75 Series Troop carrier auxiliary battery charge problem

Submitted: Thursday, Apr 13, 2023 at 13:08
ThreadID: 145528 Views:1810 Replies:4 FollowUps:2
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Hi, I run a duel battery setup in my 75 series troopy which charges a 60l engel fridge and only a few small lights in the evening. The starter battery and aux battery are near new. The alternator was only replaced a year ago too.

The problem I have is that the alternator only seems to be charging the aux battery up to 12.9V max, even after I have been on the road for hours. My battery monitor is set to shut the system down when the aux charge drops down to 11.9V to save my aux battery from being damaged. Last time I camped, I drove for three hours and I parked up at 4pm. I woke up at 6am the next morning with a warmish fridge full of food. It was reading at about 8 degrees which means that the aux battery charge would have dropped down to 11.9 and shut the fridge off quite a few hours earlier on in the night.

Has anyone had this sort of problem?
Shouldn't the alternator be charging my aux battery to a higher voltage after I have been on the road for so many hours? Surely it should be reaching up to a high enough voltage that I can at least park up in the evening and keep a fridge running for one whole night easily.

Thank you!

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Reply By: Member - LeighW - Thursday, Apr 13, 2023 at 16:54

Thursday, Apr 13, 2023 at 16:54
When you write 12.9V I assume that is the battery voltage after you ahve turned off the ignition?

What is the actual charge voltage the alternator is putting out?

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Follow Up By: Chris L28 - Thursday, Apr 13, 2023 at 17:24

Thursday, Apr 13, 2023 at 17:24
Reply to LeighW

Sorry for the confusion

12.9V is the actual charge voltage that the alternator is putting out.

The battery voltage only sits at 12.3V when ignition is turned off.

When I’m camping for days without moving, I use solar panels which increase the battery charge to over 14V.
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Follow Up By: Member - LeighW - Thursday, Apr 13, 2023 at 22:05

Thursday, Apr 13, 2023 at 22:05
12.9V is way to low unless the alternator is very heavily loaded.

I'm assuming you don't have the equipment to test the alternator properly so I would suggest you do a basic test as follows:

First fully charge the both the aux and cranker with a 240V charger. Then start the car and hold the engine speed at 1800RPM and see what charge voltage you get. I would expect the charge voltage to be between 13.8V and 14.4V. If the charge is ok then turn on the headlights on high beam, the airconditioner and the rear screen heater if you have one and again the charge voltage should remain as per above. If the charge voltage is lower or collapses with or without the accessories turned on the alternator is faulty.

Forgot to add, if voltage is low across the battery posts check it on the clamps themselves as the posts or clamps may be dirty. Also check for voltage drop across the connection between the negative battery terminal and the body work as it is not unheard of for the connection between the negative battery cable and the body work to delovope a high resistance which causes a voltage drop.

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Reply By: Allan B (Sunshine Coast) - Thursday, Apr 13, 2023 at 18:13

Thursday, Apr 13, 2023 at 18:13
Hi Chris,
We are going to need some more information to address this. Can you answer the following:

a) Did the system ever work correctly to your satisfaction?
b) After driving for a time, and with the engine still running at idle, what is the voltage at the aux battery terminals and what is the voltage at the engine battery terminals?
c) What is the size in Amp-hours of the aux battery?
d) What is the wiring connection that charges the aux battery. Is it connected to the engine battery/alternator directly or is there a relay/solenoid between them?
e) When the aux battery is charged to 14V by the solar, does the battery then maintain the fridge adequately for the overnight?

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Reply By: Mikee5 - Friday, Apr 14, 2023 at 06:58

Friday, Apr 14, 2023 at 06:58
How is the second battery connected into the charging system? I have a Redarc SBI Smart Charger. This week my second battery would not come above 11.5 for the first time ever. As a quick fix on the road I put a jump wire across the two big terminals on the Redarc. Solved the problem until I got home.
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Reply By: qldcamper - Friday, Apr 14, 2023 at 08:07

Friday, Apr 14, 2023 at 08:07
Personaly I would start at the beginning which is your alternator, as suggested earlier with increased engine speed check the voltage at the start battery, even a smart alternator should be at least low 13s, then carefully take a reading directly at the alternator, it should be just a little higher than at the battery depending on the current flowing, 0.5v ish tops.
Extreem care around the alternator, slip with the probe and you will be seeing a bright spot in the middle of your vision for a while and probably blow your main fuse.
Also that drive belt wont stop for fingers, just chew them up and spit them out in an instant.

If thats ok follow it through one step at a time till you find where the voltage drops off checking the negative side as well.
Feel connections for heat after you have been driving for a while with a flat acc battery.
Dont rule out a faulty new battery.
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