understanding 12 volt battery capacity

Hi, I am using 2 Kinetic Energy 600 Batteries as the primary source of prototype boat and am not sure how to read the capacity levels. They are 12 volts and I am running them in parallel. When I got back I put the battery on the charger and it said 51%. It showed 12.5volts. I know when this particular battery is charged it reads about 14.5 volts. Does this 51% mean I have half the battery left? Would that 2nd half of the charge deliver the same power? Thank you

The short answer - the ROT is that for a lead-acid battery, each 0.1V represents 10% capacity. (It may be more, ie, 0.13V = 10%.) If you want more info, keep reading. I'd suggest Bill Darden's BatteryFAQ (see section 19 & download the zip file). In theory, a fully charge battery is 12.67V; typically 12.6 to 12.7V in practice. A 100% discharged battery is typically around ~11.3V but may be higher (~11.9V); ie, full to flat is a range of ~1.3V. I use a general rule that 1.0V is full to flat, hence each 0.1V drop is a 10% discharge (from its fully charge voltage of (say) 12.7V). Two main "discharge" or usage types of batteries exist - deepcycle and cranking. General rules are that deepcycles should not be discharged more than 50% (though some claim 80% & higher) and crankers no more than 20%. Hence eg, from 12.7V, no lower than 12.5V & 12.2V for crankers & deepcycles respectively. (They are conservative values - if 10% is really 1.3V, then a deepcycle can go 12.7 -(5x0.13) = 12.05V, etc.) There are other "type" classifications - like stationary, cyclic, solar, UPS, etc batteries, but they are based on the 2 topologies - many thin plates for cranking/high current, and fewer thick plates for deep discharge. And then there is flooded/wet cells, gel cels, and AGMs.     The voltages above are OC = Open Circuit, and rested - ie, no loads, and long enough after a discharge or charge to be "rested". Discharge recovery may be insignificant or a few minutes, but charging causes a "surface charge" which can take 24 hours to dissipate (or use a load to remove it quicker). The OC voltages are NOT the same as charging or loaded voltages. A typical vehicle will charge at 14.4V to a max (long term) of 14.4V. [ The older traditional and "theoretical" setting of 13.8V was increased because it was not enough to remove sulfation caused whenever a battery is NOT fully charged, and hence batteries would prematurely fail. ] Some will decrease to 13.8V or even a float voltage of ~13.3V when the battery is fully charged, though this is rare. "Fully charged" may be sensed by the charge current dropping to the battery's float current - typically up to 2A. The charging voltage gives no indication of battery capacity. You may be charging at 14.4V, but the battery's internal voltage may be 12.4V (~30% discharged). Usually AC-injecting techniques are used to determine a battery's charge state (relative capacity) whilst being charged (eg, some "smart" chargers). To measure capacity in a real situation where a battery is being loaded, the same Volts per discharge% (eg, .1V per 10%) can be used, but with an offset. EG - you place your load on a full 12.7V battery and the voltage drops (almost) immediately to 12.4V, but it is still 100% full at 12.4V (there is a 0.3V drop from the internal (theoretical) battery across its internal resistance to the battery terminals).    But from that 12.4V "full" voltage, the .1V/10% rule applies - ie, the cranker should not drop to under (12.4 - 0.2V) 12.2V (20% discharge). BUT that does assume a near constant load. If the    12.4V load were halved, then the battery terminal voltage would be 12.55V. Note too that the battery voltage "capacity" is its relative capacity. EG, it may be rated as a 100AH battery, but due to its age it is only 60AH. The voltage reflects its 60AH capacity. And though the voltage is linear (voltage drop is proportional to discharge amount) for good batteries, an old/faulty battery can collapse a any stage (ie, it gets to 30% and then just dies...). Hopefully the above sums Bill Darden's relevant stuff. But hence why I like the short answer of 0.1V per 10% provided the OC rested voltage else the initial voltage drop under load (offset) and constant load is understood. Be aware that batteries should not remain connected when not in use (ie, being charged or discharged), hence the use of (automated) battery isolators. Unfortunately Bill cleverly passes that issue to other references, though all the relevant theory is contained in his BatteryFAQ. (See my posts on the UIBI (Ultimate Intelligence Battery Isolator) on the12volt and mp3car which can be used if you have some charge indicator - eg, a charge light ca control a relay. It's much cheaper and almost always superior to voltage sensing and other "smart" isolators.

Hello, I currently use 2 12 volt batteries in parallel to achieve 24volts in my system. Unfortunately this is not giving me the capacity I need. My thought is to add 2 more of the same batteries but I am wondering if that would be advised and how that would be accomplished. I believe I would need to run each of the 2 in series then connect one to a black lead of the one of the banks and one to the red connection. Any thoughts would be appreciated. Thank you

2 12 volts batteries in parallel is 12V (with about double the AH capacity). 2 12V in series is 24V. So which do you have or want? I assume you do not want to connect a 24V battery to a 12V system? (Nor a 12V bat to a 24V system.) What capacity do you need?