classic motorcycle forum

Motorcycle Discussions => British Bikes => Topic started by: GJM_T120 on July 12, 2008, 10:56:04 AM

Title: voltage drop
Post by: GJM_T120 on July 12, 2008, 10:56:04 AM

I get a voltage drop from the battery when just turning on the ignition key.  With an older batter it can be around 3 v.  

Is this to be expected or do I have a "leak" somewhere?

The bike is a BSA A65 with stock electricals.

thanks,
GJM

Title: Re: voltage drop
Post by: twolitre on July 12, 2008, 08:50:48 PM

Batteries can give problems in various ways.  One, which might be your problem, is being unable to deliver the stored power quickly enough although the off-load voltage shows normal.  Even batteries with very little active material on one or more plates can be charged to normal voltage.  Remember also that freshly charged batteries will show a "surface" charge voltage which will quickly disappear when under even a small load.
  To find out if you have a "leak", make sure that the ignition points are open when you turn on the ignition (assuming you do not have electronic ignition nor wrongly fitted points).  The system should then take negligible current except for any warning lights if fitted and the battery voltage should not change significantly.
  This, of course, assumes the bike runs in the first place.

Title: Re: voltage drop
Post by: Rex on July 17, 2008, 10:01:13 PM

You don't have a "leak" but you do have a load.
As the poster above says, most likely the coil (when the points are closed, that is)

Title: Re: voltage drop
Post by: twolitre on July 21, 2008, 04:59:22 AM

Yes Rex!

I suppose, rereading what I wrote, the next to last line would better have read "........should be not change significantly unless you have a 'leak'."

Thanks.