Hey RC - I am having a problem with my alternator overheating any time my batteries drop below about 90% SOC.
With a 140A alternator on a 420Ah battery bank, figuring approx 120A when hot, your charge rate is approx .28C or 28% of Ah capacity. With .2C charge rate from 50% SOC, on a Lifeline battery bank, bulk is approx 1:15 minutes long. With a charge rate of .4C bulk is approx 20 minutes long. You are likely 30-35 minutes in bulk from 50% SOC. If you are over heating in the 90% SOC range this is pretty tough to do because you will be in absorption and current will be declining pretty rapidly. My first question would be how are you determining SOC? Under your normal/average house loads you should not be letting your Lifelines dip below 12.2V. This will be
approx 50% SOC.
Charge rate affects the bulk/absorption transition point in the SOC curve:
Charging at .4C, from 50% SOC, the transition from bulk to absorption = 63% SOC (approx)
Charging at .2C, from 50% SOC, the transition from bulk to absorption = 77% SOC (approx)
You will be somewhere between theses two points where you hit absorption voltage unless you are de-rated down towards .2C then your bulk to absorption transition will be at approx 77% SOC...
I can reduce the temp by going to Small Engine Mode (switch at helm), but that obviates the benefit of the HO alt.
Small engine mode is a 50% cut in field potential. If your engine room is so tight that you need to go to 50% field then you simply need more air. The E-Maax alt brings air in from the front and back and spits it out the middle. If you replaced the Yanmar belt guard after the upgrade, REMOVE IT NOW! These things do nothing but cook alternators.
You will be best to force cool air at the back of the alternator where the rectifier is. Inject it right into the vents of the black plastic cover.
My engine space is very compact, especially around the front of the engine. I'm thinking of ducting my engine room blower directly to the alternator - question is, would it be better to blow fresh air in toward the front, or suck air out from behind?
This alt is a dual internal fan unit. It sucks air in the back and from the front and expels it in the middle. The best way to help cool that alt is to force feed the back of the alt with cool air. Another blower sucking the hot air out of the TOP of the engine bay will also help.
Proper set up of any alternator includes running it at full bore via a dummy load (inverter & electric heater etc.) then closing the engine room and monitoring alt temp remotely. Reduce belt manager until the alt can run in the environment. You then back this up with the Balmar alt temp sensor as added insurance.
Please don't ever assume any small case alternator can run at full rated output for more than a few minutes, especially without external cooling. Some small case alts are better than others but they all need field reductions when fed into large loads. The only way I know of to run a small case alt at full bore is to remove the rectifier and rectify the alt remotely. Belt manager and additional cooling is a heck of a lot easier.
NOTE: Please note that EM has been know to install rotors with the rotor fins purposely bent so they don't have to do any additional machining to the alternator case. They also will sometimes run low temp wires right over the rectifier cooling fins which impacts cooling negatively and in some cases have cause melted wires.. The rectifier negative contact area is also not machined well, usually not at all, and can yield poor contact to the aluminum case which can also lead to poor cooling. They also don't tend to use heat transfer paste between the rectifier & case. These four issues can really impact cooling in a negative way.
