Car battery charging current.

[Dave Plowman (News)]

Given many of us won't be driving much for the moment, did wonder just how hard an alternator charges a battery after a cold start. In amps - given virtually no car has an ammeter these days. I know you can buy clamp meters, but they're not accurate at low current on DC, so when I've needed to measure things on the car, just use a DVM, as 10 amps is enough for most. And don't want to buy one just for this. ;-)

So assuming a good battery with enough charge to start the car. Voltage on charge 14.4v No other load than engine electrics.

Obviously the charge current will taper down as the battery charges.

[Andy Burns]

They may not have an ammeter on the dashboard, but they do have one on the CAN bus.

[The Natural Philosopher]

Alternators are good for at least 15A. I wouldn't be surprised if they can't mostly manage over 50A if needs be

Oh.. treble that - apparently 45A - 200A is the range...

[Andy Burns]

yep, mine is rated 140A

[Harry Bloomfield, Esq.]

The Natural Philosopher explained :

But not charge current into the battery.

[The Natural Philosopher]

Yes, charge current into the battery if its flat enough

[Dave Plowman (News)]

So you are guessing it charges a low battery at 140 amps? ;-)

[Dave Plowman (News)]

Remember it is Turnip. Never reads or understands a question.

[Harry Bloomfield, Esq.]

Dave Plowman (News) formulated on Saturday :

No, there would need to be a rather large difference in voltage between the alternator and battery, to achieve that and as the battery attains a charge, its voltage rises quite rapidly.

I would suggest nearer an initial 20amps, which would rapidly fall to maybe 3 to 6 amps. A flat to full charge time when being driven, can require 6 to 10 hours of being driven. Which is why it is always best to put such a battery on a mains charger for 24 hours, if at all possible.

[Fredxx]

For the few seconds to a minutes of a flat batter I can well believe that may be the case. As soon as the terminal voltage goes to 14.5V or the regulated charge current will naturally tail to a more modest value.

There is very little literature on battery charging, of when a charging device might switch from constant current to constant voltage at different initial charging currents.

[Algernon Goss-Custard]

Fredxx [e-mail] posted

My car battery charger, bought second-hand forty years ago for £5, has just packed up. Can anyone recommend a suitable replacement? Doesn't have to be portable, just plug into the mains and recharge a flat car or lawnmower battery. Preferably for the same sort of price I paid for the last one ...

[Andy Burns]

Yuasa recommended rate is only 5A

[The Natural Philosopher]

The alternator will deliver whatever voltage it meeds to but of course there would not need to be a particularly large voltage to charge a battery that is capable of delivering over 700 A to starter motors without crapping out

Total crap. There is no pint in having a 150A capable alternator to do THAT

[The Natural Philosopher]

Halfords cheapest.

[The Natural Philosopher]

Oh dear. Yuasa batteries are a couple of ampere hours

[The Natural Philosopher]

"Not long ago, an 80 amp alternator was considered a high output unit. Most late model alternators produce 120 to 155 amps or more. Current output increases with engine speed, from around 20 to 50 amps at idle up to the unit's maximum output at 2,500 RPM or higher (refer to a service manual for the exact charging output specifications for your vehicle)."

[charles]

yes, Oh Dear - a very quick google shows them up to 100Ah.

[Andy Burns]

95 Ah in this case and 850 CCA

larger battery for a 2.0 petrol than the previous 3.0 diesel, as it's a start/stop car.

[Bob Eager]

Are you saying they're useless? Always found them pretty good.

[williamwright]

Yes that's all correct. I have digital ammeters on my motorhome and that's what they tell me.

Bill