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Old 09-25-12, 03:14 PM   #13
Join Date: Sep 2012
Location: England
Posts: 28
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How long will a 12v battery last with a .0228kW drain on it?
If I have say a 800cranking amp deep cycle battery?

12v Load = 1.9A
12v x 1.9A = 22.8W = P


1.Add up the hours that each Load will be on for to get the Total of Energy Consumed.
2.From that you can Calculate everything you need IE: Solar Panel / Battery / Regulator

My Calculations

1 X 12V Pump @ 22.8W
on for (SEE CHART) hours per night

-Watt/Hour Chart-
1 hour = 22.8Wh
2 hour = 45.6Wh
4 hour = 91.2Wh
8 hour = 182.4Wh

Total usage per night in Winter = 182.4Wh/d

-Load and Battery -

Consumption = 182.4Wh over 8 hours
182.4 watt-hours divided by 12 volts = 15.2 amp hours taken from system in 1 night

Because we can only use half the energy in a lead acid battery without harming the battery,
the minimum battery size is 15.2 amps / .5 = 30.4Ah.

30% Discharge rule - the minimum battery size is 15.2 amps / .3 = 50.67Ah.
20% Discharge rule - the minimum battery size is 15.2 amps / .2 = 76Ah.

choose what calculation will be best for your battery !!
10% discharge is a non no.

I want my system to be reliable if we have four consecutive days of cloudy weather,
(using the 30% Discharge rule) 4 days of autonomy x 50.67Ah = 202.68Ah for the battery. = 210Ah + battery
(Required Battery Bank = 1 x 200 or 220 Ah True Deep Cycle


-Charging Battery from Panel-
This installation is in a location that gets 4 hours of full sun per day.
Check the charts for this depending on the month in the Season.
To recharge the battery for one day of use we need 50.67 amps in 4 hours = 50.67 / 4 = 12.7amps from a 12 volt solar panel array.

-Solar Panel -
Most load calculations include a discount factor for the inefficiency of recharging the battery.
20% is typical. 12.7 / 0.8 = 15.83A

A 2 x 140W solar array that has an Impp (amps maximum power point) of 7.7A = 15.4A would be suitable


-Charge Controller Rating in AMPS-
The 2 x 140W solar array has a short circuit amp rating (Isc) of 2 x 8.2A = 16.4A
16.4A x 1.25 = 20.5A
use a 20.5A or larger charge controller with this array to charge the battery.

Cheap example - 30A Unit
For best efficiency to charge use an MPPT Type.

- estimated Cost of mentioned Materials -

Have seen 2 x 140w poly panel on Net for $340 inc delivery
Have seen 220Ah Varta Leisure Battery 12V 220Ah for around $300 inc delivery
20A Charge controller $80 - mppt prefered of course
Mounting Bracket for Panel = $20

Estimated Total = $640ish

Also required - Fuse box / switches / bits .. !!

Question ...If above calculations are based on a maximum of 8 hours use = 640
so 4 hours would mean 320 and 2 hours would = $160 for a 45.6W/h Load would all this calculate back nicely ?

-Solar Panel specification-
STK-140P6-A ...... manufactured by 3E
Related power
Open circuit voltage Voc(V)
Short-circuit current Isc(A)
Optimum operation voltage Vmp(V)
Optimum operation current Imp(A)

The above methods of calculation have all been taken from the Net over time !!
They seem to work, but if you think they are wrong then let me know.

Remember 12V Batteries should be fused and Stored in a safe manner
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