Batteries as backup - Best practice for longevity.

[Tulex]

Will be receiving my Sol-Ark 15K shortly. Building 2 304ah batteries.
It seems that hybrid with battery backup is kind of a new concept, or at least not as common. But, that's what I am doing. Could be looked at as a ready to go system if SHTF, enough power to live on, but until then, it's battery backup and to help offset my E bill.

What is the best practice in this type of setup to get the most life out of my batteries? Do they need to be cycled? Regularly or occasionally? Do I want to keep them at a different state of charge than off grid use?

This sort of matters up front before I'm even up and running because I read in the many compression threads that compression necessity depends on how the battery is used. I will go with compression if it will extend my battery, but using the batteries correctly is the first concern.

 

[SeaGal]

Many threads on that on here. So many factors, but depth of discharge, rate of charge/discharge and temperature will be the major factors to keep them lasting as long as possible.

i.e. smaller DoD is better. Lower charge current is better and keep them around 25 degrees C.

 

[Hedges]

LiFePO4 should be good for several thousand cycles 80% to 90%
Maybe that is just one decade of nightly cycle, maybe two.
Failure of BMS could be a bigger issue. Periodic reading of cell voltage, to alert you if balancer goes haywire?

Occasional charge and hold high enough to rebalance should avoid having capacity reduced by high/low cell voltage disconnect. How often, you'll learn from experience.

If not cycling, I think fully charge to balance then discharge about half so sitting around 50% SoC should reduce degradation rate. And especially keep temperature moderate, even cool.

Because they probably cost around $0.05/kWh of cycle life, you may want to cycle daily if doing so saves money vs. utility rates. e.g. if you have time of use, or poorer than 1:1 net metering. But shifting time when loads operate costs nothing at all.

We've read that compression can help, but I would avoid excessive force, e.g. hard clamping and swelling due to charge might be excessive. Spring loaded with moderate pressure seems safer. If not cycling every night or every other night, could be the additional claimed cycle life won't be used anyway, in which case unclamped might be fine.

 

[PreppenWolf]

If you are using LiFePO4, and your battery it properly sized to at least two days of offgrid support, there is no reason to keep them in standby only. I have 105kWh of storage for a house that uses 30-50kWh per day depending on time of year. I've had my system running in SBU (Priority Solar first, then Battery, last Utility) since November. I have only the equivalent of 18 cycles on each pack. The batteries will die of old age before I could ever cycle them down to even 80% capacity.

Snapshot of one of my batteries right now:

 

[Tulex]

If you are using LiFePO4, and your battery it properly sized to at least two days of offgrid support, there is no reason to keep them in standby only.

That's not my setup. I've got enough battery to last me half a day, but a full day if I'm running only critical load.
My goal is to always have battery backup. I'm guessing I can set it up as yours but raise my low limit and accomplish the same thing.

 

[PreppenWolf]

Even if you cycle once per day, in 10 years you'll still have <80% capacity.

I would just plan on future battery expansions with that in mind.

I cycle from 48V to 57V. Which also help buffer the "wear".

 

[wattmatters]

I have 10 kWh (soon to be 15 kWh) of LiFePO4 for daily cycling duties and 20 kWh of Sealed Lead Acid on standby for outage backup. The SLA are ex data centre backup units. Designed precisely for that task.