By Rusty Latenser
A DOT reviewer, a contractor, or a procurement officer needs to understand why a PCMS dies overnight — not the generic “battery bad, solar low” Stattement you see in vendor brochures. This is the field‑accurate, root‑cause analysis you can hand to a crew or use in a spec justification.
Root Cause Analysis for Real‑World Field Conditions
A PCMS almost never “dies overnight” because of a single failure. It’s nearly always a stacked chain of small degradations that converge into a full shutdown sometime between 2 AM and 6 AM. Below is the operator‑first breakdown of the true root causes.
1. Battery State of Charge Was Already Too Low at Sunset
This is the number one root cause across all manufacturers.
Why it happens
- The unit ran multiple nights in a row with no full recharge.
- The solar array was shaded, dirty, misaligned, or undersized.
- The controller was misconfigured (common with PWM).
- The operator assumed “it’s sunny enough” without checking actual SOC.
Field effect
If a PCMS enters sunset at < 70% SOC, it is already mathematically impossible for most 4S LiFePO₄ or 4× AGM banks to survive a full winter night.
2. Solar Harvest Was Lower Than Expected (Winter Reality)
Operators often assume “400 W of solar” means “400 W of charging.” Especially in winter, that is fantasy.
Real‑world drivers
- Low sun angle (first and last two hours of the day – <30 degree above horizon
- Short day length
- Heavy overcast (thick dark clouds cut solar output to < 20% normal
- Snow, frost, or dust on panels
- Opposing‑tilt arrays wired incorrectly (parallel instead of dual‑MPPT)
Field effect
A 400 W array in December in the Midwest may deliver 60–120 Wh/day under overcast.
- Light overcast: ~30–50% of normal → too high for 60–120 Wh
- Moderate overcast: ~15–30% of normal → fits the upper end (80–120 Wh)
- Heavy overcast: ~5–15% of normal → fits the lower end (60–80 Wh)
So the 60–120 Wh/day band spans moderate → heavy overcast, with the lower half being heavy.
A PCMS load is typically 300–600 Wh/night. The math simply isn’t close.
3. Load Is Higher Than Operators Realize
Most PCMS units draw more than the spec sheet suggests.
Hidden load sources
- Radar or camera add‑ons
- Cellular modems in poor signal areas
- LED drivers running inefficiently in cold weather
- Controller overhead
- Battery heaters (LiFePO₄ only). In sustained cold, 1/5 to 1/3 of your battery is burned just keeping itself warm enough to charge, not running the load or 20–30% of the battery’s usable capacity in cold conditions.
Field effect
A unit that “should” draw 25–35 W may actually draw 45–70 W. That turns a 12‑hour night into a 600–800 Wh event — far beyond what a depleted bank can support. most standard PCMS units are undersized on PV for winter reality. The PV isn’t undersized for marketing specs — it’s undersized for reality.
4. Battery Chemistry Limitations (Cold or Aged Packs)
Lead‑acid
- Capacity collapses in cold weather – The battery delivers far less usable capacity than its rated Ah — typically only 40–60% of normal once temperatures drop into the 20s°F.
- Voltage sags under load
- Cannot accept full charge in winter. Because cold temperatures slow the chemical reactions inside a lead‑acid battery, increasing internal resistance. As a result, the battery cannot absorb charging current efficiently, so it reaches a “false full” early and never actually gets fully charged.
- Sulfation reduces usable capacity year‑round. Sulfation is caused when a lead‑acid battery sits partially charged or discharged, allowing lead sulfate crystals to harden on the plates.
LiFePO₄
- Cannot charge below freezing. Always use ‘blanket heaters to prevent freezing.
- BMS should block charging entirely if battery drops below freezing.
- Heaters craw 20–80 W power while the heater is ON (not 24/7 consumption).
- Cheap packs have weak low‑temp protection
Field effect
A “400 Ah” lead‑acid bank at 20°F behaves like 200–240 Ah.
A LiFePO₄ bank with heaters may burn 200–400 Wh before sunrise.
5. Charge Controller Is the Bottleneck (PWM Especially)
PWM
- Cannot boost voltage
- Cannot optimize mismatched panels such as Spyder Opposing Tilt Array.
- Loses 20–40% harvest in winter
- Collapses under partial shading. The power drops sharply when even one section is shaded, dragging total output near zero.
MPPT
- Only works if array wiring matches its input window
- Single‑MPPT units waste half the harvest on opposing‑tilt arrays
6. Shading Events That Operators Don’t Notice
Common culprits
- Light poles
- Trees
- Buildings
- Traffic queues
- Snow berms
Field effect
Even 10 minutes of shading on one panel in a series string can collapse the entire array’s output, especially with a PWM controller.
8. Battery Bank Is Smaller Than the Spec Sheet Suggests
Manufacturers often list “nominal capacity,” not usable capacity.
Lead‑acid
- Usable Depth of Discharge (DoD) is 30–50%, Depth of Discharge (DoD) = how much of the battery’s capacity you’ve used. Do not let DoD drop below 50% since it shortens battery life.
- Cold derates another 20–40%
LiFePO₄
- Usable DoD is 80–90%.
- Heaters subtract from usable energy
Field effect
A “400 Ah” AGM bank may only deliver 120–160 Ah in winter.
A “200 Ah” LiFePO₄ bank may deliver 140–160 Ah after heater losses.
9. BMS or Controller Low‑Voltage Cutoff Triggers Early
Why it happens
- Voltage sag under load
- Cold‑induced internal resistance
- Aged cells
- Poor wiring or loose lugs
- Undersized cables
Field effect
The battery may still have 20–30% energy left, but the BMS shuts the unit down to protect the pack.
10. Operator Assumptions Don’t Match Field Reality
This is the meta‑cause that ties everything together.
Common assumptions
- “It was sunny today — we’re fine.”
- “The panels look clean enough.”
- “The battery is new, so it’s good.”
- “The controller says ‘charging,’ so it must be charging.”
- “The unit worked last week, so it should work now.”
Field effect
Operators unknowingly run the unit into a multi‑day energy deficit until it finally collapses overnight.
The Real Root Cause
PCMS units die overnight because they enter sunset with insufficient stored energy (State of Charge < 70% – lead-acid battery) and insufficient daytime harvest to recover.
Everything else — cold, shading, PWM, mis‑leveling, aging — is a multiplier on that core problem.g — is a multiplier on that core problem.
