Battery puzzle - 35Ah vs 45Ah?

[Oldmogman]

Just spotted John Waters' note in Technical Tips (MM Mar/Apr) about battery ratings - and I'm puzzled!

When I phone motor factors for prices, they always quote me for a 45Ah battery - which as John says, is the recommended spec.

However, I'm not sure I quite understand Rob Thomasson's reply to John's note. Does a 35Ah battery have a cranking current of 250 or 450amps? Or is there an option for 250/450 amps?

So is Rob saying the 35Ah battery starts his car better because it delivers its power more quickly - ie, emptying the bucket in one hit? (Which presumably means it will take a while for it to be charged up again?)

Also, surely the type of generator will have a bearing on the choice of battery? My car is still running the good old dynamo, which produces less juice than an alternator.

I'd be interested to hear what other Minor owners have found. Is 35Ah better than 45Ah?

Thanks all.

[mike.perry]

The larger battery the better, it gives you more reserve, especially with a dynamo which takes longer to recharge or if your charging circuit fails and you have to get home on battery power alone.
Besides a small battery looks silly in a large battery box.

[Oldmogman]

A quick update: I just spoke to a very helpful guy at Charles Ware's Morris Minor Centre who said they recommend the 35Ah (type 038), and explained that a 45Ah would require the dynamo to work harder to keep it topped up.

[mogbob]

Your dynamo is still going to "work" at the same rate, whenever the Battery "needs topping up ".
The bigger the Battery, the bigger the " reserve/ store " of electrical power.....i.e.doesn't need
topping up so frequently as a smaller Battery.

I'm firmly in Mike's camp on this one.
Bob

[MarkyB]

Thirded, only was has been used needs topping up.
You just have more in reserve with the bigger battery.

[bmcecosse]

" just spoke to a very helpful guy at Charles Ware's Morris Minor Centre who .......... explained that a 45Ah would require the dynamo to work harder to keep it topped up. "
What utter nonsense - dear oh dear...... where do they get these people??
I say the smaller battery will be fine - BUT - if you can get the larger battery for the SAME money - go for it! Nd in general - Costco seem to be the best source of top grade batteries for fair cost. And in practice - a dynamo actually tends to recharge a battery more quickly, because it's voltage is not as well regulated as an alternator. Just watch the ammeter on a dynamo car - it will show a good heavy charge into the battery after a cold start. The alternator tends to give a longer/slower charge - which is kinder to the battery of course.

[Oldmogman]

Thanks guys, that's great advice.

So when I shop around for a 45Ah battery, and bearing in mind Rob's comment in Minor Matters, do I specify one with 250 or 45amps? Or is there not a choice?

(Couldn't find anyone who's a Costco member, and I'm not eligible myself, so sadly I don't think I'll be able to get one of their batteries.)

[MarkyB]

This site has a guide, albeit a bit American.
http://hubpages.com/hub/Choosing-a-Car-Battery---Guide

[bmcecosse]

Just go for 'value for money' - an A series doesn't NEED big cranking capacity - so no point going overboard. A small 35 amphour battery will be fine. But if you can get a larger one for same money - go for it. MANY on ebay with free next day delivery for ~£40. Or if you know an employee in Halfords - they get a good discount!

[Rob_Jennings]

capacity = how much is in there, linked pretty clearly to physical size and weight
crank current = maximum current you can draw at once, linked to the way the battery is wired (and a little to the size)

these are roughly independant.

there is nothing wrong with puting a larger capacity in, it won't 'strain' the dynamo or alternator to charge it since you are only putting back what you use (less what gets lost standing around). Higher capacity will give you more chance of getting home if the charge circuit fails or simply can't keep up with demand (e.g. everything on).

As has been said the a-series engine is pretty light so crank capacity does not really mean much to us. It is normally quoted for modern cars with big engines where you are concerned about how hard it might be to turn the engine over (e.g. big 2 ltr diesel needs loads of crank power).

I've seen some modern cars with tiny batteries e.g. low capacity but high crank power so designed to be light and just enough to start the engine, but after that the alternator is doing ALL the work. this sort of makes sense since you now have electric power steering, electric air-con and other electric mod cons, so your alternator is massive and running all these things and they dont have to be bolted on the engine block just wired in somewhere on the car. Of course if your alternator fails then you lose everything, but there such a high electrical demand that a battery would not last long enough to get you anywhere sensible anyway!

[mike.perry]

just spoke to a very helpful guy at Charles Ware's Morris Minor Centre who .......... explained that a 45Ah would require the dynamo to work harder to keep it topped up. "[/quote]
My C39 dynamo handles the 540 amp cranking current battery on my Series MM with no trouble.

[bmcecosse]

[phurn]

People really do talk out their rear end when it comes to batteries don't they? Ignore what ever dodgy advice the guy on the phone gave you. Buy which ever you want... there's no such thing as a too large battery. I once heard a mechanic tell a guy it was normal for his rover to use water as it sits at The bottom of the engine when its not in use! And that he shouldn't worry about it being his head gasket =\ so don't trust someone just because they work at a garage!