Battery/Panel Specs and Selecting MPPT Charge Controller

[Kugel8x57]

I now have a 24V x 96Ah Battery Bank: 12 x 12V x 16Ah LiFePO4 batteries wired in a 2S6P configuration.

Also, for this battery bank, I have 6 x 250W Panels: Vmp = 30.3V, Voc = 37.6V and Imp = 8.27A, Isc = 8.85A. My plan is to wire them 3S3P for 90V x 16.5A nominal to the charge controller. This higher voltage and lower current is a good thing for wire size and/or distance to the MPPT... Yes?

Loads will be intermittent, but will be plugged into a 24vdc-120vac 2000W PSWI. There will be a steady load of about 300W and, on occassion, I will be drawing ~ 1800W from that inverter (300w + 1500w). With 85% efficiency and overhead, that's going to be about 98A @ 24vdc. However, it should only last about 15-30 minutes once or twice a day.

Based on the attached Spec Sheets for the batteries and panels, what charge controller and charge profile would you all recommend?

Many Thanks and God Bless!

Kugel

 

[gnubie]

6 x 250W panels = 1500W peak paper. 1500W into a 24V battery is approx 63A. With your array voltage and the desire to get every last watt out of the array and into the battery you'd want an 80A charger with at least a 105V input voltage rating. Your array may never reach 1500W and even if it does it may only do that at the peak of the day so you could also use a 60A controller if the loss of a bit of power should your array ever produce 1500W is acceptable.

If your battery is 96Ah 60A is a fair charge rate, 0.6C. Even though lifepo4 cells are often rated to take 1C charge rates best life span is often obtained at a lower charge rate. Check your battery's / cell's recommended charge rate. If each battery has its own BMS you also need to consider what happens if one of them trips for any reason during charging. In your configuration a battery tripping will not result in all current flow stopping so the batteries that can still accept current will be exposed to a higher charge current.

Charge profile, the lifepo4 one of course . If you are doing a custom profile my preferences would be bulk/absorption/boost, 28.4V, float 27V. Equalisation is always off since it serves no purpose and could trip the BMS and/or over charge the battery. Absorption/boost is either tail current terminated or time limited based on what you can do but what ever you do you need to ensure that your battery is not held at 28.4 indefinitely.

 

[Kugel8x57]

Thanks gnubie! Ha! I forgot to attach the battery and panel spec sheet. Please see attached.

Also, do you have any particular brand of controller that you would recommend. I would prefer one that either has a view screen, or is capable of adding a remote view screen. I am not a fan of the Bluetooth types, that require me to use my phone. I'm looking for reliability and longevity, NOT bells and whistles.

Lastly, do you think that the 2000W PSWI is appropriate? Can or should I go with a bigger one or smaller one (requires lowering my max loads)?

Thanks again and God Bless!

Kugel

 

[gnubie]

I'd look at a Renogy Rover line. It has basic operations from the front panel and if you need a remote and/or the want to go deeper into more than just basic configuration it has an optional external panel.

The recommended charge rate for the individual batteries is 10A. 6 packs in parallel gives you 60A so you are in the sweet spot for peak charging should the array ever be capable of it and if all the batteries are capable of accepting a charge, ie the BMS has not disconnected. Max charging is 40A so you go down to only 2 sets connected before you'd get into the point where the remaining batteries all disconnect on over current. Even so that'd be riding those sets hard.

Rated continuous discharge is 42A so that gives you about 250A continuous which is about 3 times the current required for 1.8kW at 24v. Each string contributes about 14A if all strings of batteries are operating and the wiring is balanced. 14A is about 0.9C. If the BMS in any battery disconnects under discharge the current from the remainder goes up to take up the slack and this increases the C rate.

If you only plan on going up to 1800 watts for 15 to 30 mins (cooking?) I'd be happy enough with that inverter and lithium battery but you will need to keep an eye on the time of use. The battery is notionally 2.3kWh fully charged. If you run 1800W for 30 mins you are half discharging the battery so you need to make sure you have enough sun exposure to replace that. Don't forget about rainy / cloudy days. On those days your solar production will be well down.

If you are aiming for battery life or even just expected life, approx 2000 cycles you won't get it. According to the spec sheet rated cycle life is at 0.2c discharge rates. Exactly how many cycles you can expect is a good question since they don't mention that but it will be less than '> 2000' cycles since you plan on doing 0.9C rates of discharge across half the battery capacity.

Also note that their end of life point is 70% of original capacity not 80%. That catches some people out. As your batteries age their capacity drops. If you were planning on 80% still being enough after how ever many cycles, you are going to fall short of the mark.

*edit to fix incorrect C rates*

 

[Kugel8x57]

Thank you, sir! ?

Kugel

 

[gnubie]

Just a question out of curiosity, why are you using so many small batteries in parallel? The wiring to get the current balanced evenly across all the batteries is possible but it's going to be a PITA. It'd probably cost less to buy less higher capacity batteries too, perhaps even just one 24V battery.

 

[Kugel8x57]

Fair question, gnubie. $/Ah for LiFePO4 was the reason. I really wanted to build my system with lifepo4 batteries for increased longevity over lead acid batteries (Sealed, AGM, etc.). I bought 16 of these for $50 each, which will allow me to make up my two systems: one at 24v-96Ah and the other at 24v-32Ah. (We haven't talked about that one. But what you have helped me with in this thread applies to that one, too; scaled down, of course.)

From watching Will's videos, I believe that I will get a good balance, if I wire them in 2P2S and make 3 x 24V-32Ah smaller banks that I then put in parallel to get 24V-96Ah. Does that sound correct? Also, I have plenty of copper busbar that should help keep connections the same length for good balance.

Thanks again for your help and God Bless!

Kugel

 

[gnubie]

If you can swing it put more batteries into the bank we have been talking about to improve the C rate at 1.8kW discharge. It will help with the longevity of the system if you plan on using this setup for the long haul.

Yes, you can do that with the arrangement of the batteries to help more easily distribute the current. If you can do it when testing the setup use a clamp meter and compare the current at all layers of the bank to make sure it's well distributed.

 

[Kugel8x57]

When you say Improve the C rate, I assume you mean lower it. 24v-96Ah = 6 pairs of batteries ==> 1800W/24 = 75A. 75A/6 = 12.5A/pair. Which is also 12.5A/battery.

If I use all 16 batteries: 24v-128Ah ==> 8 pairs ==> 1800W/24 = 75A. 75A/8 = 9.375A/pair. That is below the recommended 10A Charge Rate. Is this along the lines you are thinking?

Thanks again and God Bless!

Kugel

 

[gnubie]

Yes, a lower C rate. Your battery's cycle count is specified at a 0.2c discharge. If the number of cycles you can get out of your battery is important lowering the discharge rate from 0.9C will help.

When calculating the DC draw for a given AC wattage you have to include a loss figure for the inverter. You may find that the inverter is 90% efficient at 1.8kW so you have to put 2kW in on the DC side which is 83A. That's using the nominal voltage. The main part of the discharge graph will see the voltage around 25V, but that only moves the current to 80A so not much change there.

Lithium doesn't mind being charged at a lower rate than the recommended rate. Recommended is just where the manufacturer has determined a good balance between charge time and the cycle life of the battery.

 

[Kugel8x57]

Thank you, yet again!