Alternator charging amperage - where it actually is

[chorky]

So the JTR, max tow, and others have the 240 amp alternator.

I am curious if anyone has actually measured what amperage the alternator pushes under various loads. Like normal operating, vs a low battery, vs a winch pull, vs any other combination.

When I was swapping out the battery tray I took a peak at the alternator wire, and positive cables. They look small - possibly 4 or 6 gauge? I have a. hard time those would really handle 240 amps for any length of time.

Asking because I am considering putting a spare battery in the bed for added power when camping on some week long trips this summer (not the norm for me) and wanting to be cautious I dont blow a fuse or overload the 4ga wires currently going to the bed - rated at about 100 amps for the length of run.

But aside from high voltage I honestly don't know (but should know) what our AGM batteries will accept for amperage to charge. I seem to recall 10-20 amps for some reason. Could be thinking of something totally different

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[ShadowsPapa]

So the JTR, max tow, and others have the 240 amp alternator.

I am curious if anyone has actually measured what amperage the alternator pushes under various loads. Like normal operating, vs a low battery, vs a winch pull, vs any other combination.

When I was swapping out the battery tray I took a peak at the alternator wire, and positive cables. They look small - possibly 4 or 6 gauge? I have a. hard time those would really handle 240 amps for any length of time.

Asking because I am considering putting a spare battery in the bed for added power when camping on some week long trips this summer (not the norm for me) and wanting to be cautious I dont blow a fuse or overload the 4ga wires currently going to the bed - rated at about 100 amps for the length of run.

But aside from high voltage I honestly don't know (but should know) what our AGM batteries will accept for amperage to charge. I seem to recall 10-20 amps for some reason. Could be thinking of something totally different

A 240 will likely put out less - the rule of thumb is that if it's 10% below rating or better, it's ok.
Look to the wire coming out of the alternator - on the output terminal, for an indication of what they expect out of it - they control output by voltage, not reading amps.
They are looking more at what it can put out at reduced RPM than actually expecting a full 240 out of it.
It takes a hefty alternator to keep up with a load at idle or highway RPMs. To get there they have to use an alternator rated much higher.

A low battery they'll only need a few amps - 20 maybe 40.
It's going to be voltage regulated - the system will try to maintain a voltage.

It would take about 4 HP to run the full 240 amps at 13 volts.

 

[chorky]

A 240 will likely put out less - the rule of thumb is that if it's 10% below rating or better, it's ok.
Look to the wire coming out of the alternator - on the output terminal, for an indication of what they expect out of it - they control output by voltage, not reading amps.
They are looking more at what it can put out at reduced RPM than actually expecting a full 240 out of it.
It takes a hefty alternator to keep up with a load at idle or highway RPMs. To get there they have to use an alternator rated much higher.

A low battery they'll only need a few amps - 20 maybe 40.
It's going to be voltage regulated - the system will try to maintain a voltage.

It would take about 4 HP to run the full 240 amps at 13 volts.

Ok thats what I thought - that they didnt really push a ton of amperage even if a battery is pretty drained. I found it interesting the CTEK charger I got is fused at 15 an and maybe 14ga wire - different topic I know....

 

[ShadowsPapa]

Ok thats what I thought - that they didnt really push a ton of amperage even if a battery is pretty drained. I found it interesting the CTEK charger I got is fused at 15 an and maybe 14ga wire - different topic I know....

The fuse is 300 amp. The wires - how long are they? It's not just gauge but length as well. If only going a foot or two a smaller gauge can handle a lot compared to going 10 feet.
The fans and PS pump are pretty much the largest draws on these things.

 

[chorky]

they? It's not just gauge but length as well. If only going a foot or two a smaller gauge can handle a lot compared to going 10 feet.
The fans and PS pump are pretty

oh for sure. I think the alternator is maybe, oh, 3 feet at most... and then another few inches to the battery - although I dont like how the factory kinked those wires so much....

Thats a huge difference compared to 20' no doubt. But it still was interesting. Especially the fuse links on the factory aux cable - maybe 8ga I think. It just got me thinking that there's probably little chance, unless it's a hard winch pull, that the alternator will push more than 150 amps continuously.

It would be cool though if there was a gauge on board for monitoring that

 

[ShadowsPapa]

oh for sure. I think the alternator is maybe, oh, 3 feet at most... and then another few inches to the battery - although I dont like how the factory kinked those wires so much....

Thats a huge difference compared to 20' no doubt. But it still was interesting. Especially the fuse links on the factory aux cable - maybe 8ga I think. It just got me thinking that there's probably little chance, unless it's a hard winch pull, that the alternator will push more than 150 amps continuously.

It would be cool though if there was a gauge on board for monitoring that

The wire to pay attention to is the one coming off the back of the alternator (or front in this case?)
All energy from the alternator must come out that wire.
On "other" or "older" vehicles that wire goes to a junction and from there, typically smaller wires branch out to their destinations. It's the size and length of the alternator output wire that matters.

 

[chorky]

The wire to pay attention to is the one coming off the back of the alternator (or front in this case?)
All energy from the alternator must come out that wire.
On "other" or "older" vehicles that wire goes to a junction and from there, typically smaller wires branch out to their destinations. It's the size and length of the alternator output wire that matters.

Yeah and that wire is pretty small. I don't know for sure how thick the harness around it is but it can't be much larger than 4 ga

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