volts

gday
I was wandering if anyone knows why my main and second bat are only charging to 12.8vlts. my alternator shows 14.5 plus and i am running a 650 cca main bat and a 500cca second, i am aware that they will only charge to the least resistance, but surely the second bat should be up around 13.5 if it is still in good nik. I have a rotronics mh10 charge and isolator which is new. I have a multimeter but am unsure how to set it to check if the bats are still ok.
cheers
trev
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Reply By: ABR - SIDEWINDER - Saturday, Jul 28, 2012 at 14:42

Saturday, Jul 28, 2012 at 14:42
Trev

The main battery should show the same as the alternator.

The aux may be slightly lower at fast idle but should be very close.

I would back probe all the wiring until you find the fault, it could be a loose connection, thin wire, bad fuse or breaker or lastly the isolator itself.

Regards

Derek from ABR
AnswerID: 491825

Follow Up By: ABR - SIDEWINDER - Saturday, Jul 28, 2012 at 14:43

Saturday, Jul 28, 2012 at 14:43
Isolator Testing

Here is our isolator test page.

Regards

Derek

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FollowupID: 767413

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 15:51

Saturday, Jul 28, 2012 at 15:51
checked the isolator as per your page. both bats 12.5 car off. both bats 14.2 car at idle
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FollowupID: 767417

Follow Up By: ABR - SIDEWINDER - Saturday, Jul 28, 2012 at 15:59

Saturday, Jul 28, 2012 at 15:59
SOC-LINK

Great, here is our state of charge link so you understand battery voltages better.

Regards

Derek
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FollowupID: 767419

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 16:11

Saturday, Jul 28, 2012 at 16:11
cheers for your help. those links were very easy ,even for a dum ass like myself to understand. looks like the only thing wrong is listening to people who think they know what there talking about compared to those who do..
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FollowupID: 767421

Reply By: Tim HJ61 (WA) - Saturday, Jul 28, 2012 at 14:55

Saturday, Jul 28, 2012 at 14:55
Can you confirm that the engine is running when you are doing these tests, and at a fast idle.

12.8v seems like the voltage of a fully charged battery just after the engine has been turned off.

The best way to check if the batteries are okay is with a hydrometer that you dip into the battery acid, suck up a bit with the sucker and see what the Specific Gravity of the battery is when the indicator thingy is floating.

Given so many batteries are sealed up, my knowledge is that without a pro tester, then measuring voltage is the next best option. There are plenty of charts around that show the relationship between charge and battery voltage.

Tim
AnswerID: 491826

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 15:27

Saturday, Jul 28, 2012 at 15:27
car is running when i test what the alt is doin. 12.8 is with the car off, i was under the impresion that they should be up around 13.8vlts. the top of the main bat has always had a slight wet look on top of it like there is something leaking on it. I have never had a problem starting the car even on cold mornings.
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FollowupID: 767414

Follow Up By: Tim HJ61 (WA) - Saturday, Jul 28, 2012 at 15:55

Saturday, Jul 28, 2012 at 15:55
There is no problem.

If you use your multimeter to probe the voltage at the fattest connection on the alternator when the engine is at a fast idle and no electrical loads, then 14.5v is very good, perfect, maybe a fraction high, but essentially perfect.

If you test your batteries at the same time, they should be close to the same as the output at the alternator. There might be a small voltage drop, 0.3v for example. This is fine. Nothing to worry about.

This means your batteries are receiving the correct charge from the alternator.

Now, you turn off your car. Initially the batteries will drop back a little from 14.2v. This is where the complications of measuring battery capacity, level of charge and the limitations of measuring voltages come into it. There is a thing called a surface charge where the battery shows higher than 12.7v which is the voltage of a fully charged battery. If you leave that battery overnight with nothing connected to it, the voltage will come down to 12.7v all by itself after the surface charge has dissipated. Some people say that running the headlights for a few minutes does the same thing.

A 12v battery never reads 13.5v unless it is being charged. A fully charged 12v battery sits at 12.7v.

I'm not going to go into it any more. It gets technical. There are excellent FAQ on this site, excellent article written by Collyn Rivers and other erudite and competent people on this forum, and others. Have a google search on surface charge, charging 12v batteries, how full is my battery.

What you can stop doing, is worrying about your batterys.

If you want to DO something, go to an auto shop and buy a spray can of Battery Terminal sealer. It sprays yellow or blue goo onto the terminals and stops that wet look, and makes them look really pro. It also sticks to your fingers, but that's another story.

Tim
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FollowupID: 767418

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 16:14

Saturday, Jul 28, 2012 at 16:14
thanks for help. all good now i can stop worrying just didnt want anything to go wrong when we head to exmouth.
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FollowupID: 767423

Reply By: Sand Man (SA) - Saturday, Jul 28, 2012 at 17:57

Saturday, Jul 28, 2012 at 17:57
Trev,

You have been convinced you have no real problem with the charging process and that is good.

However, what sort of secondary battery are you using and what are you running off it?

A battery rated in CCA (Cold Cranking Amps) is a starter battery. It supplies a heavy current over a short period of time to give a high CCA.
A battery rated in Ah (Amp Hours) is a deep cycle battery. It is designed to supply a relatively lower amount of amps, over a prolonged period of time. This is the type of battery you should use as an auxiliary battery to run things like fridges and other higher current draining devices.

There are also hybrid batteries that can supposedly do both, but they will have specs for both CCA and Ah ratings.

Just make sure you have the right battery for the right purpose mate.


Bill


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AnswerID: 491835

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 19:40

Saturday, Jul 28, 2012 at 19:40
as i mentioned the second bat has a cca rating of 500.
However it dosent have a ah reading on it only rc 90. what ever that means. I find that the second bat runs low a bit quik, i run a fridge and a dvd player for the kids. I find that after a day or so the bat is below 12v which makes the fridge struggle.
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FollowupID: 767443

Follow Up By: Trev&Ness B - Saturday, Jul 28, 2012 at 19:45

Saturday, Jul 28, 2012 at 19:45
make that rc mins 90
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FollowupID: 767444

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