How low can you go?

[quentinsf]

So my new Sunsynk CATL LFP batteries advertise a '100% depth of discharge'.
What would you say is a sensible minimum charge level actually to specify in my inverter? I have 15% at present.

I don't want to unnecessarily shorten the life of my batteries, but I also don't want to waste capacity I could sensibly be using,

I'm new to home batteries, but I know that on my electric cars, the manufacturers arrange that 0% or 100% (as displayed to the user) have sensible margins to protect the batteries, and it's normally impossible either to fully charge or fully discharge them. What should I assume is the usable capacity of my 5.1kWh batteries, do you think?

many thanks,
Quentin

 

[AHTrimble]

Check with your battery manufacturer, as with my Trophy batteries, they already have a built-in to the BMS to prevent going too much DOD.
And you are not wasting capacity...over-all. You may not use that capacity until later in the battery life because you extended the battery life, but you still will use it...eventually. Look at this way...you will get X number of kilowatts of recharge not matter what...it is either going to be used in a shorter or longer period of time.

 

[DougfromdaUP]

Here’s a post that will help you:




https://diysolarforum.com/threads/b...-stress-5-000-10-000-cycles.34813/post-435243

 

[quentinsf]

Check with your battery manufacturer, as with my Trophy batteries, they already have a built-in to the BMS to prevent going too much DOD.
And you are not wasting capacity...over-all. You may not use that capacity until later in the battery life because you extended the battery life, but you still will use it...eventually. Look at this way...you will get X number of kilowatts of recharge not matter what...it is either going to be used in a shorter or longer period of time.

Yes... except that I may well move house before the batteries expire. ?

But is it actually that simple a calculation: that the total discharge capacity over the lifetime of the batteries is pretty constant?

 

[jasonhc73]

I have recorded a total of 28 cycles in 2½ years using 15% to 99% on LiFePO4 batteries.

I set the charge to 3.40V as the top and 3.125 as the cutoff on the charge controller/inverter.
54.4V - 50V

 

[MisterSandals]

What would you say is a sensible minimum charge level actually to specify in my inverter? I have 15% at present.

If you are being conservative, i would set it so that none of the cells dipped below 3.0V.
This of course depends on how well YOUR battery behaves at low SoC.

If you need to squeeze more out of your battery for daily living, i'd go down to 2.8V.

 

[time2roll]

I would allow the use of 100% capacity. This assuming day to day max use would be 50% or less. If going to the bottom I assume it is a necessity that may never happen.

 

[quentinsf]

Thanks all - lots of useful stuff there.

Q