How can I safely make sure charge controller won't over-charge battery?

[ericbakuladavis]

Hi Folks

I'm a solar newbie and recently setup my 1st system. I have been monitoring my battery's state of charge (using resting voltage at first, and later, a battery monitor). I manually shut off the solar panels when my battery gets close to full because I am not confident that the charge controller won't over-charge the battery.

How can I make sure the charge controller will prevent over-charging?

Here is some info about my system:

Design Drawing
Link to Drawing

Two 200W Panels Wired in Series
Link to Manual
Maximum power (Pmax): 200W
Voltage at Pmax (Vmp): 17.0V
Current at Pmax (Imp): 11.76A
Open-circuit voltage (Voc): 20.23V
Short-circuit current (Isc): 12.45A

MakeSkyBlue 60A Charge Controller v118
Link to Manual
Charging Voltage accuracy: ± 0.2V
Overcharge protection voltage - 12V Bat: 15V
Limited current protection: 61A
Max PV Module for 12V Bat: 720W
PV Module Open Circuit Voltage for 12V Bat: DC20V ~ DC80V
Voc of PV should be 1.5 to 2 times greater than battery voltage for best efficiency
Controller Charging Voltage for 12V battery: 12V - 17V

My settings (see manual for explanation):
D00 24H
D01 14.5
D02 14.5
D03 10
D04 01
D05 NO
D06 13.2

Ampere Time LiFePO4 12V200Ah Battery
Link to Manual (see Charging Tips)
Nominal Capacity: 200Ah
Nominal Voltage: 12.8V
Energy: 2560Wh
Charge Voltage: 14.4±0.2V
Max. Continuous Load Power: 2560W
Max. Continuous Charge/Discharge Current: 200A
Max. Discharge Current 5 Seconds: 400A

GoWISE Power 2000W Pure Sine Wave Power Inverter
Link to Manual
Continuous output: 2000W

Surge power: 4000W

Input voltage range: 10-16VDC

Input under voltage activation: 10.6VDC

Input under voltage protection: 10VDC

Input voltage: 12V

Output voltage: 120V AC ±10%

Output frequency: 60Hz ±1Hz
Output waveform: Pure sine wave (THD≤ 3%)
Conversion efficiency: 90%
Overload protection: Yes

 

[Hedges]

Charge controller document talks about 3-stage charging.
Are you able to program its voltages? 15V would be too high for lithium battery. You need to set a lower voltage.
Battery should have a BMS for backup protection, but charge controller is primary.

The battery's suggested 14.4V or even 14.2V still seems high, 3.55V per cell.

Try connecting a car battery instead, and monitor voltage to see what it does. That way, not much harm from a brief over-voltage.

 

[Hedges]

"Suitable for battery types such as lead acid, vented gel, and lithium battery, etc"
For charging a lead-acid battery, SCC should have a battery temperature sensor and adjust voltage by some volts/degree.
For lithium it should not change voltage with temperature.

"D04 01" Lithium selected
"D01 14.5" (Not used for lithium)
"D02 14.5" That's boost voltage 3.625V/cell. I think most people here use 3.5V, maybe 3.4V. Since this appears to be the one and only charge voltage for lithium, it is float too. Held at 3.625V continuously, I think it would overcharge.

I'm confused by all the voltages given in the battery manual. But I like "Boost charging voltage 13.8V", which is 3.45V/cell. That seems better.

 

[ericbakuladavis]

A big Thank You, @Hedges, for digging deep to help me

Are you able to program its voltages? 15V would be too high for lithium battery. You need to set a lower voltage.

It looks like I can set the "highest charging voltage" (D02). Right now it's at 14.5.

Charge controller manual says Overcharge Protection Voltage - 12V Bat: 15V
Battery manual recommended charge controller settings says Over Voltage Disconnect Voltage: 15V

Do these two match up?

Try connecting a car battery instead, and monitor voltage to see what it does. That way, not much harm from a brief over-voltage.

Would this be a meaningful test given that my solar battery is LiFePO4 and my car battery is LA? Are you suggesting I try charging the car battery in lithium mode?

For lithium it should not change voltage with temperature.

I have no temperature sensor.

"D04 01" Lithium selected
"D01 14.5" (Not used for lithium)
"D02 14.5" That's boost voltage 3.625V/cell. I think most people here use 3.5V, maybe 3.4V. Since this appears to be the one and only charge voltage for lithium, it is float too. Held at 3.625V continuously, I think it would overcharge.

I'm confused by all the voltages given in the battery manual. But I like "Boost charging voltage 13.8V", which is 3.45V/cell. That seems better.

I'm fine with lowering the controller's "highest charging voltage" (D02). The battery seems to charge more than fast enough with D02 set to 14.5.
Are you suggesting I set D02 to 13.8?
Could the batteries still be overcharged at that voltage?
Battery manual says 100% state of charge is about 13.5V after 15 minutes of rest.

If I were to leave the panels on all day, how would I monitor for over-charging? Periodically shut off panels, wait 15 min, and check state of charge?

 

[brewmatic]

Are you suggesting I try charging the car battery in lithium mode?

Yes, you can use a car battery to test your charger. Keep all settings as for LiFePO4 but limit charging current to 10Amp or so.

 

[Hedges]

Overcharge disconnect 15V - that's 3.75V/cell
That, like many values in the LiFePO4 battery manual, seems to high for a LiFePO4 battery to me.
Maybe, that isn't a voltage it charges to, rather a spike in voltage if inverter is drawing current, charger is delivering current, and suddenly inverter stops drawing current? If such a spike occurs, charger is to disconnect and wait until voltage drops lower before reconnect?

Yes, I suggest 13.8V for D02. 3.45V per cell.
I'm not entirely sure what voltage held continuously would reach and exceed 100% charge. But it is OK to experiment with lower max voltage for lithium batteries because, unlike lead-acid, they aren't harmed by failing to reach full charge. If you find you aren't getting enough capacity charged to some voltage, you can increase voltage (remaining under spec maximum) to access more.

Here's a chart showing SoC vs. voltage. It shows > 99% charged at 13.8V, 90% at 13.4V
That may be resting voltage. If held at 13.8V indefinitely, current would taper off but maybe get there eventually. Your PV charging during the day and drawing current at night would probably have it at some lower state. Seems like a good setting to use.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

The numbers might vary a bit between manufacturers, but I'd be nervous following your manufacturer's recommendations. They might be for a charger which charges at that voltage, then turns off at some tail current. Your solar charge controller won't do that, will hold voltage all day long.

When people here "top balance" a battery they are building, then they use higher voltage but monitor current and turn off at some tail current - a one-time process.

The manual also says to store unused battery at 100% SoC and recharge every 3 months. That also differs from our understanding, which is that storage at 50% SoC (and moderate temperature) is best.

 

[Hedges]

You should try to cycle the battery no further than 10% to 90% SoC. That would be staying within the fairly flat area of the curve. Getting the last 10% at top and bottom accelerates wear-out.
Except, it does need to be charged to high enough voltage and held there long enough for BMS to detect out-of-balance cells and rebalance them. Exactly what voltage that requires is determined by the BMS. I think 13.8V (3.45V/cell) held for a few hours will do that.

 

[ericbakuladavis]

Yes, you can use a car battery to test your charger. Keep all settings as for LiFePO4 but limit charging current to 10Amp or so.

Nice! I'm thinking I could keep the amps in that range by using 1 panel.

Overcharge disconnect 15V - that's 3.75V/cell
That, like many values in the LiFePO4 battery manual, seems to high for a LiFePO4 battery to me.
Maybe, that isn't a voltage it charges to, rather a spike in voltage if inverter is drawing current, charger is delivering current, and suddenly inverter stops drawing current?

Seems possible. I'll do some research into the battery...

Here's a chart showing SoC vs. voltage.

Battery manual has its own chart.

The numbers might vary a bit between manufacturers, but I'd be nervous following your manufacturer's recommendations. They might be for a charger which charges at that voltage, then turns off at some tail current. Your solar charge controller won't do that, will hold voltage all day long.

Interesting... Might need to think about getting a different charge controller. But first, I'll try lowering the "highest charging voltage" (D02) and see how that goes.

... it does need to be charged to high enough voltage and held there long enough for BMS to detect out-of-balance cells and rebalance them. Exactly what voltage that requires is determined by the BMS. I think 13.8V (3.45V/cell) held for a few hours will do that.

Did not know about this! Much appreciated. Makes me thinking of opening the battery so that I can check the balance myself. I don't think the BMS has any way of communicating with me other than shutting off the battery.


Thanks all!

 

[Hedges]

Some BMS use bluetooth. A quick search didn't report that, but there are threads here discussing Ampere Time.

You obviously want your inverter to do a low-voltage disconnect higher than the BMS. Just like you want charge controller to regulate lower than where BMS disconnects. Just how to implement that considering voltage drops and surge currents, I don't know. (I have AGM and let my Sunny Island worry about it.)

Please help!

Disclaimer: I’m a complete beginner. So I have a 24 V system. I am using two 200 amp hour “ampere time” lithium iron phosphate batteries in series (4800 watts). My system is built with almost every component from mobile solar power site.(...

diysolarforum.com

 

[ericbakuladavis]

Hmm... The Amazon page for my battery indicates it has 8 cells, not 4... Does that change any of your recommendations, @Hedges?

 

[Hedges]

Hmm... The Amazon page for my battery indicates it has 8 cells, not 4... Does that change any of your recommendations, @Hedges?

If so, that would probably be 2p4s using one BMS. Otherwise, 4s2p using two BMS.
But no change to my suggestions. It is shown as 12V not 24V.

Maybe BMS has communication. If you find what model you can search.