of alternators and batteries

jojowales · Mar 10, 2011

Hi All

Like to run something past you and get your opinion guys.

I recently had my Boxster S serviced at my local Porsche centre. During the work they contacted me to inform me that I had the wrong battery fitted. The battery was specific for a Boxster S but not Porsche equipment. They said it was too high rated (ampere) and could cause the alternator to burn out. Perhaps I'm missing something here but I thought 12volts was 12volts and the current drawn by any piece of electrical equipment was governed by its resistance. I can't see how this alternator meltdown would happen, am I missing something or are they just trying out the size of my wallet?

Any electrical gurus out there?

JJ

Mark_Bennett · Mar 10, 2011

The PET for the 987 lists 3 capacities for batteries:

70, 80, 95 AH

So any of these (or any rating inbetween) would be fine.

The only thing I could conceive of is that the alternator MIGHT have to work a little harder to charge the higher capacity battery, but I would have thought it would have plenty of excess capacity enough to cope. I'm not an electrical engineer mind.

bdheng · Mar 11, 2011

The electrical gurus at work have said that if the battery is rated too high (aH), the alternator could cook itself.

So if Porsche state that 95 is the largest - anything bigger could cause a problem

vitesse · Mar 11, 2011

My physics tell me :-the alternator is always charging-if the battery is at a low state of charge,then the alternator will take longer to charge it back up before it then only needs to balance the demand from the car electrics-the greater the ampere/hour capacity just means a similar situation-after all,if you use a battery charger with low/high settings-no matter what the battery capacity is,the charger will always take longer to charge it back up on the low setting than on the high setting so a larger capacity battery will also take longer than a smaller one on the same setting-it does not cook the charger & the alternator is just the equivalent of the charger except that you don't need a very long wire to use the car.[

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akwaugh · Mar 11, 2011

Hi

The current which your alternator can deliver is limited by circuitry and you cannot overcharge your battery because of this, unless there is something wrong with your alternator.

Note here - your alternator cannot charge a depleted battery, it's only there to maintain a level of charge, which is why you can never "re-charge" a battery if flat by jump starting and driving it around (you could with a dynamo but that's showing my age!) - you need a proper charger for that. It's also the case that you could run your car without a battery so long as the alternator can cope. Wouldn't recommend it though.

Think of your battery as a bath full of water (complete with plughole and overflow) and alternator as a tap which can be closed or variably open.

When you start your car the stored water (energy) in the bath is allowed to drain through the fully open plughole - hence the size of (cable) pipe used - until the starter is operated and turns the lump of metal called an engine. The bath rapidly empties. So far so simple.

When the engine is spinning at sufficient rpm the plug hole almost closes, while the tap opens to deliver water to the bath and re-filling commences to top up the lost water (amps) plus any other water (amps) running out of the plughole. The alternator must then keep topping your (battery) bath up until it reaches the overflow pipe or it reaches equilibrium with all the toys draining amps away - lights, radio, sparklers etc..

Therefore, assuming I am correct - and I stand ready for abuse if I'm not - if you have a higher capacity battery then assume it to be a bigger bath. All your (current) drains have remained the same size, all your (current) inputs (alternator) remain the same.

Therefore you needn't worry.

Rodney Naghar · Mar 11, 2011

If Carlsberg made school teachers.....
Brilliant!

colinbythesea · Mar 11, 2011

My understanding is that the battery doesn't present a load and demand electricity. The alternator supplies electricity. As said earlier like turning a tap on not sucking water out of the pipe.

GOOFY · Mar 11, 2011

I think you may need to take heed of the Porsche technician's advice. For a start an alternator will be damaged without a battery connected. An alternator needs a suitable load - hence a bad battery can damage an alternator. Secondly a battery with a huge capacity will take a longer time to reach full charge thereby presenting the alternator with more work to do. This is still the case even if not all electrics are used, as batteries do still loose their charge. I do not see any advantage in a higher capacity battery, unless the car needs it (larger engine to turn). Alternators are really sensitive things (as opposed to the old dynamos) so OPC advice may be credible.

T911UK · Mar 14, 2011

Porsche actually list 4 batteries 60, 70, 80 & 95 amphr and only 1 rating of alternator. what does that say?

grahamwhite · Mar 14, 2011

+1 !!!!!

ORIGINAL: akwaugh

Hi

The current which your alternator can deliver is limited by circuitry and you cannot overcharge your battery because of this, unless there is something wrong with your alternator.

Note here - your alternator cannot charge a depleted battery, it's only there to maintain a level of charge, which is why you can never "re-charge" a battery if flat by jump starting and driving it around (you could with a dynamo but that's showing my age!) - you need a proper charger for that. It's also the case that you could run your car without a battery so long as the alternator can cope. Wouldn't recommend it though.

Think of your battery as a bath full of water (complete with plughole and overflow) and alternator as a tap which can be closed or variably open.

When you start your car the stored water (energy) in the bath is allowed to drain through the fully open plughole - hence the size of (cable) pipe used - until the starter is operated and turns the lump of metal called an engine. The bath rapidly empties. So far so simple.

When the engine is spinning at sufficient rpm the plug hole almost closes, while the tap opens to deliver water to the bath and re-filling commences to top up the lost water (amps) plus any other water (amps) running out of the plughole. The alternator must then keep topping your (battery) bath up until it reaches the overflow pipe or it reaches equilibrium with all the toys draining amps away - lights, radio, sparklers etc..

Therefore, assuming I am correct - and I stand ready for abuse if I'm not - if you have a higher capacity battery then assume it to be a bigger bath. All your (current) drains have remained the same size, all your (current) inputs (alternator) remain the same.

Therefore you needn't worry.

jojowales · Mar 16, 2011

Well guys thanks for all your help with this and for taking the time out to explain some of the facts.

Thought I'd just show you the offending item that the Porsche dealership suggests would burn the alternator out!

Its an 80AH Bosch S4010 battery but perhaps its the wrong colour or doesn't have the word Porsche on it.

Cheers Guys

JJ

BrianJ · Mar 17, 2011

There's a Bosch catalogue here http://rb-aa.bosch.com/aa-batterien/gb/media/BA_Pkw_Kleintransporter_Anwendung.pdf identifying which battery fits which car. 986 and 987 listed as 85Ah S5010 or 80Ah S4010.