

Horns are not fussy but an alternator output voltage is set at its terminals so any resistance thereafter will make a difference to the charging voltage at the battery. Battery cable and car bodies are very low resistance so it's the connections which are usually important.

Very possible scenario; Alternator output 14.7V. Total resistance just .2 ohm to battery. At 1A battery gets 14.5V. Battery is low and tries to draw more than 1A. By the time it tries to draw 10A it is only receiving 12.7V. Clearly a point can easily be reached, even at very low resistance, where the battery will not charge. Rich.

That resistance will be between the alternator and the solenoid if it exists at all. Get resistance anywhere near that in the heavy current cables and connections and your cranking will really suffer due to the much larger currents involved. Measuring the volt-drop during cranking (coil disconnected) from the battery 12v post (not the connector) and the solenoid stud on one hand and the battery ground post and the starter body on the other should ideally only result in about 0.3v 'lost' in each case. But I have seen up to 3v lost in just one leg due to bad connections, the cranking speed was really slow, making starting very difficult as it sapped a lot more voltage away from the coil than it should. Again ideallly you should still have about 10v left at the coil when cranking, which needs a good battery as well as good connections.
Paul Hunt

This thread was discussed between 26/02/2005 and 04/03/2005

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