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  #11  
Old 10-10-2017, 04:22 PM
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bret bret is online now
 
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Originally Posted by EarthX Lithium View Post
People associate the "flames" from the lithium batteries you see on the news which are Lithium Cobalt chemistry. It is very rare for a LiFePo4 battery to produce any type of flame but it does produce a lot of smoke, which is not good in the cockpit. This is why we also do not recommend putting the batteries within the cockpit. We will have a new, vented battery for this type of application within the month, it will be the ETX900-VNT.
Another question, with a remote voltage regulator, is it best to interrupt the field current to the alternator or the alternator B+ lead with a relay when installing an overvoltage protection device?
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  #12  
Old 10-10-2017, 04:28 PM
EarthX Lithium EarthX Lithium is offline
 
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Originally Posted by rv7charlie View Post
But I have not yet seen an explanation for the alternator's current rating being a factor in any of these discussions about EarthX batteries. An alternator only supplies the current required to maintain the regulator's voltage set point. If there's an insignificant load on the system, there will be insignificant current flowing from the alternator, whether it's a 20A model or a 120A model. If voltage sags below the regulator's set point, the alternator current will continue to increase until it hit's the alternator's limit, or the regulator voltage set point is reached.

Does the EarthX BMS depend on the limited capacity of the alternator to control maximum charging current, or does the BMS control the charge current? If the latter, why the discussion (and limits) to alternator amp ratings?
The BMS does not limit the charge current from the alternator/generator which is why the alternator/generator charge current output rating must be less than or equal to what the battery models are designed to handle, such as the ETX680 is designed for a 60 amp alternator or less.

The amount of charge current a 20 amp alternator can put out is very different than what a 120 amp alternator can put out. When the regulator fails, the alternator can put out the maximum current it is designed to produce.
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  #13  
Old 10-10-2017, 04:54 PM
EarthX Lithium EarthX Lithium is offline
 
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Another question, with a remote voltage regulator, is it best to interrupt the field current to the alternator or the alternator B+ lead with a relay when installing an overvoltage protection device?
That is an excellent question that I will refer you to your engine maker/alternator company to ask as they will know the best way to do this with their system.
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  #14  
Old 10-10-2017, 06:10 PM
Bill Boyd Bill Boyd is offline
 
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Bret, if running an IR alternator, you'd best crowbar open the B lead - via a contactor in series - to be sure the alternator will respond. It may not. With external regulation, crowbarring the field lead should suffice to shut down the alternator.

The above is my understanding of the AeroElectric discussions going on since forever, and is what I recently installed in my plane after an IR alternator runaway event. In my case I was able to control the alternator at will with the field switch, but that didn't have to be the case. Not willing to risk it anymore.
Killing the alternator output by opening the B lead under heavy load might destroy the alternator from load dump, but won't necessarily do so, and alternators are cheap compared to smoked avionics and smoke-filled cockpits.

Nuckolls has promised an OV solution for IR alternators, and has removed his diagrams for IR protection via B+ contactor, but has yet to deliver a decade later. I found his old drawings and went with them anyway, knowing the risks and lack of elegance in design.
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  #15  
Old 10-10-2017, 06:11 PM
rv7charlie rv7charlie is offline
 
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Originally Posted by EarthX Lithium View Post
The BMS does not limit the charge current from the alternator/generator which is why the alternator/generator charge current output rating must be less than or equal to what the battery models are designed to handle, such as the ETX680 is designed for a 60 amp alternator or less.

The amount of charge current a 20 amp alternator can put out is very different than what a 120 amp alternator can put out. When the regulator fails, the alternator can put out the maximum current it is designed to produce.
Do you not see the problem with this? More importantly, at least for certified aircraft owners, does the FAA not see the problem with this? Will STCs for your batteries include a requirement for the owner to downsize their alternator to fit battery requirements? What is the battery management system actually managing? Does it not manage the charge current to each cell, in addition to managing the max discharge current? If it doesn't, what prevents overcharging the battery?

An overvoltage event is a potential device-killer, regardless of the current capability of the alternator. The proper way to manage an overvoltage event is to manage the overvoltage event; not limit alternator current.

Again. The current capacity of the alternator is only tangentally related to the output voltage. If the regulator fails, and there's, lets say, 20 volts on the bus, and your battery is fully charged, then the BMS is going to see 20 volts on its input. Are you saying that your battery can handle that 20 Volts at 20 amps indefinitely, but it can't handle 20 volts at 30 amps? Will the current flowing into the battery not vary with charge level?

I'm very confused by the chemistry & physics; apparently I just don't grasp how your battery chemistry works. In a lead-acid battery, the voltage is what delivers the ultimate death blow. Many AGM and flooded cell batteries have been killed by little 1 amp & 2 amp 'trickle' chargers that don't monitor charge level. There is very little current at work, but as the battery becomes fully charged, the voltage rises above the normal ~14V level to 15-16 volts. Charge *current* stays low, but it's relentless, being pushed by the excess voltage. Eventually it boils off the electrolyte, with either battery design.

So what's different about lithium iron chemistry that makes it able to handle a 20 amp alternator in an overvoltage situation, but not handle a 30 amp alternator in the same situation, meaning the same level of over-*voltage*?
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  #16  
Old 10-10-2017, 06:16 PM
rv7charlie rv7charlie is offline
 
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Originally Posted by bret View Post
Another question, with a remote voltage regulator, is it best to interrupt the field current to the alternator or the alternator B+ lead with a relay when installing an overvoltage protection device?
With an external regulator (or internal regulator, for that matter; just harder to get to the field winding), if you interrupt the current to the field winding, the alternator's output drops to zero, right now. It's much simpler, lighter, cheaper to interrupt the <8amp (usually <5 amp) field winding than the 40-80amp B lead.

Note that the permanent magnet 'dynamo' (the small alternator often seen on vacuum pads) is a completely different animal; it has no field winding to interrupt.

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  #17  
Old 10-10-2017, 06:17 PM
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Originally Posted by Bill Boyd View Post
Bret, if running an IR alternator, you'd best crowbar open the B lead - via a contactor in series - to be sure the alternator will respond. It may not. With external regulation, crowbarring the field lead should suffice to shut down the alternator.
If the regulator incorporates crowbar overvoltage protection, why would its physical location (internal or external) make any difference?
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  #18  
Old 10-10-2017, 06:31 PM
rv7charlie rv7charlie is offline
 
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Hi Bill,

That's one area on which Bob has been, for lack of a better term, less than consistent. The original reason he gave for removing the drawings was that pilots were cycling their alternator switches in flight, causing load dumps when the B lead contactor opened, and the early design IR alternators were dying from the load dump failures. He's since written in unrelated discussions that any modern alternator has load dump protection built in (and I've seen the same in independent docs from manufacturers, as well), but he hasn't adjusted his position on using IR alternators.

edit: forgot to add, if you only open the B lead when there's an overvoltage event, then the alternator's already dead.

My personal suspicion is that with the highly reliable B&C regulators/alternators available, he sees no need to re-address the issue. Of course, those of us who'd rather spend $75 on an alternator than $600 keep using the old drawings. :-)

Charlie

Last edited by rv7charlie : 10-10-2017 at 06:35 PM.
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  #19  
Old 10-10-2017, 07:06 PM
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AlexPeterson AlexPeterson is offline
 
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Originally Posted by DanH View Post
If the regulator incorporates crowbar overvoltage protection, why would its physical location (internal or external) make any difference?
Dan, the internally regulated alternators may accept the outside "field" lead, but still may use internally obtained current to "run" the field. Hence, a failure of the guts might allow an OV to continue on even when the external field control line is killed. Might be good to call these inputs "external field control" or similar.

16 years ago I tried to understand exactly how my ND IR alternator worked. I did finally get to someone at ND who seemed to know something, and he said that the external field control was indeed the only power source for the actual field. He sent me a crummy 8th generation copy of the control scheme, but most things were in some oriental language and tough to follow... One test which would help determine if the alternator has a true field supply from external vs simply a control would be to measure the current draw of this line vs alt current output.

To answer your question, if the ONLY power supplying the internal regulator comes from the field line, then only a crowbar on this field line is needed. If it can't be determined if the field line is simply a control, then one should use a contactor in the B lead as well.
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  #20  
Old 10-10-2017, 07:29 PM
BillL BillL is offline
 
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Kathy, Thanks for the sharing of such a revealing incident. The EarthX communications on this is to be commended.

Any reason the BMS did not disconnect and protect the battery? Was this failure mode (failed regulator - voltage high) tested in an abuse test? If not, is there not some alphabet agency test for this?? Do the new required recommendations include a sealed container for the battery if located within the cabin too?
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