Anyone else struggling to get accurate readings from a cheap PWM controller in winter?

I picked up a 20A PWM charge controller off Amazon for about £18 to run a small 100W panel on my shed setup. Works fine in summer but now we're heading into December I'm noticing the battery voltage readings on the little LCD screen seem way off — it's showing 12.4V when my multimeter is reading 12.7V at the terminals. Not a massive difference I know, but it's enough that the controller keeps cutting off loads earlier than it should.

I'm wondering if the cold is affecting the controller's onboard voltage sensor, or whether these budget units just drift over time. The controller is mounted inside the shed which isn't heated, so it's been sitting at around 4–6°C overnight. I've read that temperature compensation is a thing on better controllers but I'm not sure if that's related to what I'm seeing or if it's a separate issue entirely.

Has anyone found a fix short of just buying a decent MPPT unit? I don't really want to spend £80+ on an EPever or Victron just for a shed battery keeping a few LED lights and a phone charger ticking over. Wondering if repositioning the controller somewhere slightly warmer would help, or if the voltage offset is just baked into these cheap units.

@TH_OffGrid yeah this is a known headache with the budget PWM units — the voltage sensing is often poorly calibrated to begin with, and cold temps make it worse. The shunt resistors drift and you end up with readings that are basically fiction.

Honestly at £18 you're not going to fix it, you've hit the ceiling of what that hardware can do.

If accurate state-of-charge actually matters for your setup, even a basic Victron BlueSolar PWM is a massive step up — proper temperature compensation with a BTS sensor makes a real difference in winter. I run one on my narrowboat's secondary bank and the difference vs my old no-name unit was immediately obvious.

Alternatively if the panel's only 100W, a Victron 75/15 MPPT isn't much more and you'll recoup efficiency gains pretty quickly in low winter light anyway.

@TH_OffGrid cold temps make batteries show falsely high voltage so your controller thinks it's full when it's nowhere near — my static van setup had the same issue until I ditched the £18 special and grabbed a Victron SmartSolar, which does proper temperature-compensated charging via the optional sensor dongle.

PWM controllers fundamentally can't do temp compensation properly anyway — it's basically baked-in limitation of the technology, not just a calibration bodge.

If budget's tight, even a cheap MPPT will handle winter better than PWM and you'll squeeze more out of that 100W panel when the sun's doing its feeble December impression. Fogstar do decent budget options worth a look.

@TH_OffGrid had almost identical grief with a no-name PWM unit on my static caravan last winter. Ditched it sharpish.

The core problem beyond what @BorderVanLifer mentioned — cheap PWM controllers have no temperature compensation whatsoever. A halfway decent controller should drop charge voltage by roughly 3-5mV per cell per degree below 25°C. Your £18 unit almost certainly doesn't bother.

Worth asking yourself: what battery chemistry are you running? If it's AGM or gel you're potentially doing real damage with incorrect winter charging voltages.

Honestly, for shed use a Victron SmartSolar is overkill but even a Renogy Wanderer with a proper temp sensor add-on would be miles better. MPPT also makes more sense once you factor in winter low-light conditions.

What's your battery capacity? Might help work out whether it's actually worth spending more on the controller.

@TH_OffGrid the missing piece nobody's mentioned yet is temperature compensation — a decent controller adjusts the charge voltage setpoints based on ambient temp, typically around -3mV per cell per °C below 25°C. Your £18 unit almost certainly doesn't have it.

Running my garden office through last winter taught me this the hard way. Upgraded to a Victron SmartSolar (even the little 75/15 MPPT) and the difference in how the battery behaved through January was night and day — the app shows you exactly what compensation it's applying in real time.

I know it feels like overkill for a 100W shed setup, but a controller that actively damages your battery through improper winter charging will cost you far more than the upgrade. @BayTim's instinct to ditch it was sound.

@TH_OffGrid to build on what @PikeWalker is getting at — most of those budget controllers from Amazon simply don't have temperature compensation built in, or if they claim to, there's no external sensor so it's just guesswork. At £18 you're essentially getting bare-bones PWM with fixed voltage thresholds that were probably calibrated for 25°C.

Worth checking if your unit has a dedicated temp sensor port — sometimes it's there but the probe just wasn't included in the box. A quick search for your model number might turn one up for a couple of quid.

If not, you can manually lower your absorption and float setpoints in the controller menu (if it allows it) to compensate slightly over winter. About -3mV per cell per °C below 25°C is the rough rule of thumb for lead acid. Not ideal but better than nothing.

Great points from @PikeWalker and @Wez1993 already. One thing worth adding — even if your controller does have temperature compensation built in, those cheap units often have the sensor sitting inside the controller casing itself rather than actually clamped to the battery. So it's compensating for the temperature of the controller, not the battery. In winter, your battery might be sat in an unheated shed at 2°C whilst the controller is indoors or in a warmer spot, meaning the compensation is essentially useless anyway. If you can, get a proper external battery temperature sensor — some mid-range controllers like the EPever Tracer series support them. Might be worth budgeting for an upgrade rather than fighting the £18 unit through another winter honestly.