Anyone else finding MPPT controllers wildly optimistic about battery state of charge?

Been running a Victron SmartSolar 100/30 with a 200W panel on a pair of 100Ah AGMs for about eight months now, and I've noticed the SOC readout on the VictronConnect app seems to leap to 100% far too quickly — sometimes within a couple of hours of a decent morning, even when I know the batteries were well down the night before. Then by evening they're flagging low again. Feels like it's lying to me.

From what I understand, MPPT controllers estimate SOC mainly from voltage, which is notoriously unreliable for lead-acid — especially AGMs that bounce back quickly under light load. I've been tempted to add a proper Victron BMV-712 shunt monitor to get coulomb counting instead, but at £90-odd it feels like a lot just to get an honest reading. Wondering if it's actually worth it or whether I'm overthinking this.

Has anyone gone from relying on the controller's SOC estimate to using a dedicated shunt, and did it genuinely change how you managed your system day to day? Also open to whether there's something I should be tweaking in the charge profile settings that might make the voltage-based estimate a bit more honest — my absorption voltage is currently set at 14.7V and float at 13.8V, which I think is fairly standard for AGM.

@ExpertWanderer yeah this is a known quirk with AGMs tbh. Victron's SOC algorithm relies heavily on the battery reaching absorption voltage and holding it — AGMs can do that relatively quickly without actually being full.

Worth tweaking the charged voltage and tail current settings in VictronConnect. I dropped my tail current threshold to around 2% of capacity and it made a big difference on my shepherd's hut setup.

Also check your absorption time isn't too short. If it's cutting to float early the SOC just assumes 100% regardless.

A proper battery monitor like a BMV-712 measuring actual coulombs in/out will give you far more reliable readings than the MPPT alone — that's what finally sorted my readings properly.

Sam1985 | 147 posts | ⚡ Solar Enthusiast

@ExpertWanderer totally feel your pain on this one. Worth checking whether you've got the battery capacity set correctly in the app — 200Ah for your bank — and also make sure the charge current tail setting is properly configured. For AGMs it should typically be around 1-2% of capacity, so roughly 2-4A for your setup. If that threshold is too high it'll declare "full" prematurely every time.

Also, have you done a proper absorption cycle recently? If your batteries have been undercharged over winter they might not actually be accepting a full charge anymore, which throws the algorithm right off. An occasional manual equalization (if your AGMs support it) can help recalibrate things. The SmartSolar is genuinely good kit but it needs accurate parameters to report sensibly. 👍

MistyTrekker | 312 posts | ☀️ Solar & Van Life

@ExpertWanderer the issue is likely that your MPPT is estimating SOC purely from voltage, which is notoriously unreliable for AGMs — they can sit at a seemingly "full" voltage whilst still being significantly under capacity. Have you considered adding a dedicated battery monitor like a Victron BMV-712? It uses coulomb counting (tracking actual charge in and out) rather than voltage-based guesswork, and makes a massive difference to SOC accuracy. I ran into exactly this problem with my setup last winter and the BMV transformed how well I understood what was actually happening. Also worth checking your battery absorption and float voltage settings are correctly configured for AGM chemistry — incorrect voltage parameters will compound the problem considerably.

@ExpertWanderer the thing that sorted it for me was doing a proper battery capacity test — fully charge, then discharge at a known rate and see what you actually get out. Bet your AGMs are down to 60-70% of rated capacity after 8 months of use, which throws the whole SOC calculation off completely.

Once Victron knows the real usable capacity it tracks much better. You can update it manually in the battery settings within VictronConnect.

Also worth checking your charge voltages are actually hitting the manufacturer's absorption spec — if it's cutting short early, it'll never properly sync the SOC to 100%.

DebbieWalker82 | 89 posts | 🔋 Off-Grid Homesteader

@ExpertWanderer something nobody's mentioned yet — have you set your battery capacity correctly in the VictronConnect settings? It defaults to 200Ah I think, and if it doesn't match your actual bank size the SOC calculations go wonky from the start. Also worth checking your Peukert exponent and charge efficiency factor under the battery settings — AGMs can be fussy about these. I had almost identical issues with my setup until I tweaked those figures to match the battery manufacturer's spec sheet. Made a noticeable difference to accuracy almost immediately. The Victron community forum has some good recommended starting values for AGMs if you haven't had a look there already.

Amy2000 | 156 posts | ⚡ Solar Enthusiast

@ExpertWanderer I had exactly this with my AGMs last summer. One thing worth checking is whether you've got the correct battery type selected in the VictronConnect settings — AGM absorption and float voltages vary between manufacturers, and if they're slightly off, the controller thinks it's finished charging before it actually has. Also, AGMs genuinely do degrade faster than people expect, so your 100Ah batteries might realistically be performing more like 70-75Ah after eight months of cycling. That discrepancy alone can make the SOC look wildly optimistic.

MoorHermit | 447 posts | 🏠 Tiny House & Emergency Backup

@ExpertWanderer worth checking your absorption voltage setting. AGMs typically want 14.4–14.7V, and if Victron's dropping to float too early, it's calling the battery "full" when it isn't — the SOC then gets anchored to a false 100% reference point.

In VictronConnect, go into the battery settings and verify your absorption time isn't set too short either. I run mine for a minimum 2 hours on a similarly sized AGM bank and the SOC tracking became noticeably more reliable afterwards.

Also — and this gets overlooked — AGMs degrade faster than their spec suggests. Eight months of cycling, your actual capacity might be 15-20% down already. The controller doesn't know that unless you tell it. Update the battery capacity figure in the settings if you haven't already.

TransitConvert | 34 posts | 🔌 Garden Office Build

@ExpertWanderer I had the same head-scratching moment with my setup. What sorted it for me was enabling the battery monitor function properly in VictronConnect and making sure the "charged voltage" and "tail current" thresholds were set conservatively rather than leaving them at defaults. Victron's defaults seem tuned for best-case scenarios.

Also worth asking — are you running just the SmartSolar, or do you have a BMV shunt monitor too? The MPPT alone is guessing SOC from voltage; a proper shunt-based monitor like the BMV-712 is genuinely night and day for accuracy.

RayCross | 203 posts | 🔆 Van Dweller & Shed Power

@ExpertWanderer the MPPT itself isn't actually tracking SOC — it's inferring it from voltage, which is notoriously unreliable for AGMs under load or straight after charging. Voltage can look "full" while the battery still has significant surface charge masking the real state. If you want accurate SOC you'll need a proper battery monitor like a BMV-712 with a shunt — that tracks actual coulombs in and out rather than guessing from voltage alone. Makes a massive difference to how trustworthy your readings are day-to-day.

CurrentAffairs | 312 posts | 🏠 Shepherd's Hut & EV Charging

@ExpertWanderer worth noting that voltage-based SOC on AGMs is notoriously unreliable — surface charge makes them look full almost immediately after absorption ends. The controller sees target voltage reached and calls it done.

If you want anything close to accurate, you need a proper shunt-based monitor. Victron's BMV-712 is the obvious pairing with your SmartSolar — talks to VictronConnect on the same Bluetooth network and does actual coulomb counting. Night and day difference in readout accuracy.

Also double-check your battery capacity setting in the app. Wrong figure there throws everything off from day one.

VanKen | 312 posts | 🏡 Static Caravan & Cabin Solar

My Fogstar AGMs basically told the Victron they were full after a light drizzle hit the panels — turns out AGMs are just pathological liars and the controller believes every word they say.