Pre-charge resistor seems crazy

[Steve_S]

All EV's use precharge circuits within their systems and they have abused & bashed the heck out of Relays & Contactors and know what NOT to do.
Non-Precharge in the solar world of gear wasn't a huge issue with Big Lead as it can take that kind of hit but switches & relays if not handled properly got welded together... albeit far less often. With any Lithium Base what Li can deliver in a burst is HUGE comparatively and that does weld things up very quickly. Remember that LFP for example can dump @ 5C for 5 Seconds crunch the #'s and if you have a Big Bank - it is Holy Crap That's a LOT of Amps ! Dump a 30kWh Bank @ 5C and watch lightning !

Some companies have finally gotten it and are adding precharge to the Inverter/Charger - AIO devices.
Sadly documenting such Need or Requirement is an obvious afterthought for many, and do not expect low ball value products to mention such unless pushed to do so... Tier-1 companies are now addressing such, starting to anyways.

 

[Samsonite801]

All EV's use precharge circuits within their systems and they have abused & bashed the heck out of Relays & Contactors and know what NOT to do.
Non-Precharge in the solar world of gear wasn't a huge issue with Big Lead as it can take that kind of hit but switches & relays if not handled properly got welded together... albeit far less often. With any Lithium Base what Li can deliver in a burst is HUGE comparatively and that does weld things up very quickly. Remember that LFP for example can dump @ 5C for 5 Seconds crunch the #'s and if you have a Big Bank - it is Holy Crap That's a LOT of Amps ! Dump a 30kWh Bank @ 5C and watch lightning !

Some companies have finally gotten it and are adding precharge to the Inverter/Charger - AIO devices.
Sadly documenting such Need or Requirement is an obvious afterthought for many, and do not expect low ball value products to mention such unless pushed to do so... Tier-1 companies are now addressing such, starting to anyways.

Maybe it will be one of those things where in time it will just become standard and all BMSs will just natively have the precharge circuit built in as a standard feature...

 

[rhino]

I just checked the Batrium site and the "Expansion Output Functions" has a setting for this but not sure if this is using a resistor or what: s

Critical Precharge Timer

Circuit is 2 seconds - pulse relay for 2 seconds to pre-charge and it does not spark

 

[Zwy]

I just checked the Batrium site and the "Expansion Output Functions" has a setting for this but not sure if this is using a resistor or what: s

Critical Precharge Timer

Circuit is 2 seconds - pulse relay for 2 seconds to pre-charge and it does not spark

If the amperage isn't high enough, an external relay or contactor could be used triggered by the Precharge Timer

 

[rhino]

This message from @toms does state that you can use the REC pre-charge unit with other BMSs... just not clear what how to wire a Batrium at least with the REC pre-charge unit.

 

[Zwy]

This message from @toms does state that you can use the REC pre-charge unit with other BMSs... just not clear what how to wire a Batrium at least with the REC pre-charge unit.

Look at the manual here. https://www.rec-bms.com/wp-content/uploads/2021/09/UserManualPrechargeNew_Timeset.pdf

 

[HOU_Castle]

The precharge is to stop arcing on the contactor, nothing to do with the inverter.
(and yes, the contactor will have documented inrush current limits)

So, do you all recommend to do pre-charge on all contacts, which may spark?

 

[curiouscarbon]

So, do you all recommend to do pre-charge on all contacts, which may spark?

some threshold of circuit capacitance is what i’m using personally

i don’t know what Farad amount is the threshold for me.

the spark is due to capacitance

with my 300W giandel inverter, i don’t care and just attach the battery, there’s a small spark and i accept that it puts some amount of stress on the components.

with the 1500W giandel inverter, i am working towards a custom arduino precharge circuit. haven’t found a cheap pre charge circuit to my tastes yet so DIY is my choice

 

[Steve_S]

Bigger the Inverter the Bigger the Capacitors, the Bigger the spark. Remember that HF Inverters are only 2X Surge capable while LF Inverters are 3X Surge capable and the Capacitors used within Reflect that !
HF = High Frequency. LF = Low Frequency
If you have a 3000W HF the caps are for 6000W handling while if LF it would be 9000W, this equates to a hell of a lot pull for a second or two, bigger inverter longer it takes to fill caps. Only in seconds but significant.

 

[chrisski]

I am wondering if this pre-charge resistor should only be placed in line with the equipment its is pre-charging like the inverter. So is it a bad idea if the last connection is the battery to place the pre-charge resistor there with it branching off to the SCCs, 12 volt system, and Inverter?

 

[Hedges]

No need to precharge anything without a capacitor.
If a load (e.g. incandescent light) pulled current through resistor, inverter's capacitor would never get fully precharged and resistor might burn up.
So best to only precharge inverter (and maybe SCC if it has big caps). Of course need to delay inverter turn-on, time will differ between units.

 

[HOU_Castle]

No need to precharge anything without a capacitor.
If a load (e.g. incandescent light) pulled current through resistor, inverter's capacitor would never get fully precharged and resistor might burn up.
So best to only precharge inverter (and maybe SCC if it has big caps). Of course need to delay inverter turn-on, time will differ between units.

Is Victron 250/60 mppt considered to have big caps?

 

[Hedges]

Is Victron 250/60 mppt considered to have big caps?

I don't know, probably not terribly big. I would expect SCC to be maybe 1/10th the capacitance of an inverter.
Big enough caps blow fuses. Even much smaller I would think erode contacts.
You may be able to measure it. Sunny Island capacitors are more than my meter reads.
When I reconnected Sunny Island Charger after some experiments, there was a spark. Previously that would have been within a breaker.

If you connect it with resistor and record voltage a few times over seconds, should get a curve with exponential decay. T = RC is the time to reach 1/e of final voltage, about 2/3 of the way.

 

[chrisski]

No need to precharge anything without a capacitor.
If a load (e.g. incandescent light) pulled current through resistor, inverter's capacitor would never get fully precharged and resistor might burn up.
So best to only precharge inverter (and maybe SCC if it has big caps). Of course need to delay inverter turn-on, time will differ between units.

Thanks. I had not considered the other things would pull current away causing the inverter to take longer to charge.

The resistor I used a 30 ohm 20 watt resistor barely got warm.

Is Victron 250/60 mppt considered to have big caps?

I don’t have that Victron, but I have a 100/50 and two 100/30 on this system and have never seen them spark.

My Samlex inverter went into mentioned about using a resistor to pre-charge. For Victron also being a quality company, I would hope they put that in their manual if needed.

 

[corn18]

I am wondering if this pre-charge resistor should only be placed in line with the equipment its is pre-charging like the inverter. So is it a bad idea if the last connection is the battery to place the pre-charge resistor there with it branching off to the SCCs, 12 volt system, and Inverter?

My precharge applies voltage to everything. I have a light hooked to the precharge circuit. 1 sec is all it takes to precharge the inverter, so I'm not sure if that adversely effects anything else that is getting reduced voltage.

 

[12VoltInstalls]

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.

Not really identifying as extra savvy cuz I’m not but the precharge isn’t much current with a resistor AND once the capacitor is charged… well you ain’t losing anything nor is there a trickle; it’s just there, no load, I’m sure. Even if there was it won’t add to the inverter’s standby consumption.

I think fusing it and leaving in place isn’t a bad idea!

 

[curiouscarbon]

How is inverter bus capacitance chosen?

Does anyone know how the bus capacitance of an inverter is chosen? I have been told that a 6kW inverter should have 0.1F from one source, and 0.028F from another source.

diysolarforum.com

additional detailed discussion of capacitance in this thread for the curious

 

[John Schmidt]

I always wanted to understand how the pre-charge resistor that is integrated into the EG4 series of batteries works. When Will Prowse pointed out the resistor, it sure looks larger than a "typical" pre-charge resistor. Since the resistor would need to be connected to the positive terminal, but internal, I wondered how the internal battery was bypassed during the pre-charge time when current was only going through the resistor. I figured that some type of SCR must be at play, and I found a couple of circuits that made sense, but seemed a bit complex. I suspect that the device/resistor that is used in the EG4 is similar as nicely described at the link below. Maybe I'll open my second battery up and take a close look. Has anyone already looked, and found the part number?
Battery Junction Box

More info:
DC SCR Precharge Module

 

[curiouscarbon]

I always wanted to understand how the pre-charge resistor that is integrated into the EG4 series of batteries works. When Will Prowse pointed out the resistor, it sure looks larger than a "typical" pre-charge resistor. Since the resistor would need to be connected to the positive terminal, but internal, I wondered how the internal battery was bypassed during the pre-charge time when current was only going through the resistor. I figured that some type of SCR must be at play, and I found a couple of circuits that made sense, but seemed a bit complex. I suspect that the device/resistor that is used in the EG4 is similar as nicely described at the link below. Maybe I'll open my second battery up and take a close look. Has anyone already looked, and found the part number?
Battery Junction Box

More info:
DC SCR Precharge Module

thanks for bringing up EG4 precharge in this thread.

curious about what you ask about, also

already sourced cells so DIY precharge is my path, studying each implementation teaches another lesson

 

[jsmetalcraft]

This is from a Seplos BMS. Not sure if this pertains to the precharge talked about here. Can anyone expand?

2.16 Pre-charge
The pre-charge function will be activated at the moment when BMS or
discharge MOSFET powered on. The pre-charge time range is 1mS -
5000mS. This function will effectively protect BMS from short circuit.
And it is specially designed for the application of capacitive load.