Issue with inverter shutting down at 21v on a 24v system

Re: Issue with inverter shutting down at 21v on a 24v system

Welcome to the forum Ray, and sorry for the problems you are having...

First, have you measured (and logged) the specific gravity for each cell with a decent glass hydrometer (or equivalent)?

Next, measure the voltage of each cell (or each battery).

And, if you have a current clamp meter (like this DC clamp DMM from Sears--or better), to measure the charging and discharging current for each battery string?

Basically, looking for overall state of charge of the battery bank and differences between individual cells/batteries/strings (looking for bad connections, bad cells, etc.).

Assuming you have a reliable volt meter and have measured the voltage at the input of the inverter (DMM are similar to the voltage reported by the inverter/charge controllers), my first guesses are:

• Battery bank is under charged (too little charging current, bad cell, bad connections, simply too much load for amount of sun, low charging voltages/absorb time, etc.).
• Too small of wire gauge/too long of wire run from batteries to inverter (and/or charge controller).
• Possibly a bad cell battery from factory (not common, but does happen).
• Not a well balanced system (loads define battery bank size, battery bank + loads define solar array size).
• Most systems tend to have ~1-3 days of storage--A few days of bad weather and no generator backup can mean your battery bank is simply not getting charged. It is easy to "deficit charge an off grid battery bank/system--Discharge 25%, and recharge 20%, 5% down first day, 10% down second day, etc... to where the battery bank is well under 50% state of charge. Batteries that are deeply cycled often (under 50% state of charge) or taken near dead (less than 20% state of charge) are not usually going to last long.
• Another possible mistake is how the solar array is wired... Many people use Vmp~30 volt panels wired in parallel to charge a 24 volt battery bank... That is not correct. You need a minimum Vmp~35 volts to properly recharge a 24 volt battery bank... Or two panels minimum in series, 2 parallel for your setup.

So, lots of measurements and information on your install (the DC battery bus specifically) to help us debug the issues.

Also a quick check on the sizing of your system...

• 8x 235A Crown 6V batteries, a Schneider CSW4024 Inverter
• Schneider MPPT60 Controller
• 4x 295W DMEGC DM295-P156-72 panels
• Batteries are set up 4 in series x 2 for total of 235x2x24=11280Wh or so I think.
• Based on the stats I get from the COMBOX my daily averages are Load 1345Wh, PV Input 4232Wh, battery charge 4075Wh, and discharge is 972Wh. I have no grid connection and I still don't have the generator hooked up to the system.

You have a (2x235AH=) 470 AH battery bank @ 24 volts. Using some rules of thumbs for designing a "balanced/optimum" off grid power system...Generally lead acid battery bank system will have ~1-3 days of storage at 50% maximum battery discharge. Using 2 days as "optimum", I would suggest:

• 470 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max battery discharge = 2,397 WH per day "average" load

Some other rules of thumb based on battery discharge rates for flooded cell lead acid storage batteries:

• 470 AH * 24 volts * 0.85 inverter eff * 1/20 hour discharge rate = 499 Watt load (5 hours per night for 2 nights)
• 470 AH * 24 volts * 0.85 inverter eff * 1/8 hour discharge rate = 1,248 Watt load maximum continuous (till battery dead)
• 470 AH * 24 volts * 0.85 inverter eff * 1/5 hour discharge rate = 1,998 Watt load maximum for an hour or so
• 470 AH * 24 volts * 0.85 inverter eff * 1/2.5 hour discharge rate = 3,995 Watt load maximum surge (seconds to a few minutes--starting pump, etc.)

The above are rough numbers--I just carried out the digits so you can confirm my math and see where I use them in following calculations... Pretty much for solar power, anything within +/- 10% of calculated are "dead on" numbers.

Next, sizing the solar array... Two ways to size. First is based on Battery bank AH rating (bigger banks need larger array) and second is based on actual loads supported by the solar array. First battery bank should have 5% to 13%+ rate of charge. For a full time off grid cabin/home, 10%+ rate of charge is highly recommended (less day to day "worrying" about the system and managing loads):

• 470 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 885 Watt array minimum
• 470 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,770 Watt array nominal
• 470 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,301 Watt array "cost effective" maximum

And then there is sizing the array based on the amount of sun you get for where the system is installed. For example, say you are in Atlanta Georgia, using PV Watts with a fixed array tilted to latitude (roughly 34 degrees from horizontal), we would expect:

Month    Solar Radiation (kWh/m 2/day)
1      3.86     
2      4.67     
3      5.21     
4      6.17     
5      5.95     
6      5.81     
7      5.82     
8      5.83     
9      5.21     
10      5.51     
11      4.42     
12      3.72     
Year      5.18      

At least 4 hours of sun for 10 months of the year... A typical daily load would "break even" at roughly:

• 2,397 WH per day * 1/0.52 typical system efficiency * 1/4 hours of sun minimum = 1,152 Watt array minimum

The above numbers are not exact... They are a rough guide to compare your system against. Your solar array is ~1,180 Watts... It can work for your system (based on generic design rules)--But it is very close. In general, you should only use ~66 to 75% of the predicted output for your daily/average loads--You need a little buffer for bad weather and the days when you use more power (vacuuming, washing clothes, etc.).

If your daily load is "1345Wh", then your array would need:

• 1,345 WH * 1/0.52 system efficiency * 1/1,180 Watt array = 2.2 Hours of sun

That would seem to be sufficient.

And if your array produced 4,232 WH, then:

• 4,232 WH * 1/0.77 panel+controller deratings * 1/1,180 Watt array = 4.7 hours of "noon time equivalent sun" per day

So, your array seems to be producing a reasonable amount of power for this time of year--Of course, that depends on where the system is installed... I guessed Atlanta--But you are probably somewhere else. And you probably have your array setup in 2 series x 2 parallel panel configuration--The numbers look OK here.

Now--The battery bank taking 4.1 kWH to recharge and you used 1.3 kWH the last day--It sounds like the battery bank way under charged... More loads earlier, less sun earlier, or something (could even be an error in reading the display, or the software is not working like you would expect it to--And it is reporting "something else". Measuring the battery bank state of charge with a hydrometer sounds like the first step.

-Bill