Precharge circuit to protect your Lifepo4 and inverter

[Fuss]

We never had a precharge circuit in the old days!!


Just a bit of spark, without the safety glasses, just bang it on there and the spark is gone.


But now...oh dear, oh dear ... ok, lets say that we agree with the safety glasses... we don't want too big an argument here.


But now back to the other points .


yes yes, the lifepo4 is more powerful than lead and will damage your inverter capacitors... Is there really any evidence of this?
How much damage does it do? and if so then why is victron for example not building precharge into the inverters.


Oh yea, they don't want the inverter lasting forever ....


Ok, so I think that is not so major then???


Now lets move on to the real reason why you need precharge... yes that's right, its due to your namby pamby little sensitive BMS, thats what its all about isn't it, your namby pamby BMS cant handle it.



And now we have to invent some complex circuit for a new problem we never had before.


Anyway, I've got a solution to this on my boat, just in case the inverter does have its life shortened by no precharge, but that's for another thread.



Ok, so your big powerful lifepo4 with its all singing and dancing BMS is not so tough is it, its weak and sensitive, you should go and check on it now, Right now, It's probably feeling unloved, perhaps you could move it into the house living room next to the TV....

[rslifkin]

For the last Victron inverter setup I powered up from cold and drained capacitors, I just turned the AC side on first, then closed the DC breaker. That seemed to do the trick for letting the inverter charge the capacitors via its own charger circuits instead of causing a big inrush from the batteries.

[Fuss]

Ok, very good, then there are solutions to precharge from the inverters side.

[rslifkin]

Originally Posted by Fuss

Ok, very good, then there are solutions to precharge from the inverters side.

I'd give the usual YMMV disclaimer for that trick. It seems to work on the original Multiplus models, but I can't guarantee it'll work on all inverters. Some may not output any DC power (and therefore not charge the capacitors) until they see battery voltage on the DC side.

[Pete7]

I use one of these 100w 4ohm resister occasionally if we have been away from the boat for more than a few days, or before connecting the batteries. Seems to do a good job of reducing the spark on re-connection. Just dab it on for a few seconds and job done.

No idea if the power and resistance is correct, picked this after seeing Will Prowse recommend them a while back. Inverter is a 2kW PSW but no pass though or charging option.

Pete

[Fuss]

Originally Posted by Pete7

I use one of these 100w 4ohm resister occasionally if we have been away from the boat for more than a few days, or before connecting the batteries. Seems to do a good job of reducing the spark on re-connection. Just dab it on for a few seconds and job done.

No idea if the power and resistance is correct, picked this after seeing Will Prowse recommend them a while back. Inverter is a 2kW PSW but no pass though or charging option.

Pete

Yes, but as I wrote ... we didn't all used to do this...its a new fashion


I reckon I could write that all forum members have to dye their hair green and 10% would.

[Bycrick]

Simple way. Hook up the positive wire first. Then take your 12 volt test light and clip the ground wire to the negative wire terminal. Put to probe tip through the terminal, touch it to the negative battery post, then slide the terminal down over the post. No sparks.

[Fuss]

Originally Posted by Bycrick

Simple way. Hook up the positive wire first. Then take your 12 volt test light and clip the ground wire to the negative wire terminal. Put to probe tip through the terminal, touch it to the negative battery post, then slide the terminal down over the post. No sparks.

So you’re one of the 10% members that would dye their hair green

[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.

[Fuss]

So why did we not need this in the past?

[Pete7]

Originally Posted by Fuss

So why did we not need this in the past?

We did, just no one really bothered and just got used to sparks when wiring up batteries. Equally people thought about starting the engine when the lights started to dim.

[wholybee]

It wasn't needed in the past because Lead Acid don't deliver the same current that LFP do. The instant you connect the cable (or turn on the disconnect switch) is effectively a direct short, just for an instant until the capacitors begin to charge. Directly shorting an LFP battery creates a lot more current than directly shorting a lead acid.

How long that instant is depends on the size of the inverter capacitors. And the damage it will cause depends on what else is in the circuit. Mostly, I think if an inverter is <3000W then you get a big spark, but otherwise it is fine. If you have a FET BMS, you probably should be more careful, as that current is going through the FETs, but is a short enough duration it probably still isn't an issue.

I used the same resistor as above (mine might be 25ohms) , connected with a pushbutton across my main power switch. I just push the button a few seconds before I turn the switch. Super simple.

[Bycrick]

With all the electronics that have a "soft" power off, I’d like to keep voltage transients to a minimum. I don’t know how "necessary" it is to avoid sparks when connecting an inverter, but the big battery shop here is VERY careful to connect a temporary battery to the car before they disconnect the battery to be replaced.

[Fuss]

Originally Posted by wholybee

It wasn't needed in the past because Lead Acid don't deliver the same current that LFP do. The instant you connect the cable (or turn on the disconnect switch) is effectively a direct short, just for an instant until the capacitors begin to charge. Directly shorting an LFP battery creates a lot more current than directly shorting a lead acid.

How long that instant is depends on the size of the inverter capacitors. And the damage it will cause depends on what else is in the circuit. Mostly, I think if an inverter is <3000W then you get a big spark, but otherwise it is fine. If you have a FET BMS, you probably should be more careful, as that current is going through the FETs, but is a short enough duration it probably still isn't an issue.

I used the same resistor as above (mine might be 25ohms) , connected with a pushbutton across my main power switch. I just push the button a few seconds before I turn the switch. Super simple.

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.

[tanglewood]

Damaging to the capacitors in the inverter? First I have heard of that.


I agree it's about not smoking FETs, or creating too much arcing on a contactor. The smaller the voltage delta when you close/open a contactor, the longer it's life span. I can't speak to FETs - not my expertise.