How Defibrillators Charge and Deliver Energy

How a defibrillator stores high voltage, discharges it through the patient, and measures whether the delivered shock matches the selected energy

A defibrillator may be set to:

Published August 16, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

After an energy level is selected, a charging circuit raises voltage and stores energy in a high-voltage capacitor. When discharge is authorized, switching circuitry shapes the waveform and routes current through the therapy cable and pads or paddles. The device monitors voltage and current to estimate delivered energy and record the event.

Troubleshooting separates charge, storage, and delivery. A unit may fail to reach charge, lose stored energy, or charge normally but deliver incorrectly through a damaged cable, switch, connector, or output circuit. Verify charge time and delivered energy into the specified analyzer loads; the device's own event value is not an independent test. Treat all high-voltage sections as hazardous even after power is removed.

What Is a Joule?

A joule is a unit of energy.

When a defibrillator is set to:

200 J,

the goal is to deliver an electrical pulse with approximately that amount of energy, within the manufacturer's specified tolerance.

Energy Is Related to Voltage and Capacitance

A capacitor stores electrical energy.

For a capacitor:

Energy = 1/2 × C × V²

where:

The important practical idea is:

A higher capacitor voltage stores much more energy.

Why High Voltage Is Needed

The defibrillator has to push current through the resistance of the patient and electrode interface.

That requires high voltage.

Depending on design, internal capacitor voltages may be hundreds or thousands of volts.

High-Voltage Safety

Defibrillator high-voltage circuits can retain dangerous energy after power is removed.

Follow manufacturer discharge procedures and service precautions.

Do not assume a powered-off device is electrically safe.

The Charging Circuit

The charging circuit converts:

into the high voltage needed to charge the capacitor.

Possible components include:

Charge Time

The device monitors how long it takes to reach the selected energy.

A healthy defibrillator should charge within its specified time.

Charge time may vary with:

Slow Charge

Possible causes include:

Compare against manufacturer charge-time limits.

Charges on AC but Not Battery

If the device charges normally on AC but slowly or not at all on battery, look at:

The high-voltage charging circuit itself may still be functional.

Charges on Battery but Not AC

Now consider:

Again, compare operating modes.

High-Voltage Capacitor

The capacitor stores energy until the shock is delivered.

A failing capacitor may have:

Possible symptoms include:

Capacitor Leakage

If the capacitor cannot hold charge well, the device may reach the target and then lose voltage.

Possible symptoms:

Charge Ready

When the selected energy is reached, the device indicates that it is ready.

This may include:

That tells you the charging system believes the target has been reached.

It does not independently prove the actual delivered energy is correct.

Energy Dump

If a charged defibrillator is not used, it must safely dissipate the stored energy.

This may happen through:

Do not assume the charge simply disappears.

Shock Delivery

When the shock button is pressed, switching components connect the stored energy to the output.

The exact design may use:

Output Path

The shock path may include:

A failure anywhere in that path can reduce or prevent delivery.

Paddles

Manual defibrillators may use external paddles.

Paddles provide:

depending on design.

Adhesive Pads

Modern systems commonly use multifunction adhesive pads.

These may support:

A pad or cable problem can affect several therapy functions.

Therapy Cable

The therapy cable carries high-energy pulses.

Inspect for:

Use only manufacturer-approved methods for testing high-voltage output cables.

Patient Impedance

The patient's body and electrode contact present electrical resistance or impedance.

The defibrillator may measure this before or during shock delivery.

Why Impedance Matters

A given voltage produces different current depending on impedance.

The device may adjust waveform characteristics to compensate.

This is especially common in biphasic defibrillators.

Poor Pad Contact

Poor contact can increase impedance.

Possible causes include:

The defibrillator may:

Open Circuit

If no load is connected, the device may detect:

That is different from a charging failure.

Short Circuit

Very low impedance may also trigger protection or change delivery behavior.

Use a proper defibrillator analyzer load.

Do not improvise the output load.

Monophasic Waveform

Older defibrillators may use monophasic waveforms.

Current flows primarily in one direction during the shock.

Biphasic Waveform

Modern defibrillators often use biphasic waveforms.

Current flows in one direction and then reverses.

This allows effective defibrillation at lower selected energies compared with many older monophasic designs.

Biphasic Waveform Control

The device may adjust:

based on measured impedance.

That means two shocks both set to:

200 J

may not have identical waveform shapes across different loads.

Energy Selection

The selected energy is a target.

The device calculates how much capacitor voltage and waveform duration are needed.

Delivered Energy

The actual energy delivered depends on the waveform and current through the load.

The defibrillator may internally estimate or measure this.

Set Energy vs Delivered Energy

This distinction is important.

Set:

200 J.

Delivered:

194 J.

If the manufacturer's tolerance is:

±15%,

that may pass.

The device does not need to deliver exactly:

200.0 J.

Energy Accuracy

A defibrillator analyzer measures actual delivered energy into a known load.

This gives you an independent reference.

Example

Set:

200 J.

Analyzer:

198 J.

Tolerance:

±15%.

Pass.

Low Delivered Energy

Set:

200 J.

Analyzer:

140 J.

Possible causes include:

Now isolate the stage.

High Delivered Energy

Set:

200 J.

Analyzer:

250 J.

That may be outside specification and can be safety-critical.

Investigate:

Low Energy at All Settings

Example:

50 J → 35 J

100 J → 70 J

200 J → 140 J

A consistent proportional error may point toward:

Low Energy Only at High Settings

Example:

50 J → 49 J

100 J → 99 J

200 J → 155 J

The failure may only appear when the charging system is stressed.

Possible areas include:

Charge Voltage

The selected energy corresponds to a target capacitor voltage.

If charge voltage is too low, delivered energy will usually be low.

Voltage Sensing

The device needs to know when the capacitor reaches target voltage.

If the voltage-sensing circuit is wrong, the device may stop charging too early or too late.

False Charge Complete

Suppose actual capacitor voltage is low but the sensing circuit reports:

Target reached.

The device indicates ready.

Shock energy will likely be low.

Overcharge Protection

Defibrillators include protection against excessive capacitor voltage.

Do not defeat or bypass high-voltage safety systems during troubleshooting.

Defibrillator Analyzer

A defibrillator analyzer provides a controlled load and measures:

depending on model.

It may also simulate ECG for synchronized testing.

Analyzer Load

Many analyzers use a standard resistive load, often around a typical reference value.

Use the load and procedure required by the manufacturer.

Why Load Matters

Delivered waveform behavior can change with impedance.

Do not compare results taken at different loads without understanding the specification.

Multi-Point Energy Testing

Manufacturer testing may require several energy settings.

Example:

50 J 100 J 200 J

Why?

Because one passing shock does not prove the entire output range.

Repeat Shocks

Some procedures require multiple shocks at one setting.

This checks:

Follow the specified sequence.

Battery Stress

Repeated high-energy shocks can place significant load on the battery.

A weak battery may allow:

Test the battery-dependent performance when required.

Battery Voltage Sag

The battery may show normal open-circuit voltage but collapse under high charging load.

That is another example where no-load voltage is not enough.

Synchronized Cardioversion

Synchronized cardioversion uses the same general energy-delivery hardware but times the shock to a detected ECG event.

The device waits for synchronization before discharging.

Sync Marker

The monitor typically marks detected R waves.

The device then tries to deliver the shock at the appropriate synchronized point.

Sync Failure

Possible causes include:

The charging system may be completely healthy.

Synchronized Shock Test

A defibrillator analyzer can provide an ECG waveform and measure sync delay.

This tests:

as a combined function.

Pacing Is Separate

Transcutaneous pacing may use the same pads and therapy cable.

But pacing uses a different electrical output mode.

A defibrillation pass does not prove pacing output is correct.

Self-Test

Many defibrillators perform internal checks of:

A failed self-test is a useful clue.

It is not always a complete diagnosis.

Internal Test Load

Some devices include an internal test load or test plug for energy verification.

This can help confirm output without an external analyzer.

Use the manufacturer procedure.

External Analyzer Still Matters

Internal self-test proves what the defibrillator can test about itself.

An external analyzer provides independent verification.

Charge Button Works but Shock Does Not

Possible causes include:

Separate charge function from discharge function.

No Charge at All

Possible causes include:

Start with whether the device attempts to charge.

Charge Starts Then Stops

Possible causes:

Watch:

Audible Whine

Some defibrillators produce an audible charging sound.

That proves part of the converter is operating.

It does not prove:

Shock Delivered but Analyzer Reads Zero

Possible causes include:

Check the test setup before opening the device.

Pads-Off Detection

If the device refuses to shock because it senses no pads:

The charging circuit may be fine.

The issue may be in:

Paddle Contact Indicator

Some systems give feedback on paddle contact quality.

That may depend on measured impedance.

It is separate from energy accuracy.

Internal Discharge Path

If the user cancels a charged shock, stored energy needs to be safely dumped internally.

A faulty dump path can create:

Follow manufacturer safety procedures.

Energy Calibration

Some defibrillators have service adjustments or calibration routines for energy delivery.

Only calibrate after confirming:

Do not calibrate around hardware degradation.

Capacitor Aging

Capacitors can degrade with age.

Potential effects:

This can show up most clearly at higher energy settings.

ESR

Capacitors have internal resistance, often described as ESR.

Increased internal resistance can affect rapid discharge performance.

The exact diagnostic method depends on manufacturer documentation.

Output Switch Resistance

High-voltage switching components and relay contacts can introduce resistance.

If damaged or burned, some stored energy may be lost before reaching the load.

Connector Damage

A damaged therapy connector can increase resistance or become intermittent.

Inspect for:

Real-World Example: Slow Charge on Battery

200 J charge:

Normal on AC.

Very slow on battery.

Battery analyzer shows significant voltage collapse under load.

High-voltage charging circuit is functional.

Battery is weak.

Real-World Example: Low Energy at High Setting

50 J:

49 J.

100 J:

97 J.

200 J:

150 J.

Battery and charge time normal.

Capacitor or high-energy discharge path becomes more suspicious.

Real-World Example: Charge Ready but No Shock

Device charges normally.

Shock button pressed.

No analyzer output.

Therapy cable replaced.

Normal shock returns.

Failure was in output path, not charging system.

Real-World Example: Wrong Energy Display

Analyzer:

200 J.

Device post-shock display:

160 J.

Actual delivery is correct.

Internal energy-measurement/reporting path is wrong.

Real-World Example: Pads Off

Device charges but refuses to discharge.

Analyzer connected through correct therapy cable.

Pads-off message remains.

Known-good cable corrects issue.

Impedance-detection path was interrupted by the cable.

Real-World Example: Sync Does Not Fire

Unsynchronized shock:

Normal.

Sync mode:

Device waits indefinitely.

Analyzer ECG visible but no sync markers.

Problem is ECG synchronization, not high-voltage output.

Common Mistakes

Treating Set Energy as Proof of Delivered Energy

Use an analyzer.

Assuming Charge-Ready Means Energy Is Correct

It only means the device believes charging is complete.

Replacing the Capacitor Before Checking Battery and Charge Voltage

Follow the power path.

Ignoring Therapy Cable and Pads

They are part of the output path.

Testing Only One Energy Setting

High-energy problems may be missed.

Confusing Sync Failure With Defibrillation Failure

Timing and energy delivery are different functions.

Assuming Audible Charging Noise Proves the HV System Is Good

Verify charge time and delivered energy.

Working on a Powered-Off Defibrillator Without Discharging the HV System

Stored energy can remain dangerous.

A Useful Troubleshooting Framework

For a defibrillator problem, ask:

Can it charge?

Then:

Does it reach the selected energy within the required time?

Then:

Can it hold that charge?

Then:

Can it discharge through a known-good therapy path?

Then:

What energy does an external analyzer measure?

Then:

Does the failure change with energy setting or battery/AC operation?

That separates:

Another Useful Question

Ask:

Is this a charge problem, an energy-storage problem, or an energy-delivery problem?

Those are different repair paths.

What Did You Actually Prove?

If the device says:

200 J Charged

you proved:

The defibrillator believes it reached the charge condition associated with 200 J.

You did not prove:

200 J will actually reach the patient.

If a calibrated defibrillator analyzer measures the delivered shock within manufacturer specification at the required load, you have objective evidence that the complete energy-delivery path worked under that test condition.

Final Thoughts for Biomeds

A defibrillator is basically a controlled high-energy storage and discharge system.

The path is:

Power → High-Voltage Charging → Capacitor → Switching → Therapy Cable → Pads/Load.

When the device fails, figure out where that chain breaks.

Ask:

Did it charge?

Did it store the energy?

Did it discharge?

How much energy actually reached the analyzer?

Then compare different:

And never forget:

The device's own display is not the final proof.

The analyzer is what tells you what actually came out.

— Jake

Important Note

Defibrillator charging circuits, capacitor voltages, waveform shapes, impedance-compensation methods, energy tolerances, synchronized-cardioversion requirements, and service procedures vary by manufacturer and model. High-voltage circuits can retain lethal energy after power is removed. Follow current manufacturer safety and discharge procedures, use approved therapy accessories and calibrated defibrillator analyzers, and complete all required functional, energy, synchronization, pacing, alarm, and electrical-safety testing before returning equipment to clinical use.

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