Battery Voltage

Submitted: Monday, Mar 19, 2012 at 19:08
ThreadID: 92638 Views:2069 Replies:5 FollowUps:7
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Hello, I need some technical help with my cranking battery/alternator on my tow vehicle, the battery is approx 2 y/o calcium battery and the voltage at the battery is 12.8v (not running) and after I start the car it increases to approx 13.26v.
Problem: This battery supplies power to a Redarc Charger in my caravan and these chargers require 13.2v to initiate the charging cycle for the aux battery's in my caravan. So it is very much border line whether the Redarc charger will start, Is 13.26v enough output from my car alternator? I checked 2 other cars and they put out approx 14 v with the motor running.
Also if anyone knows a web site where I can get more details of alternators and how they work etc.
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Reply By: Allan B (Member, SunCoast) - Monday, Mar 19, 2012 at 19:55

Monday, Mar 19, 2012 at 19:55
If your Redarc device requires 13.2V to commence charge to your battery in the caravan then I should think that it is a Redarc SBI12 isolator, not a "charger". There is a significant difference.

Certainly the Redarc SBI12 requires 13.2 volts in order for it to connect the alternator source to the caravan battery and will disconnect when the supply voltage falls to 12.7 volts. If the voltage at the cranking battery (which is the available alternator voltage) is only reaching 12.8 with the motor running then there is little margin to engage the Redarc. However all may not be as it seems. Firstly, you may be measuring the voltage at idle speed where the alternator is not achieving its full output. secondly, are you sure that you voltmeter is reading acurately?

Try measuring the voltage with increased engine revs.
If you want reassurance that the caravan battery is being charged, apply a voltmeter to the caravan battery terminals and have someone start the engine and raise the speed a bit above idle. you should observe an increase in the voltage at the caravan battery terminals.

A much better arrangement is to use a Redarc BCDC1220 dc-to-dc charger at your caravan battery which will charge from as low as 9 volts supply from your vehicle supply. It also overcomes the voltage drop that you experience in the cabling from the alternator to the caravan. Click here.

For an explanation of how alternators work click here.

Cheers
Allan

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AnswerID: 480835

Reply By: Greenant - Monday, Mar 19, 2012 at 20:06

Monday, Mar 19, 2012 at 20:06
I have one of these and it raised voltage by 0.6 volt and everything works a treat


http://www.ebay.com.au/itm/Alternator-Voltage-Booster-Dual-battery-systems-/320867723182?pt=AU_Car_Parts_Accessories&hash=item4ab534e7ae

Greenant
AnswerID: 480838

Follow Up By: Greenant - Monday, Mar 19, 2012 at 20:10

Monday, Mar 19, 2012 at 20:10
If the link doesnt work ebay alternator voltage booster all it is is a 6 amp diode with a 0.6 voltage drop accoss it and makes the alternator charge at a higher voltage

Greenant
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Follow Up By: Allan B (Member, SunCoast) - Monday, Mar 19, 2012 at 20:39

Monday, Mar 19, 2012 at 20:39
And they cost $30 each at that link.

Nothing more than a standard fuse body containing a diode that yo can buy for less than 25 cents. What a ripoff!

You also are raising the end-voltage of the charge to the original cranking battery which may not do it a lot of good in the long run.

Cheers
Allan

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Reply By: Eric Experience - Monday, Mar 19, 2012 at 21:50

Monday, Mar 19, 2012 at 21:50
Dave.
What is happening with your alternator is the starting battery is rising as it charges, as soon as the Redarc connects it is holding the alternator down. You can test this by disconnecting the van battery and see if the starting battery rises to 14.2. If it does you are OK. Eric
AnswerID: 480849

Follow Up By: Allan B (Member, SunCoast) - Monday, Mar 19, 2012 at 22:13

Monday, Mar 19, 2012 at 22:13
Why doesn't the starting battery hold the alternator down Eric?

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Allan

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Follow Up By: Eric Experience - Monday, Mar 19, 2012 at 22:55

Monday, Mar 19, 2012 at 22:55
Allan.
The starting battery is not flat because it just started the motor. Batteries in vans with that type of Redarc are nearly always flat, Eric
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Reply By: Member - Phil G (SA) - Monday, Mar 19, 2012 at 22:48

Monday, Mar 19, 2012 at 22:48
To get a real voltage reading, run your vehicle for 10 minutes, then measure the voltage at the battery terminals with the engine doing 2000rpm. That will eliminate factors (eg glow plugs, flat battery, low idle speed, high underbonnet temp) that keep the voltage down.
AnswerID: 480855

Reply By: Puttingdave00 - Tuesday, Mar 20, 2012 at 09:17

Tuesday, Mar 20, 2012 at 09:17
Many thanks to the people who replied, I think the problem is a faulty alternator which doesn't have sufficient output voltage to initiate the charging cycle on the Redarc charger.
Allan B: The charger I a using a Redarc BCDC 1220 and after talking to Redarc they explained how they work in much more detail than with what comes with the product, so I believe now all my problems go back to low output voltage from the tow car alternator.
Again thanks very much
Puttingdave
AnswerID: 480870

Follow Up By: Allan B (Member, SunCoast) - Tuesday, Mar 20, 2012 at 11:06

Tuesday, Mar 20, 2012 at 11:06
Hi Dave, Yes, the Redarc people are very helpful. Good that you have a Redarc BCDC1220, I have two auxiliary batteries with a separate BCDC on each of them and the setup works well for me.
Looks like getting your alternator checked by an auto-electrician. New alternators can be had for less than $350 (ask me if you want a reference) and can be a sound investment if touring.

Cheers
Allan

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Follow Up By: Ross M - Tuesday, Mar 20, 2012 at 12:49

Tuesday, Mar 20, 2012 at 12:49
If your alternator voltage is down a bit you probably only require a new regulator.
Assuming the amp delivery of the alternator is ok ie. no blown diodes, then just the reg will fix it.
Previously mentioned booster for the voltage is as stated just a diode in the sensing wire to the regulator. I fitted one 20c diode into the reg sensing line on my landcruiser and it raised the alt voltage 0.6v. Two diodes in series a cost of about 40c will raise it 1.2v.
I made mine switched so I had two options of raising the voltage.
The inline EBAY ones are as mentioned just a cheap diode. 20c versus $30 for the same thing????????
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Follow Up By: milkieboy - Tuesday, Mar 20, 2012 at 21:42

Tuesday, Mar 20, 2012 at 21:42
Puttingdave have you checked the output voltage of the alternator and not just at the battery. I had low voltage at the battery on my cruiser and thought I had a dead alternator. Ended up being the cable between alternator and battery, replaced cable for $13 and all good.

Ian
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FollowupID: 756337

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