Since I'm merrily using heated everything on my moderate commutes and after a week parked up my car whined about a low battery voltage, it got me thinking.

Has anyone ever done the financial maths on how much keeping your battery topped via a charger might save you?

Assuming that a full battery means less resistance from the alternator and less likelihood of stop-start not happening due to reduced battery voltage.

 

My vague and uneducated maths suggests (all these numbers are open to challenge!)  - 

 

If diesel costs ~£1.30/l and that is ~10.7kWh/l 

If charging the battery from combustion is ~40% efficient 

Then it'd cost about 30p per kWh stored in the battery, versus ~10p from mains charging.

 

I reckon 50% charge of a 70Ah battery is ~ 0.5kWh

 

So a trivial 15p saving on topping up your battery?

Battery lifespan and replacement may be relevant too?
 

Just some musings.

Where did you get 40% from?

IMO there are far too many variables to even put a figure on it.

 

You could more accurately for a stationary engine or dedicated generator, maybe even a car just idling but when in use with varying engine speeds and loads, not to mention when inertia is turning the engine and therefore not using any fuel. I don't see how you could ever work out a real figure based on x amount of charge for y amount of fuel.

 

Good try though 

 

 

 

 

 

 

 

A diesel converting the chemical energy to kinetic is about 40% efficient I forget where I got this rule of thumb from, but it roughly matches wiki.

There will probably be losses in the charging process too, either in the alternator or battery.

Using energy to charge during braking is free energy. Its free when coasting on a trailing throttle too, other than slowing you down more than if there was no alternator load - so you might have to burn fuel to maintain speed or re-accelerate.

I suspect someone more knowledgeable can get a more reliable estimate, probably within 50% of the true value.

 

That 40% will be peak efficiency. Idling engine will be lower, engine braking charging the battery will mean infinite efficiency. This will be very difficult to the point it will vary with daily traffic variations.

 

But keeping the 12v battery topped up has 2 benefits:

1. you'll put less stress on the battery, should last longer (£££ in your pocket)

2. you'll save fuel when stop/start kicks in (£ in your pocket)

 

Charging the battery via mains will always be much more efficient, because you are not wasting energy to convert chemical fuel to kinetic then to electricity and finally back to chemical. Any energy conversion will produce wasted heat. Charging via mains only has 1 electrical conversion before the final chemical battery charge, a good AC-DC inverter is over 90% efficient.