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Author Topic: small off grid system design questions  (Read 2934 times)
mr_magicfingers
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« on: July 31, 2019, 01:49:02 PM »

I've been running an 80w pump using a 110AH battery, solar panels and cheap inverter. It's not been very successful as the inverter, a cheap 300w one, would keep beeping and cutting out.

So, I've bought a new (secondhand) system from another forum member and I'm currently putting that together. As it's just a little more complex, I wanted to ask a few questions about how best to connect the various components and where any necessary switches and fuses should be.

The basic layout is currently like this: There are two 80w panels on a roof, these are connected at a junction box in parallel. They'll then connect to the Victron MPPT charge controller.

From the charge controller to the batteries should be reasonably heavy cables, however those won't fit into the connectors on the charge controller. I have a heavy fused connector with ring terminals on it, but that won't fit into the controller either so I need to either cut that and connect smaller wires from the controller to something like a hefty connector block or find something I can screw ring connectors to and connect the smaller and larger cables. My concern is that the smaller cables would probably be lower rated that the 60A fuse in the connector that goes from the controller to the battery. I'd be interested to know how others connect these.

The batteries are 100AH SLA 12v. They'll be stacked in pairs, wired in series to give to sets of 24v 100AH I'd then connect the +ve to +ve and -ve to -ve to give a 24v 200AH battery pack. One set of +&- terminals connects to the charge controller, the other to the Inverter. The inverter has Anderson connectors and I have a set of cables already made up for that. All the diagrams I've seen suggest that should also have a hefty fuse in it but I'm guessing that it's internally fused so I don't have to connect a fuse into the cable.

If my pump is 80w at 240v, Power = volts x amps so amps = power / volts As I calculate it, 80/240 = .33A If I have a 200AH battery pack, then 200/.33 = 66 that should run the pump for 66 hours. If I say I only let it deplete the pack by 50% that would be 33 hours and allow 10% for losses that would be 30 hours continuous use. If I ran it for 5 hours a day, that would be 6 days use even if there was no sunshine. Does that sound reasonable or have I made any obvious mistakes?

As I've drawn it, it looks like this: Should there be any other connectors/switches or fuses before I start wiring it all together. Plus there's some photos of the physical layout, I'm still building the box around it and plan to put all the controllers/switches etc on the underside of the roof.






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Solar PV: 6kw (24 x REC 250w), Sunny Boy 3600TL-21, roof facing 178 at 40 slope.
300l Thermal Store, 2 x 3kw immersions via Immersun + Dunsley Yorkshire WBS
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Countrypaul
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« Reply #1 on: July 31, 2019, 07:04:48 PM »

In your calcuations on teh 80W pump you get 0.33A at 240V, the current at 24V will therefore be 3.3A (assuming no losses). 200Ah/3.3A gives you 60Hrs

You seem to have got almost the right figure, but I am not sure whether this is pure coincidence or through giving a simplified amount of imformation.

Do the batteries have any sort of rating on them for number of cycles at a given discharge level?

Will this be running all year round or only during the summer months. Winter cold temperatures could adversely affect things significantly.

Is the APC inverter a proper inverter or is it a UPS being used as an inverter? UPS' tend to have a high amount of power consumption internally.
« Last Edit: July 31, 2019, 07:07:15 PM by Countrypaul » Logged
marcus
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« Reply #2 on: July 31, 2019, 10:53:20 PM »

with 160w of solar you won't see 60a from the controller - you might get 16a bursts with cloud 'lensing' but more usually it'll top out at ~12a - so use a thinner cable and a smaller fuse. keep the cable from battery to controller as short as practical.

probably just a 'typo' but as you've drawn the batteries they're shorting out - parallel batteries should be connected +ve to +ve and -ve to -ve.

I'd assume ~4a from the battery for an 80w load resistive; maybe more for a pump due to power factor reducing inverter efficiency. I assume you're only running the inverter when you run the pump? If we assume 5a with the inverter standing losses and power-factor inefficiency that gives you 40Hrs runtime - but only by draining the batteries which will kill them quickly. I don't know what you're doing with this pump but I'd try and keep the batteries >50% dod on average if you want them to last, which gives you ~ 20hrs runtime. And don't forget to allow them to fully equalise periodically.
« Last Edit: July 31, 2019, 11:00:32 PM by marcus » Logged
rogeriko
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« Reply #3 on: August 01, 2019, 12:15:48 AM »

No unfortunately everything is wrong. Dont get mad but the panels have to be connected in series and then into the MPPT controller. Unfortuately the controller will not convert all the power as you think. Two panels in series will give you 36v at 4 amps, but you have 24v batteries so 24 x 4 equals only 96 watts. The mppt part can boost an extra 20/30% giving you tops 120w out of your panels. That is only 4/5a in bright sun for a couple of hours but on a normal day you will only see 2a. That is not alot, in fact it will only just cover the self discharge of the batteries. You need big panels. What you put in a battery you are lucky to get 50% back the next day. The batteries are not designed for what you want, they are starting batteries, they will not hold the power. Connect a 50w 4a car headlamp bulb to one of them and it will start to go dim after 10 hours, that makes them 40ah batteries. The inverter is an APC UPS which is renowned for being hopelessly inefficient, to get 80w out it will take double that on the battery input. I cant see the model number is it square wave or sine wave? Sorry to sound so negative but its not easy to make a small off-grid system. You need one of these to check the amps flowing around the circuit and you will see what I mean.
https://www.ebay.co.uk/itm/UNI-T-UT204A-Handheld-Digital-Clamp-Multimeter-DMM-AC-DC-Voltmeter-Ammeter-J0E6/123782429295?epid=15027286727&hash=item1cd201fa6f:g:vqsAAOSwpypc1xKD
« Last Edit: August 01, 2019, 12:33:08 AM by rogeriko » Logged

Tinbum
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« Reply #4 on: August 01, 2019, 09:13:58 AM »

probably just a 'typo' but as you've drawn the batteries they're shorting out - parallel batteries should be connected +ve to +ve and -ve to -ve.

No, it is correct- layout is maybe not shown as I would show it.
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Fintray
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« Reply #5 on: August 01, 2019, 09:53:01 AM »

Is this any clearer?


* Batteries.jpg (14.35 KB, 360x305 - viewed 198 times.)
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Countrypaul
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« Reply #6 on: August 01, 2019, 10:51:56 AM »

probably just a 'typo' but as you've drawn the batteries they're shorting out - parallel batteries should be connected +ve to +ve and -ve to -ve.

No, it is correct- layout is maybe not shown as I would show it.

THink you've misread the diagram (as I did initially), as shown it has + to - and + to - for each pair of batteries, which if connected that way when there charged is certainly going to give some excitement. As Fintray has drawn it is probably how it was intended.
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Tinbum
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« Reply #7 on: August 01, 2019, 11:22:01 AM »

Sorry I cant see where the excitement is going to come from.  Huh

It has + to + and - to - for each pair. The pairs are vertical not horizontal.
« Last Edit: August 01, 2019, 11:31:04 AM by Tinbum » Logged

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nowty
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« Reply #8 on: August 01, 2019, 12:22:55 PM »

Is this any clearer?


No that's wrong cos your not taking current equally from the batteries, ie, the top battery string get used more than the bottom battery string. This is because there is some resistance in the battery connector cables.

It should be like this when you connect battery strings in parallel,



« Last Edit: August 01, 2019, 12:37:27 PM by nowty » Logged

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mr_magicfingers
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« Reply #9 on: August 01, 2019, 01:20:33 PM »

Thanks for all the replies so far, I'll try and answer the various questions that have come up.

Firstly, yes this isn't an ideal set up but it's one that's kind of growing organically as money and parts come up. Last year it was a single panel, battery and controller. It wasn't doing the job and I posted about it on this thread https://www.navitron.org.uk/forum/index.php/topic,29550.0.html where it was suggested another 80w panel in parallel would be the cheapest next upgrade, so I did that.

Recently, this little system came up for sale on here and I picked it up to try expanding what I currently have. It has a better MPPT controller, a 24v ups which is pure sine wave and the 4 batteries. While the batteries are not leisure batteries, they were cheap and the idea has been to use this for now to set up the 24v system and then, as funds allow, replace out the inverter and batteries for more suitable units. As the whole lot was 200, it gave me an opportunity for more learning and trying to understand it all better without spending a lot more money. For running the pump, the plan was to have the inverter running constantly and with a timer switch plugged into it to run the pump at certain times of the day.

The calculations of load and time were something I wasn't sure of, I understand calculating power/volts/amps etc but wasn't sure how it works going from a 24v system up to a 240v system.

The batteries are stacked in pairs, plan was to make 2 stacks of 24v and then parallel those stacks. I wasn't sure how best to connect so that power is drawn equally from the 4 batteries.

First main question is, after last year being told to wire the panels in parallel, should I now be wiring them in series because it's an MPPT controller?




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Nickel2
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« Reply #10 on: August 01, 2019, 02:48:01 PM »

What voltage are the panels?
Somewhere on the label on the back will be the following information:
Voc, Isc, Vmp etc.
Ideally please give the manufacturer and part number.
At the worst, please count the cells on each individual panel please.
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mr_magicfingers
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« Reply #11 on: August 01, 2019, 05:48:38 PM »

One of them is this one, the other is virtually identical

https://www.navitron.org.uk/store/solar-panels/small-solar-pv-for-battery-charging/80w-framed-panel

Nominal Voltage - 12v
Maximum Power Voltage - 17.5v
Maximum Power Current - 4.6A
Open Circuit Voltage - 22v
Short Circuit Current - 5A
Weight 7.7kg
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Solar PV: 6kw (24 x REC 250w), Sunny Boy 3600TL-21, roof facing 178 at 40 slope.
300l Thermal Store, 2 x 3kw immersions via Immersun + Dunsley Yorkshire WBS
Woodwarm Double sided 6kw WBS
7 acres of old woodland I'm slowly bringing back into order.
nowty
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« Reply #12 on: August 01, 2019, 07:51:55 PM »

First main question is, after last year being told to wire the panels in parallel, should I now be wiring them in series because it's an MPPT controller?


As rogeriko says, in series.

I read your charge controller manual and it says, PV voltage needs to be at least battery voltage + 5 volts and with a max of 75v.

So they wont work with a 24v system in parallel as the voltage will be too low.

But you really need more PV to make this work, why not get one or two cheap 250w ones.
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Hot water storage of 15+ kWh.
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« Reply #13 on: August 02, 2019, 10:08:30 AM »

Sorry I cant see where the excitement is going to come from.  Huh

It has + to + and - to - for each pair. The pairs are vertical not horizontal.

Because as drawn on the paper sketch, each pair of batteries are shorted. The parallel connections are fine, the series connections are shorted.
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Tinbum
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« Reply #14 on: August 02, 2019, 10:29:56 AM »

Ah yes- he's taken the output across a battery not a link between the batteries. surrender banghead
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