Video question adding another battery 24v system 100ah

Been watching a few videos on expanding 24v battery banks and I'm getting a bit confused by some of the advice out there, so hoping someone here can clear it up.

I've got a 24v 100ah setup on my garden office at the moment — two 12v 100ah Fogstar Drift LiFePO4 in series. Works well for my needs most of the year but I'm struggling through winter with the solar input being so low and the office heating pulling more than I'd like.

My plan is to double the capacity by adding another two batteries in series (to make another 24v 100ah unit) and then connecting that in parallel with the existing bank. So I'd end up with 24v 200ah total.

A few questions though:

• Does the new pair need to be the same age/cycle count as the existing pair, or is that less critical with LiFePO4 compared to lead acid?
• Should I fully charge and balance both pairs before connecting them in parallel for the first time? I've seen conflicting advice on this.
• My Victron SmartSolar MPPT is a 100/30 — will it handle the larger bank without any changes needed, or do I need to look at the absorption/float settings?

The office isn't huge so I'm not adding loads of panels, just trying to squeeze more usable capacity out of what I've got. The boat setup is a different project entirely and I learned some lessons there about mixing battery ages that I'd rather not repeat here.

Anyone done a similar expansion with Fogstar cells specifically?

@GrumpyWarden post got cut off mate, no idea what you're actually asking.

That said, classic 24v expansion pitfall — people mix old and new batteries thinking it's fine. It's not. New cell in parallel with a tired one just drags the new one down to meet it.

If you're adding a second 100ah LiFePO4, both batteries need to be same chemistry, same age ideally, same SOC before connecting. Fogstar Drift cells are decent if you're going budget.

Also check your BMS can handle the combined discharge current before you just bolt stuff together. Most folk don't bother and then wonder why things get warm.

Finish your question and I'll give you a proper answer.

Looks like we've got two cut-off posts in a row here, which is a bit of an omen really 😄

@GrumpyWarden if you're asking what I think you're asking — whether you can just bolt another 100Ah battery alongside your existing one — the short answer is: only if it's the same chemistry, same age, and ideally same brand.

Learned this the hard way with my shepherd's hut build. Mixed an older Fogstar cell with a newer one and spent weeks chasing mysterious capacity drop-offs before realising the BMS units were essentially fighting each other.

If your existing bank has seen any real-world use, a brand-new battery won't behave identically to it. Worth finishing your question though — the specifics really do matter with 24v expansion.

Looks like this thread is determined to stay mysterious 😄

@GrumpyWarden when you do get your full question posted, the most common 24V expansion mistakes I see covered badly in online videos (especially US-produced ones) are:

1. Mixing old and new cells — even same chemistry, same capacity, don't do it
2. Series vs parallel wiring — getting this wrong on a 24V bank is a bad day
3. BMS compatibility — adding a second battery with its own BMS alongside an existing one needs careful thought

My own setup runs two 24V 100Ah Fogstar Drift LiFePO4 units and getting the balancing right between them took some proper research beyond what YouTube offered.

If you're using lead-acid that's a whole different conversation around matching state-of-charge before paralleling.

Post your full question and we'll get you sorted properly.

Thread's starting to look like a Victron BMS with half the cells missing.

@GrumpyWarden finish your post mate, but I'll take a punt — whatever those videos told you about mixing old and new lithium cells in a 24v bank, they're probably wrong; same chemistry, same capacity, same age or forget it, your BMS will have a meltdown before you do.