Eric, Allan's recommendations on both points are excellent, and spot-on.
I would add the following suggestions for your controller and system if budget and circumstances permit: 1) Use a MPPT model controller to enhance the recharge rate under less-than-ideal conditions. The PV is already undersized for the battery you indicated (below). 2) If you get the job .. install the batteries in a location that will keep them as cool as possible. Heat is just as detrimental to battery life as is failure to fully charge. 3) Temperature-compensated charging will enhance battery life. However, the 75W PV vs. 120 A-H battery allows for only a C/20 recharge rate at best. This is not conducive to full recharge on a regular basis unless the loads are very small and are lightly used. If the budget allows for opportunity to double the PV wattage .. it would be wise to do so. 4) A simple "stop-light" style LED voltage display won't consume much power, but can go a long way toward providing a visual indicator for state-of-charge, especially if it's adjustable, and can be be biased toward a higher SOC (in essence, make it fib a little, and indicate a low SOC sooner than than is actually the case). Worth considering as an option .. but easily omitted to reduce cost and/or simplify the system. I wish you success on winning the project. Regards, Dan -------------------------------------------- On Mon, 12/30/13, Allan Sindelar <al...@positiveenergysolar.com> wrote: Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small OGPV systems To: e...@solarnexusinternational.com, "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Date: Monday, December 30, 2013, 6:48 PM Eric, Yes, one of the classic problems with small systems and less educated users. Just a small contribution: it would seem to me that in this case the LVD setting is as key as the LVR. What if the LVD was set as high as possible - say, 12.0 V with sealed batteries, along with a 13.8 - 14.0 V LVR. The system would still be run until it shuts off, but the battery remains at a relatively high SOC. The next day the system will return to operation, but with a much higher range of SOC. The users will still experience shutdown, and will over time become familiar with when to expect it and how to live within its capacity, but the battery life will be protected. This approach seems counter-intuitive, but the more I think about it the more sense it makes. The system would also return to operation sooner following cloudy weather - in fact, it would provide a minimal amount during each of the cloudy evenings. Is the C12 still made? It would still seem ideal if so: bulk, float, LVD and LVR, all adjustable, with removable knobs and a 15-minute reserve button after initial LVD shutdown. Rock-solid reliable, too, in my experience. I'd be interested in what approach you select. This intrigues me. Allan Allan Sindelar al...@positiveenergysolar.com NABCEP Certified PV Installation Professional NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Founder, Positive Energy, Inc. A Certified B CorporationTM 3209 Richards Lane Santa Fe , New Mexico 87507 505 424-1112 office 780-2738 cell www.positiveenergysolar.com On 12/30/2013 5:00 PM, Eric Youngren wrote: > 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, 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 > _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
