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diy solar

Struggling with battery capacity

Here is a plot of battery voltage over the same day as the above graph of power.
Screenshot 2021-09-02 12.11.41 PM.png
 
Here's another curious (to me) thing, that makes me think the batteries are full (or at least the controller thinks they are). It's very sunny today, not a cloud in the sky. I check my system status on my phone and it's only bringing in 1,210 W of PV, and sending 465 W to the batteries and 827 W to the home. I'm running the 1 ton minisplit in the battery room. I know there is more power available from the panels, but the controller isn't using it.
Screenshot_20210902-120134.png

So I turn on the 1.5T minisplit in the house, which has been off all day. Sure enough the solar power jumps to 2,946W and the house is now using 2,386W of that.

Screenshot_20210902-120712.png

So clearly I can bring in more power from the powers when needed, and I believe this tells me the controller is content that the batteries don't need any more than ~500W right now. Battery voltage is at 57.7V.

And I have trouble truly believing the "battery percentage" that is displayed. For one, it only is reported in 25% increments, so 25%, 50%, 75%, and 100%. And anytime the battery voltage climbs about 51.5V, it says 100%. So it doesn't really mean 100%.

You can see the same thing in this energy plot. When more load is needed, the PV power increases.

Screenshot 2021-09-02 12.32.06 PM.png
 
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The recorded voltages suggest that the batteries are unbalanced, thus not charging equally.

See page 20,

Fitting these units may help. ( need 3 for a 48v battery)

It would also help if the batteries were connected in parallel pairs before the series connection.

Its possible one or more batteries have been damaged/lost capacity. An individual capacity test on each would be useful.
My guess is the batteries have lost capacity due to 'capacity walk down' caused by less than ideal charge and deep discharges.


Mike
 
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Clearly that batteries don't have the capacity they should have ..... the question is how did they get in that condition.

Once the batteries have been push to the point of inverter shutdown on multiple occasions .... their capacity is going to be damaged further. How many times have the batteries gotten so low?

I would set up an independent low voltage shutdown device to shut down the inverter .... or disconnect the battery from the inverter and set it to somewhere around 40-50%.
The biggest problem right now is these batteries are either going to have to be "refurbished" ... if that is even possible .... or replaced. I don't think I would replace them with the same batteries.
 
The recorded voltages suggest that the batteries are unbalanced, thus not charging equally.

See page 20,

Fitting these units may help. ( need 3 for a 48v battery)

It would also help if the batteries were connected in parallel pairs before the series connection.

Its possible one or more batteries have been damaged/lost capacity. An individual capacity test on each would be useful.
My guess is the batteries have lost capacity due to 'capacity walk down' caused by less than ideal charge and deep discharges.


Mike
Interesting.I will share my voltage values with the battery vendor and see what he says about it.
 
Clearly that batteries don't have the capacity they should have ..... the question is how did they get in that condition.

Once the batteries have been push to the point of inverter shutdown on multiple occasions .... their capacity is going to be damaged further. How many times have the batteries gotten so low?

I would set up an independent low voltage shutdown device to shut down the inverter .... or disconnect the battery from the inverter and set it to somewhere around 40-50%.
The biggest problem right now is these batteries are either going to have to be "refurbished" ... if that is even possible .... or replaced. I don't think I would replace them with the same batteries.
I had even messaged the battery vendor a month after I installed them because I could tell I was not getting the advertised capacity. He blamed it on temperature, saying the spec is at 77F and rapidly drops as they get warmer. But they have never been above 85F and I can't imagine that is too hot for a battery to operate. During the day they are around 75F or so and rise overnight when the AC is off.

They typically drop to 50% overnight, it is only a couple times that they have dropped to 25% and the inverter shut off. I can (and will) simply change the inverter cut-off voltage, it's just a setting.
 
They typically drop to 50% overnight
This is the time to measure individual battery voltages. If you have any that are drastically lower than the others, then you've found your weak link(s). I'm still betting that you'll find a couple runners.
 
This is the time to measure individual battery voltages. If you have any that are drastically lower than the others, then you've found your weak link(s). I'm still betting that you'll find a couple runners.
oh, you mean when they are low. right. I forgot this morning, will do it tomorrow. thanks for the reminder ?
 
Who recommended the initial bulk and float voltages you initially had set?

That may have been an issue from the start then it was just time and cycling without fully charging them that did them in.
 
Sterling Power Products carried out brutal cycle testing on different battery types. OK it was biased towards the lithium battery, but shows just how few deep cycles are needed to reduce the capacity of LA batteries.
Sterling Battery Test.jpg
Mike
 
Who recommended the initial bulk and float voltages you initially had set?

That may have been an issue from the start then it was just time and cycling without fully charging them that did them in.
They were the default settings on the inverter. But I think I have found the real problem....
 
This is the time to measure individual battery voltages. If you have any that are drastically lower than the others, then you've found your weak link(s). I'm still betting that you'll find a couple runners.
MisterSandals, you nailed it!! I tested them this morning when down to "50%". The system was at 45.6V, sun wasn't up yet, very small load. I measured each battery individually, and found six were above 12.5V, and one was 8.11V and another 7.26V! So...I think we have two bad batteries, which is good news!

PXL_20210903_143514455.jpg
 
I'm going to pull the two bad batteries now, and just run on a string of four that look good and see how we get by with just that. I bet much better!

And contact the battery vendor with this data....
 
Best of luck! Any guess as to if you might be able to have your battery guy warranty the two duds?

I hope your SCC settings didn’t set them up for early failure.

Keep an eye on how much charge goes into the single string wig your new settings, you may still have some permanent damage to the “good” cells.
 
And if out of warranty, you can try individually charging and then equalizing the low batteries.
The SunXtender manual has instructions for equaling their AGM batteries. I've applied that with some success to other brand automotive starting batteries, at least bringing rest voltage up (haven't checked capacity or cranking amps.)
I think the theory is that at suitably low charge current, the released H2 & O2 can catalytically recombine so H2O isn't lost.



Definitely adjust the minimum voltage/SoC you operate to such that weakest battery doesn't go below some level e.g. 50%.
It would be preferred to base that on an SoC measurement system, not just voltage. Want to allow high current (low voltage) but not discharge to the same low voltage at low current because that would be much lower SoC.
 
I contacted the battery guy this morning, and he said he will bring me two new batteries today. And he will "inspect" my system, which is fine with me. Good service, as I had expected. It's a fairly small, well-connected gringo community down here in this part of Baja, and I know he wants to keep his reputation.

I'm still a little concerned about the non-equalization of the batteries - I will ask him about that...
 
I just have to give a huge thank you to everyone here. I really appreciate all the help and feedback on this. I'm pretty handy with most things, I designed and installed this solar system myself, but I still consider myself a newbie when it comes to solar. I put a 200W system in our camp trailer last summer, then jumped right into building this off-grid home.

I have been observing this battery capacity problem for a couple months and watching it get worse, and got no help from the battery vendor or Growatt. I was stumped and frustrated that neither would confirm my calculations that I wasn't getting what I should from the batteries.

If I could I would take you all out for tacos and beer! ?
 
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