Help Calculating Ideal Charge Current Setting on Hybrid Inverters for Batteries

[Rudy1]

After a really bad situation with a solar consultant/home builder, I have had to install and figure out the existing equipment myself - with the help of an electrician who build his own home on solar - it has been a very challenging journey. Not tied to the grid, my calculations suggest I should be able to fully charge the (8) 48V 200Ah batteries tied to the (2) 10KW hybrid inverter systems to 76.8KW - having 92KW full sun exposure to the panels over 5+hours. I have (48) 370 W panels = 17, 760 Watts/Hour.

The problem is that with full sun, the most charge I have gotten on the batteries is about 30% each - with each of the two inverters stating 20KW iintake. I still do not know what Battery C and Battery D refer to on the display. It has been challenging keeping the house powered with a long streak of cloud coverage. I believe the problem lies in the inverter settings - not an issue with panel efficiency/wiring or battery health.

Question: How many amps can I set the hybrid inverters for the charge current to safety and effectively get a greater daily charge on the batteries? The default charge current on the controller/inverter setup was 40A. I have increased it to 100 amps on master and on slave. The 48V 200Ah battery instructions from Greensun were very poor - though I did see one figure that stated 0.1-0.2 (total) for charge current. If there are 4 batteries on each inverter, does that mean 10 -20% of the total Ah is an appropriate setting - namely 4 Batteries x 200Ah x 10-20%= 80-160 Amps? I do not want to create s safety risk or damage the batteries - and customer service has been all but silent in my requests for help.

Any guidance would be greatly appreciated. I am new to this forum but am "all in" as my family depends on this system - with the help of a diesel generator on loan when needed. We just went "live" in December and still need to do cable management and design a better tilt brace. My sons and I built the 8 panel systems ourselves and got through this by the grace of God. Thank you! Bob, TN

 

[BentleyJ]

Yes, your calculations are correct. Each set of 4 x 200Ah batteries can safely be charged at 0.2C (20%) which is 160A.

EDIT: The foregoing is true only to the extent that the Cable Sizes, lugs, bus bars and Over Current Protection Devices are suitable for this amount of current. If Charging was set at only 40A perhaps there is a reason. Additional investigation is recommended.

 

[Rudy1]

Yes, your calculations are correct. Each set of 4 x 200Ah batteries can safely be charged at 0.2C (20%) which is 160A.

EDIT: The foregoing is true only to the extent that the Cable Sizes, lugs, bus bars and Over Current Protection Devices are suitable for this amount of current. If Charging was set at only 40A perhaps there is a reason. Additional investigation is recommended.

Thank you for your reply - and understand the disclaimer portion and have relied on the electrician for proper wiring - along with the factory cables also included. The default 40A was straight of of factory box - and seeing the amp output can be selected into the hundreds, I figured it could be throttled higher per battery configuration. Much appreciated.

 

[Tsolar]

First off, that is a very nice setup, size wise, good ratio of PV, battery, and inverter. Most people cheap out and undersize their battery compared to the rest of the system, yours is not at all. I have worked with some very similar inverters, I believe they were rated at 8kw thou, they were "Renogy X" inverters, but just some rebranded chinese inverter that many other companies put their name on. The menu/screen looks exactly the same. Im not a huge fan of them, they are very basic, as far as settings, but they do seem to work fine, we havent had any real issues with them.

When you are totally off grid you have to take advantage of the sun whenever you can. Throttling the charge current, especially on a battery bank that big, is just handicapping yourself. Your 48 panels only have a nameplate rating of .21C for your 1600AH/81kwh of batteries, so set those inverters to their max charge settings! You will end up overworking those inverters long before that large battery bank.

What is the max charge current each inverter can output to the batteries? 180-200A? Im guessing 160A is getting close to the max. As stated already, as long as your wiring (cables, lugs, etc) are all good for it, then give them as much powrer as you possibly can. That is a very large battery bank, and you dont have enough solar/PV, or inverter, to even make them break a sweat.

What size wiring do you have from the inverter to what looks like a "combiner box" for the batteries? From the picture it looks a little small, but hard to judge really. Is it 2/0 at least? On the battery cables, it looks like you have each inverter wired out to a combiner box, then to each battery, then it looks like each inverter is also wired together? If that is what im seeing, then that seems like a decent setup.

The .2C ratings is to get the absolute maximum life of the batteries, but in my opinion it is a pretty conservative number. Most batteries are usually rated to charge/discharge anywhere between .5 to 1C and still reach their minimum cycle rating, usually 4000+ cycles for LiFePO4. At a .2C charge rate, the batteries will die of old age or the BMS failing before the actual battery cells themself get worn out from cycles. I would not hesitate to charge at .5C for sustained periods, and .7-1C for short bursts, like when the solar is spiking due to clouds and cold temps. I believe voltage on the batteries has much more effect on their lifespan then charge current. LiFePO4 batteries are somewhat self limiting in a way, when you charge them with high current, it will cause their voltage to rise faster, which in turn will cause the charger to slow down its charging earlier, while the battery is still at a lower SOC, and can still easily accept a lot of current.

It looks like you have battery comms cables hooked up. What are you using for charge settings? Are the batteries and inverters communicating? In all the systems Ive built, Ive never seen a real advantage in having the battery communicate with the inverter, especially an offgrid system, where the batteries are getting used daily. And in my experience it actually causes more trouble than good. The battery BMS will protect the battery, weather it is communicating with the inverter or not. I usually set systems up with custom battery charge settings, and no battery comms. I set Bulk/Absorb around 56-56.5v, and Float around 54-54.2v. In the inverters you have, you probably have to set the battery as "Lead Acid" in order to have "custom" settings.

A description of your setup would also help others/future diagnosis. Are the inverters 10kw total or 10kw each? How are your panels wired? I dont understand where your 76.8KW and 92KW numbers are coming from? I think some of your numbers you are mixing up KW and KWH maybe? There is a big difference between the two.