Car batteries

A flat car battery might need 400-700W/h [+-] put back into it to crank over a car engine.  

5W car charger panel x number of hours of sun, average of 4.3hrs worth [x full output] in Melb per day puts that at about a month to charge. 
10W car panel about 2 weeks. 
200W, most likely a day. 

So, they're great for keeping a car not used with a charged battery [lead acid slowly loose charge] but not so good charging dead batteries. 
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On Mr. Pete's original message, if the batteries new then you might have something discharging the battery [usual source is lights] when you're not using it.  If you have a multimeter, you can check the current by pulling off one side of the battery with the car off and seeing what the current is.  Another more basic [not accurate] way is to pull off the battery and see the spark you get when things are turned off.  It could still be the battery, not every one is perfect, though it's unusual.   5kms isn't very far, would be more so if done at night, though most alternators ought to be able to get close to the charge used back into the battery in that distance.  It could be the alternator starting to go. 

In terms of charging a battery, panels are a good option, as is a generator or even the car if you have fuel. A lot of power to move a small alternator but it will work and you can run leads to additional batteries in 'series' for daily charging.   The panels need to match your outputs, an approximate on that is the panels make 2-6.5 [depends on the time of year] x your panel output.  So a 200W panel makes;

200W x 2 = 400Wh in winter
200W x 6.5 = 1300Wh in summer

You'll have losses in inverters and regulators and a safe place to be is about 75% of the above outputs, so 300Wh-975Wh.  Mostly you want to know what you will use those panels for and work out an energy budget.  Battery size would also depend on your energy budget, the more you use in a day [or are likely to use] the more batteries you would want to have.  Car batteries aren't very happy with being discharged too much regularly, so ideally you would keep them with about 75% charge if you wanted them to last. 
 

think OL's point is that when u keep some juice running into the batteries, you don't need to worrie about it going flat...

Thanks for the great info guys!!!!!

Great stuff folks. Thanks for sharing.

If the battery is functional, it shouldn't discharge very much unless the vehicle is parked for long periods of time. It could be that the vehicle's alternator is either not charging, the battery terminals aren't making adequate contact to the wires sometimes caused by loose clamps or corrosion, or maybe the belt is loose. A good way to check is to connect the leads of a DC voltmeter to the battery's terminals and record the voltage. Then start the vehicle and see if and how much the voltage reading increases.
Also, it's not very difficult to install a dual battery system to protect against getting stranded. You can usually find room for another battery under the hood, so you ground the spare battery's negative terminal to the motor and use either a solenoid or switch to connect the spare battery's pos terminal to the main battery's pos terminal. If the main battery is dead, you either activate the solenoid or make the switch to connect the spare battery in parallel with the main battery. After the vehicle starts, you disconnect the spare battery from the circuit so the alternator will charge the main battery.
Since normal battery chargers aren't much use if the electricity goes out, I made a gas powered battery charger by connecting a 5hp horizontal shaft motor like the ones used in garden tillers to an automotive alternator. I used standard alternator pulleys on the alternator and motor and connected them using the shortest ribbed belt I could find. Both are mounted on a hardwood board with two small wheels attached since the setup weighs about fifty pounds. If I have a dead battery, instead of pulling an extension cord, I just roll the gas powered charger out to the car, connect the alternator's leads to the dead battery, and start the motor. Since the alternator is designed to charge automotive batteries, there's no danger of over or under charging.
If something causes the grid to go down, you have a system for charging batteries AS WELL as a great 130 amp 12vdc power source for lighting or whatever you need it for.

I love my solar panels for battery charging. EG the solar panel in the pic is from my BoB 24W it just charged my phone from 28% to 83% in 60 mins. I have a 120W unit for charging my deep cycle car battery.

Generally I would recharge my 14000mA/hr power bank from the solar panel and then recharge lights & devices from the powerbank as required.