FoxESS Community - Owners & Installers Forum

Hi,

So I have a system that is 3-4 weeks old and its going great other than a few tweaks that need to be done to get the import down overnight.

When the system was installed I went with the Fox ESS EC2900 - 14.4kW Li-ion Battery (1 x Master + 4 Slaves @2.88kWh each) and it's great. But with my small amount of usage during good generation days of 60-70kWh I want to store a bit more and export the rest. My kids will be home from travelling soon and this will push the day self consumption up plus there will be more demand on a night so a larger battery storage makes sense especially on coming winter days.

When is the best time to add more batteries in relation to the ages of the existing ones vs new ones? I can have 7 in the first stack, so I'm thinking of adding 2 more before September to max out the 7 limit for that stack in currently have. If this is the case, will 4-6 months later make too much of a difference as the others will have aged?

If I plan to build out a 2nd stack of 7 batteries, do you suggest doing it all over a 12 month period to avoid an imbalance of wear and tear? Cost is an issue but would prefer not to cause any imbalance to a new stack over 12-24 months of adding to it.

I also heard that it's advisable to discharge the existing stack down to 50% before adding a brand new battery? Is this still correct or is empty better even removing the reserved battery limit of 10-20%?

Any help would be great on this, thanks

The best time to add new batteries is as soon as you can, preferably in the first 12 - 18 months, you can add them later but the older batteries will have aged slightly and they can reduce the capacity from new batteries - September should be fine.

On the second stack with the H3 Pro you need to have the capacity of both stacks the same (it attempts to run them with the same SoC across both stacks) - so perhaps you should add another stack of 5 batteries in September so that you have 2 stacks of 5 (rather than increase 1 stack to 7), and then increase both stacks to 6 or 7 when you have available funds.

Alternatively if you do it later you will need to add the second stack with the same number of batteries you have in your first stack to keep the capacity the same, a 12 - 18 month difference in battery ages would be the best timescale to add new batteries to the same stack.

When you do add batteries, always make sure your existing stack is at 50%, this is because the new batteries will be supplied at 50% (+/- 3%) and so to retain balance the packs all need to be at an equivalent soc when added.
The BMS is really good at maintaining the balance of a pack but when there are significant differences in SoC within the pack, the balancing components are not rated to correct a pack that is hugely out of balance very quickly. I've seen people add new batteries when their original batteries were at 70% and it takes weeks and weeks of cycling to get the pack to balance properly and all the while you will be limited to the lowing performing battery in the stack.

Thanks Dave,

Appreciate the advice, all noted thank you.

Question: If I'm looking to just buy 2 more due to finances but instead of making a stack of 7 or a new stack of 5, go for 2 x 4 stacks. I would buy a new master and 2 slave batteries then take 1 of the existing batteries out and add to the other stack making 2 x 4 high. Can batteries be removed and moved if its at 50% and removed and then added to the new stack all at the same time of installing?

So I would in essence just be buying a new master and 2 salves batteries for stack 2 then rob one of the existing ones to balance the 2 stacks. Any harm in doing that then I can simply buy 2 at a time and add to both stacks at the same time to keep it balanced going forwards but aware that the age of the older initial batteries will be dragging the new ones down. Just trying to get to a point that's sensible but might find it easier to go for a brand new 2nd full stack of 7 when the finances allow after completing the 5 stack I have.

merango wrote: Thu May 08, 2025 5:23 pm Thanks Dave,

Appreciate the advice, all noted thank you.

Question: If I'm looking to just buy 2 more due to finances but instead of making a stack of 7 or a new stack of 5, go for 2 x 4 stacks. I would buy a new master and 2 slave batteries then take 1 of the existing batteries out and add to the other stack making 2 x 4 high. Can batteries be removed and moved if its at 50% and removed and then added to the new stack all at the same time of installing?

So I would in essence just be buying a new master and 2 salves batteries for stack 2 then rob one of the existing ones to balance the 2 stacks. Any harm in doing that then I can simply buy 2 at a time and add to both stacks at the same time to keep it balanced going forwards but aware that the age of the older initial batteries will be dragging the new ones down. Just trying to get to a point that's sensible but might find it easier to go for a brand new 2nd full stack of 7 when the finances allow after completing the 5 stack I have.

Yes you can do that no problem, the only caveat always being make sure you are at 50% when you move things about.

I should have also said that when you add more batteries they will likely be at a different firmware version that the existing so once you have set the system up, it is advisable to get the battery master / slave firmware updated (either via your install or Fox service)