Jared is right on the money with the relation between battery voltage and battery charge. I copied a chart from Practical Sailor a number of years ago and keep it on the boat.I use a digital volt meter to check when its time to charge my batteries. I wired 2 speaker jacks to a location in the cabin where i can plug the multi meter in and check battery voltages. I use the battery switch to select the battery i want to check. When both batteries are around 50% i turn on the charger. I am able to get 5 or 6 years out of a battery. My refrigeration is Norcold. It runs off 120v or 12v. I don't have to worry about leaving the batteries on charge all of the time. Here is the chart.

When we talk about a battery being at 70% charge we are not talking about voltage but about remaining AMP-HOURS. A battery that can deliver 100 AMP-HOURS will have 70 AMP-HOURS left at 70% charge. That means, and this is a very rough and non-linear approximation, you can draw 7 AMPS for 10 hours or 10 AMPS for 7 hours. The voltage will still be very close to 12 volts, probably 12.45 volts as Jared suggests. When the battery voltage has dropped to 70% of 12.75 volts (8.93 volts), it's DEAD. Hope this helps.

Would you mind...load test the batteries (turn on the lights for one minute to kill the float charge) and then measure each one with a digital multimeter. Before launching/charging in the spring, repeat the same load test, check the voltage again. If they are fully charged now (12.75V) I'll be a picture of wide-eyed amazement if they show more than 12.45V (70% charge) in the spring. It may not seem like much--but that drop indicates you are taking months off the life of the battery. And the odds are it only comes back up to 90% of capacity after you do fully charge (and load test) the batteries again.

You may leave your batteries hooked up to a charger indefinitely as long as it is a unit designed specifically to do so. Your charger must be capable of maintaining a "precise float" voltage of 2.17 volts per cell (2.7 x 6 =13.02 for typical lead acid cells). It should be noted that float voltage is lower than charge voltage. Chargers that posses the necessary flexibility to not only charge, but also to properly maintain batteries on "float tend to be very expensive. Also batteries that are being floated need regular periodic checks of electrolyte level, charge rate and overall state, i.e. no gassing or other problems. For most of us the couple of visits to the boat in the winter are not sufficient to safely leave batteries on charge. You may want to check with your marina the may , have rules against such a practice.
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The ignition switch should have three primary "legs". One is continuously hot. This leg is the battery feed. There is another leg that operates the starter selinoid. THis is the leg your hour meter is connected to. There is one other leg that operates with switch in the "on" position. This is the leg you can move your hour meter lead to. With a volt meter you can easily determine which leg is which. Good luck.