If your desire is to stick with a flooded lead acid style (of which I am still a fan and would help keep this project simple and cheap) then the response of others is perfectly accurate. Consider and recall a few things about this tried and true battery technology.
1 - Fully charged, no load voltage, with residual surface charge removed of a typical 12v, BCI group 27 battery is 12.7 V. State or charge (SOC) = 100% 2 - The same battery is considered to be fully discharged (SOC =0%) when its voltage equals 10.5 V 3 - The discharge curve is relatively straight so we can use a voltage meter to determine a battery's SOC. 12.7 - 10.5 = 2.2v over the entire 100% range. So 0.022 volts (no load) per percent discharged from the battery. 4 - To prevent damage and maintain the highest number of years of service life from your investment, discharging a battery below 50% should be avoided. Meaning that a voltage below 11.6v should be established as a critically low SOC. 5 - Changing is not so linear. Up to about 80% the rate at which the battery will accept charge power is linear. After that, the rate tapers assuming your charger is matched to charge at the maximum appropriate rate. Getting beyond 80% takes longer and is far less critical to maintaining battery health than staying above 50% 6 - The maximum rate of charge is determined by taking 30% of the amp hour capacity. For easy math let's assume your group 27 battery has 100 A-Hr of capacity. That means you should limit the charge current to 30 amps. Conveniently enough for math sake, this means that charging the battery from 50% to 80% (30 A-Hrs) can be achieved in 1 hour with an appropriately matched charger. So, since you didn't tell us what your charger current capacity is or how much time you have to charge or how low you currently discharge a single battery or your current AHr usage, there are many options that may make your upgrade more useful and efficient. Provide those details and I'll be glad to noddle through it with you. Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD Jul 4, 2022 09:41:57 Hoyt, Mike via CnC-List <cnc-list@cnc-list.com>: > Actually is for an RV but I can think of no better place to get answer to > this type of question than this list. > > Came with one group 27 deep cycle battery. Wish to add a second and have > been told that nothing needs to change re the charging systems, etc … What > is involved in adding a second group 27 battery? > > Thanks > > Mike Hoyt > > Persistence > > Halifax, NS >
