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jash8506 asked

MPPT150|35 voltage response

Hi, I have a problem with keeping a consistent charge voltage when where there is a variation of load. It is not an issue of the load being too great, but of the MPPTs not responding quickly enough. The load is only about 20A (1kW) at peak and I have 2 MPPT150|35 connected with 2kW of solar behind each.

The charge voltage is set at 57.6V, but the battery voltage ranges between 57.5V and 55.6V when the load varies from about 5A to 20A. The chart is showing the battery current (-ve for charge), not the load current.

We have 2 MPPT150|35 chargers with plenty of solar behind them, so I would have expected that they could respond more quickly to maintain the battery voltage. The load changes constantly, so this means that the batteries aren't being properly charged. Is there a reason that the voltage control loop on the MPPT's is so slow? Is there anything that can be done to improve this?

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Mark avatar image
Mark answered ·

Its hard to assess your concern without fully understanding the load profile over the same time, but I don't really see an obvious issue.

From the plot provided and based on your description - it seems that the load vs battery bank size may not be appropriate (or maybe your batteries are not performing). I say this because the load can easily/quickly drag the battery voltage from ~57.3v to ~55.6v, which is significant a significant drop (in such a brief time).

The MPPT just keeps trying to react and gradually adjusts its output to suit. At worst case the current flow into / out of the battery only fluctuates by ~+/-2A and you said that the load is ~20A.

So I say that the main issue/contributor is the rapidly changing load profile - the MPPT keeps compensating (in an attempt to maintain the voltage set-point) but just cant keep up. Typically the battery bank should 'absorb' more of this fluctuation.

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jash8506 avatar image jash8506 commented ·

The load profile is cycling between 250W and 1000W. The battery is 450Ah at 48V (22kWh), so it is not undersized. The battery has been tested for capacity and it is fine.

I'll try to explain why 57.3V - 55.6V is not abnormal when you take into consideration that the battery is transitioning from charging to discharging:

A lead acid battery is at about 2.15V per cell (51.6V for a string) at open circuit (0 current) when fully charged, but we can charge at 2.4Vpc (57.6V) and the current will be very low (because it is fully charged). So a small change in current means a large change in voltage for any battery that is full or nearly full. This means that the battery bank won't absorb more of the fluctuation

All this means that, during equalization charges especially, the voltage can change a lot even with small load changes. Equalization charges are essential for maintaining batteries, so this is where keeping a steady voltage is most important.

The load cannot be changed in my situation. It may be that these MPPTs are not capable of maintaining the battery voltage, I'm posting here to confirm whether that is the case.

So my questions are: Why are the MPPTs adjusting their output so slowly? They are switching at kHz aren't they? Is there any way to make them more responsive?

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JohnC answered ·

Hi Jash. From your graph it appears your charge ramps quite quickly in response to a drop in V, but 'holding back' while the V is rising. This could be in the charge algorithm, and deliberately applied..

Now I'm not familiar with the various algorithms that might be settable nowadays. And I don't think I've ever seen their 'mechanism' documented in any detail (understandable, counterfeiters exist).

But you could try changing them, perhaps a full manual profile first up. No guarantee, but something to test.

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