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newbie and sizing of battery bank


Wilfried
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Hi All,

I am fairly new to the solar topic and I have experience for about 6 weeks in production with our system by now. I was lucky to find this forum and being following it since a few weeks and see there is great info coming out, which one hardly gets anywhere else.

We have an Axpert 5KV (branded by Mecer) inverter, 12x250W panels and 8 x 105Ah Maximus batteries. I am very happy with the capacity of the panels during the day with running things like pool pump, dishwasher, washing machine and the rest of the house. Our geyser is converted to solar with evacuated tubes and the oven is still on ESKOM.

What I noticed is that I clearly have to little battery capacity and I am wondering whether you could give me some input to how many more batteries I need to go for. To become quite independent from ESKOM I would like to get through the night with my batteries. We have a consumption of ~3.5 kWh during the night, which I calculated with a xls I created to understand our overall consumption. Based on battery sizers like http://www.freesunpower.com/system_sizing.php I need to double to 16 x 105Ah batteries.  

What I saw on that battery sizer is that is calculates the battery capacity for 3 days, which I don't believe we need 3 days capacity locally, not even in Cape Town winter. As the batteries cost quite some money I was wondering whether I would really need 16 batteries or whether 12 batteries would be enough to get us through the night.

Thanks a lot for your thoughts/feedback and Cheers,

Wilfried

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I have a 8x150Ah battery bank which is fine to store enough power for one night, usually ending up at around 30% discharged in the morning , but I have a backup generator for when you have 2 or 3 cloudy day in a row. So although the battery bank is 'too small', having to use the generator on the odd occasion when you have a few cloudy days in a row is not too much hassle. 

We don't have an electric oven though, that may take a bit more power.

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Okay, so I typed up a whole response and then deleted it because I realised something doesn't make sense.

If you have 8 x 105Ah batteries, they are most likely arranged in two strings of 4 batteries for a 48V bank, so you have a 200Ah 48V bank?

That's good for 9.6kwh of storage. If your night-time consumption is really just 3.5kwh, you have DoD of only 37%. This conflicts a bit with your statement that you have insufficient battery storage :-)

Are you sure of 1) the insufficiency of storage or 2) that you only use 3.5kwh? :-)

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I agree.

1 days storage, 50% DOD, you need 172ah battery bank for 48v - losses and ineficiencies calculated in.

So 200ah is more than ample for 3.5kWh used per night.

It is a lot of batteries to worry about midpoint values though. 

Another thought, if they are these ones, and they are sealed, they are not true deep cycle: http://www.battery-pmp.co.za/Maximus 105.pdf in MY opinion.

Lead Acid deep cycle batteries should not be sealed.

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Hi All,

thanks a lot for your input and feedback. I was away for a few days and therefore my late reply.

Your input helped me a lot in what to investigate further. In particular I learned a bit about the issue I have with my batteries. The Maximus batteries I have are indeed not deep cycle and therefore quite finished already after six weeks. The only way to find that out was to install Watchpower (knowing that it might not be the best) which gives a good idea what's happening with the system at any point in time. I gave the info to my installer, who will replace the batteries (for free) and replace with Led-Crystal (Deltec) batteries.

I'll report back once I have those for a few weeks.

 

Some feedback specially for Plonkster and viper_za who asked some questions:

- I have an average load of 120 to 180W during the night due to security lights (all LED) and fridge (which is AA rated)

- During the time between 6pm and 11pm we might run TV and some additional lights (LED) which pushes the regular load up to ~300-350W

- There are some peaks for using the microwave for a few minutes in the evening and morning. During that time we use ~2000W.

I created an xls to understand the overall consumption and therefore I arrived at the ~3.5kW per night.

Based on that info would you still think 200Ah is enough?

 

Thanks and Cheers, Wilfried

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13 minutes ago, Wilfried said:

Led-Crystal (Deltec) batteries

Just for your own edification, look at the spec sheets for the Deltec and compare with others (eg the Beta). I was a little disappointed to see that the Deltec LCs do a mere 1600 cycles to 50%. That is (incidentally) the same as for the Trojan T105RE, and the Trojan is cheaper (or was when I checked). The Beta-battery does 3000 cycles to 50%.

 

15 minutes ago, Wilfried said:

Based on that info would you still think 200Ah is enough?

I think 200Ah is sufficient, especially if its a decent Lead Crystal battery. Let's say you do a constant 300W through the night, that is still just 3.6kwh (over 12 hours), which is a 37% DoD (ignoring all other inefficiencies for now), that is, you're still well within the 50% bracket. Of course, this allows storage for only one day, so of COURSE 400Ah and even 600Ah is going to be better... it just depends if you can afford it :-)

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Something else to keep in mind as well: When I say that a 200ah bank at 48V stores 9600Wh, I'm strictly speaking making a rough estimate. The truth is that in the top 50% of discharge you're operating at well above 48V, probably closer to 49V to 50V most of the time. So its more like 4900Wh to 5000Wh before you hit 50%.

For interest sake, when you recharge, you have to do it an an absorption voltage of say 58V, so you have to put back 100Ah, or 5800Wh to get back to 100%.

Overall efficiency 86%. And that is ignoring all other losses, but thankfully, those others are small compared to this one. Usually you manage around 70% round trip.

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ups, I think you lost me with the second part of your response ...

I get the absorption load of 58V and I also understand that the 200Ah  should be decent for my consumption. But then I can't follow up any more. Unfortunately I am not an engineer. Do you mind giving a bit more background .... :unsure:

Thanks!

 

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You put back the same 'amps' or ampere hours to be accurate, but you have to do it at a higher voltage, hence you put back more power. That has nothing to do with the first part of the calculation, it was more "for extra credit" :-P

Sent from my GT-I9195 using Tapatalk

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Hi Wilfried

You have a similar load to mine and my bank goes down to +-75% SOC every day. The only thing I do different than what you are planning on is swapping back to grid for high wattage appliances when running on batteries alone. I do this in the evenings when microwave is running and the same for the mornings with the kettle. I don't like straining the batteries with high loads when I do have other options available (Eskom).

What you will notice is depending on your type of load 350w will pulling more in the region of 400-450W from the batteries as the inverter itself consumes +-25W. If I had the funds I would have gotten myself a 400Ah+ bank and have a little more room to wiggle with during the night :)

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1 hour ago, viper_za said:

batteries with high loads

Yup, going back to my crude Watt-hour calculation again. When you work the batteries hard, they might go down to 47V under load. The load (P = VI) is the same however, so the inverter compensates by drawing more current, which comes directly out of your amp-hour kitty. In other words, it costs more in terms of amp-hours when you work them hard.

Conversely, consider the C100 discharge rate (which is 2 amps). Often a battery will gain an extra 10%-20% or so when discharged that slowly. For a 200Ah bank, that would be 2 amps, or around 100W in your case. The reason, once again, is because you have the same amp-hour kitty, but you're doing work at a higher voltage and so you get out more power.

Also, battery has an internal resistance. When it works hard, some of your power turns into warm batteries :-)

I highly recommend watching knurlgnar's videos on this topic. Please note though, he SAYS Peukert is wrong, what he means is that the way people blindly apply it (without understanding) is wrong :-)

 

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With losses en ineficiencies: 180w on for 12 hours + 180w on for 4 hours = 2880kWh per day.
48v system with 1 days backup down to 50% DOD = 141AH battery bank.
To charge that bank in Cape Town in June, you need a 2122w array, least amount of irradiance.
On average of sunny day with 5.5 hours of good sunlight, you need a 1382w array to recharge.

The above does not include powering the load daytime, this array is to JUST charge the batteries for the load.

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