Sunday, Jun 01, 2014 at 20:10
Well you have asked a few questions there Lyndon and I'm not sure I can see the whole picture.
Funnally (who has the right spelling) enough only 2 hours ago I got a call from a stranded vehicle
which had winched its twin aux batteries flat and the car won't go.
The batteries get manually switched into the main and when they thru the switch the surge of current
took out the big main fuse (120amp) with such force that all its left was two melted prongs.
Really not in the mood to do a cold rescue this evening so I told them to wrap some speaker cable around the posts
and haven't heard anything yet.
Next time they might listen and not use 32mm oversized cable.
On your issue, it comes down to a few things that need to be done properly
the first being to know more about auto elec's volts measurement.
13.2 volts charging doesn't sound right unless its all hot and alternator
temp sensor is in shutdown or batt is flat or some other big load is on.
(Also volts is revs dependant - has it stabilized)
One thing I suggest to everyone who has no permanent voltmeter installed is to get those little
Ebay / Jaycar cigarette lighter plug in digital voltmeters and leave it in all the time and get to know
how your car behaves. (
Mine $3.98 Ebay)
Mine always starts off cold at just over 14 volts cruising dropping to 13.2 only when all hot.
Cable 13.5mm is better , but 8mm is ok if its really 10amps load as resistance of wire is approx 0.002 per meter
or volts drop 0.02 for 10 amps (1/50th of volt/m).
A total volts drop of 0.5v is acceptable at you maximum real load and this would be 15m of total cable + fuses.
But some important points
1/ ensure the cable is copper - not plated.
2/ Be sure max load really is only 10amps.
3/ The voltage drop needs to be measured right.
As it happens I just fitted a bigger inverter in back of my car to run bigger
heat lamp which takes over 25amp (
well its getting cold, winter started today)
At battery I had 12.2 volt and at other end I had 11.1 volt which at first seems a lot
for 5 meter of 8mm cable (no return cable required as Nissan uses body earth at this point).
So I then measured the real volts drop in the cable positive lead only and as expected the real drop was about 0.3v.
The rest comes from internal resistance of 12v battery and fuse.
Unfortunately, for me the inverter protests a little so I will be upping my cable size,
but on the +ve side Ebat was flogging 20mm sq cable at $1 a meter which sounds like a dream until I read that postage was $2 a meter , still $3 a meter is dirt cheap.
P.S.
Check your and your Auto elecs meters for accuracy - if can't then get both to measure something at same time and at least know they read the same.
My doctor hates me because because he once told me my weight hadn't changed in years and I told him his scales were out , cutting this short he had never had them calibrated and a cross reference with
mine showed a 4kg error - he still
dam well gave me a bill.
AnswerID:
533575
Follow Up By: Member - lyndon NT - Sunday, Jun 01, 2014 at 21:31
Sunday, Jun 01, 2014 at 21:31
Hi Robin
Thanks for the reply. Just to clarify, are you saying I will have to add all 3 lengths of cable Vd together? I wasn't sure if this was so, thought I would calculate Vd from alternator to batteries as separate and then add the Vd from batteries to fuse box and fuse box to socket outlet together? I know if it was AC I would add all together but not sure how it works having the battery in the middle?
Thanks
FollowupID:
816964
Follow Up By: Robin Miller - Monday, Jun 02, 2014 at 10:07
Monday, Jun 02, 2014 at 10:07
I am still not totally clear Lyndon so I hesitate to answer but am happy to work thru any defined senario.
As far as charging goes all thats important is the total volts drop from any source (alternator solar or charger to the actual battery terminals ), when things are normal I.E. not when battery is flat .
As far as discharging into a load is concerned its voltage drop from closest battery source to the actual device which should be below 0.5 V when system really is drawing its max load.
What is wrong to do is to measure a voltage at load with device off then switch it on and measure voltage at load again and subtract to get the difference.
If you have just 1 meter and wish to do a quick measurement you first switch on the device(s) , measure volts at device terminals then go to battery nearest device and measure volts at actual battery terminals with device(s) still on.
Most important is to do above with engine off but its good to do this with all senarios e.g. alternator charging as
well as engine off , but not with multiple active volts sources.
By this I mean that a charging solar panel connected closer to your load may raise the volts reading and give you a false impression.
FollowupID:
816980