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Do you use your battery or the grid as a buffer


Tariq
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During partly cloudy days when solar production varies constantly, do you let the battery act as a buffer constantly charging /discharging or do you let the grid act as the buffer, any pros and cons

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1 hour ago, Richard Mackay said:

The argument I have heard is that the grid is cheaper than using batteries.

I don't think that's the case any longer. 

5 000 cycles from a 3.5kWh pylontech battery equates to 17 500kWh. At R19k for the battery it works out to under R1.10/kWh.

Last two days have not been very good for pv generation with batteries only charging to 70% SOC today and 50% SOC yesterday. I just increase the minimum SOC when Eskom announces load shedding. 

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@Achmat, do you, on a partly sunny day, use your batteries to supplement the constantly changing solar production or do you use the grid, meaning does continuously letting the batteries charge/discharge during the day hurt the battery SOH.

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13 hours ago, Tariq said:

During partly cloudy days when solar production varies constantly, do you let the battery act as a buffer constantly charging /discharging or do you let the grid act as the buffer, any pros and cons

I presume you are not feeding into the grid?

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6 hours ago, Tariq said:

@Achmat, do you, on a partly sunny day, use your batteries to supplement the constantly changing solar production or do you use the grid, meaning does continuously letting the batteries charge/discharge during the day hurt the battery SOH.

I don't think lithium is prone to any damage from discharge/charge cycling during the day. 

I leave the batteries to supplement the power needs on cloudy days. The grid is my backup only.

My SOC and battery power from yesterday. 

Screenshot_20210328-082442_SOLARMAN Business.jpg

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14 hours ago, Tariq said:

During partly cloudy days when solar production varies constantly, do you let the battery act as a buffer constantly charging /discharging or do you let the grid act as the buffer, any pros and cons

This depends on what hardware you have.

I found out a few things lately

  1. Grid voltage where I live fluctuates a lot and I have observed it get as low as 187V
  2. The Goodwe, running in the general mode, allows the battery to run down to the reserved SOC and will then switch to grid, but it will not CHARGE the battery, it just leaves it sitting at 40% (in my case) until there is enough solar to start recharging
  3. Set up for South African grid conditions the Goodwe will disconnect at 195V and reconnect at 205V (this info from Goodwe)

All of this adds up to not very much on a sunny day, but on an overcast day I can get a situation where

  1. SOC drops to 40% and so system switches to grid to service the loads
  2. grid drops below 195 and so it starts drawing from battery again SOC now drops below 40%
  3. grid eventually gets above 205 and so the Goodwe reconnects but it won't recharge the battery unless there is sufficient PV.

(2) and (3) may repeat several times in a day. So my battery can easily get under the 40% I have reserved for a grid outage.

So here's what I did. I suppose personality traits come into play here.

I know that if the battery is fully charged at 16:00 I should get through to 9:30 the next morning without too much drama (assuming we don't start playing silly buggers with hair driers, toasters, microwaves etc).

I know that on a sunny day my batteries are usually fully charged before 12:00 and thereafter the system just draws enough from the solar panels to service the demand from the property. 

I am comfortable with this because it gives me some headroom on days when there is some cloud about or we use more electricity than usual.

So I set my Goodwe to run in economical mode so that it charges, if necesssary, starting at 14:00. This should see the battery fully charged by 16:00. This costs me money on overcast days - but not a lot.

The problem with this is that if the grid drops below 195V between 14:00 and  16:00 then the inverter disconnects from grid and the batteries don't charge until the grid gets back to 205V. So I lose charge again.

So I have put the Goodwe into 50Hz default mode rather than the South African mode. This is more tolerant of low voltages, but also causes the Goodwe to mirror them. So if Grid is 190, everything on my property gets that 190. I don't know if this is a good or a bad thing, but I have to make a judgement that I feel happy with, or least unhappy with. Even then it has limits. It disconnects at 170V, but I have not seen things get that low.

So I suppose I have a double buffer. Grid to charge the batteries for a set period each afternoon to ensure that the batteries get to full charge. Then battery in case the grid goes down.

I also did it this way because whilst I am working from home I can be turning things on and off, tweak parameters that my installer said I can tweak, switch between general and economical mode and all sorts of other things, but I really want a system that will just work when I'm not there and there is nobody to say "hmmmm... maybe the pool pump could be turned off" or "gosh! look at the grid voltage". So this seems like the best way of ensuring that people in the house can do what they need to do (as long as they don't abuse the appliances).

Edited by Bobster
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18 hours ago, Tariq said:

During partly cloudy days when solar production varies constantly, do you let the battery act as a buffer constantly charging /discharging or do you let the grid act as the buffer, any pros and cons

Grid,

Simple then I get money back.  LxxPOE prices will soon rise as the raw material cost are increasing.  Thus if you can be grid tied and get money back then grid tied is far more economical

 

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