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Started by mikeshadow on Dec 16, 2018 8:07:04 PM
How quickly does an alternator recharge a car battery after starting?

Assume a modern small car with a lead acid battery in very good condition.

Say starting draws 150 amps for 3 seconds = 450 amps.

At what rate would the alternator recharge that amount?

For example, would it be a fairly slow charge rate of say 2 amps, which would top the battery back up in around 4 minutes?

InternationalVicar - 16 Dec 2018 20:16:10 (#1 of 68)

Variable, depending.

Why does it matter?

It may never be fully charged.

widenation - 16 Dec 2018 20:22:50 (#2 of 68)

It's not a VW Polo is it?

coshipi - 16 Dec 2018 20:23:24 (#3 of 68)

Typical car alternator produces about 30 amps into a battery that's significantly less than full, as long as the revs are significantly above idling. 10 amps at idling speed. If the battery's nearly full it'll take less current because its voltage will be closer to the ~14V that the regulator feeds it - down to a minimum of about 1A trickle charge when it's completely full.

Assuming your regulator's working properly.

Obviously if you've got the heater fan running that's going to be nicking a few amps, as will any lights, windscreen or mirror heaters, radio etc. that are on.

bossab2 - 16 Dec 2018 20:24:36 (#4 of 68)

Take it for a 20 min run at a reasonable speed.

Unless it gets up to operating temperature you'll get moisture condensing in the exhaust, which will make it rust out early.

mikeshadow - 18 Dec 2018 17:01:55 (#5 of 68)


Thanks for that info coshipi.

widenation - 18 Dec 2018 17:06:10 (#6 of 68)

Vital win for Düsseldorf Saturday at bottom of Bundesliga 1. Unfortunately they face Dortmund tonight.

HoHoHoff - 18 Dec 2018 17:13:38 (#7 of 68)

Alternating fortunes?

Pentecost - 18 Dec 2018 17:16:26 (#8 of 68)

How soon will they recover?

HouseOfLametta - 18 Dec 2018 17:37:55 (#9 of 68)

There are other things going on in a modern car, the alternator will be putting out w certain (slightly variable) number of amps, but it will be controlled by the ECU. Essentially a computer will manage the split between charging and air con, fuel pump, lighting and so on. Several starts should be well within the headroom of the battery, so although the charge lost will be replaced, it shouldn’t be crucial to operation.

If you are starting, driving a bit and then having difficulty restarting, unless you only ever do tiny journeys with the lights on, you have trouble. Most likely a worn out battery in this weather.

SaintRicolas - 18 Dec 2018 17:43:32 (#10 of 68)

A note though for those who do 1) a lot of short journeys or 2) don't use their car very often.

As an MX5 owner, I along with most of my fellow owners, fall into this category over winter. As such, the standard advice is to take the battery out* and give it a decent charge at least once over winter.

* unless you have a charger you can connect whilst the battery is still in the car, which a few of us have.

HouseOfLametta - 18 Dec 2018 17:48:28 (#11 of 68)

Could you use a little solar charger?

HoHoHoff - 18 Dec 2018 17:56:27 (#12 of 68)

I've got a nice little charger that I alternate between the sit-down mower and the motorbike during the winter months without needing to remove the batteries. It apparently does things like de-sulphuring (?) and saving batteries at death's door prior to gently charging between 0.2 and 0.8A.

HoHoHoff - 18 Dec 2018 17:57:27 (#13 of 68)

#11 that might be tough on an MX5. My sis has got one on her VW Transporter though.

coshipi - 18 Dec 2018 18:17:18 (#14 of 68)

#10 is exactly right. We have two cars in the family & both fall in this category.

I've got a long extension lead and extended the 12V leads from the charger so that I can put the charger & the socket on the end of the extension lead in the car. The rubber seals round the doors and the bonnet are flexible enough to close and lock the car with it all inside. We're lucky to be the last house in a cul-de-sac, so I can leave the mains lead running along the foot of a fence to the car without upsetting anyone.

thisonehasalittlehat - 18 Dec 2018 18:38:47 (#15 of 68)

Two cars? Are you Rothschild?

thisonehasalittlehat - 18 Dec 2018 18:39:21 (#16 of 68)

Or just seeing how far you can drive before the planet dies?

Gotout - 18 Dec 2018 18:46:57 (#17 of 68)

I recall reading that if you're charging an alternator equipped car by using a battery charger then you're supposed to remove the car's leads, otherwise you can damage the alternator. Don't know if this is no longer applicable?

coshipi - 18 Dec 2018 18:52:55 (#18 of 68)

Depends how much current your charger delivers. As long as it's not more than four or five amps you'll be fine. Ordinary small chargers are okay.

Taking batteries out of cars is a right pain these days - all sorts of things get upset.

HouseOfLametta - 18 Dec 2018 18:58:22 (#19 of 68)

There is a diode pack in the alternator that should protect it, whether you’d hurt anything upstream is another matter. Probably depends how new/sophisticated your charger is.

coshipi - 18 Dec 2018 19:50:29 (#20 of 68)

The only ones that pose any risk to any vehicle are the "booster" chargers that can delivery huge currents, by putting large voltages on the terminals. Any ordinary small charger, whatever its age, is fine - the battery will prevent the voltage going up very much above 12V by drawing pretty much whatever current your charger can throw at it.

Just make sure you connect to the battery itself, or the connector right on the battery - don't try and connect somewhere else, such as a cigar lighter socket.

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