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Best Practice for Li-Ion Battery Maintenance


Tass99
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Good afternoon

I'm looking for some advice on best practice for properly maintaining a Pylontech US3000 3.5kWh battery.  I have a Goodwe 4.6kW hybrid inverter, 18 x 330W JA panels and the 1 Pylontech battery.  I've had a lot of people tell me that I should purchase more batteries for the size of my system, to make the system more cost effective, but I'm in a rather unique situation.  I'll explain below, but essentially I don't want to know what will most cost effective - I want to know what's best for the battery.

I'm waiting for the CoCT to sign off on my installation, and have been for about 18 months now (Covid's delayed that, somewhat).  I'm not sure what the holdup is, but at this point I don't really care.  I have the old-style meter with the disc that spins, and when I generate more than I consume it just spins backwards, essentially giving me "credit".  I therefore don't need to use the battery for financial benefit - I simply need it during loadshedding, to ensure my inverter stays running.  There's no benefit to me charging the battery using solar and then using it at night when I can simply feed back into the grid at the same rather I'm charged.

My question relates to the best approach to properly maintaining my battery.  Currently I have DoD set to 5% (which I believe is the minimum).  My battery stays at 100% all day, before dipping down to 95% in the late afternoon, then charging again first thing in the morning.

During a recent bout of loadshedding it dropped down to 20%, and I noticed that the drop from 35% to 20% was very fast - about 3 minutes.  This led me to believe the battery wasn't calibrated properly.  I set the D0D to 80%, and have watched the battery drain for the last week, and it seems better now (although from 30-20% seems to drain slightly faster than the rest).

 

So - on to my actual question.  How should I configure the battery, to get maximum life out of it?

A ) Leave DOD at 5%, and only use the full battery when absolutely necessary

B ) Set DOD to 80% and use it every day

C ) Leave it at 5%, setting to 80% a few times a year to give it a few full cycles

 

And a final tangent question - is 80% the correct DOD to set, or should I consider other options?

 

 

Thanks in advance.

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12 hours ago, Vassen said:

What you need to also bear in mind is the charge & discharge rate of the battery. Pylontech recommends 37A per battery. At 48V, that translates to around 1.8kw. What is your max load during loadshedding? 
 

You have a 4.6kw inverter and that’s why it’s recommended to get 3 batteries. However if your loads are managed during loadshedding with no kettles or heavy loads coming on, it should be fine. 
 

I remember reading something about the batteries needing to be cycled once a week to 80%. Also remember, lithium batteries don’t like to be stored at 100% soc. 

OK - thanks. My DB board has been split out, so I normally sit at about 1.1kW during loadshedding on a normal evening, heading down to 700W overnight. The system has tripped a few times when I've used heaters recently, and that's when it pushes up over 1.6kW (config in the inverter to automatically do that, of some sort of protection in the BMS?).

For now I'm happy to keep my load below 1.5kW, and should loadshedding ramp up a lot over the next few years I could always add more batteries to make that more manageable. 

 

When using my battery at night I get a pretty steady discharge of 1.4 (until that last little 20-30%, which has an annoying drop, even though the load doesn't change). 

My general feeling is I need to sit at 5%, switching to 80% every few months now and again. There doesn't seem to be any official guidance on this though. 

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19 hours ago, Boerseun said:

The less and more shalow you cycle the battery the longer the lifespan so option A would be the best choice Pylontech batteries have excellent performance for shalow charge and discharge rates.

Thanks. So any need to do a discharge from time to time? 

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1 hour ago, Tass99 said:

Thanks. So any need to do a discharge from time to time? 

Technically speaking conditions that will affect the life of your Lithium Iron Phosphate battery more is high tempreture and high charge rates however your systems Bms will controll charge rates and discharge rates there is one school of thought that claims you need give your battery a deep cycle once in a while however there is no rule of thumb on exactly how often you need to do that. The other school says let the BMS manage that and cycle the batteries according to your needs this will inadvertently achieve the same goal. However if you look at the Cycle curves of Lithium Iron Phosphate batteries you get more than double the duty cycles at 60%  dod than 80% Dod and for the Pylontech batteries they claim 6000 cycles at 80% Dod @25 degrees celcius.

Here is an interesting read https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

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20 hours ago, CaTcH21 said:

Doesn't the battery have a lifespan of 10 years ?

 

The "lifespan" is measured in cycles...E.g 6000 cycles at 80% DoD will give you 6000/365 = 16.4 years... BUT at the end of 6000 cycles the battery will only have 80% of its initial charge available for use..

In real life testing, the Pylontechs reach that 80% of initial charge after about 4500 cycles = 12 yrs.

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On 2020/09/19 at 8:53 PM, Vassen said:

What you need to also bear in mind is the charge & discharge rate of the battery. Pylontech recommends 37A per battery.

Is the recommended 37A per battery the maximum allowable charging current, what if I only charge at 10A charging current or even lower would it have any bad consequences for the battery or would it benefit the life of the battery.

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  • 1 month later...

Lithium cell lieftime is a very complex issue - in general high voltage reduces life expectancy - so if you only using 30% of your battery its better to operate 50 - 20 % SOC as opposed to 100 - 70%   SOC . This can be a challendge to get your charge to charge up to a SOC level  as opposed to a voltage level .  You can do this with a multiplus and a venus using a 2 wire BMS assitant .  Read thi spaper on lithium battery life https://www.sciencedirect.com/science/article/pii/S2405896318320329

 

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