Strange battery voltage drop

[hillbillytoker]

Ok so I am having a really weird voltage drop on my 12v solar/battery set up. First I live in an apartment where I'm am limited on how I am allowed to set up my solar system. My solar system is a 12v system. I am running 2 100w renogy solar panels, a renogy Rover 30a Li MPPT solar controller, 2 12v 100ah LiTime LiFePo4 batteries, and an 2000w inverter (for the life of me the maker got it in a bundle deal).

Now for the weird voltage issue. I am unable to charge my batteries past 13.5v. While charging, it will go up to about 13.8v but then for no reason it will start dropping to 13.5 and then sit there with in an hour. At first I thought it could be the BMS's going into protection mode but after a long 2 weeks with LiTime support they determined it wasn't the BMS and thought it could be the solar system. Support made me do a deep drain cycle to make the BMS shut down to reset it but it still didn't work. Now before I got the Rover I had the Wanderer and just swapped it out believing it was the solar controller that was giving me the problem. I am still having the same issues that I was with the Wanderer. I then went through the whole system to see if I was having any parasitic draw. I found the both solar controllers where sending anywhere between. 0.05 to 0.1mv back to the solar panels when no sun was out. I placed 4 10a diodes in parallel to stop the draw. I then also found a 0.5 to 1v draw to the inverter even when the breaker was flipped off. Replacing it and now the draw is gone.

So now that I have fixed the parasitic draw issues I can still not charge past 13.5 without the voltage dropping and sitting at 13.5. Im at a loss of what it can be. I don't think its the solar panels. Only thing it can be is the wires but how can that be. Any help would be grateful.

 

[pollenface]

What are the settings of the charge controller?

 

[sunshine_eggo]

99% of the charging is going to occur below 13.8V. You're charging at low current, so MOST of it is going to happen below 13.6V.

Have you taken the system offline to remove loads and put it on an extended charge without touching anything in great solar conditions for a week?

You have 2.56kWh of batteries. Assuming you have clear skies with unshaded exposure from sunrise to sunset with a perfect tilt for your latitude, it will take the batter part of 3 FULL DAYS to charge your batteries from empty assuming you use absolutely no power whatsoever.

I fully suspect that your PV conditions in an apartment do not allow for anything near optimal performance. If you're dealing with limited times during the day, it may take 7-10 days to fully charge your batteries.

Your PV and available solar determine how much you can use per day. Your batteries determine how long you can go between charges. Two 100W panels with limited exposure in an apartment may allow very little daily usage, and you've just never fully charged your batteries (they come at about 30-50% charged). I wouldn't be surprised if you're only able to run a 60W laptop for maybe 5-6 hours per day on your available solar. Use any more than that, and you'll never fully charge your batteries.

 

[hillbillytoker]

I wish that was the problem solar panels will pull in 19v to 21v with 200w +-5 with amps in the 10 to 12 range. I've left this set up for about 2 weeks now been having full sun until today. I get about 4 to 5 hours full sun on panels where they will sit in the above range. I was having no problems with it a month ago even on cloudy days would keep the charge at 14.4 to 14.6 on the batteries. Then one day when the power company had to replace a transformer they turned off the power to my area and I had my system running things around my house like fridge and deep freezer stuff like that. I got home and my batteries were sitting at 11v. By then the power was back on and I plugged everything in and shut off the inverter. Since then the I cant get the batteries to charge passed 13.5 with out them draining. I took them off the system and charged them with a charger back up to 14.6 and left them sitting for 3 days and they kept the charge. As soon as I put them back on the solar system the voltage dropped to 13.5 in a little over an hour. This was with the old wanderer PWM solar charger. I did the same thing with the new Rover and once again it did it again. This is why its so weird. I cant locate the issues on why its draining. I even just put one battery on the system at a time to see if it was a bad battery draining the system but it wasn't. By the way thanks for trying to help.

 

[sunshine_eggo]

I wish that was the problem solar panels will pull in 19v to 21v with 200w +-5 with amps in the 10 to 12 range. I've left this set up for about 2 weeks now been having full sun until today. I get about 4 to 5 hours full sun on panels where they will sit in the above range.

4-5 hours is actually poor solar exposure, and it's not possible to get 200W out of 200W panels on a PWM controller except in some very extreme and uncommon circumstances. In IDEAL conditions, your panels could harvest about 1000Wh of energy per day. Assuming the 4-5 hours is centered on noon, you're probably only able to get about 600Wh - about 10 hours of a 60W laptop. You'd need three days of PV harvest to power a typical residential fridge for one day.

10-12A is at BATTERY voltage, so 10-12A @ 13.5V = 135-162W.

This is very simple math. 10-12A is a very low charge rate. Using your idealized numbers, it's going to take the better part of 20 hours to fully charge your batteries. It's going to take 4-5 days to get that 20 hours of charging without using any power from the system at all.

I was having no problems with it a month ago even on cloudy days would keep the charge at 14.4 to 14.6 on the batteries. Then one day when the power company had to replace a transformer they turned off the power to my area and I had my system running things around my house like fridge and deep freezer stuff like that. I got home and my batteries were sitting at 11v.

This is a very common occurrence. Things are fine, then the system is drastically over-utilized completely tapping it out. A typical full size refrigerator and deep freezer will need at least 2.5kWh/day - about 3-4X what your PV can produce. Now you're playing catch-up.

By then the power was back on and I plugged everything in and shut off the inverter. Since then the I cant get the batteries to charge passed 13.5 with out them draining. I took them off the system and charged them with a charger back up to 14.6 and left them sitting for 3 days and they kept the charge. As soon as I put them back on the solar system the voltage dropped to 13.5 in a little over an hour. This was with the old wanderer PWM solar charger. I did the same thing with the new Rover and once again it did it again. This is why its so weird. I cant locate the issues on why its draining. I even just put one battery on the system at a time to see if it was a bad battery draining the system but it wasn't. By the way thanks for trying to help.

It's very normal for LFP batteries to drop to 13.5V under a very light load, and 13.5V is a very common float voltage for LFP, so the chargers will hold this voltage AFTER attaining the boost voltage. The charger should NOT hold the batteries at 14.4-14.6V for more than your programmed boost duration, which should be about 30 minutes at that voltage.

LFP batteries sitting on a charger @ 14.6V will typically drop to the 13.5-13.8V range in a relatively short period.

Normal charge profile:

1. maximum current charging to bulk/boost/absorption voltage.
2. Hold bulk/boost/absorption voltage while current tapers for "boost duration."
3. Drop to float and hold voltage (typically about 13.5V).

All of the evidence points to one or more of:

1. Using more PV/day than you can generate.
2. Improperly programmed charge controller.
3. Lack of understanding of normal charge behavior.

 

[mikefitz]

As soon as I put them back on the solar system the voltage dropped to 13.5 in a little over an hour.

As discussed this is the normal 'full' voltage of the battery. Higher voltages reported after charging are due to a kind of surface capacitor charge effect and will be quickly dissipated with a small load.

Note with the Renogy chargers set to lithium default charge settings there is no float voltage. Charge stops after 'boost period' is completed. When you had a change in performance after a power company cut power its possible the controllers defaulted to a different charge format.
The actual charge setting used , if posted, would help the discussion.

As also discussed with a low charge current the battery is 'full' but the time it reaches 13.8 to 14.0 volts under charge, its not necessary to reach 14.6 volts. It's possible when you were charging to 14.6 the battery BMS had entered protection and the reported 14.6 volts was supplied by the charger, not the battery. Following a sucessful full charge the battery under a small load, solar charger, will show 13.4 to 13.5 volts, this is normal.

 

[hillbillytoker]

What are the settings of the charge controller?

These settings were told to me from renogy for testing to see what is the problem.

so 10-12A @ 13.5V = 135-162W

as you can see that is what my solar is producing. Like I said before 19.4v 10.42a 200+w

As discussed this is the normal 'full' voltage of the battery

I've had these batteries sitting at 14.4 to 14.6 for two years. This is how long I have had my system. I've also have a diy lifepo4 headway cell battery that has been sitting untouched for over a year and has only dropped voltage from 14.6 to 14.3 in that time. When the batteries get fully charged to 14.6 on a wall charger then left alone not plugged into the solar controller and it still 14.6 after three days. It seems to me that its a much different issue the just normal voltage drop when it takes less then an hour to an hour to drain with NO load on them at all all load lines have a breaker and are all turned off or disconnected all together. I've had this set up like this for two years but now out of no where the batteries are draining. This isn't normal it is a weird thing that just started. My system has always held 14.4+ without a drop. so everything you all are saying is not helping.

 

[sunshine_eggo]

These settings were told to me from renogy for testing to see what is the problem.

Renogy is not known for either their technical competence, support proficiency or reliability/accuracy.

as you can see that is what my solar is producing. Like I said before 19.4v 10.42a 200+w

What you may not understand is that those numbers are almost certainly a lie unless there are some very special circumstances - extreme cold, cloud edging, etc. Panels almost never put out rated power.

I wouldn't believe those numbers without verification.

So, help me understand this math:

12.04A battery charging current * 13.6V = 193.74W? Where is the missing 34W?

Oh wait... 12.04 * 13.6V = 164W not 194W...

You've posted irrefutable evidence that you can't trust what the renogy charger is reporting:

I've had these batteries sitting at 14.4 to 14.6 for two years.

They shouldn't be at 14.4-14.6V for any longer than it takes to achieve 0.05C tail current, or 10A in your case, so proper behavior would be to charge to 14.4-14.6V, hold very briefly until current drops to 10A and then drop to float @ 13.5V.

Did you have your float set to 14.4-14.6V on the PWM? If so, that's very unhealthy for them.

This is how long I have had my system. I've also have a diy lifepo4 headway cell battery that has been sitting untouched for over a year and has only dropped voltage from 14.6 to 14.3 in that time.

This is extremely atypical behavior for LFP cells. It indicates severe over-charging.

When the batteries get fully charged to 14.6 on a wall charger then left alone not plugged into the solar controller and it still 14.6 after three days. It seems to me that its a much different issue the just normal voltage drop when it takes less then an hour to an hour to drain with NO load on them at all all load lines have a breaker and are all turned off or disconnected all together. I've had this set up like this for two years but now out of no where the batteries are draining. This isn't normal it is a weird thing that just started.

Again, this is extremely atypical behavior.

You are either dealing with extremely atypical cells, bad practices or error measurements.

You need to set your experience aside and look at objective data. It is extremely normal for 12.8V LFP to drop to mid 13.XV when charging terminates and when a load - even a very small one is applied.

For the sake of argument, let's assume the battery voltage and current are accurate. The battery is accepting 12A @ 13.6V - extremely normal until the battery is nearly poked full. So this indicates you have not fully charged your battery.

Please review this thread:

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

I recorded a 20A charge to 13.6V from empty to 0.02C tail current. It attained 99.7% SoC based on measured capacity:



Following the above charge, this is what voltage did for nearly the next hour:



Again, this battery was charged to 99.7% SoC, but within an hour, it dropped to below 13.40V with no loads whatsoever.

I also recorded the voltage on a 10A charge to 14.4V with a benchtop power supply (1 hour = about 10% SoC):


Note how voltage didn't break 13.6V until > 9.5 hours of charging and about 95% SoC. This charge rate is a little faster than you can charge your battery, so yours will look very similar but have a LATER spike.

Also note how the jump to 14.4-14.6V occurs at the very end until BMS protection triggers. Then note how the battery dropped to 14.1-14.2V level in about 15 minutes.

The above IS TYPICAL of all LFP cells/batteries and is what you should be seeing.

Regardless of all your experience, you simply haven't fully charged your battery, and you have an incorrect expectation that LFP should hold 14.4-14.6V once charging terminates. I have provided objective evidence supporting the opposite of your experience. It doesn't take much searching online to find data consistent with mine, but none that supports your expectations.

We are still here:

All of the evidence points to one or more of:

1. Using more PV/day than you can generate.
2. Improperly programmed charge controller.
3. Lack of understanding of normal charge behavior.

You've confirmed #2 by showing 14.6V as a float voltage and a boost duration of 40 minutes.

My system has always held 14.4+ without a drop. so everything you all are saying is not helping.

The issue here is your experience has established incorrect expectations. You have unfortunately learned something wrong possibly due to failure to engage best practices, bad equipment, bad measurements, etc.

What is not known is if you are experiencing BMS protection events. It is possible that the deep discharge uncovered an imbalance that prevents full charging, but that would be indicated by an abrupt drop to 0A. I'm accepting your claims that these batteries can attain 14.6V on a wall charger as a LACK of this potential imbalance.

LFP is well known to behave similarly to lead-acid when charging. You charge to higher than resting voltage to attain full charge. Once charge is removed, voltage settles.

I recommend you remove all loads from the battery and continue charging it daily. You will either see evidence of BMS over-charge protection or you'll hit 14.4-14.6V.