Current Limiting on Meanwell Power Supplies.

[Hogwit]

This is in reference to a car, but I feel this category is more appropriate given that it relates to using Meanwell power supplies.

So the current limiting mods for Meanwell power supplies are well known, and it's also obvious that in most situations, if you have a string of power supplies in series, only one would need to be current limited. My question arises from the large voltage gap of a much higher voltage battery.

Essentially, imagine you have a 48V battery being charged from two 24V power supplies where only one is current limiting. The unlimited power supply being connected to the battery wouldn't pass any current, but when put with the second, current limited, power supply, it could pass up to what the power supply was limiting to.

Now assume you have a battery that will go from 300V to 350V and a string of 24V power supplies to charge it and let's also assume only the last power supply is current limited. With all but the last power supply in the chain, the system would pass a current above the rating of the power supplies. If the last power supply can limit the current, however, the current would be within the rating of the power supplies. Being that even without the last power supply, the load would draw too much power, would the last power supply actually be able to limit the current? Essentially I'm thinking of it like this, if you put a string of 10ah batteries in series, and the last one in the line was only a 2ah battery then you start to draw power from the string, up to the limit of the weakest link (2ah), you could draw the full voltage of every battery in the string, but beyond the 2ah limit (just to ease the concept, imagine that 2ah battery now has a voltage of 0 even though that wouldn't be the case), if you continue to draw power, you would reverse charge the 2ah battery, and the max voltage would be the voltage of the 10ah batteries (minus whatever amount of reverse charging you did). Just like with the power supplies, the batteries could continue to power the load beyond the weakest link that is trying to stop them from doing so.

Sorry if that is a poor description of what I'm trying to say, but I hope you can understand it.

 

[fechter]

You need to look at the voltage range expected for the big battery. A 24v meanwell can only go down to zero, so if you only have one that's limiting, you have a 24v range. This won't likely be enough for a large pack.
Imagine a 100s Li-ion pack. At full charge, you'd need around 420v. When the pack is fully discharged, it would be around 330v. So you need a voltage range of nearly 100v.

For many supplies in series, I suggest placing a diode reverse biased across the output of each one to prevent any supply from getting reversed (and damaged) when the pack voltage is low.

You would also probably need several of the supplies to limit, not just one for a higher voltage string. It wouldn't hurt if all the supplies limited at the desired current.

 

[cycborg]

You would also probably need several of the supplies to limit, not just one for a higher voltage string. It wouldn't hurt if all the supplies limited at the desired current.

I'm trying to picture what would happen here - the limits probably couldn't match exactly, so say you have 3 supplies with limits of 4.9, 5.0, and 5.1 A. The first one tries to limit, but can't, so it ends up at whatever its minimum output voltage is, then the second tries to limit, and maybe succeeds, so now you have the first supply at a minumum voltage, the second doing CC, and the third at max voltage. Am I thinking about this right? Could be different answers, I suppose, depending on the protection circuitry of the supplies.

 

[fechter]

Yes, you are right. Unless you have the supplies synchronized somehow, one of them will always go into limiting before the other ones. It might go as low as it can while the others are kicking in. This should be OK, it just doesn't share the load evenly between supplies.

As the pack charges, the voltage will rise to the point where only one is limiting, then later even that one will go CV.

I have seen some supplies with an interconnection to synchronize the PWM so they run at the same power output. This is typically used in parallel supplies, but the concept could work for series supplies with the right interface.

 

[nutnspecial]

Op, I don't understand the analogy to the batteries. Series for batteries is the same for the chargers afaik. It only multiplies voltage and doesn't affect capacity or discharge/charge rates of the individual batteries.

That would tell me the chargers act the same in their charge rate, or 'constant current' phase. From what I understand that's what current limiting is needed for, so the amps don't overshoot while voltage is climbing and trying to get to 'constant voltage' stage. So one current limiting in a whole string works fine, because it's only keeping the amperage (same on one or 10s chargers) from overshooting.

This is just what I gathered from the forum and from using my 3x MW hrp 15a 30v (which are current limiting i think) in series, and sometimes adding a 0-30v 10a gophert adjustable supply to tune voltage and limit their CC stage by 50% or more.

No idea how realistic that is for chargers applied to 300v ev systems, but am assuming it's applicable.