I've got an '04 TJ Unlimited with the piddly 117 amp stock alternator. I've got access to an '04 WJ alternator rated at 180 amps and it seems like a no-brainer to make the switch. Anyone done this swap?
Alternator Upgrade
- Thread starter LJBean
- Start date
OP
Actually, I've not had much of a problem with the stock alternator, and the more I look into it the less beneficial it sounds.
I couldn't give you any concrete info, but this is something I need to look into as well.
My dash voltage gauge drops significantly during winching, and I'm not sure whether the alternator or the old battery is the issue.
OP
Has that always been the case or a recent change? I've heard (and this makes sense) that there is no way for the alternator to keep up with the draw from a winch, so it's pulling what it needs from the battery. I've just convinced myself that my money would be better spent on a quality AGM battery.
I've had the issue since buying it stock. A Warn M8000 has a max amp draw of 435 amps, so it makes sense that the battery would be drained with a 117 amp alternator.
In my case, it's draining much quicker than on my previous vehicles, going down to ~10 volts shortly after beginning to winch. No voltage issues outside of that. So not sure, it might just be that a battery upgrade is in order.
Regardless, of your ultimate alternator choice, i've found that I've been well-served by spending good/fair amounts of money on a quality AGM battery.
I put a high amp on my YJ because of my halogen lights. I do a lot of night desert running. I get periodic belt squeal and keep a can of belt dressing in the center console. I don’t know how much HP is lost to the higher amp alternator.
If you start with alternator output and work back to the crank, this can be calculated as long as you know two things: 1) units conversion from electrical power to engine power and 2) related system efficiencies. The first is a simple multiplication factor. Electrical power is measured in watts, and engine power is measured in horsepower. There are 746 Watts to 1 horsepower. But how do you go from alternator amps to alternator watts? You multiply voltage times current to get watts.
So, if the alternator is putting out 117A at 14.4V, that's about 1,700W, or 2.3hp. Likewise, if the higher-output alternator is putting out 180A at 14.4V, that's about 2,600W, or 3.5hp. However, that's alternator output. There are system inefficiencies that require more than that from the crank.
There are two system efficiencies to consider between the engine crank (output) and the alternator output: 1) mechanical efficiency of the serpentine belt drive system, and 2) electrical and mechanical efficiency of the alternator itself. Serpentine belt drive systems are very efficient - typically about 98%. In contrast, alternator efficiencies are very inefficient. The efficiency varies with RPM/load, typically from about 40% to about 60%. Since the alternator inefficiency dwarfs that of the belt drive system, I'll just combine them and say total efficiency is about 50%.
Now, if the 117A alternator puts out 2.3hp in electrical power, divide that by the 50% efficiency of the systems, and the engine crank is putting out 4.6hp to drive the alternator. Likewise, the 180A alternator puts out 3.5hp in electrical power, divide that by the 50% efficiency of the systems, and the engine crank is putting out 7.0hp to drive the alternator. Subtract the two, and the higher output alternator will require 2.4hp more from the engine at full output.
However, it's not that simple. That's only a comparison of power at peak output. The alternator isn't running at max output under normal circumstances. The PCM is regulating the voltage output of the alternator to charge the battery, and this changes the alternator output. So, the bigger alternator and the smaller alternator will be outputting roughly the same current under the same conditions. The two will have different efficiency curves (efficiency vs. RPM), though, so there will be a slight difference in hp required from the engine, but that will be minor.
The bottom line is that a bigger alternator won't require substantially more hp until you exceed the smaller alternator's peak output current.
I put a 270A Singer alternator and a hand throttle on my LJ so that I could more easily maintain battery voltage while winching, and to run a welder, if I decide to install one.
Keep in mind how a DC motor works. It's basically a constant-power device (requires a certain amount of power to put out a certain torque), and as such, it will draw proportionally more current when voltage drops. I know from the research I did for my M8274 install, that Warn's current ratings assume 12V at the battery. So, if you are doing a max pull at 10V instead of 12V, that max current becomes 435A x 12V ÷ 10V = 522A!
Brianj5600
TJ Expert
How much output does an alternator drop as temperature goes up? What temperature are alternators rated at?
I looked at higher-output alternators but settled on the 270A because none were big enough to supply the 623A max current for the M8274 (newest model with the bigger motor). There were downsides to going bigger than 270A, but other than cost, I don't recall what those were anymore.
It looks like they have a 200A and a 270A version, and either of them would be a big help during and after winching...
Well, the resistance of copper wire rises about .2% for every 1°F, so if there's a 100°F temperature change, the resistance goes up 20%. Since current equals voltage divided by resistance, a 20% increase in resistance would result in 17% drop in output current.
Assuming you mean what temperature can it operate at before failing, I don't know the answer to this, but I'd guess it to be in the 250-300°F range.