Pre-charge resistor


Any ideas on an ideal pre-charge resistor size for starting up inverter or making things less sparky?

As recently I had to start the inverter up with out any solar or eskom and the inverter capacitors kept tripping the bms, I used a small 1W unsure of ohm rating, got burn blister hot in 5seconds however it worked!

230V incandescent lamp. Not very fast but does the trick, and there isn’t really a way to blow anything up :slight_smile:

An idea, will give that steampunk look.

There is a time constant involved. Going back to my schooling T=CR
T= the charge time (or at least time most of the charging is done).
C = capacitance
R = resistance

Things “get sparky” when R is very low because then all the charge happens fast and that means high current.
If R is too high then you’ll have to pre-charge for a long time.
So yeah, it’s a trade-off, as @plonkster is suggesting if something works then use it.

I don’t understand…
Surely the batteries are permanently connected to the inverter and you start/stop the inverter on its control panel??

Normally yes, but there are occasions, especially for us tinkerers.

If you do this frequently you’ll have a permanent resistor in place, which can be bypassed (shorted) with a big enough switch. But it’s quite a hassle, since the continuous amps are still quite high, even after pre-charging.

MLT and Microcare inverters come with a precharge button that you press to charge the caps in the inverter, then you can close the fuse breaker.

One example, when the inverter arrives in the box, it’s not connected to any battery, should you them want to connect a battery to, you would need to either pre charge the caps by using AC or by using said resister, If you don’t and you connect the battery directly, it draws quite a spark or in some cases the “inrush” current is too high and the BMS switches off.

During extended grid outages the batteries might run out and the BMS then disconnects the batteries from the inverter, the caps discharge and you sit in the same boat as when the inverter arrived in the box…

EDIT; You would never land in the same situation while using lead acids, once the caps are charged you are fine till the day you decide to pull the fuses for some or other reason.


Now looking around seems a 25W 30ohm is fine, I see locally I can get a 100W 2ohm, what would the difference be, 1) slower charge or 2) still a super fast one and back to square one?

Indeed. Which is why the incandescent lamp hack is slow. 60W at 230V, using the usual ohms law etc gets you around 900Ω.

To charge a 2200µF capacitor, then, takes RC time, that is 900 times 0.0022, about 2 seconds. But there are multiples of these 2200µF capacitors combined in all these devices…

The filament tends to be a little lower in resistance when it is cold too, so it will probably get there a little faster.

This is pretty much what I use. Cause I have one lying around. And I do this infrequently enough that a dedicated precharge is overkill.

Another thing you can do, is to let the solar chargers bring the DC-bus up to voltage before you connect the battery. That also avoids the big inrush current.

At the risk of sounding dumb, for I have absolutely no picture in mind, how does one connect this 60w 230v incandescent bulb?

An incandescent lamp acts a type of “poly fuse” in this case, the cold resistance is very low, but if you start drawing current it rises quickly, so it works really well and is great in case of a fault.

In the general case, depending on the inverter, if you can keep the electronics off, then a ~33Ohm words well (should charge most inverters in 2-3s). If it powers on when applying battery, then you probably need something in the ~3.9Ohm range. 5W should work, but I would try to get at least a 20W.

The inverters are full of capacitors. So the pre-charge resistor is applied in series between the DC busbar ( Batteries) and the inverter for a short time before the inverter is submitted to the full whack of the DC supply.
This reduces contact wear on the DC powering on device ( be it a switch, relay or fuse) and also avoids the operator getting a fright.

Equally once all the Inverter and chargers are switched off ( but the inverter is still connected to the DC busbars) and then the batteries are disconnected from the busbar, those same capacitors will keep the busbars live until they discharge. Then a discharge resistor can be used across the busbars. Alternatively, you can wait ( quite a while actually).

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It has two terminals, so that part is at least easy.

I assume you have one of those Mersen/Jean-Muller fused disconnects. So while that fuse is open, you connect the lamp across the two terminals (so the energy flows from the battery through the lamp towards your inverter and all the rest of the stuff). If you want to see this in action, you can also put a volt meter on the DC bus, and you should see the voltage climb slowly. Then when the voltage is about 80% of the way towards battery voltage, or more (it will slow down towards the end), you close the fused disconnect.

Then there is no large spark on the fuse terminal as you slam it home…

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Thank you Phil. I’m fully aware of that yes. Happens to most larger inverters, even UPS’es.

To avert that perky spark I have a Keto Fuse which nicely contains the spark. Prior to that it was interesting to say the least.

What I do find quite interesting, in this particular case, is the BMS not being happy with that instant draw. Something to be aware of into the future methinks.

I just cannot envisage how one connects the bulb into the DC circuits.

Snap … got it!!!
Thank you.

Damn, that is clever! :+1:

It’s not only about containing the spark. It also has to do with a very large current spike that goes through for a split second. This can place quite a bit of stress on some of the components, but in the current discussion the reason for this is because the battery BMS thinks that spike is a short circuit and switches off. So you are unable to start the system because the BMS keeps switching off.

The two solutions out of it is 1) precharge it slowly, 2) connect the solar chargers so that they bring up the DC-bus first.

If you choose option 2, solar chargers must be configured for the right voltage. Cannot use “auto”, because they will default to 12V in the absense of a battery. When you then connect the real battery, they will raise a high voltage alarm.

Now go look at an old national semiconductor chip known as the NE555 (the 555 timer, which is like a rite of passage for every amateur electronics hobbyist… or was). It uses an RC network running between a 1/3 and 2/3 of supply voltage to create either a mono-stable timer or bi-stable oscillator. Same RC principle, but they run it specifically between 1/3 and 2/3 because it slows down towards the edges.

Reminds me of one day.

Sparky was here to do some work so we switch the Critical DB back to the main DB.
Switched off the inverter.
Disconnect the Keto fuse which breaks pos and neg to the battery, need to figure that one out for the bulb.
Then, knowing of the energy still inside the inverter, we switch the inverter back on … just being careful see.

Low and behold … the inverter starts right up again.

We died of fright. What on earth!!!

After careful deliberation whilst standing in awe of this “ghost in the machine”, it dawns upon me, the panels.

They are connected directly into the inverter. :laughing:

Epic fail moment.