[Zack77]

I purchased this battery: http://www.napaonline.com/Catalog/Ca...94R_0006529972

And am just curious if anyone happens to know what the aH is; I've been unable to get any assistance so far.

[tke344]

Multiply the reserve capacity (minutes) by 25 then divide by 60.

[rlee134]

Originally Posted by tke344

Multiply the reserve capacity (minutes) by 25 then divide by 60.

confused by this equation as my current battery has RC of 140 and Ah of 80 ...those numbers dont add up

140*25/60 = 58.33

please let me know if im not understanding

[PDX-335i]

Read this: http://www.dcbattery.com/faq.html

You can convert reserve capacity (RC) to amp hours by multiplying RC*0.60. So 140 RC * 0.60 = 84 amp-hrs.

Differences between Amp Hours and Reserve Capacity

Another battery specification that is closely related to Reserve Capacity is called Amp Hours. Both relate to battery capacity, however they each answer a different question about the battery.

Reserve Capacity answers the question, “Given a particular current draw (or load), how long will the battery last?”

Amp Hours, on the other hand, answers the question, “What is the maximum current that can be drawn from the battery continuously and provide 20 hours of service?”

[Aashenfox]

Originally Posted by PDX-335i

Read this: http://www.dcbattery.com/faq.html

You can convert reserve capacity (RC) to amp hours by multiplying RC*0.60. So 140 RC * 0.60 = 84 amp-hrs.

Differences between Amp Hours and Reserve Capacity

Another battery specification that is closely related to Reserve Capacity is called Amp Hours. Both relate to battery capacity, however they each answer a different question about the battery.

Reserve Capacity answers the question, “Given a particular current draw (or load), how long will the battery last?”

Amp Hours, on the other hand, answers the question, “What is the maximum current that can be drawn from the battery continuously and provide 20 hours of service?”

I always believed it was 'at the amperage draw of the given number for Ah, this battery will last 1 hour.'. For example, an 80Ah battery will last 1 hour at 80A draw and with a fairly linear conversion rate, e.g. at 1A draw it will last 80 hours, etc, etc. This is well known for ni-cd, nimh, li-po, lifepo, li-ion and all the other batteries I've ever encountered in 30 years of modelling. I must confess I'm no expert on lead acid batteries but I'd be surprised if the unit is used differently. Direct reference to the pure duration of the battery at a given voltage and calculable amperage.

[PDX-335i]

Battery ratings are not too complicated but it's more of a curve than linear relationship. Lead acid batteries will act differently with varying loads.

A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate.

Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some manufacturers and vendors have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are.

A little research indicates the most common standard to be the 20-hour rate.

[PhaseP]

The OEM battery has its Reserve Capacity rating on it, it can be compared to it directly, since that NAPA is listing the reserve capacity.

[PDX-335i]

Here's a shot of the OEM Battery:
720 CCA
160 RC
90 AH



The NAPA battery looks a little smaller on RC.

FYI - Walmart actually has some of the best batteries you can buy at the best prices. Their store brand is made by Johnson Controls, who is one of only three battery manufactures in the country. Everyone's battery is made by one of these companies: http://www.carsdirect.com/car-mainte...ng-performance

I was also reading that BMW recommends staying with an AGM battery if you currently have one. Not sure what that's all about.

[PhaseP]

I had read here depending on the options, the car could have a different size and so RC/Ah battery installed from factory. The computer has the type (AGM or wet-cell) and also the Ah of the installed battery recorded. Any change on these would require update of these parameters on the computer, supposedly. Don't know how critical it is though. I think the Ah or RC if in the ball park should be fine. A change from AGM to wet-cell or vice versa maybe more important to require update to the computer. From what I remember charging of AGM and wet-cell is somewhat different.


To tell if it is AGM or not, if it makes sloshing sound when shaken it is wet-cell, not AGM. I expect an AGM battery to be marked, because it would be a selling point. They are considered superior to wet-cells. Mine OEM was regular wet-cell, 160RC just like pictured above.

[hawkoj]

There seems to be lots of confusion to determine aftermarket battery battery AH value from RC conversion. For E90, the dealerships sell two Lead (Pb) Acid, non-AGM batteries rated at 80AH/140RC and 90AH/160RC. With simple linear equation, it can be easily derived that AH~= RC/2+10. This is further validated from a Group 49 (H8) 185RC Duracell Battery from Sam's Club. Using the equation, it works out to be 102.5AH. I followed up by calling Duracell Customer Service and ask the AH for the battery and bingo! 100AH it is. The registered AH value should never exceed the rated AH value of the battery, otherwise it will overcharge and prematurely damaging and shortening the battery life. For E90, you can only code the battery with 80AH, 90AH or 110AH. Since there is no 100AH , the closest AH value lower that the battery AH should be used. In this case, 90AH should be used to code the car for the 100AH 185RC Duracell battery. If you car is originally coded with 80AH and the replacement battery is 100AH, you car will only charge the battery to 80AH and pretty soon you will see the battery warning light on the dash even though the battery is still good. Typically this happens if the registered AH value is smaller than 20AH from the actual battery. Hope this helps.