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Battery Spikes


Weasel
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So i was chatting to someone the other day that was convinced that PWM regulator killed his batteries, reason given was the spikes that gone into the bank directly from the panels. So yes, we know how PWM works and really it shouldn't be a problem unless your array and or bank is not appropriately sized, i mean the charge voltage has to be higher but how much is too much? i've always thought it would be good for the batteries to some extent, preventing sulfation and as long as there's no heat buildup or some internal arching i cant see a high voltage being an issue. He was complaining of spikes in the order of 20 volts. My thoughts at the time where that it will reach that level if your batteries are on the way out, basically they're not absorbing the power because of high IR, also using 72 cell panels for 12V with pwm isn't helping you in any way, you'll get more power with 2 100W panels in parallel but i lost him there. So ill usually tell someone if you are unsure, or you want flexibility just pony up and get an mppt, and its exactly what ended up suggesting. But the whole chat got me thinking.

An mppt will also have "spiking" and we know it will be less than a pre-set panel voltage by the nature of the conversion which means it will be "safer"; its spiking should also give some indication to the state of your batteries, basically its applying enough voltage for the available current to move while also tracking the maximum power point from the panel. Now to finally get to the point of the post, out of curiosity on the above i was testing a crappy MPPT and got the below  plots. SDS00011.png.218a9084b0e2a3636f92e6c68fd62c40.pngSDS00014.png.a3c34d6b90f3fb2734b253c90f96284e.png  

So what you are looking at here is a  charged up very new AGM GEL battery in absorption. nothing else is connected just mppt, no loads. voltages are directly on the terminals of the battery. Its busy charging at around 5A but the switching on the converter is drawing back current as well. kind of like a 10 step forward 7 step back thing going on here. its clearly ringing on the inductor, but the currents proportional to the charge rate blew my mind. i'm thinking this is thing is a battery killer, im fine with the forward transients but what about the draw? and the p-p voltages on the terminals is almost 7v for a charged battery...

moral of the story i think is get a good mppt, I i have a few lined up i want to test when i get time then the rant can continue.

    

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7 hours ago, Weasel said:

Moral of the story i think is get a good mppt, I i have a few lined up i want to test when i get time then the rant can continue.

@Weasel can you by any chance test a Victron MPPT also?

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There you @The Terrible Triplett this is a 150/35 at the same kind of charge current. There is some ringing but current seems to be mostly in the positive and 10 times less. battery voltage is showing that its not really affected, 400mv swing.  

SDS00028.png.036dd6e34d32181eb72f0d9c25cc8779.pngSDS00027.png.17a0233b2b8cbab81df2d1bd5a22a24f.png

 

This looks like a whole can of worms topic, so what do you guys think, can these high current transients like in the first one cause damage to batteries. lets keep in mind its a chemical process and a millisecond may mean nothing.

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29 minutes ago, Weasel said:

... lets keep in mind its a chemical process and a millisecond may mean nothing.

I agree.

Thank you Weasel, this has been a pet project of mine, could just never get to this level. How it affect the batts life ... interesting to see the thoughts on that.

Now, I KNOW this is "dangerous" territory but would it be too much to ask to compare to the "other" inverter/MPPT make that is favored on this forum? 

Like you said, the affect on the batteries is another matter. Just to see If there is a difference.

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1 hour ago, The Terrible Triplett said:

Now, I KNOW this is "dangerous" territory but would it be too much to ask to compare to the "other" inverter/MPPT make that is favored on this forum? 

That will be tricky, it depends on so much. it would need to be a 60A mppt, they are pretty spensive. same battery bank at roughly same state of charge. same input power, i don't have a 150V bench supply (yet anyway) but the list goes on. Then, what  about the differences, what exactly would we make of it :D

i'm sure results wouldn't be bad anyway,  its also worth mentioning the the draw from an inverter is much more shocking, like i'm talking even that 375 of yours, whats it really doing :D:D

what i find interesting is how much of a difference a cap bank will make and if it will prolong life of batteries and consumers attached to the bank etc... or wait let me rather say how much would be enough to make an arguable difference.. 

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2 hours ago, Weasel said:

... what exactly would we make of it

Settle an argument once and for ever. :-) 

2 hours ago, Weasel said:

... like i'm talking even that 375 of yours ...

It's gone. I smoked it. But I hear you. It was a cheap one compared to the 1st one I had.

2 hours ago, Weasel said:

... how much would be enough to make an arguable difference

Cool. Lets see where this all goes.

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Current- and voltage ripple on buck converters is a design parameter. Lower is better, but it depends on the load. I am not too surprised that lower-end controllers will cheap out on this.

The BlueSolar is interleaved as far as I know. In other words, it has two inductors that it drives anti-phase. That will always significantly improve the output ripple. It is something to take into account when you compare apples with prickly-pears.

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Okay, a bit more info on this. Not all the BlueSolar models are interleaved, only the larger ones. I'm not sure if the 150/35 is considered a larger one but I suspect it is. The 100/50 definitely is. This knock-off illustrates this quite clearly. So if your testing a 100/15 perhaps the results will be slightly different to the larger ones.

I don't have quite as nice an oscilloscope as you do (I have a fairly nice 100Mhz analog scope), but I do have a couple of diverse blue chargers, a 100/15, a 150/35, a 100/50, a 80Ah lead acid battery that is practically new, and I have access to a few nice power supplies to simulate some PV with, so I could possibly contribute some results if I have some spare time. Rare these days :-)

Alternatively, if you lug that nice scope up here to Cape Town you can always do some measurements here :-)

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And yet more... for this intrigued me...

My pictures are not as nice as @Weasel's, but at least somewhat informative. This is a 100/50 charger.

I also see 400mV peak to peak at the battery, but note the scale. That's at a 50ns scale, so we're up in the megahertz's here. I really doubt that matters much. The other picture is more in the 100khz range, and that shows about 100mV peak to peak.

 

high_freq.jpg

low_freq.jpg

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22 hours ago, The Terrible Triplett said:

Settle an argument once and for ever. :-) 

Shall take no part in that :)

23 hours ago, plonkster said:

Current- and voltage ripple on buck converters is a design parameter. Lower is better, but it depends on the load. I am not too surprised that lower-end controllers will cheap out on this.

The BlueSolar is interleaved as far as I know. In other words, it has two inductors that it drives anti-phase. That will always significantly improve the output ripple. It is something to take into account when you compare apples with prickly-pears.

i think you can appreciate how difficult it is to accurately compare.

The 100/50 has 2 inductors, i did a repair on one (failed RMA, and yes i it looks terrible now) but i wasn't aware they where driven that way, ill be sure to check it out, learn something new every day. 

To test of the victrons ive got pretty much what you have 150/35, 100/50, 75/15, 100/20s, sorry this scope is staying put for now :P. I unfortunately also have the same free time issue. been building a linear supply for 80V 5A to simulate panels but the heat dissipation is a nightmare, its been shelved for months.     

20180220_230320_resized.jpg.93d2b5fc3a7cf55d90955e8b2d3ec5d5.jpg

20180220_230441_resized_1.thumb.jpg.0146a75a09b63858a5b465334d705e9c.jpg

 

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I've been using these ACDC 24V supplies for some of my tests (not the above one). But that is a good point: One should perhaps test this at a range of voltages. A higher input voltage will cause a smaller mark/space ratio and a different output ripple.

The above test was done on a 12V battery with a 20V "PV" input, ie the kind of thing you'd get from a single 36-cell module.

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