Series Parallel panel wiring newbie question

[sunshine_eggo]

Something that I don't recall discussing...

Length of wire between panels and MPPT?

 

[Belzo]

Assuming you're in the Northern hemisphere, the closer we get to the Winter Solstice, the worse our available solar becomes. If you're trying to compare what you remember to what you're seeing now, it's just not going to be reliable.

If you really think it is an issue, you can revert to 4S.

Yeah I get that. This was only comparing like a week ago to now so shouldn't be that much difference but I am waiting for a really clear sunny day to really get a comparison before I make any changes back to 4S. It may just be that we had some really bright sunny days a week or two ago and the spotty sun we have had just hasn't been as good as I think it's been in comparison. Once we get a really clear day then I should have a better comparison, but I was more asking if maybe there could be some other explanation where my MPPT may like higher voltage (80v) with lower current a lot better than my new (40v) with higher current or it just could be that I need some better sun ?.

 

[Belzo]

It's slightly less than 50ft

Something that I don't recall discussing...

Length of wire between panels and MPPT?

It's slightly less than 50ft

 

[sunshine_eggo]

2S2P vs 4S has a ~3% difference in wiring losses, i.e.,

2S2P loses 3% due to wiring losses vs. 4S.

Since you were at the upper limit of the MPPT range at 80V @ 4S, it's unknown if this 3% is significant.

 

[Belzo]

Today I had my inverter off and no load on the battery. It was a good sun day and my battery was taken from a steady 26.6v resting voltage to a fully charged 28.8v and PV power was reduced during mid day by the system in order to maintain the full capacity. This all made sense to me. When I got home I turned on the inverter and plugged in a small chest freezer to the load and after only running this chest freezer for about an hour I'm back down to 26.6v.
Does this seem right?!

 

[Belzo]

Am I paying too much attention to the voltage? I'm looking at it like the state of charge percentage on my little Jackery portable battery I guess. The state of charge on my MPP display seems totally unreliable. So what do I go off of? I was expecting to see the the voltage slowly come down from 28.8v as battery power was used for my load. Should I get a good battery monitor shunt ? Would that help me to see my actual state of charge better and this would make more sense to me?

 

[sunshine_eggo]

Am I paying too much attention to the voltage?

Way too much. Nothing appears to be out of order. The moment you pull a load on the battery, it will drop in voltage. Once charge is removed, it should settle into the 26.XV range without any loads at all.

I'm looking at it like the state of charge percentage on my little Jackery portable battery I guess. The state of charge on my MPP display seems totally unreliable.

Why?

So what do I go off of? I was expecting to see the the voltage slowly come down from 28.8v as battery power was used for my load.

Nope. It will instantly drop below 3.4V/cell around the 3.35V/cell level.

Should I get a good battery monitor shunt ? Would that help me to see my actual state of charge better and this would make more sense to me?

Possibly. It depends on how the MPP reports SoC. I'm not familiar with it. If you've programmed your battery capacity and the full charge criteria, it should be pretty accurate. If it's voltage-based, it's garbage.

What is your float voltage?

It appears that you held your battery at 28.8V for around 8 hours. This is very bad. Big Battery specs are at best confusing and at worse, moronic. They often specify the same absorption and float voltage. That is NOT good for them.

Your float voltage should be set to 3.4V/cell or 27.2V.

 

[Belzo]

This is exactly what I've been asking Big battery directly and ShopSolarKits.com. I've asked repeatedly if my bulk and float both should be 28.8v. I said this doesn't seem right from what I've read and seen elsewhere. They both said it's right. Follow what is in the specs! ?

 

[Belzo]

My understanding is that MPP reports SOC via voltage. There is not even a option for defining the size of battery.

 

[sunshine_eggo]

This is exactly what I've been asking Big battery directly and ShopSolarKits.com. I've asked repeatedly if my bulk and float both should be 28.8v. I said this doesn't seem right from what I've read and seen elsewhere. They both said it's right. Follow what is in the specs! ?

It's confusion surrounding the claims that LFP don't need a float voltage - that's true if you're just storing them. However, in a solar power system, the float voltage is what is used to tell the solar when to power the loads.

Battleborn indicates their 12V should be floated at no higher than 13.6V. That's 3.4V/cell. Floating at 3.4V will hold the battery at 95-98% SoC. If it's good enough for Battleborn, it's good enough for me.

My understanding is that MPP reports SOC via voltage. There is not even a option for defining the size of battery.

Then it's junk. A shunt or hall effect battery monitor is the way to go.

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

The first 3 are good options.

 

[Belzo]

I've changed my float to 27.2v now.

 

[Belzo]

Thank you so much for clearing the float charge up. I've been worried about it and feel like I haven't gotten clear answers so I feel much better now. ?

 

[Belzo]

It's confusion surrounding the claims that LFP don't need a float voltage - that's true if you're just storing them. However, in a solar power system, the float voltage is what is used to tell the solar when to power the loads.

Battleborn indicates their 12V should be floated at no higher than 13.6V. That's 3.4V/cell. Floating at 3.4V will hold the battery at 95-98% SoC. If it's good enough for Battleborn, it's good enough for me.



Then it's junk. A shunt or hall effect battery monitor is the way to go.

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

The first 3 are good options.

I'm planning on getting a second mule 24v battery to add in a few weeks when they get them in stock. I've already ordered it. Should I get one of the Victron shunts that have the option to monitor a second battery in this case?

 

[Belzo]

Do you think my float setting caused any major damage already? ?

 

[Belzo]

If I go to having two batteries, would I ideally want two shunts to be able to see each battery individually or just one for the combo or them?

 

[sunshine_eggo]

I'm planning on getting a second mule 24v battery to add in a few weeks when they get them in stock. I've already ordered it. Should I get one of the Victron shunts that have the option to monitor a second battery in this case?

No. It just monitors voltage, and since you're paralleling two batteries, they will be at the same voltage.

Do you think my float setting caused any major damage already? ?

No.

If I go to having two batteries, would I ideally want two shunts to be able to see each battery individually or just one for the combo or them?

It's your preference, but it would probably be confusing. Easier to treat your multiple parallel 24V batteries as a single big battery bank.

 

[Belzo]

So if you have to setup a battery monitor shunt and set the point when the battery is at 100% charged to start the monitoring how would I know when my battery is at 100% SOC with my current system. Would it be as soon as my battery gets to the new float voltage of 27.2v while charging and then stays there at rest when it's not charging?

 

[sunshine_eggo]

You program the monitor with the 100% charged criteria. In the case of solar charging, due to its variable nature, you typically go ~0.2V below absorption and specify 0.05C as a tail current for LFP, i.e., when your battery hits >28.6V AND the current needed to maintain it is only 0.05C, the monitor resets automatically to 100%. You know it's set pretty close when you're indicating 100% within ±30 minutes of dropping to float.

The monitor then counts current in and out and adjusts SoC accordingly. Every time you cycle it without going to 100%, it gets a little less accurate. Victron indicates that their BMV-7XX shunts need to see a full charge at least a couple times a month to maintain accuracy.

Short video: