Solar battery not charging: causes and fixes

Three symptoms that get called the same thing

"Battery is not charging" arrives on WhatsApp several times a week through May and June. It describes three different faults, and the fix for each is unrelated to the other two. Sort yours first.

• Zero charge current. The inverter shows the battery connected but charge power sits at 0W all day. Suspect BMS lockout, a communication failure, or the DC breaker between battery and inverter.
• Charges slowly or stops partway. Climbs to 60 or 70 percent and stalls. Usually a charge profile mismatch, a temperature cutoff, or the array simply not having spare production after the house load is served.
• Reaches 100 percent but empties instantly. The pack is not really full. Cell imbalance or a badly degraded bank. This is the one people misread most often.

Note which one applies before reading further, because the sections below are grouped by cause and each one names the symptom it produces.

BMS lockout on lithium packs

Every LiFePO4 pack sold in Lahore, whether Pylontech, Dyness, BYD or a local assembly, has a battery management system that will physically open the pack rather than let the cells be damaged. When it opens, the inverter often still shows the battery as present because the voltage sense line stays connected, so nothing looks obviously wrong.

Four things trigger it here:

1. Over-discharge. A long load shedding night, or a fault that left the pack running the house until it hit the low voltage cutoff. Below a certain cell voltage the BMS latches and will not accept charge without a controlled recovery.
2. Over-temperature. Covered in its own section below, and the single biggest cause between May and August.
3. Communication loss. The CAN or RS485 cable between pack and inverter comes loose, or the pack address DIP switches were set wrong when a second module was added. The inverter falls back to a safe mode and refuses to charge.
4. A firmware disagreement. Certain inverter and battery firmware combinations stop talking after an update. Rare, annoying, and only fixable by whoever supplied the pair.

Clearing a lockout depends on the brand. Some packs recover by themselves once conditions come back into range. Others have a reset button behind a rubber cap. Deeply over-discharged packs need a low current recovery charge from a bench supply, which is not a homeowner job because getting it wrong can damage cells permanently.

Check the obvious cable first

Before assuming a BMS fault, check the communication cable. It is a standard RJ45 that plugs into a port marked CAN or BMS. Lahore installs get this wrong regularly, either by using a plain network patch cable where a brand-specific pinout is required, or by plugging into the wrong port on a multi-port pack. If your inverter reports the battery as an unknown or generic type, that cable is where to look.

Wrong charge profile in the inverter

A hybrid inverter has to be told what it is charging. Lead acid and lithium want completely different voltages, and inside lithium the settings still differ between a 48V 15-cell and a 16-cell pack.

Setting	48V LiFePO4 (16 cell)	Tubular lead acid 48V	What goes wrong if swapped
Bulk / absorption	55.2 to 56.0V	57.6V	Lithium BMS trips on over-voltage, charging stops
Float	54.0 to 54.4V	54.6 to 55.2V	Lead bank never fully charges, sulphates early
Low voltage cutoff	47.5 to 48.0V	46.0 to 47.0V	Deep discharge damage on the lead bank
Charge current	0.3C to 0.5C of pack Ah	0.1C to 0.15C	Lead bank boils, lithium charges far too slowly
Equalisation	Must be OFF	Monthly, short	Equalisation on lithium is a hard fault every time

The classic Lahore case is a homeowner who upgraded from tubular to lithium and kept the same technician, who never changed the profile from USER-lead to LI or the correct protocol. The pack charges to 56V, the BMS says no, charging stops, and the customer is told the new battery is defective. It is not. It is a menu setting. The difference between the two chemistries is explained fully in lithium vs tubular batteries.

Lahore summer temperature cutoff

This deserves its own section because it is the most Lahore-specific fault on the list. Ambient hits 45 to 48C for stretches of May and June, and the surfaces around a battery get hotter than the air.

LiFePO4 packs typically stop accepting charge somewhere between 50 and 55C cell temperature, and stop discharging around 60 to 65C. Charging below 0C is also blocked, which matters in Lahore only on a handful of January mornings and only for packs kept outdoors.

Where batteries get installed badly here:

• A closed store room under the stairs. No airflow, and often a water heater or the main distribution board sharing the space. Internal temperature runs 6 to 10C above ambient by 4 pm.
• The car porch on a west-facing wall. Direct afternoon sun on the pack casing. Surface temperature readings above 60C are routine.
• Rooftop enclosures. Some installers put the whole system on the roof to save cable. In a Lahore June that box is an oven.
• Stacked hard against the inverter. The inverter's own heat sink dumps straight onto the battery casing.

The fix is placement, not hardware. An internal wall in a ventilated utility area, 30cm clearance on all sides, no direct sun, and an extractor fan if the room is enclosed. On premium installs a small split unit or wall fan on a thermostat costs far less than a replacement pack at PKR 285,000 for 5kWh. The same placement logic applies to the inverter, which is covered in inverter overheating.

An honest caveat: repeated high temperature exposure permanently reduces lithium capacity even when the BMS protected the pack correctly. A battery that spent two Lahore summers at 50C will not deliver its rated capacity in year three, and no reset fixes that.

Cell imbalance and the false 100 percent

Inside a 48V pack sit 15 or 16 cells in series. They should track within about 0.03V of each other. As a pack ages, or if it was built from mismatched cells to hit a price point, one cell drifts. That cell reaches the upper limit first, the BMS stops the charge to protect it, and the pack reports full while the other 15 cells are at 80 percent.

Symptoms: the state of charge jumps around, backup time is far shorter than the rating suggests, and the pack sometimes drops off load suddenly rather than tapering. Cheap locally assembled packs show this within 18 months. Branded packs usually take four or five years.

A long absorption charge at low current lets the BMS balancer equalise the cells, and on a mildly drifted pack that recovers most of the capacity. On a badly drifted pack it does not, and the answer is cell replacement or a new pack. Pricing today: 2.5kWh at PKR 130,000 to 145,000, 3.5kWh Pylontech at PKR 195,000, and 5kWh at PKR 285,000. Replacement options and timing are set out in solar battery replacement in Lahore and lithium battery prices.

Tubular banks that stop taking charge

Lead acid fails differently. A 200Ah tubular at PKR 48,000 to 52,000 is still bought in Wapda Town and older parts of the city where budget rules, and these banks stop charging for mundane reasons.

• Electrolyte below the plates because nobody topped up with distilled water for six months. Lahore heat evaporates it fast.
• Sulphated terminals under a white crust, adding resistance the charger cannot push through.
• One dead cell in one battery dragging the whole series string down, so the healthy units never reach absorption voltage.
• Charge current set too high, boiling the electrolyte and accelerating everything above.

Check water levels, clean and grease the terminals, then measure each battery individually with the bank disconnected. A single unit reading 2V low against the others is your culprit. Details on lead acid pricing and life expectancy are in tubular battery prices and Osaka batteries in Lahore.

When there is simply nothing left to charge with

The least dramatic cause, and one that catches new hybrid owners in July. Charging happens with surplus production after the house load is met. On a monsoon week in Lahore a 5kW array might make 13 units a day while the house consumes 30. There is no surplus. The battery stays at whatever the grid charge setting allows and no further.

If this describes your system, nothing is faulty. The array is undersized for the storage attached, or grid charging is disabled in the settings. Enabling a timed grid charge overnight at off-peak hours is a common fix on hybrid battery builds, though with LESCO tariffs at PKR 48 to 65 per unit the economics need checking first. Bank sizing against real production is covered in battery bank sizing, and the backup-at-night version of this problem in no backup at night.

What a battery service call covers

A proper visit measures pack voltage and per-module voltage, reads the BMS log over the communication port where the brand allows it, checks the DC breaker and lugs for heat, verifies every charge setting against the manufacturer sheet, and logs cell temperature at the hottest hour of the day rather than at 10 am when everything looks fine.

That last point matters. A pack that fails at 3 pm in June will pass every test at 10 am. If the fault is intermittent and heat-related, the technician has to be there at the right hour or leave a logger. Anyone diagnosing a summer battery fault in a morning visit is guessing.

Visit fee is PKR 3,000 anywhere in Lahore, adjusted into the final bill if work follows. Office is on Mohni Road, Mughalpura. Repair pricing sits in solar repair costs, and if the conclusion is a larger bank, the calculator and repair team can size and quote it. Homes in DHA and Gulberg with heavier AC loads usually end up needing more storage than they were originally sold, which is discussed in batteries for load shedding.