Charging Batteries from PV - Balance the Relationship

Technically one could charge as many batteries as one wants. Its what is taken away from them that has to be replaced in the time allotted that would make all the difference in the world.

175 watts at 12 volts nominal would be 14.5 amps times 4 hours a day would be 58.3 amp hours a day. Personally I would like to keep a battery in the top 20% of full charge so, if I were to assume that the load on the batteries would be no more than 58.3 amp hours divided over 4 hours then - 58.3 times 5 would be 291.5 amp hour capacity. I would say 3 - 105 amp hour batteries at 12 volts nominal... nothing else considered. Hope this helps!

My math ends up looking like this:
(battery_amp_hrs / ( (PV_Watts/Battery_Volts) * Sun_Hrs ) )

so, using my 105ah battery on a 170w PV with 4hrs per day ...

(105 / ( (170/12) * 4 ) ) )

(105 / ( (14.17) * 4 ) ) )

(105 / ( 56.67 ))

(105 / 56.67 )

( 1.85 ) days to charge a single 105amp hour battery using

my earlier result was
57amphrs of a 105ah battery

so
57/105 =  .54 batteries per day
or
1/.54 = 1.85 days per battery

I hope the experts can chime in to make sure this is correct.

hi guys,

i have been busy spending time with family and preparing for this weekend's class.  since you asked i will try to help out.

so for as far as the time it takes, there is no answer.  unless you control the weather, there is no right answer. 

now that is not what you are looking for so lets do it this way.

1 grp 31 battery.  12vdc 105 ah (20hr charge rate)

we are going to turn this into watt-hours. 
12 x 105= 1260 watt hours

next i take my array, in this case 170 and multiply by average sun hours per day.  this number will change monthly.

170 watts x 4 sun hours = 680 watt hours (ignoring reality and inefficiencies)

so if i have a battery that is 1260 watt hours i divide by my array's average production.
1260 / 680 = 1.85 days to charge the battery from zero (which it will not and should not be at)

what worries me more, is that your 170w array is one 18v or 24v module and you made no mention of a mppt controller yet there is a 12v battery involved. 

see this thread for more on that.

http://forums.altenergystore.com/Renewable-Energy/Technical/Re-24-volt-panels-to-charge-12-volt-bat/index.php/topic,1276.msg4479.html#msg4479

cheers,

james Alt-E staff

yes but more specifically

battery nominal volts x battery amp hours @ C20 rate

divided by

rated pv watts @stc x annual average sun hours per day @ latitude tilt

equals

average number of days to charge a battery bank from zero to full (without inefficiencies taken into account)

glad to hear about the mppt working well.
next step is to monitor loads closely to come up with an average watt hours per day

http://howto.altenergystore.com/Calculators/Load-Calculator/a74/

or use kill-a-watt meter to identify singular 120vac loads

http://store.altenergystore.com/Meters-Communications-Site-Analysis/Meters-Battery-Monitors/Power-Meters/P3-International-P4400-Kill-A-Watt-Power-Meter/p932/

cheers,

james
 Alt-E staff