Voltage is a poor gauge of battery condition
BTW, YES, your batteries are in PARALLEL which doubles the current capacity (assumes identical batteries) and this means longer boondocking between recharges. Relying on a voltage reading (digital or analog) to determine battery condition is almost worthless, especially if you want the batteries to last (btw, if you insist on using voltage, then you really need a high accuracy digital multimeter as analog meters lack adequate resolution). By the time a lead acid battery falls below 12 volts you are already almost at zero charge left. The difference between a fully charged, resting battery under no load and 50% charged under no load is roughly .4 volts (12.64 vs 12.24). All voltage readings have to be taken under NO LOAD and after the battery has rested from either charge or discharge at least 15 minutes. You also have to know the temperature of the battery electrolyte as it has a direct impact on the voltage you will read. You should not discharge a deep cycle battery below 50% unless you want to replace them routinely. A deep charge battery drained to almost zero routinely has a life of around 350 charge cycles. That same battery recharged when it gets to 40%-50% has a life over over 3000 charge cycles. Here are some charts, note the the chart is based on battery temps of 80degrees...see following chart to get the offset for other temps. THIS IS TEMP of the ELECTROLYTE not the air.
see the attached jpg chart images
As you can see, trying to use voltage as an absolute indicator requires an extremely accurate, high resolution digital voltmeter and a knowledge of the electrolyte temperature.
Specific gravity as well as actual amp-hour consumption/charge tracking are the other ways monitoring batteries.
Using a professional, temperature compensated hydrometer (ie not one of the cheapo plastic ball things that you see in the auto stores), you can accurately determine the charge condition, but this means letting the batteries rest under no load for 15 minutes, then removing the caps and taking readings in each cell. You then compare the reading to the chart or use the relative charge scale on the hydrometer to determine the battery condition. The good thing about using a hydrometer is that it measures EACH cell and therefore, if you have a bad or weak cell, it will detect it. ALL other means of monitoring battery condition see the batteries as a single entity and therefore have no knowledge of individual cell issues.
The other way to determine charge state is to install a amp-hour monitoring system such as the Tri-Metric 2020. These devices works by tracking and recording the actual current going and coming from the batteries. When installed, you enter the total amp-hour number that you wish to be used as the monitored capacity. As 50% discharge is always the goal, you simple enter the total amp-hour capacity of the two batterys divided by 2. The monitor will then provide you with a "gas gauge" percentage reading that gives you an pretty accurate charge state of the batteries. Once the monitor reads zero, you need to recharge, and also by using the 50% rule, if the situation should arise where you can't recharge immediately, you do have a good reserve in that the batteries are actually only half discharged. This type of monitor also provides voltage, the present amps being pulled OR pushed (ie charging) to the batteries as well as the cumulative amp-hours pulled from the batteries since the monitor was connected along with some other techy data. The flaw in amp-hour monitoring is that a battery that has a small amperage, slow discharge will produce MORE amps-hours than one that is discharged rapidly (ie high amperage). The difference between the two discharge rates is dependent on the overall condition of the battery and amp-hour monitoring systems don't attempt to compensate for this effect. But if you are monitoring for 50% actual discharge, an amp-hour monitor will still be pretty darn close regardless of the rate of discharge. Installing one of these is fairly simple in that a shunt is placed in the negative line that goes to the batteries. As current flows, the shunt develops a small voltage drop relative to the current passing thru it and that is recorded by the monitor.
If you don't want to spend the $150buck or so for a Tri-Metric or something similar, then get a good professional grade battery hydrometer and learn to use it. The hydrometer is somewhat messy and if you get any battery acid on your clothing, it will eat a hole even if you immediately flush it with water. You can use a voltmeter but it is about like wanting to know the position of the sun but the day is totally overcast and you have no idea of the time. You know the sun is shining, but that is about it.
BTW, I went the amp-hour monitoring route as I wanted to see at a glance without fuss or muss the battery condition. As I also have solar, this gave me much better visibility of the actual usage so as to ensure that the solar panels had a running chance of keeping up.
david & bret
'02 Bambi LS
'99 34' Limited
Air Forums # 2159
Past President Heart of Texas Camping Unit
WBCCI # 7548