[K9Leader]

I have installed a new battery in my 2010 E91 x-drive. I used Carly to register the new battery; however, in the registration process Carly gives me the options to select an, amp hours "Ah" rating from these choices:

55 Ah
70 Ah
80 Ah
90 Ah
110 Ah

But nowhere on the battery or in the info on the seller's website does it tell me the Ah capacity of this battery. It tells me this information:

Cold Cranking Amperage: 900 A
Cranking Amperage: 1000 A
Reserve Capacity: 150 min

How do I find or calculate the Ah rating for the registration? I did find one guide that said a rule-of-thumb is to divide the CCA by 7.25, which for my battery would be about 124 Ah. If that is accurate, then do I select the Carly option that is closest (110 Ah)?

[POBEP]

K9Leader:

Assuming this is Group Size H8, Part # 49H8 is rated 90 amp hour.

See Q/A section https://shop.advanceautoparts.com/p/...49h8/2130012-P

[dpaul]

Although there are some differences, in general Reserve capacity (RC) is just a different way to describe AH (ampere-hours).

By rule of thumb used by most:
Divide the RC by 2 to get AH

So 150/2 = 75AH

By application of physics:
Multiply the RC (given in minutes) by 60 to convert to seconds

Multiply that by 25 to get coulombs (the SI unit of charge which is the quantity of electricity conveyed in one second by a current of one ampere)

Divide that number by 3600 (the number of seconds in one hour)

So 150*60 = 9000 * 25 = 225000 / 3600 = 62.5 AH

The differences between RC and AH:
RC is the amount of time a battery can supply 25 A without the voltage dropping below 10.5v. It's a more defined measurement than AH, because the amount of current drawn in the determination of AH (or put another way, the length of time it takes to reduce the voltage to 10.5v) is generally not given.

This matters because battery capacity always decreases as the rate of discharge increases. Any battery will have a higher capacity if asked to deliver 1A than it would if asked to deliver 100A. So, you cannot make a perfect comparison of AH ratings for different batteries unless you know the parameters of the test the manufacturer used. Some manufacturers use 5hr, some 10hr, and some 20hr.

Minor point about "registration":
In BMW-speak, "registration" is telling the DME that a new battery has been installed and recording a date somewhere in its memory. Informing the DME that the battery type (FLA or AGM) and capacity (AH) have changed requires "coding" i.e. selecting one of several options which have been pre-determined by the factory.

In BMW software "registration" and "coding" are separate events although I realize that this standard is not always maintained in third party software.

[K9Leader]

Originally Posted by dpaul

Although there are some differences, in general Reserve capacity (RC) is just a different way to describe AH (ampere-hours).

By rule of thumb used by most:
Divide the RC by 2 to get AH
So 150/2 = 75AH

By application of physics:
Multiply the RC (given in minutes) by 60 to convert to seconds

Multiply that by 25 to get coulombs (the SI unit of charge which is the quantity of electricity conveyed in one second by a current of one ampere)

Divide that number by 3600 (the number of seconds in one hour)

So 150*60 = 9000 * 25 = 225000 / 3600 = 62.5 AH

The differences between RC and AH:
RC is the amount of time a battery can supply 25 A without the voltage dropping below 10.5v. It's a more defined measurement than AH, because the amount of current drawn in the determination of AH (or put another way, the length of time it takes to reduce the voltage to 10.5v) is generally not given.

This matters because battery capacity always decreases as the rate of discharge increases. Any battery will have a higher capacity if asked to deliver 1A than it would if asked to deliver 100A. So, you cannot make a perfect comparison of AH ratings for different batteries unless you know the parameters of the test the manufacturer used. Some manufacturers use 5hr, some 10hr, and some 20hr.

Thanks for that information. It is helpful.

I guess I don't understand why a battery marketed for BMWs does not include the Ah capacity in the specifications. The manufacturer knows BMW owners will need it. I just left it at 80 Ah, the same as the last battery (a BMW branded battery that was a year old when I bought the car four years ago). Starting had been noticeably sluggish the past few months and the recent cold weather made it even more so.

[ctuna]

Most battery's only have the reserve hours printed on them now.
However the last Interstate I bought had both and it was reserve
170 and AH 100.
That divide by 2 and forget about it is not the right formula.

[K9Leader]

Originally Posted by POBEP

K9Leader:

Assuming this is Group Size H8, Part # 49H8 is rated 90 amp hour.

See Q/A section https://shop.advanceautoparts.com/p/...49h8/2130012-P

Good find! It is indeed the 49H8 from Advance Auto but I had not stumbled on that information, buried over in Q&A. Thanks!

[dpaul]

Originally Posted by ctuna

Most battery's only have the reserve hours printed on them now.
However the last Interstate I bought had both and it was reserve
170 and AH 100.
That divide by 2 and forget about it is not the right formula.

AH 100 @ how many hours? Guessing it's 20hrs in which case there was only a 5A draw.

If so, makes sense that AH determined from RC is less (71AH).

Perhaps the most significant question is how did BMW determine AH?

[franciscold]

Hi guys,

Maybe someone can help me.

I have code 2E3C generator, type incorrect.

Can't find anything about it.

I've realized the battery I have installed is 62AH and the car is coded to a 70AH battery. Could that be the problem?

Thank you.