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Wrenches, In the last few years I have changed my approach to off grid system sizing, primarily due to higher battery costs and lower module costs. Rather than shooting for a traditional 80-90% of winter load profile target, I will size a larger array and smaller battery bank, figuring (especially here in the sunny Southwest) that batteries will recharge more quickly, will spend more of their time full (and thus last longer), and will be at least partially charged during cloudy weather. This even allows for smaller battery banks that are cycled more deeply (as long as there's a backup gennie in the mix); the lower cost of a smaller size may allow for a set of higher-quality industrial cells. I first began to rethink my design approach after reading Christopher Freitas' Solar Pro article on large battery banks a few years ago. Being able to refill in a day was part of his strategic approach to battery bank design. I used and wrote about this approach in my tiny house article in the current Home Power. However, I'm just beginning to get owner feedback about greater water consumption and the need for more frequent watering. This is especially the case when I add array to an existing system, and the old watering schedule is suddenly not enough. When I design new systems I now encourage larger arrays and smaller battery banks, as the batteries spend more of their time happily full and recover more quickly after a discharge period. But I am thinking that I will have to begin changing charging setpoints in response. Flooded batteries have a range of recommended bulk setpoints, from 28.8 to 29.6 volts on a 24V system. When arrays were smaller we set to the higher end of the range to take advantage of topping off when available, and a two hour or longer absorption was considered beneficial, as long as the CC could transition to float at 2% of capacity or so. With a larger array a better approach may be to lower the bulk/absorption setting, as well as reduce absorption time and raise the float transition current ("done amps"). I'm writing to encourage a discussion among off grid Wrenches. Are you changing the relative relationship between array and battery capacities? If so, how are you adjusting charge controller setpoint programming? For example, how would you program a Midnite Classic, Outback FM or Schneider XW if you could achieve a C/10 or C/15 charge rate? Thanks, Allan Allan Sindelar
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