## Problem: Using 30-watt panels on a 100 AH battery with a charge controller – by Sunking

*as Salaamu’alaykum.*

**Problem: Using 30-watt panels on a 100 AH battery with a charge controller – by Sunking**

01-26-2008, 09:51 PM #20

Sunking

Solar Nut

Forum Super Regular

MODERATOR

Join Date: Dec 2007

Location: Somewhere in TX

Posts: 4,946

Indiana, technically there is no problem using a 100 AH battery provided it is charged up near 100% capacity and the system is capable of fully charging a fully discharged battery in a 24 hour cycle.

The problem is using 30-watt panels on a 100 AH battery with a charge controller. I think you have some technical knowledge so I will briefly point the problems out.

As you know 30-watt panels do not supply 30 watts, so let’s use a real number of assuming 90% efficiency. 30*.9= 27 watts at the panel’s terminals. Now we bring that power source through the wiring to the charge controller, and we have another efficiency and power loss conversion of let’s assume 80%. So 27 * .8 = 21.6 watts delivered from the 30 watt panel to the battery terminals.

Now we convert watts to amps assuming a 12 volt system, A=P/E, or A = 21.6/12.6 = A = 1.7 amps.

So 1.7 amps is the charge rate the battery receives. The optimum charge rate for a 100 AH is the 8 hour rate. Batteries charged at an 8 hour rate is 90% efficient, so the optimum charge rate for a 100 AH battery = (100/8) / .9 = 13.8 amps.

So with the 30 watt panels, 1.7 amps is the charge rate. Now determine the time the batteries will receive the charge. The OP is in Canada, I don’t know where exactly, but at any rate you have to use the worse case scenario of Winter. The OP thinks it is 4-hours per day. I don’t believe it is because 4-hours is what PhoenixAZ gets in the winter, but for argument sake let’s use 4-hours. 4 hours x 1.7 amps = 6.8 AH, or roughly 5% of capacity taking charge efficiency into account in any 24 hour cycle.

As soon as the charge current stops, the battery starts going into self discharge, and by the following morning the battery will lose most if not all the charge it received the previous day. That is not even taking into account any use by the consumer, just pure self discharge. So in effect you have a dead battery 100% of the time. If you leave a battery discharged for a prolonged period, the internal cells will short out and destroy the battery.

To prolong battery life, the battery needs to be kept at or very near 100% capacity, and the charging system needs to be capable of fully charging a fully discharged battery in no more than 24 hour cycle. So in my professional opinion a 30-watt solar panels is incapable of doing the job and would do more harm than good.

So to conclude, there is nothing wrong with using a 100 AH battery, you just need to make sure the system is capable of charging it.

__________________

Dereck, PE, MSEE

Moderator

*Last edited by Sunking; 01-26-2008 at 09:53 PM.*

**SOURCE: SolarPowerForum.net**

TenagaSuryasaid, on April 26, 2011 at 10:40 amRelated posts:

01-26-2008, 05:01 AM #12

Sunking

Solar Nut

Forum Super Regular

MODERATOR

Join Date: Dec 2007

Location: Somewhere in TX

Posts: 4,946

cizzi the problem is this. Your system has to be capable of fully charging a battery in a 24-hour period or less. Since the sun doesn’t shine 24 hours a day it has to be something less. You are under the impression you have 4-hours, that is a summer time number, but lets use it.

100/4 = 25 amps per hour for 4 hours. Battery effiecincy is about 90%, so 25/.9= 28 amps.

So you need panels capable of providing 28 amps for 4-hours

So if the system voltage is 12 VDC, then 12 * 28 = 336 watts. Now figure 70% overall efficiency 336/.7= 480 watt of panels to be able to charge a 100 AH battery in 4 hours.

Hope that makes since.

__________________

Dereck, PE, MSEE

Moderator

TenagaSuryasaid, on April 26, 2011 at 10:50 amRelated posts: