Precharge circuit to protect your Lifepo4 and inverter

[rwidman]

As a retired electronic technician, I call BS on this "issue".

An inverter (or any other device) does not know or care where or how it gets its power. Lithium, flooded cell, AGM or even a bench power supply.

Voltage is voltage. Period.

[hjohnson]

Originally Posted by rwidman

As a retired electronic technician, I call BS on this "issue".

An inverter (or any other device) does not know or care where or how it gets its power. Lithium, flooded cell, AGM or even a bench power supply.

Voltage is voltage. Period.

But what you’re neglecting is the ESR (Equivalent Series Resistance) of the power source. The ESR of LFP is vastly lower than the ESR of Lead Acid.

Either way, I just bought the precharge/delay circuit for my BMS (REC) and at $40, it was cheap insurance. Flip the BMS on, it links in its internal resistor, then a few seconds later closes the contactor. Easy peasy, cheap insurance.

That said, I can count on one hand the number of times I’ve actually shut off my electrical ssytem in the past 18 months.

[RaymondR]

Do capacitors care how fast you charge them up as long as one does not exceed their voltage rating? One assumes that the inverter is turned off when being connected to the battery.

[rslifkin]

I've always understood the concern to be around arc damage to the switch or breaker being used to turn the power on (due to the high inrush current) more than damage to the batteries or capacitor. Although in some cases concern for a BMS cut-out due to the high current is also an issue.

[hjohnson]

Originally Posted by RaymondR

Do capacitors care how fast you charge them up as long as one does not exceed their voltage rating? One assumes that the inverter is turned off when being connected to the battery.

To a certain extent, yes they do. When you first connect them, they’re effectively a dead short. The currents can (momentarily) be quite extreme. The smoothing capacitors are on the battery side of the switch typically (Victron and Xantrex are) so whether the inverter is on or off is irrelevant.

They’re basically there to deal with sudden inrush currents on the AC side of the inverter, and to smooth out noise from the inverter so it doesn’t make it back into your DC system.

[Pete7]

Originally Posted by hjohnson

To a certain extent, yes they do. When you first connect them, they’re effectively a dead short. The currents can (momentarily) be quite extreme. The smoothing capacitors are on the battery side of the switch typically (Victron and Xantrex are) so whether the inverter is on or off is irrelevant.

They’re basically there to deal with sudden inrush currents on the AC side of the inverter, and to smooth out noise from the inverter so it doesn’t make it back into your DC system.

Thanks, that's useful to know. so there is something useful achieved by pre-charging a circuit if it hasn't been used recently

We switch the main inverter off after cooking or boiling the kettle because there is always a slight current draw. However, don't bother pre-charging again later in the day when next using the invert. If we have been away for a few days, then give the inverter a quick pre-charge.

The two small cheap square sine wave inverters for the tv and laptops stays on whilst we are on board.

Pete

[Fuss]

Originally Posted by Pete7

Thanks, that's useful to know. so there is something useful achieved by pre-charging a circuit if it hasn't been used recently

We switch the main inverter off after cooking or boiling the kettle because there is always a slight current draw. However, don't bother pre-charging again later in the day when next using the invert. If we have been away for a few days, then give the inverter a quick pre-charge.

The two small cheap square sine wave inverters for the tv and laptops stays on whilst we are on board.

Pete

if the DC supply is not interrupted then there is no need to pre charge again , no matter how often the inverter is switched off
…. have we clearly established yet whether pre charge is protecting anything???

[hjohnson]

Originally Posted by Fuss

if the DC supply is not interrupted then there is no need to pre charge again , no matter how often the inverter is switched off
…. have we clearly established yet whether pre charge is protecting anything???

You're correct about not having to do it again. But what precharge is doing is preventing a significant inrush current that can happen with LFP batteries, much more so than with Lead Acid.

Whether this is useful or not is a question, but given how cheap precharge circuits are, seems like cheap insurance.

[RaymondR]

Originally Posted by hjohnson

To a certain extent, yes they do. When you first connect them, they’re effectively a dead short. The currents can (momentarily) be quite extreme. The smoothing capacitors are on the battery side of the switch typically (Victron and Xantrex are) so whether the inverter is on or off is irrelevant.

They’re basically there to deal with sudden inrush currents on the AC side of the inverter, and to smooth out noise from the inverter so it doesn’t make it back into your DC system.

I suspect that say six 100 amp hour lead acid house batteries connected in parallel do not have enough grunt to fuse out the leads on the capacitors there's not much chance of the lithiums doing so.

[wholybee]

Originally Posted by Fuss

Ok, this would work but anyone picking up a resistor and pre-charging is going to misplace it or forget at some stage.


I don't see any real evidence that this is a new problem that did not used to exist.

In my case, it is a button. The resistor and button are permanently installed. You push the button before turning the switch. With a label there, yes someone might do it wrong, but it is reasonable safe from that. Just do as the label on the switch says. Push Button First.

It is a new problem because of the low IR of lithium batteries, and the larger and larger inverters. With Lead-Acid, it is not a problem, and with inverters <= about 3000W, it isn't a problem. But with a 1000Ah lithium bank and 10kW inverter, yeah it's a problem.

I have the button there for extra security. With my 300Ah battery and 3000W inverter, if I skip the button there is a big spark, but no damage.

[wholybee]

Originally Posted by RaymondR

I suspect that say six 100 amp hour lead acid house batteries connected in parallel do not have enough grunt to fuse out the leads on the capacitors there's not much chance of the lithiums doing so.

Even 6 100Ah Lead Acid batteries have significantly higher IR than LFP, and thus less current. And also, the size of capacitor, and therefore the size of inverter, matters. A capacitor is a direct short for a very short period of time. Short enough that it won't matter unless the capacitor is very large. My educated guess is that for any inverter 3000W or less, the capacitors are small enough that not enough current will flow to damage anything. A 10kW inverter almost certainly would be a problem.

[hjohnson]

Originally Posted by wholybee

Even 6 100Ah Lead Acid batteries have significantly higher IR than LFP, and thus less current. And also, the size of capacitor, and therefore the size of inverter, matters. A capacitor is a direct short for a very short period of time. Short enough that it won't matter unless the capacitor is very large. My educated guess is that for any inverter 3000W or less, the capacitors are small enough that not enough current will flow to damage anything. A 10kW inverter almost certainly would be a problem.

Plus, it's largely moot imho. Yeah, it might be snakeoil, but, for example, the REC precharge circuit that will also control a Blue Sea bistable contactor, is only $100 USD. In the grand scheme of things, that's peanuts.

[wholybee]

Originally Posted by hjohnson

Plus, it's largely moot imho. Yeah, it might be snakeoil, but, for example, the REC precharge circuit that will also control a Blue Sea bistable contactor, is only $100 USD. In the grand scheme of things, that's peanuts.

$100 is a ridiculous amount of money for $5 in parts. That is assuming it's automatic. Only $1 of parts if you are willing to push a button.

I don't think it is "snake-oil" just not applicable for smaller systems. When I first encountered it, it was reading up on whole house solar systems, with an LFP bank of many kWh, and 10kW-15kW inverters. If you didn't pre charge, stuff blew up, contactors would weld closed, etc. Because then it wasn't just an instantaneous direct short, it was a direct short that lasted a few seconds. That was when I added the button and resistor to my system, even though my inverter is only 3kW.

Of course, once REC started offering their kit, many jump to the conclusion they need it.

[hjohnson]

Originally Posted by wholybee

$100 is a ridiculous amount of money for $5 in parts. That is assuming it's automatic. Only $1 of parts if you are willing to push a button.

I don't think it is "snake-oil" just not applicable for smaller systems. When I first encountered it, it was reading up on whole house solar systems, with an LFP bank of many kWh, and 10kW-15kW inverters. If you didn't pre charge, stuff blew up, contactors would weld closed, etc. Because then it wasn't just an instantaneous direct short, it was a direct short that lasted a few seconds. That was when I added the button and resistor to my system, even though my inverter is only 3kW.

Of course, once REC started offering their kit, many jump to the conclusion they need it.

By the time I've designed and buit a circuit that does the pre-charge, and controls a bi-stable contactor, I've spent more than $100 in my own time. I'd rather be sailing than dealing with this.

[Wotname]

Originally Posted by Bycrick

No hair, any color. I just don’t like sparks if I can help it. I’ve worked on 50k watt broadcast transmitters, big radar systems, 5volt, 1000amp computer power supplies. Sparks are only good at fireworks displays.

I kinda like the sparks!

I don't care so much for the high volts that generate some of the sparks .


Regarding capacitors, I find it hard to believe any cap can destroyed by any high current source. High voltage and reverse voltage - yes but not high current capable sources. Current does not flow through the capacitor, only through the leads which will be sized according to the power rating of the capacitor. The time constant will ensure the power developed in the leads will not be damaging.