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Does charging your car battery on fast charge damage it?

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Simon Robinson 423/11/2021 14:06:56
80 forum posts

Ive got two settings on my car battery charger. High and Low. I’ve heard that using the high charge often can distort the electrodes over time. But surely when I drive along the motorway the battery is getting a much higher charge from my alternator than any plug in battery charger can even give it.

Any thoughts?

Ady123/11/2021 14:30:23
5160 forum posts
738 photos

A healthy battery should be fine

Pete Rimmer23/11/2021 14:36:32
1255 forum posts
69 photos

Just another 'common myth' that makes sense until you think about it properly. A 15amp charger isn't going to distort plates that deliver 120amps to the starter.

Fast charging might warm the battery up and cause the electrolyte level to drop over a long period.

Edited By Pete Rimmer on 23/11/2021 14:37:34

Nicholas Farr23/11/2021 14:48:18
3413 forum posts
1586 photos

Hi Simon, I don't know the answer, but I don't think you can compare the two. The charging circuit in your car is probably controlled more precisely than your plug in one and of course the car's electrics will also be drawing current and the circuit will also be controlled to keep levels in order and balance between the battery and the car's demand.

Regards Nick.

Clive Hartland23/11/2021 14:52:41
2837 forum posts
40 photos

The principle of a lead acid battery that I was taught was that it will take what it needs to re-charge.

A battery low in charge will draw a deep current and as it starts to reach full charge it tapers off.

Beware overcharging, so take note of the time and check regularly on it's condition.

IanT23/11/2021 14:54:21
2002 forum posts
212 photos

I will just mention that newer (Start Stop) batteries do not like old battery chargers (don't ask how I know!)

I now have a new 'Start Stop' charger (from Halfords) that offers 'fast' & 'trickle charge' settings which seem to be fine. It was much cheaper than the new battery.

The old battery charger is now regulaled to rust removal (electrolysis) duties - for which it is ideal.



Juddy23/11/2021 15:10:32
106 forum posts

I think the tale of high charge rates damaging batteries comes from when dealerships would stick an 100amp charger on a battery prior to a test drive, or leave the a high amp charger on for extended periods of time which will distort the plates heating the battery drying out the fluid. Years ago buying a second hand car from a back street dealer normally resulted in the car needing a new battery fairly quickly after purchase. Probably not such an issue nowadays with regulated & controlled charging from modern chargers.

SillyOldDuffer23/11/2021 15:29:01
8859 forum posts
1994 photos

Is the battery charger being used a lot? If so, it suggests something is wrong - too many short journeys, failing battery, capacity too small for the vehicle, stuck boot-light switch etc. A healthy battery in ordinary service should recharge on the accumulator. (Assuming a modern car. They seem kinder to batteries than older models. I think because electronic voltage regulators are more sensitive and react faster than mechanical types. )

Distorting plates isn't the only way of damaging a battery. Overcharging takes years off them! Cheap chargers ignore all the warning signs if a battery is put on fast charge and allowed to carry on past full charge. Better units back-off automatically, so much depends on the electronics inside. My classic charger contains a simple transformer and rectifier and has to be supervised.

Jump start rechargers are another ball-game. The usual size might max about 1250A into a battery, which could cause bent plates if done too often. Conversely, shaking up Sulphated plates can revive a failing battery, though the recovery is temporary, not a permanent cure. Needing frequent jump-starts suggests something is wrong too.


mgnbuk23/11/2021 16:18:28
1198 forum posts
72 photos

Ive got two settings on my car battery charger. High and Low.

The rating plate on the charger should say what the charge currents are for both settings. I would guess at Low being 1-2 amps & High between 4 and 8 amps, depending on how expensive the unit was. Rule-of-thumb for charging a lead-acid battery I was taught was to charge at around 10% of the battery capacity to prevent damage, so 10A for a 100A battery. If your car has a small battery (say 45 A) & your charger is 8 A rated you may be a bit on the high side, so if you have the time charging it for longer on low may be beneficial.

In the car the alternator will be supplying all the vehicle demands + just replacing the amount taken from the battery by starting the engine - the full output of the alternator isn't going through the battery.

Nigel B.

Simon Robinson 423/11/2021 16:30:57
80 forum posts
Posted by IanT on 23/11/2021 14:54:21:

I will just mention that newer (Start Stop) batteries do not like old battery chargers (don't ask how I know!)

I now have a new 'Start Stop' charger (from Halfords) that offers 'fast' & 'trickle charge' settings which seem to be fine. It was much cheaper than the new battery.

The old battery charger is now regulaled to rust removal (electrolysis) duties - for which it is ideal.



That’s sounds a good idea. Never thought a car battery charger could be used for electrolysis. So I guess you put the negative clip on your work and the positive on something else or the other way around?


Samsaranda23/11/2021 16:50:58
1468 forum posts
7 photos

IanT can you elaborate on what your old battery charger did to your start/stop battery? I have a start/stop battery and during lockdowns have not been using the car very often consequently the battery has needed charging at various times, I don’t do much mileage. The battery is about five years old and probably reaching end of reliable life, I have a battery charger which I bought at the start of first lockdown, it has two charging rates and I only use the low rate charge. I am very interested to know what damage a conventional battery charger could do? Dave W

IanT23/11/2021 17:11:36
2002 forum posts
212 photos

Yes - I was doing very little mileage during lockdown and my 5 year old car wouldn't always start after a week or so on the drive. Tried re-charging but it increasingly didn't seem to work. Investigated and found out I was probably accelerating it's demise. This link explains the problem...

Can I use any battery charger?





Edited By IanT on 23/11/2021 17:13:17

J Hancock23/11/2021 17:34:36
843 forum posts

A 'standard ' cheapish battery charger output is not really 'dc' like the battery output , so be careful what you connect it to if not a battery .

john fletcher 123/11/2021 17:34:56
805 forum posts

Simon above, make the sacrificial ANODE the POSITIVE and the object to be de rusted Negative. I use diluted washing soda in a plastic bucket and wear some disposable gloves. You will be amazed how well it cleans things the first time you do it. Lots of scum and of course if you don't clean up the object further and protect it, it soon run again. John

Bazyle23/11/2021 17:50:49
6377 forum posts
222 photos

In an old charger the 'fast' or 'high' is an unsmoothed full wave rectifier. The 'low' setting is just turning off half the diodes so it is an half wave rectifier. Each pulse of current is the same as before just half the time it is completely off. Also part of the cycle is below the 12v so not charging then anyway. Finally do you really think they put enough copper in that cheap charger's windings to put out any heavy current.

Ady123/11/2021 18:14:36
5160 forum posts
738 photos

Overcharging is the only caveat

The old chargers needed to be turned off manually or they "cooked" the battery

Howard Lewis23/11/2021 21:44:57
6297 forum posts
15 photos

Modern lead acid batteries are much improved over those available many years ago In those days, a 2 year guarantee on a new battery just about stated it s expected life.

In those days a Lucas C40 dynamo could deliver 22 Amps maximum. Modern Alternators can deliver 100, and the charging control systems are much more sophisticated.

My first car, a Singer 9, had three charging rates; determined by the resistance between the battery and the field windings of the dynamo, depending upon the lighting load..

My last car still had the original battery in good condition, (judged by the green charge indicator ) when it was sold after 12 years!

The thing that is most damaging to a lead acid battery is to leave it discharged. That accelerates sulphation of the plates.


Bill Dawes23/11/2021 22:17:54
539 forum posts

Happy memories of my first car, a 1946 Austin 8. I think I ran it for years with a dead battery, started with a handle and then probably just ran off the generator, driving along country lanes at night was especially exciting, slowing down for bends caused the engine to die, clutch quickly in, lights off, bump start, first gear to keep revs up, lights back on just in time to negotiate bend.

A bit of 'expert' tuning of the regulator to increase charging the next day kept me going for a little longer.

Bill D.

Ian Mellors24/11/2021 14:15:47
70 forum posts
17 photos

Not really answering the question, but my car (a WRX Impreza) does 10 mins to work and back so it is plugged into an Optimate trickle charger over the weekend. It was also so connected during lockdown last year and during my enforced stay in hospital over this summer.

Can't recommend such a device enough. Will do it's best to resurrect a deep discharged battery too

Samsaranda24/11/2021 15:05:15
1468 forum posts
7 photos

My first car was a 1954 Ford Popular with the 1172 cc side valve engine, the electrics were 6 volt and the headlights were dire at the best of times. Was glad of the starting handle on many occasions when it was cold and frosty and the battery was sluggish to respond. Because it was 6 volt electrics it lacked electric windscreen wipers, it had vacuum operated wipers, you didn’t put your foot down in heavy rain because the wipers would stop dead no vacuums to run them. For all its faults I loved that old car, when I was overseas the wife sold it to her brother and he blew the engine up on it, I still haven’t forgiven him for that and that was 1967. Dave W

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