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Hello, this is my first post and I am looking for help. I had recently bought the 100 W solar panel kit on Amazon. It came with a charge controller. I had purchased 2. 6V Duracell golf cart batteries. During the daytime my charge controller says it is charging about 12.5 V and my batteries eventually charge up to 13 V. But after the sun goes down my batteries charge goes down to 12.5, 12.3 V. I do not have my inverter hooked up to my battery bank. I am very interested in solar panels and help to make a Solar battery bank. I just cannot understand why my batteries are not holding a charge. My batteries are new.

Welcome to the forum FFNF!

Ok... Guessing that you might have gotten the Renogy 100 watt panel + 30 amp charge controller. The panel should be fine. The rated current should be around:
Renogy 100W Monocrystalline Solar Panel
Maximum Power: 100W
Optimum Operating Voltage (Vmp): 18.9V
Optimum Operating Current (Imp): 5.29A

Don't know anything about the charge controller--But lets assume it is working.

Next, if these are 6 volt ~ 200 AH lead acid batteries. Lets call 12.3 volts about 1/2 charge (for round numbers).

This is winter, using the SolarElectricHandbook for Tulsa OK, fixed array:
[h=3]Tulsa
Average Solar Insolation figures[/h] Measured in kWh/m2/day onto a solar panel set at a 54° angle (from Vertical):
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun

3.61

4.01

4.74

5.20

5.22

5.37

Jul
Aug
Sep
Oct
Nov
Dec

5.84

5.62

5.28

4.75

3.73

3.36

With a PWM controller the maximum charging current Imp~5.29 Amps. With 4.01 hours of sun average for February:

5.29 amps * 4.01 hours of sun = 21.2 AH of current per day

The battery:

200 AH * 50% = 100 AH
100 AH / 21.2 AH per day = 4.71 days of average sun for February

The 200 AH battery bank with ~5 amps of charging current is about the minimum size solar panel we would recommend for charging a lead acid battery. That is OK for weekend/seasonal usage... For daily usage, a 10% to 13%+ solar array would be nice.

If you would like to learn more about your system and how it is operating, you might want to look at getting a DC Current Clamp Digital Multi Meter, such as this one from Sears (pretty inexpensive and "good enough" for our needs).

You should also get a decent Glass Hydrometer to measure the specific gravity of your battery bank (assuming flooded cell). If you want a really nice Hydrometer, take a look at this one.

Read about batteries... They are the "critical part" of your system. If you leave it set at less than ~75% state of charge, it will sulfate faster (and could be dead within 6 months).

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Electrically speaking, you want to see the charging voltage of the battery to be held at 2-6 hours or so at around 14.4 to 14.8 volts to be fully charged.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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