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How many and what type of batteries for this system?


Ryanwb1
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Devices wanting to run:

1 x 12000 btu split aircon unit inverter type

12 x ceiling/lamps lights all have energy saving globes @ 14W each

2 x ceiling fans

2 x 40” full hd led tvs

2 x Dstv decoders

2 x laptops

1 x Fibre router

1 x single door fridge

 

What I don’t need on the inverter system:

Stove/oven

Geyser

Kettle

Toaster

Washing machine

 

At the moment I don’t want to run on solar but rather off a battery backup system.

Wnat to run the a/c for a 2 hr period over load shedding.

How many batteries do i need & what ah  and are AGM batteries sufficient for this.Cannot afford Li On at the moment.

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On 2020/02/14 at 1:48 PM, Ryanwb1 said:

How many batteries do i need

Rough estimate, your total load is about 4000W per hour that you want to draw for 2hours on batteries. 

4000W per Hr x 2 = 8000Wh required

I would go for 400AH x 48V = 19200Wh This should be ok if you don’t want to discharge batteries more than 50%. 

Anybody please rectify me if this does not look right.

 

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I estimate 2152W for that system, so just about half of the above.

image.png.f6eb2220bb9678b75e1ec07906657dde.png

Notes: The aircon will likely draw less than that in the worst case, but I have assumed it is not a Samsung (some 12000btu samsungs are only 9000btu in cooling and use less than 1000W) and that it is way undersized for the room. I have also assumed that the laptops aren't both gaming laptops - I think my estimate would hold if one is low-mid-end-gaming and the other is watching netflix or browsing or doing office tasks. All the other estimates should be relatively conservative.

Edited by P1000
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I think you are right there P1000...load is around 2100W.

Therefore in your opinion what and how many batteries would I need.

My calcs say 4 x 200ah AGM batteries connected to a mecer axpert 5kva mks2 inverter.

I don't know what the difference is between agm and agm/gel batteries btw

Edited by Ryanwb1
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On 2020/02/14 at 1:48 PM, Ryanwb1 said:

1 x 12000 btu split aircon unit inverter type

Before you get an error here working things out aren't you NON inverter type aircon ? or you changing it?. Maybe its just the start up load that makes a differance between the 2 anyway - i don't know but someone else here will

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It's an inverter type,the older tech.Just changed but was recommended to go inverter type as they less problematic with this load shedding

2 hours ago, 2una said:

Before you get an error here working things out aren't you NON inverter type aircon ? or you changing it?. Maybe its just the start up load that makes a differance between the 2 anyway - i don't know but someone else here will

 

Edited by Ryanwb1
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14 minutes ago, Pietpower said:

2.1kW x 2 hours = 4.2kWh required.

4 x 12V x 200ah x 50% = 4.8kWh 

I would also think it to be enough.

 

If you tally up the other losses:

 2.1KW AC  = 2.3KW On the DC bus (92% inverter efficiency) and 2.7KW from the battery (At 85% battery efficiency) 

So that's 5.4kWh

Also discharging at that steep a rate (4 hour rate essentially) will mean his batteries won't be 200Ah, more like 150Ah-180Ah.

Unless of course they are rated at 200Ah at a 4 hour rate, but most of the batteries I have looked at are rated over 10 hours, and the real cheats give 100 hour discharge ratings.

Edited by PJJ
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55 minutes ago, Ryanwb1 said:

It's an inverter type,the older tech.Just changed but was recommended to go inverter type as they less problematic with this load shedding

 

I think your wires are crossed there ,that's why i brought it up...the older tech is non inverter type.

Startup load will be greater but i'm not sure about running load

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12 hours ago, Ryanwb1 said:

Thanks Piet thats exactly what i needed to know and yes dod values are right

Ryan if I where you I would buy a single US3000 now.

In terms of price it would be the same as the 48V 200Ah Lead acid bank (It might even be cheaper when you consider the added components like Balancers and a BMV)

Drop the Aircon for now, keep powering your other loads, where a single US3000 would be more than sufficient.  

Save up and when you buy your second module you can add the aircon back into the mix.

If you do 50% DOD cycles I can't think of many LA banks that would give you more than 700-1000 cycles with that usage.

Seeing as a loadshedding event could = 1 cycle (Not a guarantee of course single load shedding could happen while you don't have high loads actually taking it down to 50%)

If we go with the conservative estimate of 700 cycles at 50% DOD, if we had daily stage 1 load shedding, your bank would need to be retired after only 2 years.

Sure daily loadshedding for 2 years on end might not seem likely now, but its entirely a possibility.

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16 hours ago, 2una said:

Startup load will be greater but i'm not sure about running load

Not a very big difference. As you say, the startup is typical induction motor stuff, around 5 times nominal (so as much as 6kW, but remember that the inverter can peak at 200% nominal, so you should be fine). The running cost is a little less on the inverter unit, because they usually have some smarts for determining the dew point (that point where you start condensing water out of the air) and they can control the cooling rate in a manner that avoids wasting energy around a point of inefficiency. I don't know precisely how it works... I just know there is something about it. But I think it's maybe 10% more efficient... maybe. It's not going to make a big difference in your calculations. You can get the conventional unit to do the same... just don't be an idiot and crank it down to below the dew point (usually in the high teens, eg where i live it is usually around 18-22°C).

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17 hours ago, PJJ said:

Also discharging at that steep a rate (4 hour rate essentially) will mean his batteries won't be 200Ah, more like 150Ah-180Ah.

Yup, that's one thing people tend to forget, ye olde Peukert (that's the name of the dude who made up the rule for calculating it). C/4 is a very heavy discharge rate for a Lead Acid. While some batteries are designed to take it (eg Trojan golf cart batteries), I really wouldn't do it to it to the average AGM used for backup.

And about those who cheat... yeah ... *cough* Omnipower *cough*...

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27 minutes ago, plonkster said:

that's one thing people tend to forget, ye olde Peukert

Yup, Peukert, inverter and battery losses are often not taken into account, and also very often omitted is inverter self consumption, which can also be quite a load on some models.

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