Alternator Selection
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Alternators generally seem to be sold as 12V, 24V and a few in the 48V-58V range. When selecting an alternator to charge a non-standard voltage battery (32V, 22.5V, 45V, etc) with WS500 regulation, is it generally best to select an alternator that was designed for a higher voltage than your battery? Or does this not matter? For example, say you want to charge a battery which has a nominal voltage of 22.5. Could you select a 12V alternator and regulate its output up to around the 24V required to charge this battery, or would you do better to select a 24V alternator and regulate it below its design voltage? I have read about people charging batteries as high as 48V with 12V alternators through regulation, but perhaps this requires higher alternator rotational speeds which will shorten the life of the alternator? Or perhaps the rectifier diodes have upper voltage limits? Thank you for your help.
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Richard OGM Support StaffIn general most 12v alternators will put out about 15 volts, as it must be somewhat higher or else it won\'t even charge. I too have heard of people who have used voltages up to 4 times the battery rated voltage but I wouldn\'t try it myself. For non standard voltages I would feel comfortable going up to the next standard voltage. @al-thomason
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Interesting questions today... There are two dimensions to this question. The 1st is: will it work? The answer is yes it will work. One is able to overdrive a lower-voltage alternator to support a higher voltage battery, but there is a trade off: A loss of low RPM output. In the early 2000\'s there was a bit of research done on this, and one can find some papers, but in the end: If you want to support a 24v or say 48v battery using a 12v alternator it will work, you need to set the MAX_FIELD_DRIVE % to a level that will not smoke the alternator rotor, but you will also have to spin it fast to get the needed output, and you might not get much output at low RPMs. A side benefit is increased efficiency: Look for the white papers. The other dimension is surge suppression. There are several things which can cause voltage spikes in a DC system, perhaps the most common is the stopping of a large DC motor (e.g., Windlass). Another is if the battery is disconnected while the alternator is charging (Someone opens the battery disconnect switch, or one of these \'Drop-in\' Lithium batteries triggers their internal disconnect). These events can cause spikes over 100v, and several 100mS in duration. Short and Hot so to speak. A 100v spike will often cause damage to electronic equipment, no matter how short. In the transportation sector a common technique to address this is the use of \'Avalanche\' diodes in the alternators - makes sense, the alternator is a place with lots of high current wires and already has big diodes. These diodes will clip the voltage spike at around 28-32v (for a 12v alternator). And there is the rub: If you want to use a 12v alternator to get 24v, you need to have the diodes replaced with higher voltages ones - or non-avalanche diodes. So in summary: Yes, over driving an alternator can work: With the loss of low RPM output, and the need to adjust the diode pack. BUT: A side note, these voltage spikes are an issue. Worst case example I know of was an install where an undersized fuse was used. It opened, disconnecting the battery while the alternator was begin driven hard. Alternators did NOT have avalanche diodes and the resulting spike destroyed the alternator plus about 1/2 the equipment on the vessel. Was not our regulator, and I know know of this because one of our distributors was brought in to make a more robust install. And another note: With the increased adoption of \'drop in\' batteries, the risk of a load-dump (i.e. battery disconnect) is very real: With no way for these drop-in batteries to communicate to the regulator they are about to disconnect, and lacking any other mitigation such as a keeper battery, the results can be equally as dramatic and costly. ================================================= Another answer to your question: When we talk of 12v, 24v, 48v, this is kind of a nominal system voltage. With a \'12v\' lead-acid batteries one can expect to see normal operating voltages anywhere from 11.2v to as high as 15.2v, maybe even more in cold climates. So, in your example of 22.5v your best bet would likely be a \'24v\' alternator. Unless you were looking for higher efficiency, and willing to give up low RPM output, then a over-driving a 14v alternator might be a good choice. Hope this helps, -al-
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Thanks for the informative reply! I hadn\'t considered the efficiency aspect. If overdriving a lower voltage alternator results in reduced low RPM output and higher efficiency, is it safe to assume the opposite would be true for underdriving a higher voltage alternator? i.e. higher than advertised low RPM output but also at lower than advertised efficiency? From a reliability standpoint underdriving must be the better option.
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Not sure about the other way, never seen a need where someone did that. On reliability, perhaps overthinking a bit. As long as one does not overheat an alternator, the likely failures are then mechanical (bearing, brushes) - which tend to be rather long term. Do you have a specific project in mind, or is this more of a though exercise? -al-
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Sorry, I didn\'t see your reply. I have considered using a 6s battery (Tesla module) which has a nominal voltage of about 22.2v. Your post about overdriving got me wondering if charging a 22.2v battery with a externally regulated \"24v\" alternator would result in a boost in idle output (and perhaps lower efficiency). In this example, I imagine the difference vs stock operation would be small. It would be more interesting if the battery voltage was lower (say 5s), then you would really have to consider overdriving vs. underdriving.
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I would say for your application, either an overdrive 12v alternator (with care about the diodes) or a standard 24v alternator would work. Much depends on how you are driving it, and the expectations for energy output at low RPMs.
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