[Vwdiesel] HELP-Battery voltage when glow plugs kicking in.

Roger Brown r.c.brown at ieee.org
Mon Oct 6 18:32:08 EDT 2003

Mike & Coreen Smith wrote:
> Hello Diesel Dudes,
> Tested my battery voltage today (about 12' C outside at the time)
> 12.65VDC (no load, car been parked 6 hours)
> Ran glow plugs (just 1 cycle) and voltage went down to 11.6VDC while under
> load.
> After the single cycle, battery voltage was sitting back up @ exactly 12VDC
> Started car (of course, one more cycle automatically) and voltage slowly
> crept up to 12.04, 12.05. etc. to 12.1VDC after around 30-40 seconds. (then
> I had to stop "test" & run back into work, LOL)
> Does that voltage seem a bit low to you all?
> I checked the voltage directly on the glow plugs 2 weeks ago and could've
> sworn it was around 9.75 volts or something. I posted it, but now can't find
> the message.  Must be a lot of voltage drop in my busbar, etc.
> Think I have a soft battery or is it about right?
> It's the biggie. Group...71?  Rats....I forget exactly.  Whatever that
> recommended group # is I think I have....
> Sorry for my rambling. Been an extremely hectic day and my head hurts.  Car
> goes in shop tomorrow for a block heater install, and I'll get a new battery
> based on your comments and a possible load test.  If you all think it sounds
> about right, I'll skip the load test.
> Mike in NB...home of whitetail deer, Atlantic salmon and cold temps !...brrr
> P.S.- Car won't be driven all winter. (have a 4x4 Pathfinder for that.), but
> I'd like to get OCT/NOV out of it if possible.

I think 11.6-12V would be OK under the ~40-50A glow plug load.  Internally you have an open circuit voltage of around 12.4-12.9
volts, but as current flows through the battery, you pick up the internal resistance dropping the useable voltage as noted.
Generally, the higher CCA rating on the battery, the lower the internal resistance.


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