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      03-04-2008, 02:20 PM   #28
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by rvacha View Post
I'll answer it - there's very little difference, although AGMs are definitely a little heavier. However, they are virtually guaranteed not to outgas, rupture or explode and as a result there are gains in other areas of the vehicle such as trays, vent tubes, alternator size and weight, etc. AGMs are essentially the same as “maintenance-free” batteries but instead of letting the acid just slop around it is held in place between the plates with glass mats, i.e. fiberglass. The fiberglass weighs next to nothing, but the plates are sometimes packed closer together leading to more weight.

Batteries in BMWs are simply big and heavy no matter what kind they are. This is driven by the cold crank current required of the starter and the plethora of electrical gadgets (navi, heated seats, defrosters, high speed injectors, high compute power DMEs, etc). A modern car can easily top 3KW if all the stuff is turned on.

There's also more power being used for performance too. Now we have to have somebody operate our clutches for us. Other applications begging for more power are individual solenoid driven engine valves (instead of cam shafts and VANOS), pre-heated cats, electronic braking, per-wheel controllable torque, direct injection, blah blah blah. Like it or not big-ass batteries are here to stay.

The S65B40 outputs 313KW. If we assume the electrical stuff is pulling 1KW this means that the alternator steals about 0.3% of the available WOT engine power. If the AGM battery were to add 10lbs (and it doesn't) then the car would be 0.27% heavier than it would otherwise be. If you disconnect the alternator you are ahead of the game at WOT. However, most people will not drive WOT all the time and engine output will be lower, let's say 50KW, and a 1KW load would steal 2% of the available power. Here disconnecting the alternator yields even better returns
Nice post but isn't it a bit simpler than this? I could not find the current rating on the M3 alternator but the current M6 is 170 amps. I bet the M3 is very close to this if not identical.

P = V * I = 12.5 x 170 = 2.1kW ~ 3 hp

If the battery difference is 2 kg as south posted then the power (gain) to weight of this BER system is

~ 4.5 lb / 3 hp = 1.5 lb/hp

When the car is about 8.8 lb/hp this is an awfully impressive ration. Sure it is only 3 hp but the weight "penalty" is absolutely meaningless. Even if the alternator is drawing 50% of its rated load it still offers a phenomenal power to weight ratio.

Technically there may be other weight penalties other than just the batter but I doubt they are significant enough to change this argument.

Finally can anyone say what TIS says?? It seems there still is not certainty about this system in the US.