[Kizelio]

Originally Posted by PhaseP

The RC spec is found on the original oem battery, as op had. If you match the RC of an after market battery to it rather than amp hours it will also be a better match to the amp hour configuration in the computer. OP had an exact matching RC but not amp hours, but it meant he had exact match.

Thank you, that puts my mind at ease. The cable won't be here until Friday or Saturday, so I'll let you know how it works out.

[Wilt]

The problem is that OP stated, "900cca AGM, and both have 160 reserve capacity. The only difference is that the OEM has 90 amp hours, while the aftermarket has 95."

According to multiple sources, the conversion between RC and Amp-hour is RC*0.4167= A-h
But doing the conversion 160 * .4167 = 66.7 ?!

Another source says conversion RC/2 = Amp-hours ; 160 / 2 = 80 ?!

But like I said before, the BMW battery choice menu as no selection for RC or CCA, no selection but Amp-hours.

[gjm120]

Originally Posted by Wilt

The problem is that OP stated, "900cca AGM, and both have 160 reserve capacity. The only difference is that the OEM has 90 amp hours, while the aftermarket has 95."

According to multiple sources, the conversion between RC and Amp-hour is RC*0.4167= A-h
But doing the conversion 160 * .4167 = 66.7 ?!

Another source says conversion RC/2 = Amp-hours ; 160 / 2 = 80 ?!

But like I said before, the BMW battery choice menu as no selection for RC or CCA, no selection but Amp-hours.

This will get nearly spot on:
RC * 0.473 + 14.492 = Ah

[PhaseP]

The conversion formulas between RC and amp hour are approximations. And the reason they don't always work out is same amp hour spec has loose definition as I tried to explain in previous posts.
Car computer has amp hour configurations to select from, but those correspond to the amp hour values of the oem batteries car uses. Oem battery may have used how many hours it takes to discharge the battery drawing 10 amps, and the aftermarket battery may used how many hours while drawing 1 amp. Results won't be exactly comparable. The RC is more tighter spec and so it is better way to compare total charge capacity of one battery to another, which is what really matters. The car computer could be using part numbers instead if amp hours. You are trying to match the charge capacity to the oem in the end and RC is a better spec for that than amp hours.

[PhaseP]

Originally Posted by Wilt

The problem is that OP stated, "900cca AGM, and both have 160 reserve capacity. The only difference is that the OEM has 90 amp hours, while the aftermarket has 95."

According to multiple sources, the conversion between RC and Amp-hour is RC*0.4167= A-h
But doing the conversion 160 * .4167 = 66.7 ?!

Another source says conversion RC/2 = Amp-hours ; 160 / 2 = 80 ?!

But like I said before, the BMW battery choice menu as no selection for RC or CCA, no selection but Amp-hours.

OP knows the RC value of the factory battery in his car which is printed on it. He doesn't need to do any conversions, just match it. If he didn't know the RC of the OEm battery in his car then he had no choice but go with amp hours.
All this actually is somewhat nitpicking. Car will still work and all that matching may provide who knows how many months of battery life.

[Wilt]

Just found this, for the OP 2007 328xi...

Exide Edge Flat-Plate AGM Battery
Part #: FP-AGML5/49
48 Month Free Replacement Period
CCA: 850
RC: 160
Ah: 90
Length: 13.875". Width: 6.875". Height: 7.5".
Computer must be reset. Battery must be vented outside of vehicle. OE CCA: 720

Interstate MT-5 AGM
30 Month free replacement warranty
Product ID: MT5-49/H8
Cranking Amps: 1000
Cold Cranking Amps: 900
Voltage: 12
Termination: Common Code A
Weight (lbs): 58.8
Length (in): 13.94, Width (in): 6.94, Height (in): 7.50
ReserveCapacity-25: 160.00
WET/DRY: D

Duracell Item Number: SLI49AGM
Brand: Duracell Ultra
Voltage: 12
Format: BCI Group 49
Lead Acid Type: Dual Purpose (Starting/Cycling)
Cold Cranking Amps: 850
Battery Type: Premium
Capacity 20hr: 92AH
Chemistry: Lead Acid
Cranking Amps: 975
Lead Acid Design: AGM
Made in the USA: True
Terminal Type: SAE, Automotive Post
Warranty: 48 Month Free Replacement
Weight: 57.75 lbs
Length: 13.9375 in, Width: 6.875 in, Height: 7.5 in

Autozone Duralast Part Number: H8-AGM
Weight: 58.0lbs
Warranty: 3 years
Notes: *** 900 cold cranking amps (1000 cranking amps) ***. 160 Reserve minutes.
Warranty consists of 3 years free replacement. SBattery is vented to outside of vehicle. Vehicle computer system must be reset, contact your service specialist.
BCI Cold Cranking Amps (CCA): 900
BCI Designation: 49
Battery Length: 353 mm - 13 15/16 in, Battery Width: 175 mm - 6 15/16 in, Battery Height: 190 mm - 7 1/2 in
EN / DIN Designation: H8/LN5
Reserve Capacity (min): 160
Voltage: 12.0

[Wilt]

A complication to the selection process is little consistency of parameters:

For my own 2011 328i, source is battery manufacturer information:

EXIDE GLOBAL EXTREME, PART #: L5/49X, Gr 49, 107 Ahr, 900 CCA, RC 185
EXIDE EDGE FLAT-PLATE AGM, Part #: FP-AGML5/49, Gr 49, 90 Ahr 850 CCA, RC 160
Bosch S5 Premium, Gr H8, unknown AHr, 900 CCA, unknown RC
Bosch S6 AGM High Performance S6588B, Group L5/H8, unknown AHr, 720 CCA
Delco AGM Automotive 94RAGM, Group 94R, unknown Ahr, 640 CCA
Delco Gold BCI Group 94R Battery, unknown Ahr, 640 CCA
Interstate MT5 AGM MT5-49/H8, Gr H8, unknown Ahr, 900 CCA, RC 160
Interstate Powerfast PF-49/H8-6, Gr H8, unknown Ahr, 850 CCA, RC 150
Interstate MEGA-TRON II® All Seasons MT-49/H8, Gr H8, unknown Ahr, 900 CCA, RC 150
Autozone (retailer), Duralast Platinum H8-AGM, Gr 49, 92 AHr, 850 CCA, RC 170
Autozone Duralast Gold H8-DLG, unknown Ahr, 900 CCA, RC 185

So we have for the same CCA different values of RC
900 CCA RC 185
900 CCA RC Unknown
900 CCA RC 160
900 CCA RC 150
900 CCCA RC 185

Or for same RC, different CCA
850 CCA RC 160
900 CCA RC 160
900 CCA RC 160

Or for same 90-92 Amp-hour, different
850 CCA, RC 160
850 CCA, RC 170