Changing an alternator from 12 to 24 volts

[BjarneK]

I recently changed my house battery bank from 800 AH LiFePO4 at 12 Volts to 400 Ah at 24 V. This of course involved a lot of changes to the electrical system. I will maybe start another couple of threads about the changes, but this one is about the alternator.

The short version is that I managed to change a 12 V alternator to make it deliver 24 V. It turns out that this can be done with the original stator and rotor in place (at least for the alternator I have).

The alternator is the original 12 V, 100 Amp alternator that came with the Nanni N4.60 enigine that I have. It looks quite similar to a Mahle MG 581. I have never seen it deliver close to 100 Amps. I have removed the original regulator and now use an external one (home made). I also removed the rectifier which turns out to be limited to 17 V output (I think the diodes are Zener diodes to protect 12 V circuits connected to the alternator). I made an external rectifier (Ixys VUO110-08NO7) mounted on an aluminum profile (Wakefield-Vette 125325 cut to 130 mm length) cooled by a fan (Orion Fans OD9238-24HBVXC10A, 24 V capable but supplied with only ~14.5 V). The only reasonably temperature resistant cables I had available at the time for making the rectifier external was 4 mm2, so I limited the output to 40 Amps.

Since it is a 12 V alternator, I made sure to limit the field voltage to 12 V even though I connected the rectifier output to a 24 V LiFePO4 battery. Obviously, more rpms are needed to get 24 V than 12 V, but I get an approximately linear increase from 0 Amps to 40 Amps at 24 V as the engine rpms go from 1,400 to 1,800 (the pulley ratio is about 2.2).

The other day I did the first long(ish) test of 100 minutes of continuous 40 Amp output to the 24 V battery (27 V in reality). During that period, the alternator output an average of 1.06 kW for a total of 1.77 kWh of energy. The top of the engine room above the engine reached 50 deg C and the rectifier located there reached 63 deg C. The alternator itself was located on the engine lower in the engine room (and thus at cooler surroundings) and reached 56 deg C. The alternator temperature was measured at the back of the alternator and the front got 10-15 deg C hotter (measured with IR thermometer on the outside of the alternator). I will work on getting the engine room temperature down with more air circulation, but the alternator and the rectifier performed great without too much heat generated.

I have attached a curve of the alternator output as a function of engine rpms with a constant field voltage of about 12 V. I have also attached a curve of the alternator output as a function of the field voltage (the coil is about 3.1 Ohm) when the engine is at 1,850 rpm.

[more]

why you dont buy new 24v 60-120A alternator and sold your.
rest time watch how rain drop on cockpit with hot cup coffee in hand.this danish huge cup.

[Pete7]

Originally Posted by more

why you dont buy new 24v 60-120A alternator and sold your.
rest time watch how rain drop on cockpit with hot cup coffee in hand.this danish huge cup.

Agreed. Also what is happening about the starter motor? and the VHF which don't normally come in 24v versions.

Pete

[Pete7]

Originally Posted by more

why you dont buy new 24v 60-120A alternator and sold your.
rest time watch how rain drop on cockpit with hot cup coffee in hand.this danish huge cup.

Agreed. Also what is happening about the starter motor? and the VHF which don't normally come in 24v versions.

Pete

[BjarneK]

I was just interested in what was possible and thought others might feel the same. Having an external rectifier and regulator can never be wrong even if I do end up with another alternator in the future.

[more]

Originally Posted by Pete7

Agreed. Also what is happening about the starter motor? and the VHF which don't normally come in 24v versions.

Pete

nothing , still use 12v starter battery . totaly different from house battery,simply solution.
24v alternator charge 24v house battery.12v 70-110 ah starter charge by 5A dc-dc charger .
emergency yump starter possible 24v in reality only faster spin starter.

but if i be boored best way is made air powered starter motor. small bootle and small compressor for charging air bootle. in reserve hand high pressure pump.

https://www.youtube.com/watch?v=LBa-...hannel=KT3406E

https://www.youtube.com/watch?v=R2Ag...trialSupplyInc