Inverter pre charge + alternator protection from BMS disconnect in one solution

[Fuss]

Hmmm, might be a bit early for this but here goes...


So you want to precharge your inverter to reduce damage (not sure I agree with this but anyway...)


And you want to protect your Alternator against BMS cutoffs


You have the usual BMV712 and external alternator regulation and temp sensor. (just being politically correct here, not sure you need the temp sensor unless you have a plastic alt, however, this is not the subject of this thread.)


Don't run around with some silly inverter resistor in your hand, this only works until you lose or forget to use it. Don't rely on the alternator spike protection device...some are only one time use!!, you knew that though right, one time use, you know that might be right?



So this is what I did.
Install an AGM mini battery that can be switched on/off , its 1.3ah and behind an isolating diode so the alternator can never be HOT by accident when you think all is switched off. (can anything be HOT from 1.3ah...)

Now when you switch on your batteries after "whatever", the procedure is switch on the mini AGM first (1.3AH, so please wear safety glasses!) and then switch on the main batteries switches. Inverters are now precharged, and alternator protection energised



You now have a completely protected alternator against BMS cutoffs (I'm not totally sure of this but it seems to be true on the limited testing I've done, However, I also have 3 lifepo4 banks so a bit of safety in numbers additional protection so to speak)


Ok and now for the extra bit, you can do a whole lot of testing using the mini AGM without using your main lithium banks. For example, are the alt regulators behaving as expected?...just leave the main banks off and run your alternator using the 1.3ah AGM, Bulk voltage, constant voltage, float voltage are all available to be seen within a very short space of time. Then you can feel much happier when connecting the real batteries.


So there you have it the 1.3ah mini AGM
no more lost pre charge resistors, no more alternator damage.
Almost better than sliced bread!

[sailorboy1]

forget all the extra trouble and just have a acid start battery in the circuit

BTW you should NEVER trip a BMS

[Fuss]

Yes of course there should be no BMS disconnect but this is not the main thread topic and not the concept some people are using, The wakespeed for example will switch your alternator off before... but lets stay on topic and not thread drift.

[tanglewood]

Originally Posted by Fuss

Hmmm, might be a bit early for this but here goes...


So you want to precharge your inverter to reduce damage (not sure I agree with this but anyway...)


And you want to protect your Alternator against BMS cutoffs


You have the usual BMV712 and external alternator regulation and temp sensor. (just being politically correct here, not sure you need the temp sensor unless you have a plastic alt, however, this is not the subject of this thread.)


Don't run around with some silly inverter resistor in your hand, this only works until you lose or forget to use it. Don't rely on the alternator spike protection device...some are only one time use!!, you knew that though right, one time use, you know that might be right?



So this is what I did.
Install an AGM mini battery that can be switched on/off , its 1.3ah and behind an isolating diode so the alternator can never be HOT by accident when you think all is switched off. (can anything be HOT from 1.3ah...)

Now when you switch on your batteries after "whatever", the procedure is switch on the mini AGM first (1.3AH, so please wear safety glasses!) and then switch on the main batteries switches. Inverters are now precharged, and alternator protection energised



You now have a completely protected alternator against BMS cutoffs (I'm not totally sure of this but it seems to be true on the limited testing I've done, However, I also have 3 lifepo4 banks so a bit of safety in numbers additional protection so to speak)


Ok and now for the extra bit, you can do a whole lot of testing using the mini AGM without using your main lithium banks. For example, are the alt regulators behaving as expected?...just leave the main banks off and run your alternator using the 1.3ah AGM, Bulk voltage, constant voltage, float voltage are all available to be seen within a very short space of time. Then you can feel much happier when connecting the real batteries.


So there you have it the 1.3ah mini AGM
no more lost pre charge resistors, no more alternator damage.
Almost better than sliced bread!

At least for the pre-charge function, I do the same with a house-to-starter bank parallel switch. I start the DC system using the starter bank, then turn on the LFP bank, then disconnect the starter bank. The primary purpose of paralleling is as a backup in case of complete LFP failure, but it serves well for jump starting teh system.


As for alternator disconnect, my BMS shuts off the alternator well before it disconnects, so that's not a problem.

[rwidman]

Originally Posted by Fuss

Hmmm, might be a bit early for this but here goes...


So you want to precharge your inverter to reduce damage (not sure I agree with this but anyway...)


And you want to protect your Alternator against BMS cutoffs


You have the usual BMV712 and external alternator regulation and temp sensor. (just being politically correct here, not sure you need the temp sensor unless you have a plastic alt, however, this is not the subject of this thread.)


Don't run around with some silly inverter resistor in your hand, this only works until you lose or forget to use it. Don't rely on the alternator spike protection device...some are only one time use!!, you knew that though right, one time use, you know that might be right?



So this is what I did.
Install an AGM mini battery that can be switched on/off , its 1.3ah and behind an isolating diode so the alternator can never be HOT by accident when you think all is switched off. (can anything be HOT from 1.3ah...)

Now when you switch on your batteries after "whatever", the procedure is switch on the mini AGM first (1.3AH, so please wear safety glasses!) and then switch on the main batteries switches. Inverters are now precharged, and alternator protection energised



You now have a completely protected alternator against BMS cutoffs (I'm not totally sure of this but it seems to be true on the limited testing I've done, However, I also have 3 lifepo4 banks so a bit of safety in numbers additional protection so to speak)


Ok and now for the extra bit, you can do a whole lot of testing using the mini AGM without using your main lithium banks. For example, are the alt regulators behaving as expected?...just leave the main banks off and run your alternator using the 1.3ah AGM, Bulk voltage, constant voltage, float voltage are all available to be seen within a very short space of time. Then you can feel much happier when connecting the real batteries.


So there you have it the 1.3ah mini AGM
no more lost pre charge resistors, no more alternator damage.
Almost better than sliced bread!

What is this "So you want to precharge your inverter to reduce damage" thing? Is it real? How do you "precharge" an inverter and why would you do it?

Please explain, in technical terms, what this is all about.

[Fuss]

Originally Posted by rwidman

What is this "So you want to precharge your inverter to reduce damage" thing? Is it real? How do you "precharge" an inverter and why would you do it?

Please explain, in technical terms, what this is all about.

It’s all the rage, the latest fashion…apparently Lifepo4 damages big inverters when initially connected so you have to run around with a resistor to pre charge the capacitors.
Look in this forum, they are all saying it…

[Fuss]

They might be right… it might be similar to how long it took to realise that 12v spikes damage instruments

[rwidman]

Originally Posted by Fuss

It’s all the rage, the latest fashion…apparently Lifepo4 damages big inverters when initially connected so you have to run around with a resistor to pre charge the capacitors.
Look in this forum, they are all saying it…

As a retired electronic technician who understands theoretical and practical electricity and electronics, this makes no sense to me. That is why I asked for a technical explanation.

A device (an inverter, in this case) does not know or care where the power is coming from. Power from one type of battery is no different than power from another type of battery or even a power supply.

[Pete7]

Originally Posted by rwidman

As a retired electronic technician who understands theoretical and practical electricity and electronics, this makes no sense to me. That is why I asked for a technical explanation.

A device (an inverter, in this case) does not know or care where the power is coming from. Power from one type of battery is no different than power from another type of battery or even a power supply.

As a nearly retired desk jockey my understanding is there are two types of inverter. Low frequency inverters using coils to change DC into AC. Victron and Sterling are examples. But they are big, heavy and expensive.

The alternative is high frequency inverters using MOSFETs? just as a BMS does. Its these that are at risk from the initial spike particularly by LFP with potentially a large inrush of current. As said earlier I noticed Will Prowse pre charging circuits before making the final connection and decided perhaps I should follow suit. For £6 I now have a pair of widgets which remove the big spark you sometimes see when connecting large DC circuits. What's not to like.

If you think this is nonsense, well I am happy to be educated on how to avoid the big spark, be it fitting a battery cable or turning on a battery master switch. The alternator bit doesn't apply to us because we don't charge LFP directly from the engine.

Pete

[sailorboy1]

Originally Posted by Fuss

Look in this forum, they are all saying it…

I don't know who "they" are, but reading it here (I never have) definitely doesn't make it true.

[rwidman]

Originally Posted by Pete7

As a nearly retired desk jockey my understanding is there are two types of inverter. Low frequency inverters using coils to change DC into AC. Victron and Sterling are examples. But they are big, heavy and expensive.

The alternative is high frequency inverters using MOSFETs? just as a BMS does. Its these that are at risk from the initial spike particularly by LFP with potentially a large inrush of current. As said earlier I noticed Will Prowse pre charging circuits before making the final connection and decided perhaps I should follow suit. For £6 I now have a pair of widgets which remove the big spark you sometimes see when connecting large DC circuits. What's not to like.

If you think this is nonsense, well I am happy to be educated on how to avoid the big spark, be it fitting a battery cable or turning on a battery master switch. The alternator bit doesn't apply to us because we don't charge LFP directly from the engine.

Pete

There won't be a bigger inrush of current from a lithium battery than from any other source of the same voltage and capacity. An inverter doesn't know or care where its power comes from.

This is why I was asking for a technical explanation from the person who was promoting this "pre-charge" idea.

[Pete7]

Originally Posted by rwidman

There won't be a bigger inrush of current from a lithium battery than from any other source of the same voltage and capacity. An inverter doesn't know or care where its power comes from.

This is why I was asking for a technical explanation from the person who was promoting this "pre-charge" idea.

So if I have this right, the difference in internal resistance between a LFP battery and say a flooded lead acid battery, not exactly known for supplying big thumbs of power, doesn't apply?

Why then the big DC spark on first connection of a battery cable and presumably the same inside a battery master switch? Do the other boat circuits absorb therefore cushioning the inrush of power.

At stake is a newish £800 PSW inverter, which I would really like to protect from any catastrophic event.

Pete

[GreenWave]

Seems the answer here is to buy a REAL inverter and not one of these mosfet cheapos

[Fuss]

Originally Posted by rwidman

There won't be a bigger inrush of current from a lithium battery than from any other source of the same voltage and capacity. An inverter doesn't know or care where its power comes from.

This is why I was asking for a technical explanation from the person who was promoting this "pre-charge" idea.

Here is a thread where I asked the same question,
https://www.cruisersforum.com/forums...ml#post3860824

[hjohnson]

Originally Posted by rwidman

As a retired electronic technician who understands theoretical and practical electricity and electronics, this makes no sense to me. That is why I asked for a technical explanation.

A device (an inverter, in this case) does not know or care where the power is coming from. Power from one type of battery is no different than power from another type of battery or even a power supply.

What does matter is the nature of the powered circuit, and the equivalent series resistance of the power source. Nothing is perfect in the real world, so different power sources can have significnatly different performance depending on how they're assembled.

The internal resistance of an LFP batter is an order of magnitude lower than an AGM, which is an order of magnitude lower than a traditional FLA. this means when you connect them to a dead short (which is what very large capacitors appear as when they're discharged) you get dramatically higher currents with LFP vs AGM vs FLA.