Charging question (R model)

[Lmackattack]

  Been having what I think is an on going charging issue with my RS700 that I decided to dig into. I only have two 12v batterys installed in the truck (made by Interstate). One is brand new the other is 4 years old but load tested good. When the truck is running (2000RPM) the dash gauge shows I only have 12v (no lights on). Testing voltage at the battery cables again with engine running at 2000RPM I see slightly higher 12.8V.

I pulled the Alternator and had it tested at a reputable rebuilder (not a big box auto store) and its putting out a respectable 14.8v . He suggested I run a wire from the Alternator direct to the battery. Doing so I was able to get 13.2v at the battery cables, still well below the 14.8 it can put out. 

When I unhook both positive cables from the batterys with the truck running at 2000RPM I have 14.8 at the alternator. the dash gauge is showing 14.8v and 14.8v at the cable ends. As soon as I connect to one battery it drops to 14v at the cable ends. and when I connect to the 2nd battery im back down to 13.2v at the cable ends. 

 

To sum it up I have a alternator that can put out 14.8v at 2000rpm

with batterys connected I only see 13.2v at the cables

with batterys connected I only see 12.8v on my dash gauge

my dash gauge says im lousing 2 volts somewhere in the charging system with no lights on

with lights on my dash gauge is showing 12v

 

 

 

Thoughts?

 

 

[Underdog]

Not an electrical expert, but I would replace that 4 year old battery. If you have another good battery to test in it's place before you buy one, but at 4 years that battery doesn't owe you anything.

Sent from my XT1254 using Tapatalk

[j hancock]

Also check your cables and cable ends.  I had an older jumper cable between two batteries that seemed good but come to find out it was all green corrosion and disintegrated on the inside causing lots of electrical resistance and sending the ohmmeter sky high.

[Vladislav]

I would say you have everything about Ok.

Actually your alternator gives 14.8. What I would say is a bit much. As I remember from theory the optimal charging voltage is 13.8-14.2.

To explain the matter of your issue I have to lay out some theory again. Any current goes over a circle. Here we speak about the charging current. Starting from "+" pole of the alternator it goes to the "+" of the battery terminals, than passes both batteries (parallel), passes "-" terminal, follows through the negative cable to the chassis and the engine block to the alternator's body where alternator's coil provides it to the "+" pole again. And from this point to the next circle.

So you noted the current passes every cable and connection included. Every part of the cycle has some resistance. The current passing a particular part affects loss of voltage. By Ohm's law I=U/R, so U(Volts)= IR.. As you can see the loss would be more if the resistance is greater. Or if the current is greater also. Taking this to attention we have: If you measure the voltage at the alternator's pole (and it's body) on a test bench you see 14.8V. In this case the current from alternator goes through your tester only and is minor. The voltage loss is minor either so you see full volts. Than you attach the alternator straight to the batteries. The resistance of the circle consists of those straight (and good) wires. So there's a drop from 14.8 to 13.2. I'd say the cables you used in that case were not fatt enough. When you tested your truck originally with the stock wires (and long unserviced connections) you got 12. Than when you disconnected both the batteries (and interrupted current through them) you saw 14.8. About neither current went through the alternator excepting very small which passed the gauge coil so you saw almost all what alternator could produce.

There's one more point (if you still have some power to follow my brain-burning tale). Your tester shows voltage (or a loss of voltage) exactly between the points of the circle you put test wires to the ends of. What this mean. When you put your tester onto the alternator you see the alternator's voltage (actually including its internal loss on it's coil). But it produces so you see all it produces. When you check on battery(s) you see the voltage loss on it (the part of alternator's produced voltage which actually PROVIDES CHARGE). Talking formally this voltage is also a loss. Just a useful loss.  If you put your tester between the battery pole and alternator pole you would see the loss along that part of the cables. Interesting question is where the stock gauge is attached to. Usually closer to the battery poles to show how batts do. But if you pretend to know WHAT IT ACTUALLY SHOWS you should learn the points it is attached to.

One more side of the story. If you look again at Ohm's law you can see voltage (loss of voltage in our case) depends on current either. This explains for example why you see a drop on V gauge when batts are discharged. Alternator produces more current to charge them up and they're "hungry" to accept. But all the charge circle has its own resistance of cables and (poor) connections. Having existing resistance and higher current at the same time the voltage loses more on those resistance. As a follow you see lower Volts at the batteries - the rest of what alternator gave minus those increased losses.

Sooo... If you'd like to see higher number on the clock check out all the wire connections along the charging circle. Including connections of the alternator to the engine block. Minor but important note. If you noted I mentioned battery's internal resistance. As long as the current goes through a battery, there's some voltage loss in it. And it gives some drop in what gauge shows. So you will always saw less Volts on your gauge with batteries connected (and being charged) as on a "naked" alternator.

Now on a positive note (from which I started) - there's not such hard matter of the issue. When you batteries are good (and being not deeply discharged) they don't require strong charging current. Less current means less loss of voltage. So as higher the batts are charged as less meaning in the low voltage you observe.

Hope you haven't damned me seven times reading all that this can help

 

[Lmackattack]

Thank you Vlad... I had the truck on a solar power trickle charger for the last few days. its keeping voltage at 12.5 during they day. When I started it voltage at the battery was a little higher than 13.5 so I guess its doing what it should.

after some research I also noticed that my unit does not have a 2nd + post like some Delco Remy units have. the delcos have a "smart sense" option. It seams the delco units have a wire that goes to the starter and another wire directly to the battery. the delco units will self regulate making sure that 14.5 is always going to the batterys. I guess it can better see if the battery is seeing all that the unit is really putting out?

The truck starts fine and in excites as soon as I touch the pedal I just fount it odd that I could see 14.7 when the battery was disconnected but 13.5 when they were in the system