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Old 11-25-2019, 11:05 AM   #1
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Charging with a (dumb) power supply?

Looking for ways to charge my batteries from the wall since the threat of snow looms and I already need to beef up my panels. I ran some tests today with a 20A, 24V power supply and want to run this by you folks.


So to begin with, I'm using 24V Tesla packs. Each pack connects to a set of 1000A bus bars via 250A breakers on the positive side and through a 250A winch disconnect.
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Similarly the Victron Energy MPPT charge controller connects to the bus bar via a 100A breaker on positive, and there's a 60A breaker that the panels go through into the charge controller, allowing me to disconnect the charger from the system on both ends, and a 100A shunt between the negative bus and the charger.



I have a native 24V, and stepped-up/down 12V, 48V DC fuse panels, which connects to the positive bus bar via a breaker, and the negative via another 100A shunt. I've made it easy to isolate components, measure performance, and any appliance is run behind several safety mechanisms.


So to begin my test I wired in the power supply by connecting the 24V positive output to the 24V fuse panel, via its own 20A fuse. It shouldn't exceed that, but to guard against the batteries/solar backflowing current into the supply/etc. I then put the negative from the power supply upstream of the solar charge controller's 100A shunt, so that the load on the power supply would show up as charge current.


I then disconnected the batteries and charge controller from the bus bars. System went dead. Took a while for the voltage to drop to zero in the meter output with nothing running- with a light on or something it cuts out immediately. Once out, it did not come back- there's no rogue device supplying power.


I then popped in the 20A fuse for the power supply and plugged it into shore. System came back up, 20W usage. Lights came on, etc. Good.



So lets see what blows up when I reconnect one of the batteries... nothing. Whew. 380W! The power supply is charging~! Hmmm, still kind of nervous. Disconnect the battery, back to 20W. As a kid I always thought that battery chargers were doing something magical- some special waveform or whatnot to convince a battery to charge. Thus, a charger for one chemistry wouldn't charge another battery, etc. This is of course nonsense, although some (lead acid?) chargers use some "magic" like this to try and "reawaken" or "bring back to life" cells.


Anyway, I was worried that the power supply would just continue to pump in electrons at full tilt without regard to the state of the battery. So to test this I adjusted the voltage on the power supply to closer to that of one of the batteries in isolation. 23V ish. Reconnect the battery- 50W. Seems like, as the battery charges, the power supply output will taper off.. and I assume at whatever voltage I set the supply to- stop. Its what is beyond that, which worries me.


Next test, I reset the voltage to 24V and reconnected. 380W. Lets connect the solar charger.... alright, 780W. Great. Seems like they wont interfere with eachother, at least not yet.



Then I pulled the plug on the power supply. Well, it has a fan and an "I'm on" LED- neither shut off. Charge current dropped to whatever the solar output was (I disconnected the 20A fuse to verify). I assume that the fan and LED are DC and tap off the 24V output, which is still hot because there's 24V coming from the charge controller/batteries.


Anyone see a problem with me leaving a power supply connected in parallel with the batteries to charge them? Any pitfalls there? I do wonder if it should be cut off altogether after the batteries reach the set voltage, I probably could do this with my MPPT charge controller.



https://www.amazon.com/gp/product/B01HTF1Q06


I also wondered if it would be possible to put the power supply "north of" the MPPT charge controller, and have it manage it like a panel, which connects only at night when plugged in.

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Old 11-25-2019, 11:20 AM   #2
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Depends on the SC, Victron MPPT apparently works.

As far as the packs are concerned, volts is volts, amps is amps, the only difference is a PSU has no automatic stop-charge, and may not limit current well.

But either can burn your house down, not minor issues.

What makes you think your PSU terminates charging?

Do **not** charge unattended!

You are literally playing with fire.

Learning such basic stuff hands on would be much safer with LFP.
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Old 11-25-2019, 11:30 AM   #3
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Taking for granted there isn't any sort of BMS logic happening in your lipo packs, this is a bad idea. Lipo wants a CC/CV charge, and a dumb power supply would just be CV. CC/CV applies a constant current (amps) until the batteries reach their optimum voltage (4.2v per cell) and then hold that voltage and reduce current until full.

if your MPPT controller is lipo compatible, I would go the route of running your charger through it as if it were a panel, and let it do the magic for you.

If you had a lead acid bank and had your voltage set properly this would work just fine.
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Old 11-25-2019, 11:39 AM   #4
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Quote:
Originally Posted by john61ct View Post
a PSU has no automatic stop-charge
Right.



Quote:
Originally Posted by john61ct View Post
and may not limit current well.
Seems to me based on tests, that charging current will be dictated by the differences in voltage of the PSU to the batteries. I imagine (good) battery chargers would work by upping their output voltage to keep current high until the battery voltage got to certain levels, then change their voltage or shut off altogether at a maximum, based on chemistry.



Quote:
Originally Posted by john61ct View Post
But either can burn your house down, not minor issues.
Yeah...



Quote:
Originally Posted by john61ct View Post
What makes you think your PSU terminates charging?
I don't think it does, I think it puts out current to maintain a desired voltage. I have confirmed that the load on the PSU fluctuates with the demand from the fuse panel, and that the batteries act as a load that varies by their voltage.


Quote:
Originally Posted by john61ct View Post
Do **not** charge unattended!
Yeah no way in hell.


Quote:
Originally Posted by john61ct View Post
You are literally playing with fire.
I'll push back on this- I haven't started a fire... yet. And there's lots of safety equipment in play here, not to say there aren't conditions I'm not sufficiently guarded for (battery over voltage, cell over voltage (much harder), potential current going back into the PSU).


Quote:
Originally Posted by john61ct View Post
Learning such basic stuff hands on would be much safer with LFP.
I think it would probably be good to build something with a 6S RC lipo I could test things with, but if anything those would probably be much more sensitive to problems than these packs.


Speaking of which, I suppose I could just use RC chargers. Hmm. I wonder if you can run the balance leads all in parallel like the load leads? Would be hard otherwise.
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Old 11-25-2019, 11:47 AM   #5
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Taking for granted there isn't any sort of BMS logic happening in your lipo packs, this is a bad idea.
Thing is, my MPPT charge controller is in the same situation. It has no awareness of cell balance- this is quite annoying. Balancing solar charge controllers seem like unicorns.

See the picture? Below the breakers there's an RC balancer. These constantly balance the cells in the packs to 0.01V, each pack. They're very much insufficient to get the job done with large current draws or high rate charges, but they've kept my test loads/charges under control.

And without being able to run the balance leads in parallel I don't know how to scale these up. If possible I'd like to run them all to a common "balance lead bus" where I could run some really beefy balancers against all the packs at once, but there's problems with that design. It would mean that Cell 1 in Battery 1 would need to be kept close to Cell 2 in Battery 2 at all times, and so on for each cell. I think this isn't done this way for a variety of reasons- what if B1C1 is 4.0V and B2C1 is 4.2V? In parallel, they'd measure 4.1V (ish) which could make it harder to detect charge proper cutoff. OTOH, any current would likely go to B1C1 anyway since it is lower voltage, but you can't completely guard against overcharging B2C1 this way even if parallelizing the balance leads worked.

I will be looking into beefier balancing systems in the near future, but what I seemingly can't avoid is the charger being unaware of what is going on under the hood.
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Old 11-25-2019, 12:11 PM   #6
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While I agree with the points about balancing, that's ultimately different from CC/CV charging. I'd need both of those problems to be solved before I would go with a lipo solution for house power.
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Old 11-25-2019, 12:32 PM   #7
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Quote:
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While I agree with the points about balancing, that's ultimately different from CC/CV charging. I'd need both of those problems to be solved before I would go with a lipo solution for house power.
Its been a year of testing these packs with that MPPT charge controller and the balancers I have. So far, they've worked fantastically... far beyond the performance of the AGMs I used to use. The first... month was pretty scary because it was all new to me but system behavior has been consistent.



You can fully control bulk/absorption/float voltages of this charge controller, and I set bulk/absorption/float to the same value (24.75V-25.00V I think- I don't go for maximum which is ~25.25V). I don't think you can expect to get "real" CC/CV from any solar-based solution- you can't control your solar input after all, and charge current is going to vary internally per cell, and even within each cell. Even at 100A charge you're dividing that by 444 individual cylinders (* number of packs) at some point, current is going to go where it will, and _never_ will it ever be "Constant".



Most likely I'll end up with some beefier balancers on each pack in the long run. Plenty of Chinese / bluetooth ones out there that do the trick, but there is also a company I've been talking to ( https://stealthev.com/ ) that is hoping to solve "my" problem with running Tesla packs in parallel and balancing. If they come up with a good solution I'll probably adopt it. I ordered some balance pass-through boards from them and when I did the subject came up.
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