I did some search on this topic but could not find anything helpful.

I have a FishMark 320 that display the voltages. Can I use this as a battery meter?

percentage of

charge      12v bat. voltage       24v bat. voltage      specific gravity

100                  12.70                        25.40                  1.265

95                  12.64                        25.25                  1.257

90                  12.58                        25.16                  1.249

85                  12.52                        25.04                  1.241

80                  12.46                        24.92                  1.233

75                  12.40                        24.80                  1.225

70                  12.36                        24.72                  1.218

65                  12.32                        24.64                  1.211

60                  12.28                        24.56                  1.204

55                  12.24                        24.48                  1.197

50                  12.20                        24.40                  1.190

40                  12.12                        24.24                  1.176

30                  12.04                        24.08                  1.162

20                  11.98                        23.96                  1.148

10                  11.94                        23.88                  1.134

info. from ...http://www.otherpower.com/otherpower_battery_metering.html

What do you guys think? Do I need a real battery meter?

All of the battery meters i have seen are just voltmeters with an appropriate scale. If the VM in your fish finder is accurate, is should be as good as any other meter. The trick is to determine the voltage vs the state of discharge. The scale you have provided looks wrong to me, because the battery voltage at 100% discharged is 10.5 volts, and you show about 11.90. The voltage will vary a little with battery type (not brand). Flooded batteries according to the Trojan website measure 12.9 at 0% Depth of Discharge charge, 12.66 for 20% DOD. 12.38 for 40% DOD, 12.06 for 60% DOD and 11.7 for 80% DOD and 10.5 for 100% DOD. These are the voltages measured across the battery terminals with no current flowing to the load. As soon as you turn on the motor the voltage will drop significantly, and that is normal.  

Thanks Sneaker,

Do you have a link to the website with the discharge list.  The table I listed is from website I linked.

I know we have several battery expert on this forum.  Your opinion is highly appreciated.  

tight line!

More info. this one from http://www.batterystuff.com/tutorial_battery.html#8

6. Battery Testing can be done in more than one way. The most popular is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer and measure voltage, use a digital D.C. Voltmeter. A good digital load tester may be a good purchase if you need to test batteries sealed batteries.

You must first have the battery fully charged. The surface charge must be removed before testing. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must experience a load of 20 amps for 3 plus minutes. Turning on the headlights (high beam) will do the trick. After turning off the lights you are ready to test the battery.

State of Charge      Specific Gravity      Voltage

                                               12V/6V

100%                        1.265            12.7/6.3

*75%                        1.225            12.4/6.2

50%                         1.190            12.2/6.1

25%                         1.155            12.0/6.0

Discharged                  1.120            11.9/6.0