Bank voltage

[Markj]

I have a 6 300w panel system tied into a conext mppt controller and 4048 inverter. I have 8 6v rolls surrette s550s wired to get 48v. I was looking to confirm my absorb float and charging settings but think I'm close enough. Anyone know what they should be? I'm just starting to understand the draw and how batteries work. Most mornings I'm sitting at 50.4 volts but that's with the fridge (efficient inverter type) constantly drawing. I don't know if I get enough sun to fully load them each day. Is the only way to check battery state of charge with a hydrometer? What should I program the low shut off at? I don't have a backup generator yet. If I do, what size- I just want to ensure the batteries are charged enough I don't need more power. Other than the fridge I have a water pump to a pressure bladder and a few led lights. Not much.

I'm sure this is all here somewhere but I searched and just confused myself more...

[krez]

Hi Markj

Just my 2 cents through my amateur experience of living off grid for 5 years with the same scenario as you, Water pump, fridge, lights. I however have half your battery capacity and have been doing just fine with 2.5kw of solar connected. Whats does rolls recommend for the charge profile? And SG of the batteries i.e do you know what s.g the batteries came with? To my memory (and please double for 48 volt system) rolls was recommending a 29.6 v bulk/absorption charge followed by a 27 volt float, for solar/daily cycling service. Waking up to 50.4 seems like a pretty healthy number. Using voltage as an soc that would suggest 90ish plus charge (especially under load with no solar) I would seek to confirm this with a hydrometer reading. In winter when I only have 4 'good' solar hours I leave set float at 29.6 volts as well, as the batteries never really reach a complete charge. These are the numbers I have been working off and my already used/abused 24v scrap forklift set have seen 5 years of service, and I have 2 kids, running a washing machine, domestic fridge, pumps, lights, stereo, pressure cooker, water heater, water filter etc. I often wake up to anything between 24.8-25.2v in the morning and drop to 60-70% (24.4-24.6v) charge maybe half a dozen times a year, After 3-4 rainy days in a row. It needs a lot of current and time to get it back up, I have done without a generator this far, however if I have forked out good money on my batteries I would be considering a generator. I don't have any experience In this area, but I would suggest a voltage cutoff (continuously below say 48.4 without load) as a conservative cut off level and maintain cycle life. Ideally this would not be reached very often. Hope I have helped with my experience in this all, regards Kristijan.

[krez]

You should be able to tell how full the batteries are getting through the current required to keep them at float, given you can isolate the loads, the SG will also provide the most accurate reflection of each cells capacity. Most of my understanding of these things has arises from sunkings sticky threads!! Good luck

[Markj]

Thanks krez . Learning the soc from the voltage readings seems to be an art. I guess I'll have to prove with hydrometer settings to know. You have to test each cell in each battery right? Do you ever do an overcharge?

[krez]

Yes ideally you would test each cell, especially when identifying when and how often to overcharge (Equalize) which is indicated when cell SG deviates more than .03 points from another cell. I think overtime you tend to get to know your bank, you get to know which is the weak cell (there is always one) and the others that hold charge best, therefore over time, testing just a few cells will give you a good idea about roughly where you are at and what voltage corresponds to roughly what state your bank is in, because every bank is unique yet somewhat predictable with experience. Lead acid banks are like having a pet hamster, and is really not a set and forget type situation if your looking for longevity unfortunately. SG is also at the whim of temperature coefficients that need to be applied to get an accurate picture of the cell, these can be found on Google as my memory fails. For example, in winter my SG reads 1.285 where in reality it is 1.265 (when temp adjusted) which for my bank is 100% or 25.2v under slight load. Remember, even the inverter is a load on the bank, so your batteries are never truly at rest and voltage will be slightly lower than at actual rest. Given these points, the voltage you wake up to is a great state for your bank to be in, hopefully you can keep it up. What I keep the closest eye on is how much my voltage sags and recovers through a large load, to confirm the capacity is still there as that is what you want to retain the longest. Thanks Markj

[krez]

There are some generic charts on Google that compare SOC and flooded lead acid voltages, they can give you a good ballpark idea of where your are at