I've been running off-grid solar in Central Queensland long enough to know that a charge controller is not the place to guess. Wrong setting, wrong sequence, or wrong battery type and you can cook a battery bank, pop a controller, or worse. Below is what I actually use as a reference sheet when wiring up or reconfiguring a system. It's written for PWM controllers in the 10A/20A/30A class, which are still the most common units I see in domestic off-grid sheds and small pump setups around here.

By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD

“After blowing a battery bank in a 2018 QLD heatwave, I learned that 14.4V is the absolute ceiling for lithium, not a target.”

Controller Basics

A PWM solar charge controller sits between the solar panel(s), the battery, and the DC load. It automatically manages and regulates voltage and current from the panel to the battery. It also protects itself and the battery against short-circuit, open-circuit, reverse polarity, and overload conditions, and it prevents power from being drawn backwards from the battery into the panel during shaded or dark conditions. Most consumer units also include a USB port for charging phones or small electronics directly from the controller.

Two important caveats from the field:

• The controller's internal circuitry runs entirely off battery voltage. Connecting a live solar panel to an unpowered controller can permanently damage it. This is the single most common failure I see on new installs.
• These controllers are only suitable for lead-acid batteries — Open, AGM, and Gel. They are not suitable for nickel-metal hydride, lithium-ion, or other battery types. If you're running LiFePO4, you need a controller that explicitly supports that chemistry.

Sizing the Controller to the Panels

Match the controller current rating to the panel array, not the battery. The rule of thumb from the manufacturer guidance is that the maximum panel output current should be about 25% less than the controller's maximum input current rating. That headroom prevents the controller from running at its absolute limit on the hottest, brightest Queensland afternoon.

Quick examples:

• A single 6A 100W panel pairs with the 10A model.
• Two 6A panels in parallel (12A total) need the 20A model.
• If you're unsure, round up. For 9A maximum panel output, use the 20A rather than the 10A model.

Voltage Matching: Panels to Battery Bank

Voltage matters as much as current. The panel's open-circuit voltage (Voc) has to land in the right window for the battery bank you're using, and you must not exceed the controller's upper input limit.

Going above those maxima will generally kill a PWM controller. If your array runs hotter than those numbers, you need an MPPT unit with appropriate PV input ratings — and you still have to do a low-temperature Voc check, because panel Voc rises as temperature drops, and our clear winter nights in Central Queensland can push strings well above their STC rating.

Maximum Input and Output Power

Power ratings depend on both the controller model and the battery bank voltage. The verified specs for the PWM 10A/20A/30A range are:

Maximum discharge current to the load is 10A regardless of which model you use — that's 120W with a 12V battery bank and 240W with a 24V bank. Bigger controllers do not give you a bigger load port. If you need more DC load capacity, use a separate switching device rated for the load and let the controller do charge management only.

Connection Sequence (Read This Twice)

This is the part where new installers most often get it wrong. The order is non-negotiable:

1. Battery first. Connect the battery to the controller and confirm the LCD turns on. The battery must have enough voltage (typically 10V or more) to power on the controller and recognise the battery type.

2. Solar panel second. Only after the controller is powered, connect the panel.

3. Load last. Connect the DC load last.

Disconnection is the reverse:

1. Load first.

2. Solar panel second.

3. Battery last.

Keep battery cables as short as possible to minimise voltage drop and losses. Long battery cable runs are a quiet killer of system performance — the controller sees a higher voltage than the battery actually has, and your absorption/float targets end up wrong.

Battery Type and Settings

Battery manufacturer specifications should take precedence over any generic preset in the controller menu. The presets exist as a starting point, not as gospel. If the maker publishes a specific absorption voltage, float voltage, equalisation voltage, and temperature compensation coefficient, use those numbers — not the menu default.

Lead-acid chemistry notes from long practice:

• Flooded lead-acid equalisation can be necessary under some conditions, but it must be voltage-, current-, duration-, and temperature-limited with adequate ventilation and PPE. Equalising in an enclosed shed without venting hydrogen is asking for trouble.
• Equalisation must not be applied to LiFePO4 or sealed gel batteries unless the manufacturer explicitly permits it. Most don't.
• Temperature effects on lead-acid charging generally require the battery maker's specified temperature compensation. The controller's temperature sensor (if fitted) should be attached to the battery case, not to the controller or a random spot in the cabinet.

PWM controllers regulate by switching the panel connection on and off, which makes them less effective when the panel's operating voltage is poorly matched to the battery voltage. That's the structural reason MPPT exists.

MPPT vs PWM — When It Matters

MPPT controllers perform DC-DC maximum-power-point tracking and can usefully employ higher-voltage panel arrays than the battery bank. They're not interchangeable with PWM units in every situation. If your array Voc runs higher than the PWM windows shown above, or if you want to harvest more energy from a fixed array, MPPT is the right tool.

An MPPT controller must still be selected with adequate PV input-voltage and current ratings, including the low-temperature Voc check I mentioned earlier. The two are not a like-for-like swap — they're different controller categories.

Settings, Logs, and Diagnostics

Controller data logs — current, voltage trends, temperature, and fault codes — are useful for diagnosing system performance. If something's not right, the log usually tells you whether the problem is on the panel side, the battery side, or the load side.

Low-voltage load disconnect protects against excessive discharge, but it must be coordinated with the battery, any BMS present, wiring, switching device, and load ratings. Setting the disconnect too low ruins batteries; setting it too high leaves capacity on the table.

Safety — When to Call a Licensed Professional

The safety notes from the manufacturer and from long experience:

• Always connect the battery to the controller first and confirm the LCD turns on before connecting the solar panel or load.
• If the LCD display does not turn on after connecting the battery, do not connect anything else. The battery may be below ~10V and must be charged first — usually with a bench charger or a known-good source.
• Disconnect in reverse order: load first, solar panel second, battery last.

Call a licensed professional (electrician) if:

• You're working on the AC-coupled side of the system, any 230V wiring, or inverter AC output.
• You're unsure about battery bank isolation, breaker sizing, or fault current ratings.
• The battery bank is large, the cables are heavy, or you're dealing with parallel strings where a wrong torque on a terminal is a real hazard.
• There's any sign of a swollen battery, a hot cable, a tripped breaker you can't explain, or burnt insulation.

Solar DC voltages above extra-low voltage can be lethal under certain conditions, and Queensland electrical licensing rules apply to fixed wiring work.

FAQ

Q: Can I use a PWM controller with lithium batteries?

A: No — at least not the PWM 10A/20A/30A units described here. They're only suitable for lead-acid (Open, AGM, Gel). For LiFePO4, use a controller that explicitly supports that chemistry and its specific charge profile.

Q: My panel is rated 100W but my controller is 10A — is that fine?

A: Yes. A 6A 100W panel on a 10A controller sits within the rule that maximum panel output current should be about 25% less than the controller's rating.

Q: Why won't the controller turn on?

A: The battery is likely below ~10V. Charge the battery with a separate charger before connecting the controller to the panel. Do not connect a live panel to an unpowered controller — it can permanently damage the unit.

Q: Can I run a 24V load from a 12V system through the load terminals?

A: No. The load terminals supply battery voltage — they don't boost or step down. Match the load to the battery bank, or use an appropriately rated DC-DC converter.

Q: My controller shows USB output — can I leave a phone plugged in overnight?

A: The USB port is powered from the battery through the controller. The controller's low-voltage disconnect should protect the battery, but verify the disconnect setting is correct for your battery type and capacity before relying on it.

Q: How do I know if I need MPPT instead of PWM?

A: If your panel array's open-circuit voltage falls outside the windows in the table above (e.g., grid-tie style panels with Voc above 25V on a 12V bank), or if you want better harvest from a fixed array in our high-irradiance conditions, MPPT is the better match. Make sure the MPPT unit is rated for the array's voltage and current, including a low-temperature Voc check.

Sources

• https://envistiamall.com/blogs/learn/pwm-solar-charge-controller-10a-20a-30a-guide

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