A car battery usually does not last long and is durable only for 3-4 years. In my experience, if its efficiency starts dropping, it would soon accelerate the pace of worsening its quality. In this system, it plays 2 major rolls; to save electricity during day time and use electricity especially during night time.
However, if we concentrate its function on charging smart phones and cell phones in the system, it would be much simpler. It means less costly and smaller.

Then the system is as follows.

The system consists of the following materials
.
1. 20W solar panel
2. Cigar socket
3. USB output(12V as well as 24V)
4. USB cable

As above, you should choose a USB output for a cigar socket with dual mode of 12V and 24V. This is because the maximum input from the 20W solar panel stands at around 21V. You should not use this output only for 12V.
Without a cigar socket, you can wire both of the panel and the USB output directly.

The specification of the panel is as follows.

Maximum Power: 20 watt
Maximum-power Voltage: 17.3 volt
Maximum-power current: 1.16 Ampere
Open-circuit voltage: 21.4 volt
Short-circuit current: 1.23 Ampere
Tolerance: + / -5%

You can use an internal checker of the charge through the applications. You can find and use freely various applications from a particular shop such as Google Shop and Apple Store. They will tell you 1) a present status of the battery by % and 2) while charging, a status of the battery by % as well as the approximate time to finish the charge.

The below shows a monitor of the status of charge. The monitor tells you the time of the finishing of charge.

The direct connection of a smart phone with a cigar socket may harm your machine. It is recommended that at the beginning you had better install the checker and confirm the current values of V/A. Then, you should get used to the practice.

The right photo shows charging the iPod Touch. You can see the current V and A values on the checker.

For the sake of safety, you can use a USB voltage/ampere (V/A) checker in the above photo. If you can install it between the USB output and the object to be charged, you can know of the current V/A values.
In the photos below, the blue one is an ordinary device to confirm V value which shows around 5V. The right one has the same functions and additionally it can measure the accumulate value of the ampere as photo. It shows 298 mA accumulated so far. It means that it helps know the charging volume and projecting approximate time of finishing of the charge.

For your reference, the below is the battery capacity of smart phones.

iPhone 5 : 1,440mA
HTC : 1,810mA
iPad 4 : 11,560mA
iPad: mini 4,440mA

On the contrary, it of cell phones is a half of the smart phones.

System without Car Battery

Still do you want to have light at night ?