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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11A DIY external SLA flash mod replaces a flash’s internal AA battery supply with a cable to a sealed lead-acid battery, commonly a nominal 6V unit. The concept is real, and it was demonstrated with a Canon 550EX-class flash, but it is not a universal modification. A 6V SLA battery can measure roughly 7V when fully charged, while some flash external ports are designed for high-voltage capacitor charging rather than battery-level input. For most photographers, a compatible commercial power pack or fresh NiMH batteries is safer and more reliable.
What the original modification does
The historical project removes or bypasses the flash’s normal external-power arrangement and connects a replacement connector to the battery contacts, often using dummy-cell contacts. A cable then runs to an external sealed lead-acid (SLA) battery. The original Canon-oriented project is documented by DIY Photography and was also reported by Make.
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This is best understood as a reverse-engineering project, not a construction recipe that can be transferred from one flash to another. The flash model, battery topology, connector function, polarity, charging circuit, thermal protection and battery-door interlock all matter.
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What SLA means
SLA means sealed lead-acid. In this project, the intended battery is generally a small rechargeable 6V SLA battery—not a 12V automotive battery and not an unregulated lithium pack.
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- ML12-6F2 SLA is a 6V 12AH Sealed Lead Acid (SLA) rechargeable maintenance free battery - UL Certified
- Dimensions: 5.94 inches x 2.01 inches x 3.94 inches. Terminal: F2. Listing is for the Battery only. No wire harness or mounting accessories included.
- SLA / AGM spill proof battery has a characteristic of high discharge rate, wide operating temperatures and long service life.
- Rechargeable battery that can be mounted in any position, resists shocks and vibration. Long lasting high performance in high and low temperatures.
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- Nominal voltage is not operating voltage. A 6V SLA can be close to 7V after charging and decline toward approximately 5V as it discharges.
- Capacity is measured in amp-hours. A larger Ah rating mainly provides longer runtime; it does not automatically force that current into the flash.
- Fault energy still increases. A larger battery can sustain a short circuit longer and make wiring faults more dangerous.
- Voltage sag matters. A battery with low internal resistance and adequately sized wiring may maintain voltage better while the flash charges its capacitor.
SLA batteries are inexpensive and capable of delivering substantial current, but they are heavy, bulky and require a suitable charger. They are generally more practical in a stationary or off-camera setup than on a camera-mounted flash.
More shots is not the same as faster recycling
An external battery can reduce voltage sag and extend the number of shots before the flash slows down. It does not guarantee a dramatic reduction in recycle time.
| Power arrangement | Typical purpose | Main qualification |
|---|---|---|
| External battery-level pack | More endurance and less voltage sag | The flash’s existing charging circuit still determines performance. |
| High-voltage capacitor pack | Potentially faster recycling | Compatibility is model-specific and tube, capacitor and thermal stress may increase. |
| Higher-voltage hack | Attempted faster charging | High risk of damaging the flash and exceeding its design limits. |
The original project reported performance broadly comparable to fresh ordinary batteries, rather than the aggressive recycling associated with some dedicated high-voltage packs. Treat that as a model-specific report, not a guaranteed result.
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Do not assume the original external connector is suitable
A connector’s shape does not reveal its electrical function. Depending on the flash, an external port may connect to:
- the normal battery input;
- a high-voltage capacitor-charging circuit; or
- a manufacturer-specific circuit with additional switching or signaling.
A low-voltage SLA battery must not be connected to a port intended for high-voltage charging. Identify the exact flash model and trace the circuit using service documentation or a trustworthy schematic. Visual inspection alone is not enough.
This is also why official accessories cannot be treated as generic adapters. Nikon’s SD-9, for example, uses four or eight AA batteries and requires batteries to remain installed in the Speedlight. That is a different topology from a modification that replaces the internal battery supply.
Which flashes might be candidates?
The concept is most relevant to older AA-powered flashes with a four-cell or similar low-voltage battery compartment, accessible battery contacts and a documented circuit. A disposable older flash may be a reasonable experimental platform.
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- Long life and environmental protection: Designed for 500+ charge cycles and low self-discharge rate. Recyclable and complies with RoHS standards
- Ready-to-use and packing list: Includes pre-attached F1 terminals. Ships ready to use with full packaging protection (Battery Only, No Accessories Included)
It is a poor candidate if it:
- uses an integrated lithium battery;
- requires battery authentication or individual-cell monitoring;
- has only a high-voltage external port;
- cannot safely isolate its internal battery compartment;
- has a difficult-to-replace battery-door interlock; or
- is expensive, new or essential for paid work.
Commercial compatibility lists reinforce the point. The Godox PB960, for example, specifies particular flashes and cables rather than offering universal compatibility.
Replacement, augmenting and high-voltage topologies
Before opening a flash, determine which arrangement you are designing:
- Replacement pack: the external battery substitutes for the internal cells through the battery contacts or dummy cells.
- Augmenting pack: external and internal batteries work together as the manufacturer designed. Nikon’s SD-9 is an example where the internal batteries must remain installed.
- High-voltage pack: an external unit charges the flash capacitor through a dedicated circuit while the internal batteries may still power the controls.
Never infer from one manufacturer’s accessory that another flash uses the same arrangement. Whether internal AA cells should remain installed has no universal answer.
Safety comes before disassembly
Flash capacitors can remain dangerous
A flash can retain high voltage after it is switched off and its batteries are removed. The voltage, capacitance and stored energy vary by model. Do not open a flash unless you are trained to work around high-voltage capacitors.
Follow the service documentation. Use a properly rated discharge tool or the manufacturer’s procedure, then verify the voltage with a meter rated for the expected voltage. Never discharge the capacitor through your body, and do not use an improvised direct short such as a screwdriver. Nikon’s official guidance warns against user disassembly and says to stop using equipment that becomes hot, emits smoke or produces a burning smell.
An SLA can deliver dangerous fault current
Install a fuse close to the battery’s positive terminal. The fuse primarily protects the battery and wiring against a short circuit; it does not make an incorrect voltage or polarity safe for the flash.
Use a covered battery enclosure, insulated terminals, strain relief and a connector rated for the expected current. Protect the cable from crushing, abrasion and sharp edges. Do not carry an unfused SLA with exposed alligator clips in a camera bag.
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- ML7-6 SLA is a 6V 7AH Sealed Lead Acid (SLA) rechargeable maintenance free battery - UL Certified
- Dimensions: 5.94 inches x 1.42 inches x 3.94 inches. Terminal: F1. Listing is for the Battery only. No wire harness or mounting accessories included.
- SLA / AGM spill proof battery has a characteristic of high discharge rate, wide operating temperatures and long service life.
- Rechargeable battery that can be mounted in any position, resists shocks and vibration. Long lasting high performance in high and low temperatures.
- Full One Year Warranty
Choose the fuse from the actual wire gauge, connector rating, flash demand and fault-protection objective. Do not select it from the battery’s Ah label alone, and never replace a blown fuse with a larger one simply to keep shooting.
Safety-oriented parts and tools
- Exact-model flash service documentation or a reliable schematic.
- Correct-voltage SLA battery and a charger intended for that chemistry and voltage.
- Fuse holder and correctly selected fuse mounted near the battery positive terminal.
- Polarized, preferably locking connector pair.
- Flexible cable with adequate current and insulation ratings.
- Dummy-cell contacts or another mechanically secure battery-contact interface.
- Insulation, heat-shrink tubing, strain relief and a protective battery case.
- Multimeter rated for the flash’s high-voltage section.
- Proper capacitor-discharge equipment and, where available, a current-limited bench supply.
“Tip positive” is not a universal rule. The original project used that polarity convention for its replacement connector, but polarity must be verified for the specific flash and clearly marked on the complete harness.
Historical Canon-style workflow
The following summarizes the original Canon 550EX-style project only. Do not copy its screw count, dimensions, connector, polarity or voltage assumptions to another model.
- Identify the exact battery voltage, polarity and circuit topology.
- Confirm that the original external connector is not being used as a low-voltage battery input.
- Remove the batteries and follow a safe, documented capacitor-discharge procedure.
- Open the housing carefully. The historical article describes removing bottom screws, rotating the head to access additional screws and separating housing sections that may also use adhesive.
- Remove the original connector and fabricate a replacement panel for the chosen connector.
- Route the cable to the battery contacts or dummy-cell assembly without interfering with the tilt head, battery door, insulation or controls.
- Secure all solder joints, insulate them and provide strain relief.
- Reassemble only after completing electrical and mechanical checks.
The historical author inferred that the particular flash’s internal electronics and converter could tolerate the SLA’s voltage range. That was not a general manufacturer specification. Do not treat it as proof that another flash can accept a fully charged 6V SLA.
Commissioning and testing sequence
- With the battery disconnected from the flash, check continuity through the harness.
- Check that positive and negative are not shorted.
- Verify connector polarity with a meter.
- Measure the SLA’s open-circuit voltage after charging.
- Compare that voltage with the flash manufacturer’s specified input range. If no range exists, label the modification experimental rather than assuming compatibility.
- Use a current-limited bench supply or another current-limiting method for the first power-up where practical.
- Start at low flash output and observe current, sound, smell and temperature.
- Test recycle behavior at low power before attempting full-power operation.
- Increase repetition cautiously and respect the flash’s thermal protection.
Stop immediately for abnormal heat, smoke, burning smell, erratic operation, excessive current or a blown fuse.
Troubleshooting without making the fault worse
The flash does not power on
Disconnect the battery. Recheck polarity and connector pinout, inspect dummy-cell contacts, verify the battery-door interlock and look for a broken wire or solder bridge. Do not increase voltage to see whether it starts.
Recycle time is slow
Measure voltage at the flash while the capacitor is charging, not only at the battery. Check cable length, wire gauge, connector resistance, solder joints, fuse condition and the SLA’s state of charge. Compare with fresh cells at the same output level. A higher-voltage battery is not a safe default solution.
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The flash becomes hot
Stop firing and allow it to cool. Repeated overheating is a design failure, not a nuisance to work around. Never defeat an overheat cutoff.
The fuse blows
Do not fit a larger fuse. Look for reversed polarity, a crushed cable, solder bridges, failed components or an unsuitable input topology. Replace the fuse only after the cause is understood and only with the same type and rating.
How to measure whether the modification helps
Do not claim a percentage improvement without measuring the complete system. Record:
- recycle time at 1/1, 1/2 and 1/4 power;
- voltage at the battery and at the flash during charging;
- the number of shots before recycle time increases materially;
- flash-head temperature during a defined firing pattern;
- battery, cable and case weight; and
- time required to recharge the SLA.
Use the same flash settings, distance, battery state, cable and firing interval when comparing fresh NiMH cells with the external pack. More available capacity, lower voltage sag and faster recycling are separate outcomes.
Alternatives to a DIY SLA modification
| Option | Advantages | Limitations |
|---|---|---|
| Fresh low-self-discharge NiMH AAs | Portable, reversible and widely compatible. | Voltage sag and charging discipline still matter. |
| External AA pack | Avoids opening the flash and is easier to replace in the field. | Requires exact flash and cable compatibility. |
| Commercial high-voltage pack | Designed for specified flashes and may recycle faster. | Can increase thermal stress and may be discontinued or model-specific. |
| Modern lithium power pack | High energy density and potentially long runtime. | Requires correct protection, charger and compatibility. |
| Newer integrated-lithium flash | Modern thermal management, support and compact packaging. | Higher purchase cost and less convenient battery replacement. |
Godox lists the PB960 as an 11.1V/4500mAh lithium pack with dual outputs and manufacturer-rated performance of approximately 1,800 full-power flashes and approximately one-second full-power recycling with specified equipment. Those are manufacturer claims for named flashes and conditions, not results that apply to every speedlight.
Canon’s CP-E4N is an example of a manufacturer-designed alternative, although Canon’s U.S. shop page currently indicates that it is no longer available despite showing residual pricing information. Nikon’s SD-9 demonstrates the same principle: use a supported pack and follow its exact battery requirements.
When the DIY mod makes sense
This project may be reasonable when the flash is inexpensive or disposable, weight is unimportant, the goal is education or experimentation, and you have high-voltage electronics experience plus reliable documentation for the exact model.
It is a poor choice when the flash is needed for paid work, the reader cannot safely service a charged capacitor, a compatible commercial pack is available, or the expected result is a guaranteed dramatic reduction in recycle time.
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