Should I upgrade my alternator?

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DillonOnTheCoast

10 Year
Joined
Mar 17, 2011
Threads
26
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269
Location
N. California
Subjectively speaking, I've been having a bit of trouble keeping my batteries charged. I think my problem is a slightly worn charging system and increased demand from my accessories.

Vehicle:

  • HJ60
  • Central American
  • 12v
  • dual Optima batteries on a PowerGate isolator


Often times I will run the auxiliary battery down low and it doesn't always charge back up to 12.5v very easily. The factory voltage gauge often reads below 12v, especially when I have a lot of accessories running. At highway speed without too many accessories it will sometimes read almost 14v. It reads below 12v even when the engine is off and I know the batteries are above 12v, so I don't trust the factory gauge. I am assuming a bad ground in the dash somewhere.

My fridge has a low-voltage cut-off. Right now it is set with the low voltage cut-off at 11.6v and the turn-on at 12.5v. It is constantly flashing low power and refuses to start up.

I used a multimeter and came up with the following (raw data is below). The alternator puts out ~13.7v at 600rpm and 14v at 1500rpm. After the regulator it is ~13.2v/13.7v.

As for accessories, I the fridge draws ~3amps when it is running, a set of round eye headlights (assuming I have 55watt bulbs) that I figure draw up to 9amps, a JL XD 700 class D amplifier that I figure could draw up to 10 amps. I don't know what the heater blower draws but say it's 5 amps and another 5 for the wipers. I have 4 chargers hardwired to the aux battery that draw a theoretical max of 8.5a but they would not likely be all going at once.

This means that on a cold rainy night I could be drawing as much as 43.5 amps, not including dash lights and other minor things. In good daylight conditions it's more like 15 amps.

Questions:

  • Is the voltage on that alternator low enough to justify a rebuild? It's pretty close to being in-spec (13.8-14.8v @ 2000 RPM).
  • Is the voltage drop after my regulator (0.5v) normal? Any point in adjusting it?
  • Do you think my current draw estimates are reasonable? Do they justify a higher-output rebuild?
  • If I do rebuild the alternator, would the wiring upgrade be as simple as a heavier gauge wire from the alternator B output to the regulator and from the regulator to the power isolator?


Voltage Measurements (The vehicle was driven with all accessories off for 15 minutes on the highway and then parked overnight before I started.):

  • Aux Battery Engine Off: 12.01v
  • Aux Battery 600 RPM: 13.12v
  • Aux Battery 1500 RPM: 13.43v
  • Main Battery Engine Off: 12.55v
  • Main Battery 600 RPM: 13.17v
  • Main Battery 1500 RPM: 13.48v
  • Alternator B Terminal Engine Off: 12.01v
  • Alternator B Terminal 600 RPM: 13.69v (grounded to body)
  • Alternator B Terminal 600 RPM: 13.72v (grounded to engine)
  • Alternator B Terminal 1500 RPM: 14.00v (grounded to body)
  • Alternator B Terminal 1500 RPM: 14.05v (grounded to engine)
  • Power Isolator Aux Side Engine Off: 12.01v
  • Power Isolator Aux Side 600 RPM: 13.20v (grounded to body)
  • Power Isolator Aux Side 600 RPM: 13.25v (grounded to engine)
  • Power Isolator Aux Side 1500 RPM: 13.50v (grounded to body)
  • Power Isolator Aux Side 1500 RPM: 13.53v (grounded to engine)
  • Power Isolator Main Side Engine Off: 12.55v
  • Power Isolator Main Side 600 RPM: 13.20v (grounded to body)
  • Power Isolator Main Side 600 RPM: 13.25v (grounded to engine)
  • Power Isolator Main Side 1500 RPM: 13.45v (grounded to body)
  • Power Isolator Main Side 1500 RPM: 13.53v (grounded to engine)
 
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I just took the following measurements, this time including load (headlights but not brights, wipers, heater on high, stereo on, fridge plugged in). All measurements taken at the battery terminals. The vehicle was driven for 15 minutes at highway speed earlier in the day and the radio had been used with the engine off for ~1 hour before I started.

Aux Battery
  • Engine Off: 12.41v
  • 600 RPM no load: 12.80v
  • 600 RPM loaded: 12.01v
  • 1500 RPM no load: 13.62v
  • 1500 RPM loaded: 12.38v

Main Battery
  • Engine Off: 12.68v
  • 600 RPM no load: 12.84v
  • 600 RPM loaded: 12.57v
  • 1500 RPM no load: 13.67v
  • 1500 RPM loaded: 12.42v

That seems to indicate my alternator is not up to the task. Am I correct in assuming I am of the rare breed who needs a higher-output alternator?
 
From what i aware or have been advised ,

A 12v battery is flat & needs recharged below 12v , a fully charged 12v battery is 12.7-8v, i find alot of diesel 4x4"s i have had wont crank over at 12v ( unless u have relay connected to ur starter solenoid ), needs 12.1 min . Different batterys require different charge ,lead acic =14.4volt , calcuim =14.8volt , etc ,etc . Heaps different type batterys out there that require
different things.

The more assesorys u run , the more amps they draw , i notice alot of car makers are putting high amp alternators on thier vehicles , seen hummers run 350amp alternators , performance cars & 4x4s now have 175 - 250 amp alternators to run all the electricals . I think its better to have more amps than less , ur regulator controls it anyway how much charge ur batterys get , but i know if u charge calcuim batterys ar 14.4v that the charge drops off & the batterys hold less charge & within a period of time 1-2yrs ur new battery wont hold enough charge to start ur car .
 
Sorry for the confusion. Was logged in under the wrong name.

Anyhow, if I get something higher output, say in the range of ~80 amps, what would I do with the wiring?
 
Hi Dillon,

First, set your DVM or DMM one position higher so you read 1 digit behind the dot. That makes life a lot easier. Right now you're calculating wit 1/100 of a Volt which is pretty useless.

Second, loosing 0.5 V over 40Amps draw is nothing to worry about. But if you loose 0.5 V over 5 Amps draw it's time to check your wiring (thickness), connections, connectors and so on.

Third, when you go for a 80 amp alt you have to upgrade the Alt to Bat wire and the Bat to fuse panel(s) wire(s) to let's say AWG10 or better AWG8. Maybe you have to get another fusible link(s) too but you can check that easily by checking the temp of that link under full charge or discharge. The rule is: A fusible link is 4AWG sizes higher then the wire it's protecting. So if the wire is AWG10 the fusible link should be AWG14.

Good luck,

Rudi
 
Would you agree that I'm on the right track? I don't really want to disrupt the "equilibrium" of the electrical system unless I have to. If I'm understanding the numbers I probably need to right?
 
Let's do some battery basics.
There are much better write ups about this subject but I'll do a quick 101 here.

A good new, fresh battery (fully charged) and left alone with no load for more then 2 hours measures 12.6V
12.4 is pretty good, 12.2 is halfway dying and 12 or less means it can't hold the charge.
A good new, fresh battery is 12 Volt under a certain load for so many hours. This all depends on the capacity of the battery.
If the battery voltage drops under a normal load, say the parking or running lights, within an hour the battery is bad. It should be capable of delivering 12V for a certain time (several hours).
Now keep in mind that we are talking a cranking/starter battery and not a traction battery used in electric wheelchairs or fork lifts which are made for that specific task.

Let's go to the Voltage Regulator and the Alternator.
The alternator re-charges the battery. It's controlled by the Voltage regulator. The VR sends a signal to the Alt depending on the condition (voltage) of the battery. The VR does not regulate the current. The VR regulates the Voltage and (by electric laws) the current. The maximum Voltage you can measure is equal to the set point of your VR. So if the set point of the VR is 14.2 Volts that's all you get.

The highest value I see in your table is:

Alternator B Terminal 1500 RPM: 14.05v (grounded to engine)

So that's the limit while driving with no load other then the ignition system and the cluster gauges.

Everything between 12V (new battery) and 14.05 (your VR set point) is called "headroom" which means that that energy is available for accessories without dis-charging the battery. So if your alt is capable of supplying 30 Amps you have 14.05 x 30 = 421.5 Watts available from the Alt before you start using energy from your battery. So if all your stuff is switched on and is using more then 421.05 Watts you're draining your battery.

Are you with me so far? Are we one the same page?

This is all theory. In daily life it's different. You drive (re-charge) and wait at a traffic light (idle, draining your battery) and so on. The VR is taking care of this matter. You won't notice unless you're monitoring your ammeter.

What I'm trying to say is this:
Drive your truck for several days, switch on the stuff you have on board, daytime, nighttime and don't think about charging and dis-charging. Just use it as a user who has no clue about this subject. Like you have a modern car with only a "no charge" light on the dash.
If you can cranck your engine every day with no problems your charge system is OK. If not....

Rudi
 
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Cranking the engine is never a problem. Fridge/lights/stereo are all on the aux battery. I suppose if I drove around with everything running for many days in a road the main battery might be affected.

My aux battery, however, routinely falls below 12v and doesn't easily charge back up.
 
DillonOnTheCoast said:
My aux battery, however, routinely falls below 12v and doesn't easily charge back up.
Click to expand...

Fridges kill cranking batteries.Cranking batteries are designed to give lots of power for short periods.
Your aux battery is probably sulphated,sometimes a good pulse type charger can restore them over 24 hours or so.

Deep cycle batteries are designed to dribble out small amounts of power for long periods.
Proper AGM batteries or the gel type are best. The hybrid marine batteries are not much chop.

If you dont want to buy a good deep cycle type battery,the next best thing is to swap the cranking and aux battery around as often as possible.
 
The aux battery is a Blue Top Optima, hybrid deep cycle battery. Even if it isn't ideal, I don't think it's my immediate issue.

Don't voltages I measured when the system is loaded indicate that the alternator isn't supplying enough current to power everything and keep the batteries charged?
 
I think you're approaching this from the wrong direction. You really can't look at voltage alone in a system thats under load and know anything about where the power is going. IF you had perfect knowledge of every load at that point in time and the resistance of every connection and the state of every regulator then you could start sorting things out.

You said your worst case draw is 43A, average case is 15A.

Now ask yourself, in a worst case, how long would you want to have to run the truck to recharge a fully discharged battery?

Lets say you decided it should be 2 hours. I assume you have 150Ah of batteries, so you'd need to supply 43A+75A = 118A... i.e. you'd need a 120A alternator worst case, or 90A on average.

Lets say you were comfortable with it taking 4 hours. Then you'd need 43+37 = 80A worst case, 55A average case.

Now, in reality, a battery at 100% discharge is damaged... you've got bigger problems if that happens. 50% discharge is more reasonable for a "dead" battery.

In that case, if 2 hours to recharge is the goal, then you need an 80A alt worst case or a 55A alt on average.

If you can live with 4 hours to recharge, you need a 60A worst case or a 35A on average.

You may be wondering... how reasonable is this? Well, my BJ42 came from the factory with no real electrical loads except for the headlights, 2x 75W bulbs = 13A. Lets call it 20A to account for two taillight bulbs, instrument cluster, etc. I have an 80Ah battery and the stock alternator (40A). Ergo, at 50% discharge, under worst case loads, I need 2 hours to recharge my battery at 50%.
 
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What's the procedure for pulling the alternator? It's under the a/c compressor.

Regarding the wiring, thanks bj40green for correcting my misconception on current flow & the regulator.

My understanding then is the wire from the B terminal of the alternator to the battery (or the power isolator in my case) may need to be upgraded. Correct?

I already have heavy gauge wire from the isolator to the batteries and from the batteries to my accessories.
 
DillonOnTheCoast said:
What's the procedure for pulling the alternator? It's under the a/c compressor.

Regarding the wiring, thanks bj40green for correcting my misconception on current flow & the regulator.

My understanding then is the wire from the B terminal of the alternator to the battery (or the power isolator in my case) may need to be upgraded. Correct?

I already have heavy gauge wire from the isolator to the batteries and from the batteries to my accessories.
Click to expand...

The alternator acts as the belt tensioner. You have to losen the 12mm bolt on top that locks it in its slide, then take out the 14mm long bolt underneath the alternator. Then it comes out, not sure anymore if to the top or to the bottom.
J
 
Im not familiar with the 2H alt ,but sometimes its easier to lie on your back and do it from underneath
 
Do y'all suppose it's worthwhile to crack open the regulator and adjust the voltage upward a bit? The FSM makes the procedure look pretty easy. I could go from 14.0 to 14.6 or so.

Worth the effort for more headroom?
 
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