Battery Testing

Car in question is a Mk V 3.5L saloon. Ok, troops, time to wear thinking
caps as I'd be grateful of some opinions. I have an awful suspicion that my
battery is on the way out, but can't be sure. It is about 8 years old and
not of authentic period pattern. I'm not bothered about that as I won't be
going in for concours, a working vehicle is the goal. All other electrical
parts of the car check out (generator brushes, nice fat spark at plugs, new
coil, condenser, leads, plugs, points, rotor). Carbs properly set etc and
everything points to the battery being tired. I get 13-14v across the posts
and it always takes a 6-8A charge on connecting the charger dropping to a
steady 2-3A after about 3-4 hrs. And then it turns over
e-v-e-r-s-o-s-l-o-w-l-y. Before I go out and buy a new battery, I was given
to pondering whether anybody had ever considered using a couple of modern
small physical size batteries in parallel rather than an old style
larger-than-life model. This would give extra cranking amps and the
possibility of using just one battery to feed the ignition circuit (so that
its voltage didn't get dragged down by the drain on turn-over with the
starter). Or would a better alternative be to simply replace the battery
with one of adequate capacity and then use a second auxilliary battery for
the ignition feed? Or am I COMPLETELY off beam here and missing something
very obvious? All opinions/comments welcomed! - Dick Clements, '51 Mk V
3.5L Saloon

Try checking your grounds...where it mounts tot he frame..engine, etc.
These should spin right over. Also look at the cables..do they have white
stuff inside..this increases resistance.  This problem will only get worse
so fix now.. Sorry we didnt touch base in London.  We get there
periodically, but we got busy this time.  Check out the Globe theater..very
nice.  We took a river cruise up to Greenwich.  also check out the Clink
prison museum on the riverside by the Globe.. thanks  john shuck..beijing

All good suggestions that will be followed up, so thanks John. Equally
sorry we didn't get a chance to meet up in London.... I could have
introduced you to a few really drinkable beers.... You might like to know
that all the major timberworks for the Globe Theatre were hewn out of green
oak trunks by an artisan not three miles from where we live. Won some kind
of national prize for it. Back to the Jaguar.... - Dick Clements

Dick, Batteries usually last 3-5 years so you have definately received your
money's worth. Don't even hesitate to buy a new one. - George Badger

Have you measured the battery voltage at several points and across
connections while cranking  to see what the batery voltage drops to or if
there is resistance across the connections? If the battery is good, there
should be adequate cranking amps to spin the starter fast enough.  Does the
starter spin faster with a known good battery? If not, double check the
connection loss. If none there, it's possible the starter windings have
shorted, reducing starter torque. As for starting voltage while cranking,
one solution is to install an after-market Capacitive-Discharge (CD)
ignition system which will put out good spark (30-40,000 volts) down to
about 5.5VDC battery voltage. Parallel batteries will work, but they should
be isolated from each other with a diode isolator so one battery doesn't
drag the other one down and the charging circuit charges only the "low"
battery.  Those battery isolators are available at most Recreational
Vehicle (RV) Accessory stores. 8 years is a long time for a battery. I'd
certainly try a known good one and go from there.  Your system must have
been converted from 6 to 12V somewhere along the line. When was the first
12V Jaguar system? - Glenn C, 53 XK120 FHC

Remembering back (way back) to when my 50 MKV DHC was running last...the
motor turned over speedily with the 'normal' battery. I suspect you may
have a :
Bad Battery
Bad cables or connections
Bad starter
the one battery will do fine all by itself..... Good Luck! - Dave Drenzek

Glenn: You asked when Jaguar changed from 6v to 12v. I thought all XK's
were 12v.  Mine is and it was built in 1951. The two 6v batteries were
connected in series. - Carl Hanson

..... I knew it. One seemingly simple question taps into a whole mass of
experience/skill and reveals all the things that I thought I could take for
granted and about which I SHOULDN'T have made any assumptions..... Thanks
ladies and gents, I'll be back to the drawing board in my just above
freezing garage tonight with my tail tucked away and feeling rather
embarrassed .......Dick Clements, '51 Mk V 3.5L Saloon

Carl - My ignorance showing again. I didn't know that. My assumption
(wrong) was
that Jaguar shifted from 6 to 12V about 1953. - Glenn C

Dick, Why not just jump your (very) old battery with one that you know is
up to strength. Good luck. - Sam Bell

This might work under normal circumstances, but if you a shorted cell in
the battery, this will drain away power. If you have a bad ground, this
will also mask the real problem.  Here's a hint, after you have cranked it
for a while..1 minute..no longer..never crank longer, feel the cables..if
they are hot, you could have a ground problem or at least a
high load situation.  john shuck..beijing

I'm not sure whether this reply overlaps with others, but in past years
when I was keeping the 120 DHC going on a student budget, whenever I
appeared to have a complex electrical problem, usually resulting in
difficult starting, just about every time it was the battery.  Apart from a
trolley jack, I would suggest the essential XK tool is a battery charger. I
now replace the batteries whenever there are those telltale signs, and the
elimination in frustration has been enormous! In the past I had even
replaced a starter solenoid as I thought the problem could not possibly be
the battery (it was). - Regards, John Elmgreen

The use of a 12v system goes back at least to the 1936 SS Jaguar saloons,
and perhaps earlier (though probably not on the Swallow sidecars ;-)
Thumbing through a few picture books it looks to me like they always used a
single big 12v battery on saloons and the SS90/100, so the XK120 may have
been their first model to use two 6v batteries. So the British electrical
system was ahead of the Americans in that respect. Hey Dick, there's
something to counter the Prince of Darkness and warm beer jokes. - Rob
Reilly

Hi all -- I'll join this debate on when Jaguars were 12V -- my good friends
Bruce Brown and Bruce Carnachan confirm that our Cats were 12V all the way
back to the first SS1 in '31. My own knowledge goes back to '35, because I
used to take my friend Roy Helm's '35 SS1 tourer to shows for him, and it
was 12V. So the Mk V was 12V! I think the confusion comes because some
Jaguars postwar (XK120, I think XK140 and XK150 and my own Mk IX) used twin
6V batteries, wired for 12V. I hope this info helps -- Larry Martz

If you have any questions or comments send e-mail to: ted@jag-lovers.org
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