Monday, May 26, 2014

Driving two alternators -

I was asked about the ability to drive two alternators on one engine and charging one one battery bank.  Here is what I replied:

Driving two alternators.  There is a massive capability in the Field FET driver ckt.   Each FET has an Rds of only 8 mOhms, and is base rated for 80A.  I used two in parallel just because they are not that expensive and two will assure a massive current capability with very little heat.  I think the biggest limitation is the current capability of the PCB, even with large copper areas.  If you note I am leaving a gap in the solder-mask on the top and bottom from the Field connectors to the FETs.  On this one can solder heavy wires (like 16g or even 14g solid) to increase the current capabilities of the copper PCB.   For two alternators I would suggest you do that to beef up the PCB.

Then I would simply wire the alternators in parallel.   If they are the same type / model there will be some level of current mismatch between the two, but not sure that really matters too much.  I just now thought:  there is a spare temperature probe connector on the PCB, you could use that to monitor the temp on the 2nd alt.  I will modify the code to sample both alts temps when deciding if things need to be backed off.  But as long as you do not need abs matching, I think just drive them in parallel…  

If one is much larger than the other, the miss-match could become large.  For example if you had a 200A and a 100A, perhaps the small guy will drive harder for a given field PWM setting.  In that case we could be driving the stuffing’s out of the small guy while the big one loafs along.   You could measure the current of each with a clamp-on amp meter and if you need to pull back the little guy some just add in a couple of diodes into the field for the small guy, to drop its field voltage and try to bring things more back in line.  (ala, thinking at any given PWM drive level you would want to have the Amps out of each alt be at about the same % of their respective max capability.  If you have a 200A and a 100A alt, then when delivering 150A into the battery you would want the alts delivering 100A and 50A respectively.  Not say 60A and 90A -  thereby working the small guy so much harder).

Hope this makes sense.  In short, I think all you need to do is add the copper beef-bars to the PCB, connect the Alts in parallel, connect up a 2nd temp probe to the 2nd alt.  Then check with a clamp-on amp meter to see if the load splitting between the two seems reasonable.


Today I modified the code adding in a spot for a 2nd alternator temperature sensor.  To enable it you just need to add in the address of the one-wire probe in the source:
#define OW_ALT2_TEMP    {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
                                             // 2nd Alternator Temp (if present)

Note that by default I have this filled with zeros, in effect disabling that temp probe.

In operation the code will look at both alternator temperatures and pick the hottest one to 'manage' things with.  I have not added anything into the LCD display for the 2nd probe, need to figure out where to put that at...

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