Anyone else found that cheap Amazon PWM controllers absolutely murder your batteries in winter?

Just replaced a pair of 100Ah AGMs that I reckon died well before their time, and I'm pretty convinced the culprit was a no-name 30A PWM controller I'd been running for about 18 months. Even with a decent 200W panel on the roof of the van, I kept noticing the batteries never seemed to get a proper full charge — voltage would sit around 13.8V and the controller would just call it done. Bulk, absorption, float — all a bit vague and rushed, especially when the sun was weak.

Swapped everything over to a Victron SmartSolar MPPT 75/15 last month (modest setup, I know, but it suits my 175W panel and the leisure battery I've got now — a 100Ah lithium from Fogstar). The difference has been genuinely surprising. Proper absorption cycles, accurate voltage curves, and I can actually see what's going on via the VictronConnect app. In the last few weeks of pretty grim November weather I'm still getting the battery up to 95–100% most days.

Has anyone actually done a proper side-by-side comparison, or kept records of battery health over time with PWM vs MPPT? I feel like a lot of folks on here still default to PWM just because it's cheaper upfront, but if it's quietly wrecking your batteries it's false economy, isn't it?

JohnShaw86 | 847 posts | ⭐ Regular

@LDVBuild Yep, been there. The thing nobody tells you is that most of those cheap PWM units don't do proper temperature compensation. In winter your batteries genuinely need a higher absorption voltage to charge fully, and without temp comp they're just plodding along with summer settings, chronically undercharging month after month. Sulphation builds up quietly and by spring you've got half the capacity you started with.

I switched to a Victron MPPT two years ago and the difference in winter state of charge was immediately obvious. Not cheap, granted, but when you're factoring in the cost of replacement AGMs every couple of years it's a no-brainer really. What size system are you running? Might be worth looking at one of the SmartSolar units if you haven't already.

RussOliver64 | 312 posts | ⭐ Regular

@LDVBuild The temperature compensation issue is the real killer in my experience. Most of those cheap PWM units have a fixed absorption voltage with no temp sensing whatsoever. Come winter, your batteries need a higher charge voltage to compensate for the cold, but the controller just plods along at its summer setting. You end up with chronic undercharging month after month, which sulphates the plates gradually. By spring they're already half ruined and you don't even realise it. I switched to a Victron BlueSolar with a proper temp sensor last year and the difference in how my batteries are behaving is night and day. Worth every penny of the extra cost compared to replacing batteries every couple of winters.

DorsetExplorer | 1,203 posts | ⭐ Senior Member

@LDVBuild welcome to the forum, great to have you here! Loads of knowledgeable folk who've been exactly where you are 👍

Switched to a Victron SmartSolar MPPT on my motorhome build last year and honestly it's night and day. The Bluetooth app alone is worth it — you can actually see what's happening with your charge cycles rather than guessing.

The winter gains from MPPT over PWM are genuinely significant too, especially on those low-angle December days when you're scraping every watt you can get.

Fogstar do decent value AGMs if you're replacing, or worth considering LiFePO4 if budget allows — pairs brilliantly with Victron kit.

What panel setup are you running?

ExFarmer21 | 2,156 posts | ⭐ Senior Member

Switched to a Victron SmartSolar and my Fogstar batteries stopped ageing in dog years.

NoPlanB77 | 847 posts | ⭐ Senior Member

@LDVBuild Gutting about the AGMs, but honestly not surprised. The issue I've found beyond temperature compensation is that cheap PWM units often have wildly inaccurate voltage setpoints from the factory. I tested three different budget controllers with a decent multimeter and not one of them was hitting the absorption voltage they claimed. One was nearly half a volt low, which means your batteries never fully charge, sulphation builds up over months, and come winter when capacity naturally drops anyway you're already working with a compromised bank.

Worth grabbing a basic multimeter and checking what your controller is actually doing versus what the display claims. Often quite revealing. What solar panel wattage were you running through it?

BoxerProject | 156 posts | ⭐ Regular

@LDVBuild The Amazon PWM special — I swear they come pre-programmed to murder batteries slowly enough that you can't prove it in court 😄

Jokes aside, I ran one in my garden office for about a year before switching to a Victron Smart

NicolaTaylor72 | 43 posts | 🌱 New Member

Going through something similar on my boat build right now, actually. I've got a cheap PWM from Amazon running temporarily and I'm paranoid about it after reading threads like this.

Quick question for the more experienced folk — is there a meaningful difference between budget MPPT controllers versus budget PWM ones, or are they both equally risky? I was looking at the Renogy Wanderer as a slightly less terrifying option, but I genuinely can't tell if that's just a better-branded version of the same problem. Would a Victron 75/15 be overkill for a small 200W panel setup?

DodgyRigger | 312 posts | ⭐ Regular

@LDVBuild Worth checking whether it was actually doing temperature compensation. Most of these cheap units claim it on the listing but without the external temp sensor actually connected and working, they'll just blast the same voltage regardless of ambient temp. In winter your batteries need a higher charge voltage to hit full capacity, so you end up with chronic undercharging and sulphation sets in quietly over months.

@NicolaTaylor72 on a boat especially, watch this - bilge temperatures swing massively. Even a half-decent MPPT with a proper temp probe makes a noticeable difference to battery longevity.

BazBurns | 89 posts | 🌱 New Member

@DodgyRigger nailed it — temp comp is the big one people miss. On the boat I had a cheapo PWM that claimed temp compensation but the sensor was just floating loose inside the unit, nowhere near the battery bank. Was basically useless.

Swapped to a Victron SmartSolar and the difference in winter charge quality was immediately obvious, especially on cold mornings. The Bluetooth monitoring helps too — you can actually see what the controller's doing rather than guessing.

AGMs are pretty unforgiving if you're repeatedly undercharging them through winter. Once they sulphate that's largely game over.

MoreTeaVicar | 156 posts | ⭐ Regular

Had almost this exact story play out in my static caravan setup two winters running before I finally wised up. The cheap PWM was cheerfully pumping in bulk voltage on a -4°C February morning like it was a balmy August afternoon. Batteries absolutely hated it.

Swapped to a Victron SmartSolar MPPT — yes, it cost proper money — but the Bluetooth app meant I could actually see what was happening for the first time. That alone was worth the price of admission. Sometimes you don't realise how blind you've been until you can suddenly read the data.

HeathOllie | 847 posts | ⭐⭐ Veteran

The temperature compensation point has been covered well, but there's another angle worth raising: PWM controllers fundamentally cannot drive a partial charge to absorption properly when cell voltage is sagging in cold weather. The panel voltage just collapses under load because PWM is essentially a direct connection — no voltage boost whatsoever.

My garden office setup had exactly this problem. Switched to a Victron SmartSolar MPPT 75/15 and the difference in winter charging behaviour was immediately visible in the VictronConnect app. The MPPT actively tracks the panel's optimal operating point regardless of battery state or ambient temperature.

For AGMs specifically, chronic undercharging is arguably worse than overcharging — sulphation is irreversible without serious intervention.

JulieAllen | 312 posts | ⭐ Regular

On my narrowboat I ran a cheapo PWM for ages and noticed the same pattern — batteries never seemed to fully charge through November to February. Swapped to a Victron SmartSolar and the difference was immediately obvious, you can actually watch it hit proper absorption voltage on the app.

The other thing nobody's mentioned yet: those no-name units often have wildly inaccurate voltage sensing if there's any cable resistance between controller and battery. You think it's charging to 14.4V but measure at the battery terminals and it's actually 13.8V. Proper unit with a remote voltage sense wire sorts that completely.

FinnThomas | 203 posts | ⭐ Regular

Worth mentioning something I haven't seen touched on yet — a lot of these cheap PWM units have wildly inaccurate voltage sensing, so even if the charge profile looks right on paper, the controller might think it's hit absorption voltage when it's actually 0.3-0.4V short. In winter that's genuinely catastrophic for a battery that's already struggling with cold. I tested mine with a decent multimeter against the controller's display and found exactly that discrepancy. Switched to a Victron SmartSolar MPPT and the difference in how fully my batteries actually charge is night and day. @LDVBuild your replacement AGMs will thank you for the upgrade!