Calculating temperature compensated control setpoints

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Solar Expert Posts: 100 ✭✭
I've been discussing the impact of a cold battery bank and CC on my inverter in another thread: Inverter high voltage shut down. It appears that the batteries are now cold enough so that when the CC compensates for the battery temperature it allows a charging voltage that exceed the HVD for my inverter, which is 15.5 volts.

To clarify what's going on so that even I can understand it, and to make sure my CC is working correctly, I would like to calculate the compensated setpoints for my CC given the the current temperature of the battery bank, which is 48F/ 8.9C. The CC and battery bank are both located in the unheated cellar under my cabin where the temperature is currently 48F.

Here is the information on temperature compensation from my CC manual:

"Temperature Compensation: Four control setpoints (25°C reference) are compensated for temperature (PWM regulation, float, equalization, high voltage disconnect). The charging is compensated by –5 mV/°C /cell (–30mV/°C for a 12V battery). Compensation is limited to minus 30°C."

Would someone explain how to use the above to determine what the control setpoints should be at 48F or at any given temperature?

Thanks a million - Bill
Two 140 watt Kyocera panels, wired in parallel; Ironridge top of pole mount; two 6 volt, 242 AH US batteries, wired in series; Morningstar ProStar 30 charge controller and SureSine 300 inverter; Trimetric 2025-A meter; IOTA DLS-45 charger, Honda EG3500X generator; Aermotor 702 water pumping windmill.

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Re: Calculating temperature compensated control setpoints
kansas wrote: »
Here is the information on temperature compensation from my CC manual:

"Temperature Compensation: Four control setpoints (25°C reference) are compensated for temperature (PWM regulation, float, equalization, high voltage disconnect). The charging is compensated by –5 mV/°C /cell (–30mV/°C for a 12V battery). Compensation is limited to minus 30°C."

0.005 volts = 5 mVolts
Tref=25C

Temp Comp Voltage = (-0.005 V/°C/Cell) * number of cells * (Tbatt-Tref) + Setpoint Voltage

(-0.005 V/°C/Cell) * 6 cells (for a 12 volt system) * (8.9C-25C)* = 0.483 volt offset for 48F or 8.9C

So, if you want a 14.8 charging voltage at 25C (room temperature/standard conditions), the you need:

14.8 volts + 0.483 volts = 15.283 ~ 15.3 volts for charging @ 48F temperature corrected

If the battery bank temperature is >25C, then the offset "subtracts" from the charging voltage.

Note that for equalization voltage--The voltage you want is that which gives you around 2.5 to 5% of the battery banks AH capacity (the 20 Hour rate). If the battery bank becomes too hot during equalization, stop equalization and let the battery bank cool... Start equalization (if needed, later in the day or the next day, after bank is "fully charged").

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 100 ✭✭
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Re: Calculating temperature compensated control setpoints

Thanks, Bill. You're calculation confirms that my CC is functioning as designed. It appears that my inverter's disconnect of a couple of days ago occurred during bulk charging, when the CC exceeded its nominal HVD limit of 15.2, by producing a temperature compensated voltage of 15.6+volts. That was high enough to trigger the inverter disconnect, which is set at 15.5 volts.

So, why not make temperature compensated inverters?
Two 140 watt Kyocera panels, wired in parallel; Ironridge top of pole mount; two 6 volt, 242 AH US batteries, wired in series; Morningstar ProStar 30 charge controller and SureSine 300 inverter; Trimetric 2025-A meter; IOTA DLS-45 charger, Honda EG3500X generator; Aermotor 702 water pumping windmill.
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Re: Calculating temperature compensated control setpoints

High volume (i.e,. inexpensive mass produced) inverters were originally aimed at the automotive market. Nominally, the inverter runs from ~12.0 to ~14.4 volt.

Higher voltage FETs / Transistors cost more money. And probably it costs more to make an inverter that can take cover a DC input range of 10.5 to 16.5 volt DC input range and give you a stable ~117 VAC output voltage (more copper and iron?).

24 volt inverters are probably used a lot of trucks and marine... And the narrower input voltage range was, in general, "good enough".

Now that off grid is used around the world with deep cycle lead acid batteries and are not installed in homes/RVs conditioned spaces, where the batteries may NOT see a more even 60-80F environment (office, home, etc.)--The bad habits have been carried through for decades now. Even for companies that make solar charge controllers with temperature compensation that can go >15.x volts, and the inverters that connect to the same battery bus fault at >15.x volts.

For the most part, it is the folks that live in cold climates (through the winter) that have these issues--A much smaller market.

Anyway, those are my guesses.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 100 ✭✭
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Re: Calculating temperature compensated control setpoints

Interesting - thanks. Bill
Two 140 watt Kyocera panels, wired in parallel; Ironridge top of pole mount; two 6 volt, 242 AH US batteries, wired in series; Morningstar ProStar 30 charge controller and SureSine 300 inverter; Trimetric 2025-A meter; IOTA DLS-45 charger, Honda EG3500X generator; Aermotor 702 water pumping windmill.