Confused about battery capacity – am I losing half my usable power with AGM?

I've just finished wiring up a fairly basic 12V system in my Transit camper – 200W of solar through a Victron SmartSolar 100/20 MPPT, feeding into two 110Ah AGM leisure batteries wired in parallel. On paper that gives me 220Ah, which I thought would be plenty for a long weekend. Fridge, a few USB charges, some LED lighting – nothing mad.

But I've been reading about the 50% depth of discharge rule for AGMs and I'm starting to think I've massively miscalculated. So in practice I'm only working with around 110Ah usable? My 12V compressor fridge (a Dometic CFX3 35) seems to pull somewhere around 3-4A on average once it's up to temperature, so that's roughly 72-96Ah over 24 hours just from the fridge alone. That feels like it's eating most of my budget before I've even plugged anything else in.

Should I have gone lithium from the start, or is there a way to squeeze more reliable use out of the AGMs? I've seen people mention keeping them above 12.2V as a rough guide but I genuinely don't have a solid grasp on what voltage corresponds to what state of charge under load versus resting. Is the Victron app giving me accurate readings or is it just measuring voltage at the controller rather than at the batteries themselves?

@NotAnElectrician48 really good first setup! Quick question though — have you looked into what depth of discharge you're actually getting from those AGMs? With lead-acid you're typically limited to around 50% DoD to protect battery life, so your 220Ah effectively becomes ~110Ah usable. Are you finding that's enough for your daily usage, or are you hitting the wall earlier than expected?

Genuinely curious because this was exactly the point where I started seriously looking at lithium — Fogstar Drift cells give you 80–100% DoD, so a single 100Ah LiFePO4 can actually outperform your whole 220Ah AGM bank in real-world terms.

What are you running off the system? That'd help work out whether you're undersized or just need to manage expectations around AGM limitations.

@NotAnElectrician48 great setup to start with, but yes — this is the AGM reality that catches everyone out. Your 220Ah nominal gives you roughly 100-110Ah usable at 50% DoD before you start damaging the batteries long-term. Push them to 80% regularly and you'll be replacing them within a year.

I ran AGMs on my narrowboat for two seasons and the cycle life is genuinely painful once you factor in proper DoD limits. Switched to a Fogstar Drift 100Ah LiFePO4 and suddenly 80-90% DoD is perfectly acceptable — your usable capacity nearly doubles from the same physical space.

Your Victron MPPT will handle lithium charging profiles natively too, so the controller investment is already made. Worth seriously considering if you're planning any extended off-grid trips. The upfront cost stings but the maths over 2-3 years is compelling.

LakelandSolar | ⚡ 847 posts | Cumbria

@NotAnElectrician48 welcome to the forum! Yes, unfortunately this catches a lot of people out. With AGM you really want to stick to 50% depth of discharge maximum to protect battery life — so your 220Ah total gives you roughly 110Ah of usable capacity in practice. Push deeper regularly and you'll dramatically shorten their lifespan.

As @Cleggy alluded to, it's worth programming that limit into your Victron — the SmartSolar can handle low voltage disconnect settings, and if you pick up a SmartShunt or BMV-712 monitor you'll see exactly where you are in real time rather than guessing.

For a Transit van setup 110Ah usable is honestly workable for weekends, just be mindful of what you're running overnight. What loads are you planning to power?

WillHall | ⚡ 312 posts | South Wales

@NotAnElectrician48 worth adding to what others have said — the 50% DoD rule isn't just about capacity, it's genuinely about longevity. Push those AGMs below 50% regularly and you'll likely be replacing them within a couple of years rather than getting five or six years out of them. So your effective bank is around 110Ah usable, which sounds disappointing but is workable with 200W of solar on decent days.

One practical tip: set your Victron SmartSolar's low voltage disconnect (if you're running one) or at least monitor your battery voltage closely. Around 12.1V under no load is roughly your 50% warning sign with AGM. The Victron app makes this really straightforward to keep an eye on.

QG_Marine | ⚡ 203 posts | Norfolk Broads

@NotAnElectrician48 one thing nobody's mentioned yet — that 50% DoD isn't just about protecting capacity, it directly affects cycle life too. Push your AGMs regularly to 80% DoD and you might see 200-300 cycles before they degrade noticeably. Stay around 50% and you're looking at 500+ cycles. So ironically, being conservative actually gets you more usable energy over the battery's lifetime.

With your 200W solar and 220Ah bank, also worth keeping an eye on charge rates — AGMs like a decent absorption charge to stay healthy. Your SmartSolar handles that well, just make sure you've selected the correct AGM profile in the settings.