--- In [email protected], "peteuk" <peter.jea...@...> wrote: > > > Hi Tony, > > Thanks for that - Iknew you would be lurking with the answer somewhere! > > Could you explain why the 80% problem is far worse on boats than cars. > My car averages say 30min. journeys per start and the battery is about 10yrs > old and still going strong ! > > Regards > Pete > www.thecanalshop.com >
On cars etc. the battery should very rarely supply the loads. The alternator does that with a but left over to charge the battery. If you look at the back of an A127 alternator you will see two 9mm blade output terminals. On cars typically one of these would run to the battery and the other to feed all the other loads. The battery will "back-feed" to the second terminal when there is no alternator output or when alternator output is less than the load, but fundamentally its the alternator that supplies the loads. Think about how long you can leave the hazard lights on and still start the car! Also it should only take a few amp hours to start a vehicle. I know this sounds odd because of the high current but the engine should be started within 30 seconds of cranking and usually far less so lets assume 150 amp starter draw average during cranking. 150 x 0.5 (30 seconds = half a minute) divided by 60 (60 minutes in and hour) = 5 amp hours. Now even a severely compromised battery can probably deliver 5 amp hours over a few seconds so the capacity required for actual starting is far less than we tend to think. Hence the car battery being of perhaps only 35 to 45 Ah capacity (I know it is no longer measured in that way) is recharged faster after starting and really does very little work when compared with the domestic bank. Tony Brooks
