Proposed goal: estimate time to charge an FLA battery bank from 50% SOC to 100%, to determine generator run time after several cloudy days.
Assumption: there hasn't been sun, and no sun is coming for another day or two.
Please remember - I have no experience with solar, and am trying to estimate a system and visualize modes of operation and real world consequences.
I found a graph that purports to represent a charge curve for FLA batteries, attached below. My particular bank would be 800Ah @ 48V, and I would charge at C/10 or 80 A in bulk mode. Now I am at 50% DOD, and I want to charge. From the graph, which may or may not represent an actual battery, it appears the energy transferred in the absorb mode is about half that in bulk mode. The energy transferred in float by comparison is relatively small.
An armchair analysis of this generalized graph would be I need to add 50% of capacity, 400Ah - about (2/3)400 in bulk and (1/3)400 in absorb. In bulk, the time would be (2/3)400 / 80 = 10/3 or 3.3 hours. In absorb, (1/3)400/ (2(80)) = 5/6 hours = .83 hours
So that is about 4.1 hours +/-. Would I extend this time by maybe 10% to account for battery efficiency? That's about 4.5 hours. Is this a rational estimate of the run time needed?
Now the question of float comes up. It doesn't seem efficient at all to run a generator for hours to float the batteries. Might I plan to start early in the morning with generator charge from 50%, and then by noon or early afternoon, shut off the generator and allow the PV at 10% or less output to float the batteries?
Am I anywhere near a reasonable estimate, or have I made serious errors? I did ignore loads on the system, which won't be an issue other than once the generator is shut off. The only question I see there is if the PV in low output would generate enough to float the batteries and power needed loads. Is full float critical? If by evening when I start loading the batteries with lights, short runs of a microwave for cooking, etc, the fact I didn't float fully an issue in terms of cycling? It doesn't seem you can ever count on a full uninterrupted charge cycle unless you have two banks.
Thanks in advance!
Assumption: there hasn't been sun, and no sun is coming for another day or two.
Please remember - I have no experience with solar, and am trying to estimate a system and visualize modes of operation and real world consequences.
I found a graph that purports to represent a charge curve for FLA batteries, attached below. My particular bank would be 800Ah @ 48V, and I would charge at C/10 or 80 A in bulk mode. Now I am at 50% DOD, and I want to charge. From the graph, which may or may not represent an actual battery, it appears the energy transferred in the absorb mode is about half that in bulk mode. The energy transferred in float by comparison is relatively small.
An armchair analysis of this generalized graph would be I need to add 50% of capacity, 400Ah - about (2/3)400 in bulk and (1/3)400 in absorb. In bulk, the time would be (2/3)400 / 80 = 10/3 or 3.3 hours. In absorb, (1/3)400/ (2(80)) = 5/6 hours = .83 hours
So that is about 4.1 hours +/-. Would I extend this time by maybe 10% to account for battery efficiency? That's about 4.5 hours. Is this a rational estimate of the run time needed?
Now the question of float comes up. It doesn't seem efficient at all to run a generator for hours to float the batteries. Might I plan to start early in the morning with generator charge from 50%, and then by noon or early afternoon, shut off the generator and allow the PV at 10% or less output to float the batteries?
Am I anywhere near a reasonable estimate, or have I made serious errors? I did ignore loads on the system, which won't be an issue other than once the generator is shut off. The only question I see there is if the PV in low output would generate enough to float the batteries and power needed loads. Is full float critical? If by evening when I start loading the batteries with lights, short runs of a microwave for cooking, etc, the fact I didn't float fully an issue in terms of cycling? It doesn't seem you can ever count on a full uninterrupted charge cycle unless you have two banks.
Thanks in advance!
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