FoxESS Community - Owners & Installers Forum

Hi,

My system: 7 x 440 PV. H1-3.7-E-G2. 1 x EP11.

In terms of battery health and longevity, I'm seeking guidance on the merits of trickle charging the battery v. deeper discharge then force charging from the grid.

I've been running my system for a couple of months since the install in self-use mode. The battery has never been below 90%.

In the meantime, I'm wondering if I should instead change to feed-in and deplete the battery more fully and force charge it overnight using the low rate offered by the tariff?

I've very recently changed to the e.on next drive tariff including their export rate. We have an ASHP so the battery will need grid charging when we start using the heat pump for heat and not just for hot water, as now, but that's a few months away (hopefully!)

In this household, the jury is out on the economics of self-use v. feed-in given we have a small system so really the post is about maintaining battery efficiency.

As a separate, but still battery related issue, I get the impression from the forum that force discharging the battery to maximise export may not be good, long-term, for the battery?

Paul

Your daily self discharge rate will change dramatically as the winter draws in which makes it a slightly more complex answer.

In general the batteries will last longer if not being regularly cycled hard but then they will also lose some capacity through ageing, so it's best to use them as effectively as you can to get the best return.
I'm not a big fan of force discharging hard (unless the commercials make sense) as that will provide more stress (and rapid internal temperature rise) on batteries than under normal use, but I know a lot of people that use the batteries simply as a commodity and dump as much charge during the higher export tariff to get the best possible return.

The batteries are warrantied to provide a certain throughput but the reality is that if you treat them better (fewer cycles, limit deep/fast discharge, take time to charge them, maintain temperatures within 20-30C etc..) they will last considerably longer; sadly the jury is out as to how long because the cell technology (LFP) doesn't have enough track record yet.

But all of that aside as long as you stay within the specifications of max charge / discharge current and temperature range for your battery type - they will provide you long service and so it's best not to get too caught up in the management of them, instead use them as effectively as you can to minimise the cost of your grid import.

With home assistant you have a lot of control over your batteries and it is easy to get a bit myopic in the protection of them, so the best advice is from a practical point of view the BMS needs the batteries to be fully charged and discharged occasionally to maintain accuracy of it's state of charge calculation, preferably do this weekly but at least monthly otherwise you may get unexpected jumps in SoC when the BMS learns the hard way what the actual residual power is in the battery.

SunnySpot56 wrote: ↑Tue Jul 01, 2025 1:19 pm The battery has never been below 90%.
Paul

I've just bought a couple of the same battery and am very much at the bottom of the learning curve. I did find this paper though which relates to what you just said (there is an annoying human check)

The Operation Window of Lithium Iron Phosphate/Graphite Cells Affects their Lifetime

Lithium iron phosphate (LFP) battery cells are ubiquitous in electric vehicles and stationary energy storage because they are cheap and have a long lifetime. This work compares LFP/graphite pouch cells undergoing charge-discharge cycles over five state of charge (SOC) windows (0%–25%, 0%–60%, 0%–80%, 0%–100%, and 75%–100%). Cycling LFP cells across a lower average SOC results in less capacity fade than cycling across a higher average SOC, regardless of depth of discharge. ........
Operating LFP cells at lower average SOC can extend their lifetime substantially in both EV and grid storage
applications.
© 2024 The Author(s).

So according to this you would be substantially better going 25% -> 35% -> 25% than going 90% -> 100% -> 90%. I would assume only using 10% of capacity is also going to significantly increase your payback time.

Thanks Dave and Chapeau for your responses. What a good resource this forum is!
Interesting to note that one conclusion from the iop.org paper is that charging the battery up to 80% SOC from a low starting point, with occasional full charging, might be better for longevity.