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

[CaptainRivet]

Originally Posted by hjohnson

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.

Exactly and that is basically what you doing when you use a resistor or like FUSS does with a 1,3AH AGM, you raise the internal resistance from a near dead short of an LFP to several ohm to limit the inrush current into the large capacitor every inverter has (LF and HF) also to just simply filter the power. If the inverter is connected the capacitors are normally kept charged but if you eg cut power with the disconnect switch then not.
If you use a 2way switch and connect on 1 a precharge resistor glued into the switch and 2 is then connect you won't forget to pre-charge or loose the resistor. Thats common practise...
Some inverter have a build in soft start (like my chinese Edecoa boost inverter) which first switch on and charge the capacitors and then connect the load.
Instead a 1.3AH AGM you coukd also use a 10AH AGM and use it as backup for the Navstation if BMS cuts off all power due to whatever reason.

[tanglewood]

And it was answered here https://www.cruisersforum.com/forums...ml#post3861310


This is NOT about damaging the inveter. It's about arcing and shortening the life of whatever switch or contactor is being used. And it's possibly about smoking FET BMSes with fast current surges, but I don't know about that personally.

[CarinaPDX]

Strange stuff. It does make sense that contacts could arc and be damaged; damage from charging the capacitors seems very dubious.

I question the wisdom of using such a small battery to protect the alternator. My alternator can easily put out 100A and some larger boats output 200A or more. If (in a failure mode) the BMS cut off charging to the lithium batteries the little AGM could be hit with an initial 100A (on my little boat), and even though it would instantly drop to a lower charge level I am pretty certain that there would be a voltage spike that could damage any electronics currently connected and operating. It seems so much wiser to just put in an appropriate AGM starter battery, permanently connected to the alternator and starter motor, then charge the lithium batteries from that point.

N.B.: I haven't installed lithiums yet, although given the likelihood that my carbon foam batteries are soon to die I am starting to study the issue. It seems to me that we are still early in the game, where there are many solutions of varying "goodness".

Greg

[CaptainRivet]

As i said use a 10AH AGM instead a 1.3AH one and that can withstand the short 100 or even 200A with no spikes whatsoever.for sure
That 10AH is nothing elae then a buffer battery in case the lithium shuts off so the nav electronics run even when LFP is cut off. Know many sailboats that have that setup, incl. my old one too. Never an issue and it serves as spike protection too.
On my new cat i just done it right and cut the field wire to shut off the alternator and have an LTO buffer battery (that can also emergency start the engine) is always there as a load too. Done right the first time and piece for the next 20years with 0 maintenance.

[stgermain45]

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.

When I first looked into this and built our pre-charge circuit, I found this whitepaper from Sensata/Gigavac (contactor manufacturer) helpful and easy to follow:

https://www.sensata.com/sites/default/files/a/sensata-how-to-design-precharge-circuits-evs-whitepaper.pdf

Obviously we aren't talking EVs but the theory is still relevant.

[Pete7]

Originally Posted by stgermain45

When I first looked into this and built our pre-charge circuit, I found this whitepaper from Sensata/Gigavac (contactor manufacturer) helpful and easy to follow:

https://www.sensata.com/sites/defaul...whitepaper.pdf

Obviously we aren't talking EVs but the theory is still relevant.

Thanks, that's useful to read.

Pete

[rwidman]

Originally Posted by hjohnson

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.

We are not talking about discharging a battery into a dead short, we are talking about powering an inverter. The inverter is designed to be connected to a 12 volt (nominal) power source.

Please post a schematic of this inverter that will be damaged by being connected to a lithium battery bank and we can discuss what happens. Or a warning from an inverter manufacturer to not connect it to a lithium battery.