Do most builders/users not use inverter precharge?

[Skypower]

I would not have known about this issue unless I had stumbled across Will’s video and further reinforced it landing on the related threads on this forum. But I probably would have figured it out by myself if connected something and seen the pop, blown a fuse, faulted the bms and/or inverter or wrecked something. 12 volts inverters had a little spark or snap but I know the big system 48 volt jobs are real serious.

 

[stienman]

It is the inverter's problem, and they should be the ones to handle it.

The inverter is rated for a certain maximum amperage it will draw during operation. All they need to do is state their surge current rating so the user can design their system around that OR have their own soft start (ie, timed pre-charge) so they never go above a certain current draw.

If they want to keep a capacitor bank tied to the battery input without protection, they should at least provide an accurate peak current rating and the duration.

It won't happen until/unless customers demand it, and right now inverter companies are more than happy to let battery manufacturers deal with it, since the fault that the inverter caused appears on the battery BMS.

But it's not the right place for the solution. So ask your inverter manufacturers for their surge current, and keep asking until they document it.

 

[HarryN]

At some point, you are going to to need to do maintenance / changes on your system.

If you include the ability to individually breaker protect each parallel battery string, then this allows you easily / safely do changes, as well as re-set the BMS if it / when it trips.

So just as an example arrangement, suppose that you have 4 battle born 100s arranged 2 S 2 P in 24 volt.

Put one of these 80 amp, blue sea 187 breakers in series with each parallel string.

187-Series Circuit Breaker - Surface Mount 80A - Blue Sea Systems

Thermal circuit breakers provide heavy duty circuit protection for 25 to 200 Amp loads when switching and circuit protection are both required.

www.bluesea.com

Those batteries are nominal rated for 100 amp discharge so running them conservatively at ~ 50 amps, and then protecting the string with an 80 amp breaker reduces the risk that some will have a BMS trip and the pack will be operating on just part of a pack for very long.

To re-set a BMS, just flip off the breaker and turn it back on - very simple - anyone can do it safely and remember how to do it, even at night in the middle of no where.

If you want to include a pre-charge circuit - just add another similar / lower amp breaker and a resistor off of one of the battery strings.

I do the exact same thing with AGMs - not to re-set a BMS obviously, but it make working on and starting up a system so convenient. If there are issues, it is easy for a customer to isolate a problem / run on 1/2 power / deal with it later vs ruining their vacation.

I do this all of the time.

It is a massive gain in convenience for a small cost of a few more parts.

 

[Hedges]

It is a system integration problem, and you're the system integrator.

Older inverters have specs and warnings regarding use with lead-acid and other banks. If lithium delivers 5x the surge current, it might damage capacitors.

One forum member just got SS to send him two replacement inverters as his both had failed. There may be a common cause, and if not addressed I'd expect the replacements to experience same fate. Inrush blowing capacitors is one possibility.

It may be everything which closes needs precharge, or careful sequencing. If I put LiFePO4 on my 4x SI, the BMS will have a contactor and each inverter has a breaker. REC offers a precharge solution, which takes care of BMS. Either I close all SI breakers first, or make a precharge system for each of them. Now that I realize I should be able to backfeed their 0.75A PTC battery fuses I may do precharge that way.

MOSFET BMS has an opportunity to do precharge, gradually enabling gate voltage, but only if inverter doesn't start applying load before precharge is complete.

 

[BobCollins]

"drop-in" batteries tend to be cheap and they lack an on/off switch.

And not-drop-in batteries are not appropriate for mobile use.

Here you go:

I don't find the requirement of a strict power up sequence an acceptable solution. This is a campervan, not a power station.

Yep. It's why inverters don't have a pre-charge feature. They were designed to be hooked to sources (lead acid batteries) that could deliver a very brief surge with higher internal resistance that also served to limit the surge.

Essentially, we have decades of products designed a specific way. Now we have a market flooded with power sources with a built-in limitation that can't meet the demand. Should the inverter manufacturers tailor their design to the limitations of cheap products, or should the new products meet the need of the existing industry?

"Drop-in" is misleading. Lead-acid can deliver ridiculous surge currents at the expense of voltage. LFP has a piece of circuitry that prevents the battery from meeting the need of pre-charging the capacitors of an inverter.

I see this as a battery problem, not an inverter problem.

Pointing to legacy design situation (lead acid batteries) is a reasonable reason why we are where we are today.

It is not necessarily the better design for LFP battery systems.

Good news. You get to be the Guinea pig. Pretty simple... install without a pre-charge circuit. Keep a #2 pencil with both ends of the "lead" exposed (about a 6Ω resistor) handy for a quick fix.

Let's say the 12V battery has a resistance of 2mΩ (pretty typical for a decent 12V 100Ah LFP):

12V / 0.002Ω = 6000A

Doubt your BMS is rated for 6000A, and it's probably going to see that as a short circuit and trigger protection.

Alternative: Victron MPPT can operate w/o battery, run directly off PV and will provide voltage/current to the battery terminals. THIS can actually pre-charge your capacitors. The MPPT is current limited and will only output what it can. If you have PV and sunshine, you don't need a precharge with a Victron MPPT... unless it's at night.

As I said initially, I can construct a appropriate precharge circuit. What I was curious about was why so little is written about users experiencing problems in mobile setups when they are clearly not using one.

 

[BobCollins]

At some point, you are going to to need to do maintenance / changes on your system.

If you include the ability to individually breaker protect each parallel battery string, then this allows you easily / safely do changes, as well as re-set the BMS if it / when it trips.

So just as an example arrangement, suppose that you have 4 battle born 100s arranged 2 S 2 P in 24 volt.

Put one of these 80 amp, blue sea 187 breakers in series with each parallel string.

187-Series Circuit Breaker - Surface Mount 80A - Blue Sea Systems

Thermal circuit breakers provide heavy duty circuit protection for 25 to 200 Amp loads when switching and circuit protection are both required.

www.bluesea.com

Those batteries are nominal rated for 100 amp discharge so running them conservatively at ~ 50 amps, and then protecting the string with an 80 amp breaker reduces the risk that some will have a BMS trip and the pack will be operating on just part of a pack for very long.

To re-set a BMS, just flip off the breaker and turn it back on - very simple - anyone can do it safely and remember how to do it, even at night in the middle of no where.

If you want to include a pre-charge circuit - just add another similar / lower amp breaker and a resistor off of one of the battery strings.

I do the exact same thing with AGMs - not to re-set a BMS obviously, but it make working on and starting up a system so convenient. If there are issues, it is easy for a customer to isolate a problem / run on 1/2 power / deal with it later vs ruining their vacation.

I do this all of the time.

It is a massive gain in convenience for a small cost of a few more parts.

Harry,
Are you saying here that you generally do not use a precharge circuit? You keep your discharge rate conservative for the battery's BMS and use a CB to catch any potential faults due to precharge?

 

[sunshine_eggo]

As I said initially, I can construct a appropriate precharge circuit. What I was curious about was why so little is written about users experiencing problems in mobile setups when they are clearly not using one.

Alternate sources are more common in mobile applications - PV, DC-DC and shore.

The presence of a MPPT can provide enough current to the inverter supply circuit (when the inverter is off) to eliminate the need for pre-charge when it's sunny. Mobile applications also frequently have DC-DC chargers, and that's another potential way to eliminate the need for pre-charge. Both of those solutions require the devices to provide current and voltage even if none is sensed due to the battery being off. An inverter/charger powered by shore power may not need pre-charge as it's charger can pre-charge the capacitors.

I think this thread has collectively consumed about 500 #2 pencil pre-charge events worth of time in man-hours.

It's a real issue. It's not made up. You've argued about it much longer than it would have taken you to fix it.

 

[OffGridInTheCity]

I have AIMS 12,000w and SGP 12,000w inverters. These each have their own 250a circuit breaker to the battery bank buss. I've never used pre-charge and haven't had any trouble. The battery bank BMS is Batrium and not setup to care about amps.

 

[timselectric]

Alternatively
2 - 200a BMS's can't start up 3 of my Growatt's without tripping. Haven't tried since my bank grew. But my final install will include a precharge circuit for each AIO. No point in worrying about it. It will never be an issue, again.

 

[sunshine_eggo]

I have AIMS 12,000w and SGP 12,000w inverters. These each have their own 250a circuit breaker to the battery bank buss. I've never used pre-charge and haven't had any trouble. The battery bank BMS is Batrium and not setup to care about amps.

I've deliberately avoided talking about the Batrium...

 

[OffGridInTheCity]

I've deliberately avoided talking about the Batrium...

Yea. There were some BMS 'is the issue' comments and I do have a BMS, but I don't use the current monitoring feature as it's not needed.
The battery bank can do 900a but is protected by a 400a breaker and my highest charge/discharge to date has been <300a.

 

[timselectric]

It is a combination issue.
Lithium based batteries can dump an enormous amount of current, quickly. Capacitors can absorb an enormous amount of current, quickly. BMS's can monitor and control current flow, quickly.

 

[HRTKD]

I asked on the Victron forum if my planed Phoenix 1200 W inverter needed a precharge, and I was assured that it did.

The answer comes down to how much battery you have. If you have a dinky battery, with a dinky BMS, the capacitors in the inverter could very well consume more amps than the battery/BMS can supply.

I built two 280 Ah, 12 volt batteries with 120 amp Overkill Solar BMS. If I had built a single battery, either 2p4s or just 4s, with one of the 120 amp BMS, that may not have been enough to bring up my Victron Multiplus 12/3000. But my system has no complaints when I turn on the main power switch.

Perhaps you're not seeing this issue in mobile environments because more batteries are used in that case. Boondockers may be installing more medium sized batteries (which work well for charging capacitors) than one big battery.

The higher the voltage, the bigger the potential spark. Keep in mind that some switches are rated to 48 volts, but not beyond that. A 48 volt system lives 99.99% of its life well above 48 volts.

 

[HarryN]

Harry,
Are you saying here that you generally do not use a precharge circuit? You keep your discharge rate conservative for the battery's BMS and use a CB to catch any potential faults due to precharge?

Hi Bob - you have been to my shop - and my typical customer is not a power engineer. Some are EEs but really none power engineers. Some are full time users, some occasional.

They all have a manual that they don't bring with them on their vacation - neither the printed nor e version.

But they do have my cell phone number to help them through whatever is going on, so I design in a "plan", "back up plan", and a "secondary back up plan" to minimize the number of times that they have to call me.

The other thing that they have in common is that they know how to fix a windows computer by re-starting it and that is what the breakers allow them to do either on their own or on the phone with me. It is all there and tolerant of small errors because that is what people do.

You have to decide on your own if you will be the only one running your power system, or if others will also be using it when you are not there and don't have your knowledge to overcome challenges that happen at inconvenient times.

Harry

 

[BobCollins]

Hi Bob - you have been to my shop - and my typical customer is not a power engineer. Some are EEs but really none power engineers. Some are full time users, some occasional.

They all have a manual that they don't bring with them on their vacation - neither the printed nor e version.

But they do have my cell phone number to help them through whatever is going on, so I design in a "plan", "back up plan", and a "secondary back up plan" to minimize the number of times that they have to call me.

The other thing that they have in common is that they know how to fix a windows computer by re-starting it and that is what the breakers allow them to do either on their own or on the phone with me. It is all there and tolerant of small errors because that is what people do.

You have to decide on your own if you will be the only one running your power system, or if others will also be using it when you are not there and don't have your knowledge to overcome challenges that happen at inconvenient times.

Harry

Exactly! Even though I understand the issues, I don't want a design that requires a fussy sequence to control.

What I was looking for, was to understand of what I could "get away with" for a simple design. The fact that so many builders and users survive while being ignorant of the precharge issue, leads me to believe that there must be circumstances where the lack of a precharge system must "just work."

Your mention of effectively derating the BMS might actually be what is unintentionally happening with many systems out there any why they keep working.

Your thoughts?

 

[HarryN]

Exactly! Even though I understand the issues, I don't want a design that requires a fussy sequence to control.

What I was looking for, was to understand of what I could "get away with" for a simple design. The fact that so many builders and users survive while being ignorant of the precharge issue, leads me to believe that there must be circumstances where the lack of a precharge system must "just work."

Your mention of effectively derating the BMS might actually be what is unintentionally happening with many systems out there any why they keep working.

Your thoughts?

It is not unusual for battery BMS to trip in van battery packs and people just don't even realize it until they have problems with over heated battery terminals. There is no easy way to detect it. There is an easy way to re-set them using the method that I outlined - if the breakers are in place. So a couple of breakers provide great peace of mind. Just flip them individually on / off every once in a while.

 

[Vigo]

I couldnt tell you why, but my combination of a small amount of experience and a large amount of filter-feeding the internet like a baleen whale, is that you seem to do some kind of aging or stress to the capacitors of the input section of the inverter if you repeatedly make no attempt to precharge. I think its not a noteworthy issue for most because the battery doesnt get disconnected all that often. But in my mind its a valid reason not to switch the battery circuit as a control scheme, without a precharge circuit. If you did that i think youd have blown capacitors in the inverter in fairly short order.

My question is, is the leakage current of solid state relays enough to obviate (with a little patience) the need for a separate precharge circuit? Because those things are pretty cheap and available in fairly large ampacities. I think it’s a possible solution for smaller inverters IF it works.

The way ive typically done precharge is just with a tiny power supply, like a wall wart charger kind of thing. 12v of those are common, 24v less likely to already be in your house but still readily available online, and 48v semi common and readily available. The POE for my wireless internet radio has a 48v wall wart for example. Im not saying thats the most practical approach but i already have those things kicking around and i always have multiple ac power sources available, so i just hook the output of a tiny power supply to the inverter in question, plug in the ac side, let it charge that circuit a bit, then make the battery connection. Been working for me but havent had the need in a good while either.

 

[sunsurfer]

If avoiding the arc is so important, why isn't there a commercial solution? I have explored the problem space, and a stand-alone design is quite feasible. My suspicion is that living with the arc is such a minor issue for most users that there is no market for a solution.

I use an automotive test light. Put the ground clamp on one end, then the probe on the other. The light will dim as the caps charge up. Make the connection while its still touching. I have not tested it on 48v (bulb could blow?) yet but it does work on my 12v and 24v stuff.

Works perfectly and the light is nice for feedback.

 

[Tomthumb62]

I use an automotive test light. Put the ground clamp on one end, then the probe on the other. The light will dim as the caps charge up. Make the connection while its still touching. I have not tested it on 48v (bulb could blow?) yet but it does work on my 12v and 24v stuff.

Works perfectly and the light is nice for feedback.

Thanks. This is what I’ll do next time as I seem to fudge it up using a regular 60W lightbulb.

Also, I’ve seen most videos doing the pre charge on the negative battery cable. Does it matter? I’ve been doing it on the positive cable and maybe that’s why I’m getting such a big spark even though I’m using resistor of a lightbulb.

 

[timselectric]

Thanks. This is what I’ll do next time as I seem to fudge it up using a regular 60W lightbulb.

Also, I’ve seen most videos doing the pre charge on the negative battery cable. Does it matter? I’ve been doing it on the positive cable and maybe that’s why I’m getting such a big spark even though I’m using resistor of a lightbulb.

Positive or negative.
It doesn't matter.