Exciting a 12V Alternator with 24V?

[GeoffHammond]

My house is powered by a stand-alone energy system: 1kw wind turbine, ~2kw solar, 1050AH 24V battery and an inefficient 240V generator/battery charger. I originally thought I would buy a 24V alternator and baby-sit it (or cobble something via the control circuits of my inverter) through bulk charge and let the solar deal with equalisation charge, but have been convinced otherwise.

I've read mobile_bob's paper on using a 12V alternator to charge a 24V battery and understand enough of it to get excited. It will probably be read several times in the near future.

I have purchased a cheap electric start engine and am now considering my alternator and regulator options. Here in Australia, I can get a second-hand 12V alternator for much, much less than a 24V equivalent. The cell-cell interconnects on my battery would get in the way of using it as a voltage divider as described in mobile_bob's paper and while I could change that, it is my preference not to.

I'm looking at the 24V Sterling regulators and wonder if I could discard the alternator's regulator and replace it with just the Sterling so that the entire thing is operating at 24V rather than the alternator's designed-for 12V. I figure that there will be double the current flow in the rotor field of what it was designed for and this might generate excessive heat and/or annoy the regulator, but will this be excessively annoying?

Imagine that it were doable - with a dual-voltage regulator such as the Sterling, both 12 and 24 volt batteries could be charged with the same unit and just a small amount of fettling, making both my house and my LandCruiser chargeable from the one unit.

[mobile_bob]

cell to cell interconnects?

can  you take a picture and post showing how your battery bank is setup, or alternatively
what the bank is made up of? such as 4 six volts, 2 twelves, 24 two volt cells or?

from the size of the bank i am thinking it is made up of multiple cells with accessible interconnects

the white paper relates to tapping the bank only for a sense voltage, this is in the milliamp range generally.

i would not suggest center tapping for field current, rather derive  your 12volt field current from the starting battery and its charging system
instead, this would be nothing different than driving any other accessory load for the starting battery/charging system, and basically makes
for a separate excitation buss not unlike what the big boys do at the power company on mega generators.

study that white paper, you system is exactly what the paper was written to address,
and would be a perfect match for a small cogen system, where you could charge batteries and make hot water in an amount
appropriate for an average family.

1000amp hour battery bank, being used and charged under the 50/80 regime would return about 300amp/hrs per cycle
and provide for maximization of battery life vs system efficiency and overall life cycle cost.

if you can get your hands on a 12 volt version of the leece neville/prestolite 110-555jho and one of the sterling regulators that has the sense
terminal you could put together a very efficient charger for that voltage and battery capacity. using 300amp/hrs per cycle the 555 alternator could recharge in about 3 hours of run time (going from 50 to 80% state of charge, where the batteries are very efficient at taking a charge).

of course this leaves you with having to recharge to full state of charge once every week to 10 days, and equalization perhaps once every month or so.

so far the system is dollar for dollar the most efficient and cost effective of anything i have ever seen, with most not even coming close to matching the performance.

thats not to say that there is not something more efficient out there, but i certainly have not seen or heard of anything even close, so
if you find something better i would really like to know about it.  talking about engine driven alternator/generator charging systems, whether
direct charge or via some discrete charger.

bob g

[GeoffHammond]

Bob: Thanks for your time and the response.

The battery is 12x2V cells (Raylite M-Solar series - just a shade under 600kg total weight) and the proprietary interconnects between the cells have a waterproof seal that helps to inhibit corrosion of the terminal and the connector. I can replace one of them with something more accessible, but would prefer not to.

I understand your point about taking a 12V field current from the centre tap of the battery bank versus a sense voltage. My thinking was to use 24V for the field to avoid taking too much current from the small engine's electrical system in case it couldn't cope; in essence, everything would operate at 24V, but with an alternator that has 12V written on the side. Is there anything stopping me from exciting the field with 24V?

I purchased the battery (as a replacement to the pre-existing battery that had died) with the intent of very shallow daily discharge to allow for several days without solar/wind input. Assuming a fully charged battery, we tend to see SOC drop to 90-93% before the sun comes up most of the time. Our major power draw is an excessive number of fridge compressors. In the majority of cases, my solar/wind system brings the battery through a 4-stage charge to float each day. But, in the depths of winter there is an occasional need to ensure absorb and equalise charging happens. I've got too much invested in the battery bank to mistreat it and the current petrol-based charger is simply inappropriate.

[buickanddeere]

  The field winding in the rotor is current limited for 12V. If you want more stator voltage. Just excit the rotor with 12V and adjust rotor rpms to obtain the 28V required to charge a 24V battery bank.