Question about battery sizing

[h_1985]

Hello,

In off grid systems sizing batteries is done using watt-hour method taking into account all parameters like (battery efficiency, autonomy days, ..... )

my question is about installed panels watt-peak >>> is that affecting size of batteries ??

another question about what is the status of battery during day light >>> charging or discharging or both & and how i can involve this in battery sizing??

Thanks

[SunEagle]

Usually the panel wattage is determined after the battery system is sized which is after the daily watt hour usage is calculated.

If you already have an installed pv array then you can work backwards to determine the size of the battery that will stay happy based on the pv wattage and type of charge controller. The problem is that if your panel wattage is lower then you need then your battery will also be lower then your load usage and will be over discharged.

The answer for your second question is that the batteries can be doing both (charge and discharge) during the day light. While you can utilize the "extra" panel output to run your loads during the day time it might let you reduce your battery size. But if you do not get enough sun (clouds, low insolation hours, snow, dust, etc.) what you expected to get from the panels, now has to come all from the battery system. Which again can easily deplete it and shorten the battery life.

So first step is to determine your daily watt hour usage. Then size your battery for no more than a daily discharge of 20 to 25%. Then determine the panel wattage to charge the battery which requires you to know your lowest insolation hr period based on your location and if you get any shade on the panels. Then size your charge controller to get the most amps out of your panel system to fully charge the battery each day.

Hope that helped you understand the process better.

[Sunking]

You must know two data points before you start.

1. Daily Watt Hours
2. Solar Insolation for your location in the shortest winter months.

From those two data points everything is calculated. First determination is what battery voltage you will be using. For battery voltage anything below 1500 watt hours can be 12 volts, although 12 volts is very inefficient, expensive to implement, and can only input up to 1000 watt Solar Panels.

So let's run an example. Say you need 3 Kwh per day and in January your Solar Insolation is 3 hours. First try 24 volt battery. For the battery size AH = [Daily Watt Hours x 5] /Battery Voltage], so [3000 wh x 5] / 24 volts = 625 AH..

Panel Wattage = [Daily Watt Hours x 1.5] / Sun Hours, so [3000 x 1.5] 3 = 1500 watts.

Now for the MPPT charge controller Amps = Panel Wattage / Battery Voltage, so 1500 watts / 24 volts = 62.5 amps.

Lastly you need to do a sanity check for the batteries as batteries, FLA Batteries, have a window of charge current they can accept of C/8 to C/12 where C = the AH capacity, and the number is Hours. We determined it would take a 625 AH battery. To make that battery happy means we must charge it with no less than 625 AH / 12 hours = 52 amps, and no greater than 625 / 8 = 78 amps. So the range is 52 to 78 amps for a 625 AH battery. We calculated 62.5 amps @ 1500 watts. We have a perfect C/10 charge current.

So the answer to your question is do the math, do it correctly, and everything works out just right.

[h_1985]

Usually the panel wattage is determined after the battery system is sized which is after the daily watt hour usage is calculated.

If you already have an installed pv array then you can work backwards to determine the size of the battery that will stay happy based on the pv wattage and type of charge controller. The problem is that if your panel wattage is lower then you need then your battery will also be lower then your load usage and will be over discharged.

The answer for your second question is that the batteries can be doing both (charge and discharge) during the day light. While you can utilize the "extra" panel output to run your loads during the day time it might let you reduce your battery size. But if you do not get enough sun (clouds, low insolation hours, snow, dust, etc.) what you expected to get from the panels, now has to come all from the battery system. Which again can easily deplete it and shorten the battery life.

So first step is to determine your daily watt hour usage. Then size your battery for no more than a daily discharge of 20 to 25%. Then determine the panel wattage to charge the battery which requires you to know your lowest insolation hr period based on your location and if you get any shade on the panels. Then size your charge controller to get the most amps out of your panel system to fully charge the battery each day.

Hope that helped you understand the process better.

Thanks for detailed answer

in my case i have already installed array of 390 Wp (2 panels in series) , 700 W 24 Vdc inverter & MPPT charge controller (Steca solarix 2010 12/24V 20A)

watt-hour required is about 1200 Wh daily

batteries went so bad >>> it was wet flooded ... even it was starter battery

now i wanna choose good battery for this system >>> after calculation i found that i can buy two 12v batteries (to connect in series) from Fullriver 193 AH @ C10 (10 working hours) as shown in link below

https://www.civicsolar.com/product/fullriver-sealed-dc215-12-12v-215ah-agm-battery

i wanna know your opinion about my selection ?

Note: peak sun hours (4.5-5) hours

Thanks for your time

[SunEagle]

Those batteries should be able to get you 1200 wh daily (215Ah x 24v = 5160wh / 4 = 1290wh) using 25% of the capacity although I did not see any cycle count or life time rating on the battery which probably means 3 years at best.

The issue is you really do not have enough panel wattage (390w) to generate 1/10 the AH rating of those batteries. At best it can get you about 16 amps (390w / 24v = 16.25a) which is just on the low side of the charging criteria at ~ 1/13 (215ah / 16.25a = 13.2hr)

Also is that 4.5 - 5 hr time frame your worst case or average across the year. If it is average you will definitely be short of sun hours and using solar never put back what you take out on a daily basis without the aid of a generator and battery charger.

[h_1985]

First, basic issue here is that this system is already installed over our company roof as a pilot project only to run two fluorescent light fixtures about 120 w for about 10 hours >>> also no enough space to add mores panels

Task required is to choose another batteries ..... that is it

The issue is you really do not have enough panel wattage (390w) to generate 1/10 the AH rating of those batteries. At best it can get you about 16 amps (390w / 24v = 16.25a) which is just on the low side of the charging criteria at ~ 1/13 (215ah / 16.25a = 13.2hr)

That means battery needs about 13 hours to be fully recharged again ?? .... so it will not recharged during day?? .... why 1/10 of its AH ??

Also is that 4.5 - 5 hr time frame your worst case or average across the year. If it is average you will definitely be short of sun hours and using solar never put back what you take out on a daily basis without the aid of a generator and battery charger.

No 4.5 hours is the worst ... it may even reach 5.5 peak sun hours sometimes

So we have to choose another battery to be recharged using those panels? >>> OR I can use FullRiver batteries mentioned

Thanks for your patience

[SunEagle]

First, basic issue here is that this system is already installed over our company roof as a pilot project only to run two fluorescent light fixtures about 120 w for about 10 hours >>> also no enough space to add mores panels

Task required is to choose another batteries ..... that is it



That means battery needs about 13 hours to be fully recharged again ?? .... so it will not recharged during day?? .... why 1/10 of its AH ??

That 13 hours is the time period to "fully" charge your battery. The time you will use to charge it depends on how much you discharge it.

The best charge rate for a flooded lead acid battery (FLA) is between 1/8 and 1/12 of the battery Ah rating. That is due to the battery chemistry and how fast or slow is should be charged. The 1/10 is the middle or prime spot. So if you take the battery Ah rating and divide by 10 you get the prime amount of amps required to charge the battery.

Once you calculate the prime amp value along with the number of good sun hours, you then size your panel wattage and charge controller.


No 4.5 hours is the worst ... it may even reach 5.5 peak sun hours sometimes

So we have to choose another battery to be recharged using those panels? >>> OR I can use FullRiver batteries mentioned

Thanks for your patience

Answer to your first question is in bold above.

The battery is first sized based on the daily watt hour usage needed. Then the panels are sized to re charge what you take out of the battery. That 390 watt is not enough for your battery system to get the prime charge rate. Adding another 195watt panel to your system will get you about 24 amps of charging which is better than 1/10 and not faster than 1/8.