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Pre-charge resistor seems crazy

And then what happens when your BMS decides to turn off on safety protection? You have to add that ritual into your routine every time you reset it?
This is a very good (shall I say obvious) Point that no one raised. Manual Button needs a body to push it. that's OK BUT what if the Body is away.

Now you cross another area of Foibles as well.
* Relay/Contactor BMS' obviously can be handled by using the relays and even a Timer Relay that can bypass and trickle that precharge in. There are a few methods posted in the forum and on YT that can work. This is another advantage of having a Relay Based system, gives a few options to "be creative". (Ohhh boy Deja-Vu 2019). This is partly why Chargery developed the DCC Solid State Contactor with the precharge circuitry built-in.

* FET Based BMS' on the other hand, cannot be accessed in the sense of taping into its circuitry, and who would take a Soldering Iron to their new BMS ? So we are left with a Manual Switch or a Commercial/Retail auto-switch (crazy cost) and not a simple solution to just go buy a "quick kit". And just to further complicate things, if you have Multiple Packs within a Battery Bank now you have to deal with it at the BUS Level as opposed to the Battery Packs individually.

A Question for the Extra Savvy, some on YT just leave the resistor in-place all the time so there is always a constant "trickle" if you will, I suppose that may be OK if it's fused but I dunno how I feel about that myself.

I am now in process of my final redesign / build which also involves replacing my entire fleet of BMS', including my Utility Packs to the JK-BMS with Active Balancer (no relays/contactors) and of course no Pre-Charge circuitry. I have reached out to https://www.jkbms.com/contact/ asking about how they suggest handling the PreCharge requirement and asking as to why this "essential" feature is not Built-in already.
 
This is a very good (shall I say obvious) Point that no one raised. Manual Button needs a body to push it. that's OK BUT what if the Body is away.

The reason it remains manual is due to the BMS shutting down the system due to a fault. Unless one is willing to develop and program an automated system with AI, then it still requires the human input of troubleshooting.

In the case of a low voltage disconnect by the BMS, that means to me the system was not designed properly or not setup properly.

A Question for the Extra Savvy, some on YT just leave the resistor in-place all the time so there is always a constant "trickle" if you will, I suppose that may be OK if it's fused but I dunno how I feel about that myself.

That would be a waste of time actually as the fuse will blow if the system ever has a load when BMS prevents discharge. Result would be no different than the BMS shutting down.
 
We did have another thread on this forum not long ago, where a guy was going to build a circuit using an Arduino to tackle the task of providing a precharge resistor connection, prior to bringing up the main relay connection between BMS/battery to inverter (using the Arduino automation).
 

In case you're curious (carbon), I just went back and found the thread I had been thinking of (my memory was very weak and was fading fast)...

The thread had originally been started to address using an Arduino with a relay to build a low-temp disconnect circuit. At some point the discussion came up about the 'dreaded' spark when reconnecting back up, so had talked about how it wouldn't be hard to add logic and another relay in for that if you wanted too. Never got into specifics on how exactly it would be done or anything in detail. All in all it's just a... nother brick in the wall...



EDIT: Oh you're already following that thread (and posting), just figured that out again now duh hehe. Nevermind, I guess if anyone else would like to check it out they can with the above link :geek:
 
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haha.. your mentioning it spurred me work on it this afternoon, soldered some parts.. too much time on backburner :)
parts: 1) Arduino compatible microcontroller (µC) adafruit qtpy samd21 2) 5V up/down regulator for microcontroller 3) adafruit power relay Normally Open (NO) type 4) INA219 voltage meter 5) 72V 3A resettable fuse on both leads to/from resistors 6) 30Ω(ohm) 100W resistor 3x in series for 90Ω(ohm) 7) OLED for display 8) single computer keyboard switch 2x for On / Off request.

basically, the INA219 would observe the voltage @ Battery Pack Positive Terminal and also the voltage @ Inverter Positive Terminal.
when the "On request" button is pressed, if safety check passes, then Activate the Relay which Connects The Resistor
voltage difference between Pack Positive and Inverter Positive will indicate when PreCharge Complete
e.g. PreCharge Complete -> Connect Main Contactor -> Disconnect PreCharge Resistor

Some May want to Disconnect the PreCharge Resistor before the Main Contactor Activates. Arduino makes this really simple moving one line of code ?
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BTW the INA219 is appropriate for monitoring 4S LiFePO4 only, due to +26V max input. no higher than 7S LFP max (7*3.65=25.55<26)
 
haha.. your mentioning it spurred me work on it this afternoon, soldered some parts.. too much time on backburner :)
parts: 1) Arduino compatible microcontroller (µC) adafruit qtpy samd21 2) 5V up/down regulator for microcontroller 3) adafruit power relay Normally Open (NO) type 4) INA219 voltage meter 5) 72V 3A resettable fuse on both leads to/from resistors 6) 30Ω(ohm) 100W resistor 3x in series for 90Ω(ohm) 7) OLED for display 8) single computer keyboard switch 2x for On / Off request.

basically, the INA219 would observe the voltage @ Battery Pack Positive Terminal and also the voltage @ Inverter Positive Terminal.
when the "On request" button is pressed, if safety check passes, then Activate the Relay which Connects The Resistor
voltage difference between Pack Positive and Inverter Positive will indicate when PreCharge Complete
e.g. PreCharge Complete -> Connect Main Contactor -> Disconnect PreCharge Resistor

Some May want to Disconnect the PreCharge Resistor before the Main Contactor Activates. Arduino makes this really simple moving one line of code ?


That's pretty sweet, very creative!
 
haha oops. i totally messed up the resistance calculation. should have eaten lol. was aiming for like 20-50W precharge power, but with this resistor arrangement it’s like 2.5W ?

calculated the power in watts but interpreted it as amps, so that 3x 30ohm resistor thing will only pass like 0.1 Ampere at 12V lol. oops!! anyways passing 2-3W of power through 300W worth of resistors they don’t heat up much

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edit: added photo
 
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thanx.
That works but I want to minimize the need to unbolt cables. I have an Anderson quick disconnect. I guess I could stick a pencil in there and use a wire on the other side before I push it back together..

This is what I use for my Anderson connectors. Just a piece of scrap wood, a resistor, and a properly spaced wire.
 

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Also of some interest is the Gigavac MX14 is rated for 100,000 switch events at 24V and 400A. So is this an issue for that relay even if you don't pre-charge?
 
Rated for 400A, but what about 4000A, up to 10kA or 20kA?
That's where REC's pre-charge unit could help.

I would consider a pre-charge relay, followed by time-delay (or voltage measurement) controlling power relay.

The resistor always connected is an interesting idea. Would want a manual disconnect switch to let caps discharge before working on system. If BMS opens contactor, inverter still powered by resistor and might cycle on each time caps get up to voltage, then off again.
 
Just wondering if the REC pre-charge unit will work with non-REC BMS like a Batrium. There is no specs on what the BMS+ connector is expecting on the pre-charge unit.
 
For the REC precharge, you also need to buy a $177 contactor for it to work.

I had to have a precharge when I had a single SOK 12V 206Ah battery going to a single 1200W inverter. The SOK BMS would shut down if I just flipped the CB on. When I added another battery, I no longer needed the precharge to turn on the batteries, but I kept it (just a momentary switch hooked to a 25W 6 ohm resistor). Now I have the 2 SOK batteries and a Victron 12/3000 and I don't precharge anymore, even though I still have the circuit installed. I probably should, but I haven't found a reason to. Lots of grousing, but no one has made me think it is required for my system. I may remove the circuit. Or may not. AHHHHHH!!!!

DC wiring 290RL install labeles.jpg
 
For my boat build I purchased the rec bms precharge contactor controller. Simple to setup and wire. Popped it open and it's coated inside to avoid corrosion as well as having some vibration reduction silicone on some components.
 
I am now in process of my final redesign / build which also involves replacing my entire fleet of BMS', including my Utility Packs to the JK-BMS with Active Balancer (no relays/contactors) and of course no Pre-Charge circuitry. I have reached out to https://www.jkbms.com/contact/ asking about how they suggest handling the PreCharge requirement and asking as to why this "essential" feature is not Built-in already.
Are you no longer going to use the chargery?
 
Are you no longer going to use the chargery?
The fleet of Chargery's, the JBD & other JK BMS' are all going, so that I only have One Type that covers it all. I'm going with the JKBMS with 2A Active Balancer, BT, CANbus & RS485 right across the board for all packs (Production & Utility). That eliminates the "assorted" BMS' the QNBBM-8S & Heltec Capacitive Balancers as well. SADLY, the Folks at JKBMS Tech Support are Non-Communicative with clients but they Communicate through Vendor and that muddles up stuff...
 
Doesn't it seem like if the step is really necessary to precharge an inverter, the manufacturer would put that warning/step into their manual for that model?

And then what happens when your BMS decides to turn off on safety protection? You have to add that ritual into your routine every time you reset it?

I don't ever precharge on my own inverters anymore (unless if I were to see it in the manual, which I haven't yet). I used to always do it when I worked with car audio amp installs a long time ago, but at some point I just stopped worrying about it.

I might do it if I am using someone else's equipment, like a borrowed unit or something (except I don't really borrow stuff anymore), but on my own stuff I don't really care anymore, and so far it hasn't bitten me yet.

I do remember doing it on an install I did for a friend while back with his 12v AIMS inverter (more like I didn't have time to read the whole manual, and I had a 12v testlight right there to use as a resistor already so I just did real quick, and since it wasn't my equipment)...

Honestly, I say just read the manual, see if it says they think you should precharge, if not, I follow their routine and wouldn't do it either. Or you can call your inverter manufacturer and ask them if it really matters to them. Seems like if they built it using quality components, then it should be able to resist a potential blowout caused by simply connecting the battery bank, or they would get a lot of come-backs on their warranties (since most people probably don't do it)...

DISCLAIMER: This is me on my stuff, maybe you should keep doing it if you're worried about blowing something up. :geek:
Thank you. Your response makes the most sense. I'm thinking people just want to avoid the "Big Spark" when they make that connection. (Can't blame them, It is scary) The YouTube experts use the resister the first time they hook up but I've never seen a dedicated charging circuit on the schematics they offer or even a warning to always use the resister every time the capacitors might have discharged.

My Growatt manual does not mention precharging.

if this was a real problem I'd expect you'd see a lots of Posts about it. I see nothing about fried inverters and precharging.

Bottom line... I will use the resister I have taped to my batteries whenever it is convenient but won't stress out about it.
 
Thank you. Your response makes the most sense. I'm thinking people just want to avoid the "Big Spark" when they make that connection. (Can't blame them, It is scary) The YouTube experts use the resister the first time they hook up but I've never seen a dedicated charging circuit on the schematics they offer or even a warning to always use the resister every time the capacitors might have discharged.

My Growatt manual does not mention precharging.

if this was a real problem I'd expect you'd see a lots of Posts about it. I see nothing about fried inverters and precharging.

Bottom line... I will use the resister I have taped to my batteries whenever it is convenient but won't stress out about it.

Yeah that is kind of like my notion exactly. If it's convenient and easy to do for you, go for it. Even now that we have some turn key solutions for precharging with automation even, it's looking more and more practical to just add something in to do it if you got the time. I consider it to be in that scope of industry 'best practice' as they call it, but I don't think it is really something to generally worry about too much.

If I was building an EV car or something I would probably be more likely to put in a precharge circuit as EVs will be getting cycled multiple times daily (a spark may hammer on the main relay contacts after awhile, some might say they can weld together). On a home setup it will be a lot more rare the battery ever even gets disconnected very often.

I don't worry about it on my portable inverters for my cars and my Prius APC UPS 5kW inverter as I have an Anderson connector on it and it's more than I want to worry about with it, I just turn the car on first (to engage the main factory lithium battery relay first), and plug in the Anderson connector to the UPS, turn on the inverter and been doing that quite a lot for a couple years now and it still works fine, same as it always did.

I figure if it is quality equipment it shouldn't be a problem, or the manufacturers would say to do it.
 
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