[dpaul]

Amp hour rating is not meaningful UNLESS the manufacture tells you the rate of discharge. You cannot compare batteries from different sources without that information.

That's why Reserve capacity is a more useful number - because it is standardized to a given discharge rate and endpoint.

Those "95-110" AH batteries won't have reserve capacities suggesting anywhere near that.

Forgive the rant

[Welcome to NBA Jam]

Originally Posted by dpaul

Amp hour rating is not meaningful UNLESS the manufacture tells you the rate of discharge. You cannot compare batteries from different sources without that information.

That's why Reserve capacity is a more useful number - because it is standardized to a given discharge rate and endpoint.

Those "95-110" AH batteries won't have reserve capacities suggesting anywhere near that.

Forgive the rant

The only reason why it matters so much is because you have to code your car to a specific Ah rating if you deviate from the factory setting.

[ewicky]

Originally Posted by dpaul

Amp hour rating is not meaningful UNLESS the manufacture tells you the rate of discharge. You cannot compare batteries from different sources without that information.

That's why Reserve capacity is a more useful number - because it is standardized to a given discharge rate and endpoint.

Those "95-110" AH batteries won't have reserve capacities suggesting anywhere near that.

Forgive the rant

Ah ratings on lead batteries are at a 20hr rate unless otherwise specified. It's pretty standard.

Not accounting for Peukert’s Law, the RC*25/60 conversion is spot on. Every battery has a different Peukert's constant, and at different temps and wear levels. A 90% efficiency at 25A can be reasonably assumed. So divide your Ah rate by .9ish if you want to pretend to account for that.

For those not great at math, I'd say RC*0.47 is a good estimation of Ah.

[Jim_B]

Originally Posted by gurusan

I'm sure the actual manufacturer should be somewhere on the battery?

You've raised a really good point. I didn't previously look at the battery in my wife's car because it's a bit of a pain to access. (I definitely didn't feel like removing it to look for information I kinda figured wouldn't even be there.) I happened to be in Walmart today and pulled one off the shelf to have a look:



Sure enough, the manufacturer is listed right on the top. I'll e-mail or call Johnson Controls and see if they have the information.

[ctuna]

https://www.walmart.com/ip/EverStart...up-H8/16782659
specs say reserve capacty is 140 at the above link.

Amp/Hours = (Reserve Capacity / 2) plus 16

140/2 +16 =86 amp hours close enough to 90

Debate RC or amp hours Ok
But amp hours is what BMW uses to set up its charging
profile if you believe there profiles actually does anything.

https://www.interstatebatteries.com/products/mtp-49-h8

[dpaul]

Originally Posted by ewicky

Ah ratings on lead batteries are at a 20hr rate unless otherwise specified. It's pretty standard.

Not accounting for Peukert’s Law, the RC*25/60 conversion is spot on. Every battery has a different Peukert's constant, and at different temps and wear levels. A 90% efficiency at 25A can be reasonably assumed. So divide your Ah rate by .9ish if you want to pretend to account for that.

For those not great at math, I'd say RC*0.47 is a good estimation of Ah.

Good point but I've seen some rated @5 hr and @10 hr rates, which can change the assessment significantly, as per Peukert's law.

[ewicky]

Originally Posted by ewicky

... 20hr rate unless otherwise specified...

Originally Posted by dpaul

Good point but I've seen some rated @5 hr and @10 hr rates, which can change the assessment significantly, as per Peukert's law.

Yup. The UPS batteries I buy state they are rated at 1hr since they are designed for that application (higher rate than normal deep cycle)

[Jim_B]

I called Johnson Controls and was told "there is no Amp Hour rating for that battery." I may just see if I can return it...:

[ewicky]

Originally Posted by Jim_B

I called Johnson Controls and was told "there is no Amp Hour rating for that battery." I may just see if I can return it...:

You could put a load on it and time it until fully discharged. Pretty simple experiment. A halogen headlamp would be ideal for an almost 20hr rate. Just need a timer and a voltmeter.

[Jim_B]

Originally Posted by ewicky

You could put a load on it and time it until fully discharged. Pretty simple experiment. A halogen headlamp would be ideal for an almost 20hr rate. Just need a timer and a voltmeter.

Or I could (hopefully) return it, buy an AGM battery rated for 90Ah, and be done with this nonsense once and for all.

It's very odd to me they don't rate these batteries for Ah. It seems they're used in a lot of European cars and Euro manufacturers use Ah instead of CCA.

[Sgop335]

Originally Posted by Jim_B

Or I could (hopefully) return it, buy an AGM battery rated for 90Ah, and be done with this nonsense once and for all.

It's very odd to me they don't rate these batteries for Ah. It seems they're used in a lot of European cars and Euro manufacturers use Ah instead of CCA.

if you can code it, sure an agm w 90ah. What was the original battery? sorry if it was already mentioned.
If you are going to return it because JC says there is no rating what are the other guys going to do?

[PhaseP]

Originally Posted by ctuna

https://www.walmart.com/ip/EverStart...up-H8/16782659
specs say reserve capacty is 140 at the above link.

Amp/Hours = (Reserve Capacity / 2) plus 16

140/2 +16 =86 amp hours close enough to 90

Debate RC or amp hours Ok
But amp hours is what BMW uses to set up its charging
profile if you believe there profiles actually does anything.

https://www.interstatebatteries.com/products/mtp-49-h8

Amp hours is a loosely defined parameter. Discharge time at 1 A is different than 10A and different than 100A, even though simple modeling of battery says it should give same result of amp hours, it doesn't.

Reserve Capacity is well defined:
"It is a time measurement that explains how long a fully-charged battery can deliver 25 amps of current in an 80°F-environment, before the battery is discharged down to 10.5 volts."

So to compare apple to apple battery capacity RC is more useful.

BMW used amp hours in the car computer parameters, what to do?
If you know what the OEM battery RC was on your car, then you look for same or close RC battery as replacement. My OEM was 90 Amp-hour 160 RC if I remember correct, don't quote me on it writing from memory. I bought one that has close to 160 RC, replaced and initialized it done.

[asquared]

Originally Posted by Jim_B

Or I could (hopefully) return it, buy an AGM battery rated for 90Ah, and be done with this nonsense once and for all.

It's very odd to me they don't rate these batteries for Ah. It seems they're used in a lot of European cars and Euro manufacturers use Ah instead of CCA.

Check out advanced auto parts if you have one nearby you. I just swapped my H6 lead acid to a H8 AGM. Used a 25% off online coupon they usually always have floating around. Battery came out to $159 out the door with the core charge refunded. It's rated 95AH. I used bimmergeeks protool and coded/registered it at 90AH which was closest setting. Worked flawlessly. Def worth the little more money versus an unknown Walmart lead acid.