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If I want to charge my deep cycle 12v battery from a discharged state (5v or so), how do I figure how much current I need from solar cells to charge it (solar cell is rated at 21v max output and has an external regulator limiting out put to 14v). Will it charge it as long as I'm putting more current in that is being drawn and the only drawback is the time it will take to charge?

If you can avoid it in the future, please don't take that deep cell down below 9.5 volts or so. They are not bulletproof, and can lose some capacity when very deeply discharged.

Could you describle this battery in terms of amp-hours? How about pounds of weight? They are roughly interconvertable.

The amps you will be charging at will depend on the power source and the internal resistance of the battery (which will change). Not sure I can respond to the question exactly as asked. I would say that any PV can recharge any 12 volt battery eventually, but there must be exceptions.

DAS

It is an AGM 75amp hours. Weights 50lbs or so. I left the car alone for about 2 or 3 weeks and it drained. In hind site I probably should have disconnected it.

Here are the specs on the battery.
http://www.svrbatteries.com/battery_page.php?bid=24

Tom,
Yes the time to charge will be determined by the difference between the background load and the output current available from the solar panel.

The external charge limiter should prevent overcharging.]

As Tochatihu indicated, letting the battery get that low can do some damage, so you would be better off leaving the solar charge on the battery, and keeping the thing topped off.

Do you have a large background load on the battery? A battery that size should not drop to that low level of SOC with the car off, even if sitting for several weeks.

To figure out the time it helps to know just how low the current Ah reading is. For example, if you're at 25 Ah remaining you need to replace 50 Ah at 12V, or 600Wh. If you have a 100W panel that outputs at 12V it will take 6 hours at maximum output to replenish the battery.

I have a few large amps and car audio stuff, as well as a few gauges. While they are off when the power is off, who knows how much current they are using to keep their memories from reseting.

I'm wondering if my battery is now dead. I drove it around for about 20 minutes the other day after jumping it, but then the next morning the voltage was down to 8v and wouldn't start again.... I did have to turn the headlights on, but I would have thought it would still be charging the battery.

Sounds like your battery may already have some damage.
You need to start by measuring what the 12V current draw is when the car is off.
When the car is running with no loads, what is the 12V system voltage.
Have the battery tested.
Once you know what the car off current requirement is, and the condition of your 12V battery, the original question will be easy to answer.