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

[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

[Rerouter]

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.

[splin]

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

https://www.eevblog.com/forum/renewable-energy/hi-guys!-need-help-choosing-the-right-battery-type-for-my-solar-powered-project/msg1192652/#msg1192652

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.

[Seekonk]

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.

[Chris Wilson]

Thanks Rerouter for the reply, it is looking to me as though my idea is financially unsound and potentially a lot of hassle and work, but the jury is still out until I can see some very round figures to make a judgement. Appreciated!

[splin]

For prices you'll struggle to get much better than here for batteries and panels:

http://www.bimblesolar.com/batteries/2v/2v1000ah

£175 2V, 1000Ah flooded cell, 1500 cycles @ 80% discharge.

They sell new panels @ £620/kW, but also sell used panels at £360/kW which should be perfectly good:

http://www.bimblesolar.com/solar/individual/canadian245w-used

So £10,500 for 60V, 2000Ah battery + £6120 for 17kW of panels + mountings £2k? + cable etc.

Charge controllers could be hundreds to a few thousands depending on how sophistication, quality etc. But you could probably make a simple, very cheap charger using MOSFETs to switch off charging when the batteries are full. To do this you'd need to match the panel voltage to the battery voltage.

But as Seekonk pointed out it is likely to be uneconomic. The battery cost alone would be £10,500/(60 * 2000 * 80% / 1000)/1500 cycles = £.073/kWh. In practice you'd be very lucky to achieve that battery life - solar storage is not generally a good match with their characteristics - mostly shallow discharge cycles and a lot of time on float during the summer and prolonged periods of time partially discharged in winter with the risk of them sulphating.

Also, if anything goes wrong it's quite easy to damage one or more (or even all) the cells. Set the charger up incorrectly and you could lose the lot in a few weeks! There is some scrap value though.

Lead acid batteries also need periodic maintenance including topping up water levels and monitoring their specific gravity, periodic equalization etc. so you'd need to factor in your time as well.

Li-ion batteries are becoming available which are pretty much fit and forget but are a lot more expensive at present.

[Seekonk]

Who needs batteries, you have the grid to fall back on.  I live off grid for four months a year and have only a single car battery (from the vehicle I don't bring) to power everything.  The battery is there just to provide starting current for loads like the refrigerator, not storage.  The whole house runs off a single micro so it knows what is needed where.  Disconnect the fridge and other loads from the grid when sun is plentiful.  With the excess heat water or the room or pump or ventilate. It is tricky pulling loads on and off, but not that hard. Change your lifestyle a little.  This year my wife wants me to add a dishwasher.  If a cloud passes over or the fridge needs to come on, it shuts down for a while. It is all time share power. People tell me I can't be running a house on what few panels I have and no battery. Be creative. Most big dumb solar systems I see make me want to gag.

[splin]

Who needs batteries, you have the grid to fall back on.  I live off grid for four months a year and have only a single car battery (from the vehicle I don't bring) to power everything.  The battery is there just to provide starting current for loads like the refrigerator, not storage.  The whole house runs off a single micro so it knows what is needed where.  Disconnect the fridge and other loads from the grid when sun is plentiful.  With the excess heat water or the room or pump or ventilate. It is tricky pulling loads on and off, but not that hard. Change your lifestyle a little.  This year my wife wants me to add a dishwasher.  If a cloud passes over or the fridge needs to come on, it shuts down for a while. It is all time share power. People tell me I can't be running a house on what few panels I have and no battery. Be creative. Most big dumb solar systems I see make me want to gag.

I can't disagree with any of that but what relevance does it have to this thread? The OP was:

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.

We aren't told what the purpose is, but it sure isn't for off-grid living.

[Chris Wilson]

With our mains electric at 13.6 pence Sterling per kW hour and a worst case 2kW hour load for this device it's a none starter for solar power, I had no idea just how huge the costs would be for storage of solar energy after so long in its development, and it's been an education! Thanks for your patient description of the costs and needs for this, much appreciated.

[bazza]

In Australia most grid slaves pay about 500 AUD per year (about 290 GBP) just for the privilege of an electricity connection. Usage Charges are on top of that. Talk about a scam. Anyway, these need to be taken into account, not just the KWh prices.

What device are you powering, by the way?



 

[Chris Wilson]

We would need to pay the connection charge come what may, as there's no way I would make all the hassling changes to try and be independent of it, although i do know two people here in the UK who claim running a modern diesel 3 phase generator on red diesel (tax subsidized) diesel by night and solar / storage by day is cheaper, but i am skeptical). I am running an RF amplifier most nights, and it uses a minimum of 1kW input. But with those costings I'll just keep sending the electric company a cheque each quarter