Sizing a battery bank (small purpose)

Re: Sizing a battery bank (small purpose)

You really need to define the battery first... For example, what min/maximum voltage (6 volt lead acid would run from ~5.8 volts to ~7.5 volts; discharged voltage/maximum charging voltage).

Next, how many amps/milliamps per 24 hours will you be using...

For example a 100 milliamp for 24 hours would be:

• 0.1 amps * 24 hours = 2.4 Amp*Hour per day.

Then, how many days of "no sun" would the application need power. For example we generally size a battery bank for 1-3 days of "no sun" and 50% maximum discharge. For 2 days (example):

• 2.4 AH * 2 days * 1/0.50 max discharge = 19.2 AH @ 6 volt battery pack

You should design your Vmp to be ~ 9 volts (18 cells in series) and around:
19.2 AH * 1/0.77 panel eff * 0.10 rate of charge = 2.5 amp array Imp

-Bill

Re: Sizing a battery bank (small purpose)

You can read about batteries typically used in solar applications here:

Deep Cycle Battery FAQ
www.batteryfaq.org

Solar panels are not "perfect" solar batteries in any sense of the word... We can start with some approximations to roughly size a system.

For example, under normal conditions (typically warmer weather), Vmp tends to fall with increasing panel temperature (solar panels tend to get pretty hot in full sun and not much wind--upwards of 35C/65F temperature rise in full sun).

So, we tend to assume that at ~25C/77F "standard test condition", that Vmp~1.7.5-18.0 volts or so is the minimum voltage required to recharge a 12 volt battery to ~14.6 volts or so during normal charging and ~15 volts for equalization. Plus allowing for a bit of wiring and controller drop.

Usually we assume a solar cell is ~0.5 volts Vmp, or 36 cells in series to properly recharge a 12 volt lead acid battery bank. A 6 volt battery bank would be ~1/2 the number of cells in series, etc...

Vmp=~17.5 volts is the "optimum" tradeoff of charging voltage vs solar panel charging using a simple charge controller (PWM or other type).

You can have higher cell voltages, but there will not be any additional "power"... Remember that POWER=VOLTAGE*CURRENT...

Solar panels are, more or less, current sources (they output constant current in proportion to the amount of sunlight hitting the panel). So with P=V*I, the V is set by the battery bank and I is set by the amount of sunlight on the panel.

The reason for the 1-3 days of storage "no sun" has to do with the overall sizing of the battery bank. If it is too small, you will need a backup generator or need to turn off your loads on a cloudy day. If you have more than three days of storage, then you need very large solar panels just to quickly recharge the battery bank (and batteries that set for days/weeks/months below ~75% state of charge will sulfate and have less capacity--eventually failing).

And discharging a battery bank below 50% state of charge (20% state of charge is the maximum discharge before damage may occur) will limit the number of cycles of the battery bank.

Many of your battery choices depend on your needs... If this is a summer only project and you will throw the battery in the recycle bin at the end of the season--then you could design for 1 day of life and 80% depth of discharge, or 1/0.8 = 1.25x your daily load.

I can go on an on about all the rules and suggestions about your system, but it is probably much easier for you to describe your loading needs and the environment... and then we make suggestions on how one may approach the problem.

-Bill