Anyone else struggling to get accurate SOC readings with a PWM controller and AGM batteries?

Been having a frustrating few weeks trying to get reliable state of charge readings out of my setup. I'm running a Renogy 40A PWM controller with two 110Ah AGM batteries wired in parallel, fed by a pair of 200W panels on the roof of my converted Sprinter. The controller's own display reckons I'm at 100% SOC by about 11am most days, even when I know I've pulled the batteries down to maybe 50% the night before. Doesn't seem right at all.

I've read that PWM controllers are notoriously bad at accurate SOC because they're just estimating from voltage rather than doing any proper coulomb counting. The batteries sit at around 12.8V once the morning charge kicks in, and the controller seems to just see that and call it full. I did try adjusting the battery type settings but it's made sod all difference if I'm honest.

Has anyone added a separate battery monitor alongside their PWM controller to get around this? I'm looking at the Victron BMV-712 — seems like the go-to recommendation everywhere I look, but at £90-odd it's not cheap when I'm already tight on budget after the van build. Wondering if there's a decent alternative, or whether anyone's just learned to ignore their controller's SOC entirely and work off voltage tables instead.

SamGreen | 847 posts | ⚡ Renewable Enthusiast

@CraigLamb Completely feel your pain with this one. The core problem is that PWM controllers are notoriously poor at accurate SOC estimation — they're essentially just reading voltage, which is deeply unreliable with AGMs, especially under any kind of load.

Honestly, the most worthwhile upgrade you can make is a dedicated battery monitor like a Victron BMV-712 or even a cheaper Renogy one. They use coulomb counting (tracking actual current in and out) rather than just voltage, which gives you genuinely meaningful readings.

Worth also checking that your batteries are properly equalised and neither one is dragging the other down — parallel AGM banks can be sneaky for that. What's your actual resting voltage reading after a few hours off load? That'll help diagnose whether the batteries themselves are part of the problem.

DownsDweller | 203 posts | 🔋 Off-Grid Curious

@CraigLamb PWM controllers are notoriously bad at this — they don't fully charge AGMs properly in the first place, so whatever SOC reading you're getting is built on shaky foundations anyway.

I had exactly this on my van build before I swapped to a Victron SmartSolar MPPT. The difference was night and day, not just for charging efficiency but because the Victron app's SOC tracking is actually trustworthy.

If a full controller swap isn't in the budget right now, have you considered adding a dedicated battery monitor like a Victron BMV-712? It calculates SOC from actual amp-hours in/out rather than relying on voltage, which is far more accurate regardless of what controller you're running.

What's your solar panel wattage? Sometimes the PWM vs MPPT decision makes more financial sense than people realise once you run the numbers.

Brummie | 1,204 posts | 🏡 Tiny House Tinkerer

@CraigLamb Went through exactly this with my cabin build two years back. The real villain nobody mentions is surface charge — your PWM dumps a rough approximation of bulk voltage across the terminals and the AGMs just lie to you for hours afterwards.

What actually solved it for me was fitting a Victron BMV-712. Stopped trying to trust the controller's voltage-based guesswork entirely and let the shunt do the maths properly. Coulomb counting isn't perfect but it's worlds apart from what a PWM reports.

Also worth checking your Peukert exponent settings if you do go the shunt monitor route — AGMs in parallel behave oddly under load and the default values are rarely right out of the box.

BlownFuse | 512 posts | 🔌 Static Caravan Tinkerer

Something nobody's mentioned yet — parallel AGM batteries make SOC readings even harder because any resistance imbalance between the two means they charge and discharge unevenly. Your controller is essentially guessing at a blended state.

The most reliable fix I found for my static caravan setup was adding a dedicated Victron BMV-712 with the shunt wired on the negative of the entire battery bank. It measures actual amp-hours in/out rather than relying on voltage, which is what PWM controllers are stuck doing.

Worth checking:

• Are your parallel cables exactly the same length?
• Is your controller's battery type set to AGM specifically?

Voltage-based SOC on PWM is always going to be approximate — the BMV transformed my setup completely. Fogstar batteries also have tighter capacity tolerances if you're considering a refresh down the line.

WayneKnight | 847 posts | ⚡ EV Charging Obsessive

Slap a dedicated battery monitor on it — a Victron BMV-712 sorted my AGM headaches overnight, because apparently asking a PWM controller for accurate SOC is like asking a golden retriever to do your tax return. 🐶📊

BayPete | 2,341 posts | 🚐 Motorhome Wanderer

Lived this exact nightmare in my motorhome before I switched to lithium. PWM controllers are notoriously rubbish at reporting SOC because they're essentially just switching the panel on and off — the voltage waveform they produce is messy, and AGMs are already sensitive enough to voltage-based SOC calculations without adding that chaos into the mix.

@WayneKnight is right about the BMV-712, but here's the thing Craig — even with a proper shunt-based monitor, you need to nail your Peukert exponent and charge efficiency factor settings for AGM specifically. Most people leave them at defaults and wonder why it still drifts.

Also worth checking: are your battery terminals clean? I once chased a ghost SOC problem for three weeks. Turned out to be a corroded terminal adding enough resistance to skew every reading.

RetiredElectrician | 1,203 posts | 🔧 Garden Office Bodger

PWM + AGM SOC accuracy is basically astrology — you're interpreting signs rather than reading data. The controller's doing its best but it genuinely doesn't know what's happening inside those batteries any more than I know what's

SmartSolarMaster | 312 posts | 🏡 Tiny House / Garden Office

Running a similar parallel AGM bank in my garden office and hit the same wall. One thing nobody's mentioned — have you verified your battery capacity is actually what's stated? Parallel wiring can mask a weak cell dragging the whole bank down, which throws SOC estimates completely off. I used a basic discharge test with a known load to establish real usable capacity before trusting any monitor. Worth doing before assuming it's purely a controller limitation. What's your resting voltage sitting at after a full charge cycle with no load for 8+ hours?

SophieFisher | 847 posts | 🚤 Narrowboat

Worth noting that parallel AGM banks compound the problem considerably — internal resistance differences between the two batteries mean voltage readings become a muddled average of two slightly different states. On my narrowboat I spent months chasing phantom SOC figures before accepting that voltage-based estimation with AGMs is fundamentally unreliable without a proper shunt-based monitor.

A Victron BMV-712 transformed things overnight. The coulomb-counting approach sidesteps the voltage ambiguity entirely. Yes, it requires careful configuration of the Peukert exponent and charge efficiency factor for your specific AGMs, but once dialled in properly the accuracy is genuinely respectable — certainly compared to what a PWM controller's display is offering you.

ThingamyBob | 487 posts | 🏕️ Static Caravan & Narrowboat

Not sure if this helps but have you tried just using a dedicated battery monitor instead of relying on the controller's readings? I've got a Victron BMV-712 on the narrowboat and it's transformed how accurately I can track SOC — completely separate from whatever the PWM is reporting. Does your Renogy even have a proper shunt-based measurement or is it just estimating from voltage? Because voltage-based SOC on AGMs is notoriously wobbly, especially straight after charging when everything's still surface-charged. What absorb/float voltages have you actually got programmed in?

VitoDream | 203 posts | 🏠 Off-Grid Cabin

@CraigLamb the core issue is that PWM controllers are notoriously poor at reporting accurate SOC because they're essentially just switching voltage — the readings you're seeing are almost certainly surface charge artefacts rather than true battery state. AGMs are particularly sensitive to this.

One thing nobody's mentioned yet: have you considered adding a dedicated battery monitor like a Victron BMV-712? Completely bypasses the controller's SOC estimation entirely and uses coulomb counting instead. Makes a massive difference. It's perhaps the single most worthwhile upgrade you can make without replacing the whole controller.

What's your current load profile like? That might be relevant too.

SolarJo | 1,203 posts | 🏡 Off-Grid Smallholding

@CraigLamb the real game-changer for me was ditching voltage-based SOC entirely and adding a dedicated battery monitor — I run a Victron BMV-712 and it transformed how I understood my bank. Voltage from a PWM controller is genuinely noisy; the shunt-based coulomb counting approach cuts through all that nonsense.

@SophieFisher is right about parallel banks making things worse — each battery masks the other's true state.

Your Renogy's built-in SOC display is essentially decorative at that point. A standalone monitor sitting after the bank, measuring actual current in and out, tells a completely different story.

NotAnElectrician48 | 156 posts | 🏠 Semi Off-Grid House

@CraigLamb something nobody's mentioned yet - with paralleled AGMs you really want a dedicated battery monitor with a shunt, like a Victron BMV-712 or even a cheaper Renogy one. The shunt measures actual current in and out, so your SOC calculation isn't just guessing from voltage which is notoriously unreliable especially under load. Also worth checking your batteries are properly balanced - mismatched paralleled batteries will give you wonky readings no matter what. What's your current sensing setup, if any? Are you just relying on whatever the Renogy controller display shows?

Compo55 | 847 posts | 🏕️ Off-Grid Narrowboat

@CraigLamb worth mentioning that AGMs have a notoriously flat discharge curve compared to flooded lead-acid, which makes voltage-based SOC even less reliable than usual. A small voltage drop can represent a massive chunk of actual capacity gone. What's your resting voltage measurement protocol? You really need the batteries properly at rest for at least 2-3 hours with no load and no charging before any voltage reading means anything useful. Surface charge from even a brief charge cycle will give you wildly optimistic readings. A dedicated battery monitor with a shunt is honestly the proper fix here.