On Friday the install of my new off-grid system was completed. I was very pleased as it is a sterling job and very well delivered.
However, I have a question about the performance of my new BYD Lithium batteries as something seems a little strange (at least to me, as a novice).
I’ve attached 4 screenshots. Two are from the 1st of January and data from my legacy system for which the batteries were 14 year old (!) lead-acid ones. As you can see, at 10pm the SOC was 100% and overnight they lost about 3%. One of the other screenshots is the consumption, which when I am in the place on my own is generally the fridge and freezer so the average usage is about 300w at any given point.
So I was very surprised to see that over the first couple of nights of having my new system in place, that the drop was about 15%. The consumption is pretty much the same - fridge and freezer so about 300w.
When installed (14 years ago) the lead-acid batteries were 12kWh, the new ones are 15kWh.
Does this seem correct? Scaling up the 300 to 1kW then they would have lost 45% of their charge in that 8 hours. I’m slightly worried that for what I have paid come winter when there is far, far less PV that I will be using my generator as much as I was before.
Maybe I am comparing apples and pears but I would have thought that if my old, old batteries only dropped 3% overnight in the depth of winter then I would have expected the new ones to have dropped much less.
Or am I barking up the wrong tree and lacking some understanding? I am, as it is maybe apparent, no expert!!
So from your 15kWh BYD batteries, this is about 100% x 2.4/15 = 16% so that seems about right.
For your lead acid 100% x 2.4 / 12 = 20%. Your figure of 3% is completely wrong.
Lead acid batteries have a feature called the Peukert effect whereby at low rates of discharge you can get more energy out but not by a ratio of 7:1.
I can only conclude that your shunt was set up wrong, probably the calculation of the battery capacity in Ah. This is a common fault if people have 4 x 100Ah 12V batteries in series to make a 48V bank, they then tell the battery monitor that the battery is 400Ah. This is wrong, batteries in series do not multiply the Ah, it is a 100Ah bank. A large battery capacity will increase the calculated Peukert effect, hence the massively optimistic rate of SOC use.
I too lived with lead acid for over 20 years before installing a diy lifep04.
I’ve gone from a 700ah lead acid to a 200ah lithium
As you said, they are apples and pears
The lithium have so much more usable power and recover so much quicker too.
If you’re at all concerned about the fridge a small timer that turns the fridge off between 10pm - 4am may be the answer.
I have a small freezer that I run like that.
Hey! Thanks for the info. Yeah the more I look into it, the more I realise that I’d misled myself by using the old stats as a guide. But it’s not a massive issue - I just wanted make sure that the new batteries are performing as they should, and never having had them before I wanted opinions from others who had. They are scalable, so if needs be, I can always add to them.
Plus there is scope to add in a little wind turbine (if I can find something suitable and a good enough charge controller) and given where I am, wind is abundant!
I was just using the fridge and the freezer as examples but there is relatively little that runs overnight, but obviously in the depths of winter there’s no PV from about 3pm, so that 8 hours becomes 16.
Maybe I just wait and see what happens over the next 8-12 months. I am a great believer that we should only use energy when it is available and I have done that well on my old batteries.
After 20 years with lead acid, before offgrid became a fad, we still use power sparingly with the new batteries.
Only because we conditioned ourselves. We still turn everything off at the power point and monitor heavy users.
Now we’re getting the hang of it, the washing machine gets used on cloudy days, as does the sewing machine.
We also installed a electric frudge freezer and sold the gas fridge.
I really struggle to comprehend the power system sizes people are purchasing.
