jontm avatar image

Inrush current Quattro 230V 48/10000VA for BMS dimensioning


I am currently looking to build or buy a battery pack for my old Quattro 230V 48/10000VA unit. Talking to some manufacturers they are a bit hesitant to recommend battery packs due to the fact that the Quattro apparently have a quite high inrush current due to the large inductor/capacitor inside the unit. They are a bit vague on it, but are recommending me mush larger battery packs than what i want at this point.

Are there any recommendations regarding what peak inrush currents I can expect with the Quattro 10000 VA unit ? I guess that I will need to find a BMS with sufficient inrush rating based on that.

Or is a precharge recommended/needed when using the Quattro 10000VA together with lithium packs with BMS ?

MultiPlus Quattro Inverter ChargerLithium BatteryBMSquattro 10kvadiy
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3 Answers
wkirby avatar image
wkirby answered ·

A 10kVA Quattro needs quite a decent battery pack as it is, something capable of supplying 200A+ at full chat. That's the continuous current though, not the inrush current.

Inrush current is the initial current that flows when you first connect the battery to the Quattro. It's several thousand Amps for a fraction of a second, but it's enough current for long enough to trip a BMS.
It's standard practice to limit the inrush current with a high power resistor prior to connecting large systems to batteries. A competent engineer performing such installations will have such a resistor in his toolbox, next to his terminal screwdriver. I'm not sure why the dealer is recommending an oversize battery just to cope with a one-off event which should be managed appropriately anyway.
Once the batteries are connected, then the inrush current concern is overcome and there is no need to worry about it until the batteries are disconnected and need to be connected again.

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Makes sense. However a BMS normally have protection functionality that can disconnect and connect the battery again in case of overcurrent, overvoltage, under voltage and in some cases temperature. If one of these events occur would it not cause a potential harmful situation where the BMS would trip on overcurrent immediately on reconnect after a error event is cleared. Causing a "infinite lop" (not really infinite, i guess each reconnect will charge some of the DC-link capacitors in the unit causing it to finally be at a high enough level.

A BMS with a good precharge circuit can also do it. But as @WKirby mentioned a big resistor is used by most decent installers to help with the inrush. It prevets the skid marks on the terminals that void warranty anyway.

I have seen guys precharge from AC as well, I have not personally done so though so cannot say if it is a good idea.

And yes battery safety, and at least a big fire extinguisher of the right type is also needed in the area especially if you are building your own pack

jontm avatar image
jontm answered ·

Makes sense, i guess there is no relay or switch between the input capacitors and inductors on the battery DC input ? Based on what i was told, i assumed that there was some sort of battery disconnect or something like this that was the reason behind the inrush current being a issue.

I sort of assumed that there was a option to set the maximum output current in software ? Tripping the system if the value was exceeded ? I might be mistaken though, had a look at the configurator and did not find an option for it.

The plan was to purchase a minimal amount of batteries this year and to expand with more batteries later. Figured it would be good to start with a large charger/inverter, so that i did not have to change it down the road when i am able to purchase more batteries. Might have been a mistake :)

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It is certainly possible to limit the output power of the inverter and also limit the charge power to suit the battery bank size.
It's not normally recommended to add new batteries to an existing bank, but with lithium chemistry this is a lot less of a problem. Just don't leave too much time before the expansion, a year is probably the most. Also try to make some assurances that the batteries you buy today will still be available when the time comes to expand. Mixing different brands or models of batteries would not be a good idea.

jontm avatar image jontm wkirby ♦♦ ·

Ok. Thanks. I will look into it.

I am planning to purchase a battery pack made out of repurposed EV lithium batteries. I assumed that it might be OK as long as the battery chemistry is similar and that each paralell battery pack have its own BMS to manage it. I would also need to make sure to equalize the voltage between the packs before connecting them together and i assume that i should make sure that each of the packs are able to withstand close to the full output current/charging current so that an uneven current balance is not that big of a issue.

If you're going to use repurposed EV lithium batteries, don't forget that these are almost all lithium-ion batteries using chemistries like NMC, which need *much* more stringent safety precautions than LFP (LiFePO4) to guard against overcharging/shorts/damage which can lead to catastrophic battery fires.

EVs use NMC because of the higher energy density, but the manufacturers also spend millions of dollars on testing and safety systems -- and still these don't always work, and vehicles occasionally burn out.

This is the reason that most home/boat/RV systems use LFP batteries, energy density is not so important as for EVs and safety is much easier to guarantee. Some insurers (e.g. marine) will not cover systems using EV-type batteries (e.g. NMC) for this reason, they're simply too dangerous in the hands of amateurs.

Elimac avatar image
Elimac answered ·

I think the manufacturers you spoke to are talking about the Peak Power capability of the Quattro: 20 kVA which translates to over >400A inrush current at DC side, and not so much about the initial capacitor charge issue that this topic has discussed (which yes, could also be an issue, but it is a 1-time event only).

Be sure that the BMS of the new pack can deliver at least 500A peaks without tripping to be on safe side, or, if you know you will not need such high Power Surges (for example, no motors on your installation) just live with lower spec battery/BMS and try to limit the power the Quattro can deliver on the AC side to the corresponding value you have (safely) available on the DC side.

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