Hi Wrenches, We are bidding on a project to provide several hundred small (75Wpv, 120AH battery) 12V DC off-grid solar home systems for rural villages in West Africa. The project planners have requested that the systems be designed and built to provide 7 years of battery life. That seems crazy optimistic to me but I'm trying to design the systems that will give the batteries a fighting chance of lasting that long.
In my experience with these types of installations<http://solarnexusinternational.com/images/documents/shortcuttofailure_ghtcpublished.pdf>, the system users have little or no understanding of how to properly manage their batteries and usually no metering or SOC indication to help them even if they knew what to look for. So, the default control strategy becomes: run the loads until the Low Voltage Disconnect (LVD) turns them off, then wait until the Sun returns and the voltage rises to the Low Voltage Reconnect (LVR) setpoint (around 12.5V is a common default) , upon which point the cycle repeats, with the result that the battery bounces between LVD and LVR, almost never reaches a full SOC, and the batteries are lucky to survive for maybe two years. After that the system will provide a little power during the day while the Sun is shining but the batteries will quickly crash below the LVD after the sun goes down. So, we want to offer a controller with a high LVR setting that will ensure the batteries reach a full recharge after each and every LVD incident. I know the C-12 has an adjustable LVR setpoint and I see one Chinese brand (Manson) that can do it. Does anyone have any other suggestions of small (<20A) PWM controllers that can be adjusted to not reconnect until 14V or so? Price is going to be a factor in this project so they need to be low cost. Any other suggestions? What would you use for the optimal LVD and LVR in this situation? These are rural household, DC only systems with primarily lighting and small device charging loads. I know that this strategy will mean that they might not have any access to battery power for up to a few days if they hit the LVD during the rainy season and there is not enough sunshine to get the battery charged in a single day. My thinking is that experience will provide good feedback that will encourage them to conserve and manage their batteries to avoid the LVD situation as much as possible. That's the idea, anyway. I'd appreciate any advice from you all. Thanks! Wishing you all a happy and productive 2014! Best energy, Eric
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