RAC battery replacement

[wimorrison]

Because repeated short journeys that are using significant amounts of power when a battery has already been hit cranking over a cold engine prevent the battery from becoming fully charged again before the engine is switched off again. The result of this is that a good battery is gradually discharged and as the battery is called upon to operate at less and less peak condition it suffers more and more until eventually it doesn't have the capacity to start the engine ...

[cheshired5]

Originally Posted by wimorrison

Because repeated short journeys that are using significant amounts of power when a battery has already been hit cranking over a cold engine prevent the battery from becoming fully charged again before the engine is switched off again.

By your rationale, if you were to switch off your lights and heater, the battery would then be charged better and recover faster.
In reality, what actually happens when electrical loads are added is the alternator output is increased accordingly. That's the purpose of voltage regulation.

The battery charging rate remains the same regardless of other electrical loads and this can seen when using a current clamp.

[wimorrison]

Not quite correct as the regulation is within limits to ensure that the battery isn't not damaged and on dark winter mornings it isn't safe (or legal) to run without lights and the additional drain from the other devices means that the car is switched off before recharging is complete.

Well known issue for school run cars, especially older ones with smaller capacity batteries that haven't been replaced since car was built.

[stephend]

Originally Posted by cheshired5

Why is driving with lights and the heater on hard on a battery?
Isn't the alternator taking care of electrical demands so there is zero load/drain on the battery?

I think the answer is that if the car's only run for a short time, there isn't time for the alternator to completely replenish the charge drained from the battery. So each time you do it, you're reducing the charge left in the battery. Cumulatively, if you don't do a long run from time to time and get it fully re-charged, the battery ends up low on charge.

Edit: sorry, cross-posting...!

[cheshired5]

Originally Posted by stephend

I think the answer is that if the car's only run for a short time, there isn't time for the alternator to completely replenish the charge drained from the battery. So each time you do it, you're reducing the charge left in the battery. Cumulatively, if you don't do a long run from time to time and get it fully re-charged, the battery ends up low on charge.

Yes, the length of charging time will have an impact over a long period of time but having lights and the heater on won't make any difference.

The biggest impact is making sure your car is a good starter and making sure your charging system is functioning correctly.
If you nail those two things, 15-20 years out of a battery is easily achievable.
In 27 years and 1.5 million miles, I've bought 1 battery so they're definitely not consumable items in my view.

[stephend]

Interesting! So why do you think so many batteries fail in the winter? Temperature?

[wimorrison]

Essentially yes, it is the temperature that is the killer.

As the temperature falls the ability of the battery to provide the stored charge drops dramatically - almost exponentially and you are asking this device with reduced capacity to then preheat a Diesel engine (often 50+ amps for 10-20 secs) then turn over a cold engine which even with a low viscousity oil is still difficult to get moving. Even petrol engines have difficulties in winter, though less so.

You are asking a battery to perform to the maximum extent when it is least able and then giving it a reduced charging cycle due to all the ancillaries drawing power with a short journey and when you add all this up you get the all to normal winter problems of a 'flat battery'

Never worked out why we say flat though

[cheshired5]

Originally Posted by stephend

Interesting! So why do you think so many batteries fail in the winter? Temperature?

The battery isn't at its best at cold temperatures but I would say that it's a combination of not addressing poor starting, not monitoring charging and folk not understanding that using electrical loads with the engine off has a surprisingly high drain will kill a battery prematurely.

Small things can make a big difference too like popping the door latches and not having the radio on when hoovering your car.
In my parasitic drain test video, you can see how easy it is is to increase battery drain just by unlocking the car and "waking it up" so the longer it's in that state without the engine running, the faster it will discharge.

I also have a smart charger so maybe twice a year around summer and Christmas, I'll give the battery a full recharge off the car which can take up to 24 hours.

A battery is a bit like a leaking bucket and how you use it determines the size of the hole.

[stephend]

I have to say, I've only had to replace one battery in many years: it was at least 10 years old, and was completely discharged twice, once due to alternator failure and again due to an interior light left on. I think that counts as abuse!

[Marty Dolomite]

There is only one way to test a battery properly and that is to do a 'drop test' which requires the battery to be fully charged and this can't be done on the car at the roadside.