Necessity of a Pre-Charge Resistor

[sunshine_eggo]

I do not know what "the fees in a bms" are.

FETS (MOSFETS)

 

[Supervstech]

I do not know what "the fees in a bms" are.

I thought the idea of a pre charge resister is to protect things from getting destroyed. If I want to prevent the fantom load from the inverter, I will have to disconnect it from the battery when I am not using it, or switch the inverter off. I can see that happen multiple times a day in a camper for example. Surely a manufacturer solution would be preferable and cheaper, regardless if it is in the BMS, the inverter or both.

Sorry, I didn’t notice the autocorrect… Fets. As in mosfet switches.

Turning off he inverter should effectively stop the standby draw. Disconnecting the batteries shouldn’t be needed, unless you just want to be thorough for travel.

 

[DerBuchner]

Big difference between turning inverter off and connecting the inverter to the battery.
A precharge limiter is needed when connecting the battery to inverter.

I am always worried what happens when the battery is disconnected unintentionally and/or automatically. For example, when there's undervoltage in the battery and the BMS shuts it down.

Question:
- is the battery then really disconnected - or is the inverter, physically, still connected?
- do the capacitors (I assume that's the most vulnerable part) of the inverter discharge of time, and when the BMS brings the battery online again blow up?

 

[eXodus]

I am always worried what happens when the battery is disconnected unintentionally and/or automatically. For example, when there's undervoltage in the battery and the BMS shuts it down.

Question:
- is the battery then really disconnected - or is the inverter, physically, still connected?
- do the capacitors (I assume that's the most vulnerable part) of the inverter discharge of time, and when the BMS brings the battery online again blow up?

Depends on the BMS design. Some are just shutting of for a brief period and then self reset after a few minutes.

Capacitors bleed of over a period which could be days.

the BMS is not a switch - there are MOSFETs on the circuit board which are the shutting off the current. They ramp up very quickly when activated - but are slower then connecting a cable.

 

[DerBuchner]

Depends on the BMS design. Some are just shutting of for a brief period and then self reset after a few minutes.

Capacitors bleed of over a period which could be days.

the BMS is not a switch - there are MOSFETs on the circuit board which are the shutting off the current. They ramp up very quickly when activated - but are slower then connecting a cable.

I intend to use a JK B2A20S20P BMS.

> ... there are MOSFETs on the circuit board which are the shutting off the current.

So the battery and the inverter always stay connected. I assume this is a good thing here because it might prevent a huge surge of current, no?

 

[curiouscarbon]

So the battery and the inverter always stay connected. I assume this is a good thing here because it might prevent a huge surge of current, no?

The cells will be "physically" connected to the inverter via BMS MOSFETs at all times, unless a physical switch is used to disconnect.

Without a physical switch, the MOSFETS can disable discharging which will result in the inverter capacitors gradually draining down.

Most BMS, when put into Disable Discharge mode, should be able to reach a condition where the inverter capacitors become drained.

The mosfet are odd in a way, the BMS can independently disable Charging or disable Discharging. Disabling both should effectively disconnect the inverter.

 

[rhino]

Just for the record since people are questioning need for pre-charge, I have personally blown up a 110A JLLN T Fuse when I connected a 48V LiFePO4 battery bank to a 5kVA 48V Quattro. Yes, the 110A T fuse is undersized but there was no issue with this same fuse when I was using 48V lead acid battery.

 

[OffGridInTheCity]

I have dual AIMS 12,000w inverters and have not used pre-charge. When I got my first one (4 yrs ago), doing an Initial test connection produced a modest spark when putting the lug in place for bolt-on but it wasn't major. For regular operation after that, I have 250a Midnite breakers and just turn them on/off as needed for maintenance with no problems observed. My overall battery bank has a 400a shunt-trip breaker.

 

[DerBuchner]

The mosfet are odd in a way, the BMS can independently disable Charging or disable Discharging. Disabling both should effectively disconnect the inverter.

I have no deeper understanding of MOSFETs. Wouldn`t it be possible to implement a "connect to inverter" mode in the BMS where it gradually ramps up and allows the MOSFETs to transfer more current? Or do MOSFETs have no chance to "defend" themselves against high currents?

 

[rhino]

I have dual AIMS 12,000w inverters and have not used pre-charge. When I got my first one (4 yrs ago), doing an Initial test connection produced a modest 'spark' but I wouldn't call it a major spark. For regular operation I wired in 250a Midnite breakers and just turn them on/off as needed for maintenance with no problems observed.

I'm also considering doing this instead.. I delved into the Midnite breakers in this post: https://diysolarforum.com/threads/class-t-fuse-or-dc-circuit-breaker.44249/post-575402

 

[curiouscarbon]

I have no deeper understanding of MOSFETs. Wouldn`t it be possible to implement a "connect to inverter" mode in the BMS where it gradually ramps up and allows the MOSFETs to transfer more current? Or do MOSFETs have no chance to "defend" themselves against high currents?

In short, yes that is possible, but very few BMS seem to implement it.

MOSFET behave like resistors that can change their own resistance. One moment it is super high resistance (off/disconnected) and next moment it is very low resistance (on/connected). Precharge needs an inbetween resistance.

By giving the MOSFET a voltage inbetween on and off, there is a range where it can act as a pre charge function.

The major downside of this operation mode is that it would generate additional heat in the MOSFETs due to letting current through with higher resistance. But that can be dealt with by metal heat spreader or heat sink etc.

Basically a MOSFET makes much more heat when half on as compare to completely on.

Battleborn apparently sells a dedicated device for this which uses FETs https://battlebornbatteries.com/product/current-surge-limiter/

The CSL500 Current Surge Limiter is a FET-based current limiting device [...] in conjunction with large (greater than 3.5kW) inverter/chargers.

The device is mounted permanently in series between the negative pole of the battery bank and the negative DC input terminal of the inverter/charger. The device protects the battery management system (BMS) from damage caused by the initial current spike that is created when connecting directly to the large capacitors (> 5 milliFarads) that are typically on the DC input side of the inverter/chargers. The device allows the batteries to slowly charge the capacitors (within 1 ms). This soft start avoids the high-current shutoff inherent to the BMS of the batteries.

 

[SolaroSsaurius]

for the 3000 W inverter, i shall delay the power up until automated pre charge circuit declares bus has been pre charged.

Thanks for all your info.
Having just bought a MultiPlus-II 24/5000/120-50, and seeing people on the web using such procedure, I'm right now trying to also understand why, and how.
How do you know your Multiplus is charged? Voltage comparation? How one do it manually?

 

[Hedges]

Surely a manufacturer solution would be preferable and cheaper, regardless if it is in the BMS, the inverter or both.

American consumers, by and large, buy according to price, and price alone.
If a company includes precharge circuit resulting in price 5% higher than the competition, they go out of business.

Couple years back there were many SMA Sunny Island being liquidated for about $1700 (MSRP is $5800). A number of people looked at that and said, "But for $400 less I can buy an MPP or GroWatt".

"The bad drives out the good".

I know that according to your world view, companies should just pay the extra $2 ~ $3 and include the extra feature.
Make you a deal - you can mandate that just as soon as you mandate consumers paying the increased retail price (with its percentage markup), which will also help cover using green energy, paying prevailing (i.e. union) wages, etc. etc.

In the case of this precharge circuit, need varies considerably between various batteries and inverters. If a company provides an all-in-one solution, would expect them to make it play together. But another goal in both consumer protection and marketing is open systems, so mix-n-match has varied requirements. For a price, you can purchase a configurable precharge device, e.g. from REC.

Multiple hardware configurations also cost money, and you may not find the model you need on the shelf.
One solution is to drive down the cost, include it in all units, but sell an unlocking key for a price. Same as enabling software features, enabling signal generator in an oscilloscope, heated seats in a car.

But let's make sure we have "right to repair" and DIY is not made illegal.
Ultra-liberal California still has not banned DIY manufacturing of "ghost guns", but they do require affixing a serial number now.
However, DIY performance enhancements to cars (e.g. turbo) are illegal. Can only do that by purchasing a CARB licensed product.

 

[Sustainable Bloke]

were can you actaully buy these? where did this image come from? google lens gets it wrong
View attachment 116035

Anyone actually found one of these yet? cant find anywhere online