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Old 09-19-2009, 07:38 PM   #1
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Join Date: Jan 2009
Posts: 1,215
Alternator Output to House Batteries - Chev Express

I posted this on an other forum but it would also be of interest here.

Be patient in your reading..... The first post(test) had some problems that were corrected in the second post(test)..


My class 'B' (Chevrolet Express van) has 100 amp Delco alternator and that feeds both the chassis battery and the two house batteries (230 Ah total - new batteries). I am using the 6 ga. cable that runs from the battery isolator to the house batteries as a CRUDE shunt so that I can get a feel for the amount of current flowing from the alternator to the house batteries.

The 6 ga. house battery charging cable is 18 feet long (about...wish a thought to do this when I was outfitting the van 8 years ago) and therefore has calculated resistance of ABOUT 0.00712 ohms (from the look up table I have).

So here is my first check .... will repeat when we have dry camped and have pulled the house batteries down to 12.2 or so volts.

12.59 volts on house batteries prior to starting van

Voltage across the 6 ga. by 18 foot shunt:

0.562 volts engine on +5 seconds........ 79 amps
0.510 volts engine on 1 minute...............72 amps
0.471-0.491 volts engine on 2 minutes......67-69 amps
0.446-0.462 volts engine on 3 minutes......63-65 amps
0.419-0.442 volts engine on 5 minutes......59-62 amps

13.84 volts on house batteries prior to shut down at 5 minutes

Surprised me that topping off an already charged battery (12.59 volts) that the alternator would be sending that many amps to the house batteries. THAT IS OF COURSE ASSUMING THAT MY SIMPLE TEST IS NOT TO FAR OFF IN LEFT FIELD.

Any one laying under the shade tree got any feelings on this or any measurements yourself? Maybe some of the meters you have in your rig measures this directly??


You were right Gary......My fully charged was not fully charged.

Had not driven van or charged house batteries in three weeks but had used "some" lighting while working on van in that period. I also ASSUMED that as the max. battery voltage (fully charged) was approached that the alternator would look like a tapper charger. I was all wet.

Did a FULL charge on house batteries last night and re-ran the test this afternoon.

After 14 hour charge (starting at 12.74 volts):

7:45 am 13.38 volts after 5 minutes of charger being off
2:30 pm 13.19 volts after 6 hrs and 45 min. of charger being off

Let the retest begin

0.027 volts 5+ seconds 4 amps
0.019-0.021 volts 1 minute 3 amps
0.017-0.021 volts 2 minutes 2+ amps
0.015-0.017 volts 3 minutes 2 amps
0.013-0.017 volts 4 minutes 2 amps
0.013-0.017 volts 5 minutes 2 amps

14.52 volts on house battery just prior to turning van off 5+ minutes

13.28 volts on house battery 3 minutes after turning van off

The alternator output difference from the house batteries being at 12.59 volts and at 13.19 volts really surprised me!!!

I was thinking (wrongly) that the alternator would behave more like a taper charger. It appears to put out the max right up to the moment the batteries are very near fully charged.

Live and learn

This would also indicate to me that if my house batteries are 50% discharged (down 115 Ah) that the van alternator putting in ~60 Ah into the house batteries would effectively recharge the house batteries in about 3 hours.
Len & Joanne

The Green TARDIS
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