diy solar

diy solar

Adding battery capacity later on

GoRun

New Member
Joined
Jun 10, 2022
Messages
5
I searched the forum and couldn't find a clear answer to the following question(s):

Suppose a system is installed with several units of server rack style lithium batteries with proper communication standards, is it ok to add a new unit after 6 months? 1 year? 2 years? What conditions should be met for this operation to be successful? What should one be wary of when adding a new unit?

Since a battery starts to lose health/total capacity as it gets cycled, the idea of hooking a new unit (or more) to an older set of batteries doesn't seem to be a good idea, I imagine the older batteries could overdischarge and the new battery could overcharge, but I don't know if having the batteries "talk" to each other can work around the issue, or perhaps the only way to do any addition is to do it within a few months of use. I've seen people just add a new battery no problem on youtube, but there is no context there, could be a week old system or something.

I'd appreciate if you can help me avoid a costly mistake ?
 
If you add the new batteries while the "old" ones are still in the first (say) 1/3 of their life, I wouldn't expect any negative concerns. What happens as the batteries are degraded, all cells will have somewhat less capacity. Not all cells will degrade the exact same amount. So cell imbalance starts to become a factor, even without new batteries. If the peak charge voltage is reduces a little, the imbalance issue is not as noticeable. When you do this, you reduces your capacity a tiny bit, but the battery is somewhat degraded already, so no big issue. If you now add new batteries in parallel with the old ones, charging them to something less than ideal voltage, then you may not see 100% of their capacity. We normally don't try to get 100% capacity from the batteries anyway, so that is not a real concern.

I have two old degraded LiFePo4 batteries and a new larger one, a little larger than the two old ones combined. If I draw a heavy load, they discharge a little differently and then if I remove the load, one battery helps "charge" back to the other one. This is often called something like "internal micro cycling". It is not ideal but not bad. My battery system is for emergency and special testing/experiments and such. So mine doesn't get worked hard every day.

If you do this, there is a concern when you connect the new battery. You want the old battery to be fully charged or discharged, and the new battery to be in the same state of charge/discharge AND at the same voltage as the old battery. That way when connected there is not an effort of one battery to charge the other battery.
 
With LFP this is NOT a Problem.... With Lead Batteries, it is a Major Issue with regards to aging & capacity (very small window to add more).
The Catch with using Rack Batteries (Example) if you install 4 EG4's today they will have matching BMS, Communications Protocols etc. But things evolve and in 6 months or a year the newer ones "MAY" be a little different and if done properly, the newer ones should be "backward compatible".

Even mixing Capacities, ie: 4x 200AH go in today and next year you get a 300AH (same brand) pack you should still be able to add it without issue. The one Obvious & Noticeable difference you will see with different Sized Packs will be how they discharge & charge as it IS PROPORTIONAL TO CAPACITY.

The gotcha with Rack Batteries is that you are NOT guaranteed "Compatibility" between different brands as they may use Different BMS, Protocol settings etc.. So If you go with EG4 then best to stick to that One Brand. Applies to all of them.

My Bank has 2x 174AH (Used EV LFP) cells and 3x 280AH Packs, 1 Bulk Cells, 2 A Grade Cells with one being "Properly Matched & Batched". I watch this game play out daily and zero issues. These have been put together & implemented over 3 Years, no real issues but ONE, I've just had to swap all the BMS' to be Identical & the Same Version with BlueTooth/RS485 etc as well. Even doing the same with my Utility Packs.

Hope it Helps, Good Luck
 
As long as the original batteries are not cycled into the knees. Then capacity is related to calendar aging. I don’t know if that is completely linear or not though.
If you need more capacity you are probably pushing into the knees though.
 
I think the issue of most concern is that the battery industry is changing very fast and vendors will come out with new models, new chemistries, new capacities, new BMS's, new communications standards, etc and you won't ever get a perfectly matched system like you do if you buy the capacity you need all at once. Just look how much the server rack battery market has changed in a year...
 
Very interesting information, thank you all for being so helpful!

My system is new and I have only experimented with it during these long summer days and I know what battery capacity will be more than enough for the summer nights, and I estimate that it will be enough for winter nights but I may be off by 20-25% in which case I'll need to add a unit to the 4 I am planning to install.

I can't ask for recommendations since I don't have many options, I am going for Dyness B3 batteries, this battery looks decent but if you know that I should steer clear of it, let me know.
 
Last edited:
Back
Top