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Solar panels to recharge 5 off in series 12V accumulators during the day?

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Chris Wilson:
Toying with using 5 off 12V lead acid accumulators in series to power a device that draws about 30 Amps at 60V from about  7PM to 8.00AM every night. What sort of array would be needed to have a good chance of recharging them during the day in the Midlands, UK? I have no other interest in the panels, and they could be roof or field mounted. Thanks

well i cannot say much for the charger itself, but 5 lead acids in series would work, if your planning this to last 6 months or more, you will want to over rate the batteries a little as there life is related to depth of discharge, so for 30 Amp Hour, (for lead acids you want a DOD of less than 50%), so you need atleast a 60Ah battery for each that you are running in series, and they must be deep cycle, or gel, a cranking battery will not last through deep discharges,

as for recharging them, you are discharging 1.8KWh, the batteries themselves have a charge efficiency of lets say 80% (some of the charge current is converted to heat), so up to 2.25KWh, lets say your charger is only 90% efficient at using its input, up to 2.5KWh, (the losses might not be as high, but this gives wiggle room for things like wires and such)

ok so as a ball park you want 2.5KWh going in to them every day, this is where you may want to oversize your batteries again, as it comes down to how many rainy days in a row will it need to keep up with. and oversizing the panel wattage to counter cloudy days and making up for rainy day shortfall.  the panel math i will leave to another poster as its out of my area of knowledge.

Chris, take a look at my reply to a similar question here:


Putting Leicester into PVGIS (Stand-alone PV tab), panels at 35 degrees, 23400Wh daily consumption, 60V 1000Ah betteries, 40% discharge cutoff limit:

10kW of panels will keep the batteries from going flat only in June and July, and by December they are flat (ie. down to 40%) 86% of the time.

20kW of panels gets you through most of the year with batteries flat 17% of the time in January. Increasing the panel inclination to 70 degrees reduces that 17% to 4% as you capture more of the low winter sun at the expense of less output in summer when you usually have much more than you need anyway.

To ensure the batteries never run flat (ie. to cover that last 4% in January) you'd need:

44kW of panels with 1000Ah batteries or
17kW + 2000Ah or
12.5kWh + 3000Ah.

That's a big investment. Is that what you were expecting?

You could consider allowing the maximum discharge to approach 100% for flooded lead acid cells as this won't hurt them, but may damage other types. The vast majority of the time the batteries will be almost full.

Covering the worst case, but relatively rare extended winter dark spells is disproportionately expensive in panels/batteries and generators are usually a much more economical solution. Fuel is expensive but they only get used a few days each year.

Also remember that if this is a critical application then you need to add in large margins for battery and panel capacity loss with time and temperature, exceptional weather events (getting less exceptional it would seem), dirt/snow/ice on panels etc.

Chris Wilson:
Splin, thanks for the very detailed reply. You say it's a big investment,a and ask if it was what I was expecting. I have to say I can't answer that unless you can put a very ballpark figure on the hardware cost, assuming it is fitted by myself. I then have to work out the off grid energy cost of electricity over say a year and see how long it would be before the solar system would be ahead. It's not something I would do for altruistic purposes, to be Green. It would have to bring a significant cost saving within a fairly short time frame for me to even consider it :) I appreciate the info, it's a subject I have never looked at other than very peripherally.

Once you add batteries, there is no payoff.  Typically batteries cost 14 cents per KWH of use. Find ways to use the panel power directly and there is a payoff.


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