Anyone else running a Victron SmartSolar on a leisure battery that's way too small for it?

So I picked up a Victron SmartSolar MPPT 100/30 earlier this year thinking I'd upgrade the battery bank to match eventually, and that "eventually" has somehow turned into six months of running it with a single 110Ah AGM. I know, I know. The controller is rated up to 30A charge current and the battery really wants no more than around 11A at the 0.1C rate, so on a decent sunny day in summer I was basically throttling a perfectly good bit of kit.

What's surprised me though is it's actually been fine in practice. The Victron app shows it regularly hitting absorption around 14.4V and the battery seems happy enough — no obvious signs of overcharging and it's held capacity well. I've got a 200W panel on the roof (Renogy mono) and realistically in the UK the full 30A was never going to happen anyway, so maybe I've been worrying over nothing.

Planning to add a second 110Ah AGM in parallel before winter to make a proper 220Ah bank, which should suit the controller much better. Has anyone else been in a similar situation — running a controller that's technically oversized for the battery? Did you notice any actual damage over time, or is it mostly theoretical if the MPPT is doing its job properly?

@SolarOwen ha, sounds very familiar! I ran my 100/30 on a single 100Ah AGM for nearly a year before I finally sorted a proper bank. Honestly it wasn't a disaster — the Victron just absorbed quicker and sat in float mode most of the day.

Main thing to watch is if you're getting decent solar input, that battery is going to hit absorption voltage pretty fast and cycle more than ideal. Not great for longevity.

Worth setting a proper absorption time limit in the VictronConnect app if you haven't already — made a noticeable difference for me.

When you do upgrade, Fogstar Drift lithium is worth a look for the money. That's what finally got me sorted. Your MPPT will handle it no bother 👍

@SolarOwen the Victron is genuinely quite forgiving in my experience — I ran a 100/20 on a single 100Ah AGM for about eight months before I sorted out a proper bank for my garden office setup. The MPPT will just throttle back once the battery's full, so you're not really damaging anything as such.

What I would watch is that a small battery will hit absorption voltage quickly and you might be cycling it harder than you'd like if you're drawing decent loads. That'll shorten the AGM's life over time.

If you're not ready for a full bank yet, honestly consider grabbing a second matching battery in the interim — two 100Ah in parallel makes a noticeable difference and won't break the bank. Fogstar do decent AGMs at reasonable prices if you're watching the budget.

Hey @SolarOwen, classic "I'll sort it eventually" situation! One thing worth keeping an eye on that I haven't seen mentioned yet — with a small battery the controller will hit absorption quite quickly on a sunny day, then float, then potentially cycle back through bulk again repeatedly. Over time that can be a bit hard on AGM batteries compared to a proper lengthy bulk phase. Worth checking your VictronConnect history to see how long it's actually spending in each charge stage. If you're seeing very short bulk periods most days, it might be worth tweaking your absorption voltage settings slightly. Not a disaster by any means, just something to be mindful of while you're waiting on that upgrade! What size battery are you running with it currently?

Chiming in here as someone who ran almost exactly this setup for a while — the main thing I'd add that nobody's touched on yet is to check your charge current settings in the VictronConnect app. The 100/30 can push 30A which on a small battery is quite aggressive, especially towards the end of a charge cycle. Worth capping it manually to something like 0.2C of your actual battery capacity. So on a 100Ah battery, I had mine limited to around 20A. Made a noticeable difference to how the battery was handling the bulk stage. The Victron app makes it dead easy to adjust. @ThistlePaul is right that it's generally forgiving kit, but a bit of tweaking goes a long way. Good luck with the eventual battery upgrade — once you do it you'll wonder why you waited! 😄

Similar story here @SolarOwen! I ran my 100/30 on a single 110Ah leisure battery for about four months before I added a second one.

One thing I'd add that nobody's mentioned yet — keep an eye on your absorption and float times in the VictorConnect app. With an undersized battery the Victron will hit absorption voltage surprisingly quickly, which can look like everything's fine and dandy but doesn't necessarily mean the battery is properly full. I was convinced mine was getting a good charge until I actually started logging it properly.

Not a disaster by any means, especially if @Tina1971's eight months without issue is anything to go by, but worth knowing what you're actually looking at in the app rather than just glancing at the status LED. The data logging is genuinely one of the best things about Victron kit.

Been there with my motorhome — ran a 100/30 on a single 100Ah AGM for ages "temporarily" 😅

One thing worth adding: check your absorption/float voltage settings in the VictronConnect app actually match your battery chemistry. Easy to overlook when you're just chucking it in quick, and a mismatch will quietly cook a small battery faster than the oversized controller ever would. Caught mine set wrong for months.

Also if you do eventually expand, Fogstar Drift lithium is worth a look — good value and plays nicely with Victron's charge profiles.

Great thread! One thing I'd throw in that hasn't been mentioned yet — keep an eye on your battery temperature if you're doing this over winter. A small battery being pushed by a 30A controller on a decent solar day can warm up more than you'd expect, and cold-soaked leisure batteries don't appreciate being hammered when they're already stressed.

The Victron app is your friend here — set up some alerts and actually check the charge cycle history occasionally rather than just assuming all's well. @SolarOwen how are your absorption times looking? That can tell you a lot about battery health.