I have a 67 reg Skoda 3 with a 1.4 petrol engine.  I'm currently using it little so I am concerned about the battery condition.  What is the ideal rpm for the alternator to put maximum charge into the battery?  I currently try to do a 20 mile journey about once a fortnight keeping to 2000 rpm.  Is this about right to keep the battery in good condition?

The alternator will give its full 90 amp plus output from virtually tickover.

 

Thats the short answer, you will soon get the long low slow chapter and verse from battery man.

Maybe try one of those solar AA battery maintainer devices. Got one myself recently. 

Get a C-Tek MXS 5 charger. It charges and conditions your battery. You can get them from Tanya Batteries for about £63. Plug it in once every couple of months. At todays fuel prices, it would save you money and keep your battery in good order.

Measure 'turns ratio' (crank pulley diameter/alt pulley diameter) then pick an engine rpm after a quick browse here: Experimental study on the effect of alternator speed to the car charging system (matec-conferences.org)

The situation is made less predictable by the fact that you will presumably have start/stop and a battery management module that will 'tinker' with the alternator output voltage according to its whim, which isn't likely to prioritise battery lifetime.

An alternator is self managing. It uses feedback voltage from the battery to work out how much current it needs to generate. Just drive it normally for over 30 minutes regularly. 

 

Consider a battery maintainer that you plug in to keep it in best condition

True for traditional alts, not so simple with newer set-ups where the alternator is controlled by external electronics.

As above, the BMS is set up for economy and tends to only charge the battery on the overrun. Driving 20 miles @ 2000 rpm  to charge the battery is just wasting fuel. Buy a C-tec or similar charger suitable for AGM/ EFB stop/ start batteries if you need to.

11 minutes ago, Kenny R said:

As above, the BMS is set up for economy and tends to only charge the battery on the overrun. Driving 20 miles @ 2000 rpm  to charge the battery is just wasting fuel. Buy a C-tec or similar charger suitable for AGM/ EFB stop/ start batteries if you need to.

At todays fuel prices, payback time on the cost of the charger will be very short.

A point worth noting. If you do invest in a charger, do make sure you connect the -ve lead to the dedicated point on the bulkhead and not to the battery -ve terminal as per warning in owners manual.

4 hours ago, Kenny R said:

A point worth noting. If you do invest in a charger, do make sure you connect the -ve lead to the dedicated point on the bulkhead and not to the battery -ve terminal as per warning in owners manual.

Always something people seem to forget. Same with jump starting - if it’s done from the bulkhead point it skips the amp clamp on the negative terminal and the car will loose track of battery capacity and current charge levels.

 

regarding rpm for optimal charging, everything said above is spot on - the alternator has a variable charge rate that doesn’t depend on rpm alone and can vary magnet field strength to increase charge rate at idle speeds.

 

 

On 17/07/2022 at 08:01, Breezy_Pete said:

True for traditional alts, not so simple with newer set-ups where the alternator is controlled by external electronics.

That is a very good point that I should have pointed out. I'm used to more traditional alternators but modern LIN ones are a whole different kettle of fish! I've only just got a car with a smarter alternator and so still learning. 

 

I do remember being sat in a friends recent S60 with my USB charger (which shows voltage) and watching it switch between battery voltage and charging voltage. 

You also need to be wary that some recent systems also charge at higher voltages for AGM batteries. The Ford Transit uses a 15V system for instance.  

On 18/07/2022 at 16:13, Nathanio said:

That is a very good point that I should have pointed out. I'm used to more traditional alternators but modern LIN ones are a whole different kettle of fish! I've only just got a car with a smarter alternator and so still learning. 

 

I do remember being sat in a friends recent S60 with my USB charger (which shows voltage) and watching it switch between battery voltage and charging voltage. 

You also need to be wary that some recent systems also charge at higher voltages for AGM batteries. The Ford Transit uses a 15V system for instance.  

That Ford system has been around since the early 2000 it can use peak voltages of 18V so can easily fry another cars components if used to jump start another car. The days of the Lucas 17ACR are long gone. 

Thanks for all those very helpful replies.  I park the car too far away from a mains socket to use a mains battery maintainer.  I shall look into a solar charger.