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A red/green flash every ~2 seconds on your GARDENA 25.2 V / 500 mA charger normally means the charger is not entering a valid charging state. The most likely causes are:
Also note: “D-8909 Ulm” is not the charger model number. It is related to GARDENA’s company address/location marking. The useful information is the charger’s output voltage, current, polarity, and model number printed on the label.
Your charger is most likely a Li-ion battery charger, not a simple DC power supply. A Li-ion charger checks battery voltage before applying charge current. If the battery voltage, temperature sensor, or terminal connection is outside the expected range, the charger may repeatedly try to start, detect a fault, stop, and try again. This can appear as red/green flashing every few seconds.
Typical LED behaviour on many GARDENA-style chargers is approximately:
| LED state | Likely meaning |
|---|---|
| Green steady | Standby, no battery, or battery full |
| Red steady | Charging |
| Red/green alternating or flashing | Fault, bad contact, battery protection trip, or charger error |
The exact code can vary by model, but alternating red and green while not charging should be treated as a fault condition.
This is the easiest and most common thing to check.
If the charger starts to deliver current but the contact resistance is high, the charger may see an abnormal voltage and stop immediately. It then retries, causing the red/green cycle.
Check:
Clean with:
Do not scrape aggressively, because the contacts may be plated.
If the battery has been stored flat for a long time, the individual Li-ion cells may have dropped below their safe voltage threshold.
For Li-ion packs:
This distinction matters. If your charger label says Output: 25.2 V DC, it is probably intended for a 6S Li-ion pack. If the battery is marketed as 25 V or 25.2 V nominal, the correct charger may instead output around 29.4 V. So check the label carefully.
A deeply discharged pack may cause the charger to refuse charging for safety reasons.
Approximate guide:
| Battery voltage reading | Likely condition |
|---|---|
| Near full charger voltage | Battery may be full or charger not detecting load |
| Normal mid-range voltage | Battery probably electrically alive |
| Very low voltage | Deep discharge or failed cell group |
| 0 V or near 0 V | Battery protection circuit/BMS has opened, fuse failed, or pack is dead |
For a 25.2 V maximum 6S pack, anything much below about 15–18 V is suspicious.
For a 29.4 V maximum 7S pack, anything much below about 17.5–21 V is suspicious.
The battery pack may contain a protection circuit, sometimes called a BMS. It monitors:
If one cell group is too low or faulty, the BMS may disconnect the pack internally. From the charger’s perspective, the battery may appear open-circuit or abnormal. The charger then tries, stops, and flashes.
If the battery reads 0 V at the output terminals, the BMS may have shut down or the pack may have an internal fault.
The charger itself may be failing. Common faults include:
A failing switch-mode charger can enter hiccup mode, where it repeatedly starts and shuts down. This may also produce rhythmic LED flashing.
If the charger still flashes red/green, continue below.
With the charger plugged into the wall but not connected to the battery:
With the charger unplugged:
Then retry charging.
If you have a digital multimeter:
Compare with the label:
Important: check polarity. Many barrel chargers are centre-positive, but do not assume.
Measure the voltage across the battery’s main positive and negative terminals.
Possible interpretations:
| Reading | Interpretation |
|---|---|
| Normal voltage range | Battery is probably not deeply discharged; suspect contact or charger |
| Very low voltage | Battery is deeply discharged or has a bad cell group |
| 0 V | BMS lockout, blown internal fuse, broken connection, or failed pack |
| Voltage rises briefly then collapses | Battery may have high internal resistance or failed cells |
If the battery is old, has been unused for months/years, or was stored discharged, the battery is more likely to be the problem than the charger.
Do not try to “boost” or jump-start a Li-ion battery with another battery or an uncontrolled power supply. A deeply discharged Li-ion cell can become unsafe if forced to charge.
A controlled recovery charge is sometimes possible using a bench power supply with strict current limiting, but I would only recommend that if you are experienced with Li-ion battery repair. For most users, if the pack is deeply discharged or reads 0 V, the safer answer is:
Stop immediately if the battery:
Recycle damaged Li-ion batteries properly.
Most probable fault order:
The quickest test is this:
When replacing the charger, use only a proper Li-ion CC/CV charger with:
Do not substitute a generic DC power adapter unless it is specifically a Li-ion charger with the correct voltage profile.
User question
I have a Gardena 25.2V charger Model 04025-25.640.00 240VAC input. Normally 28.7 fully charged 500mA it flashes red, green and double red cyclically and does not charge the battery which is at 24V state of charge.
Artificial Intelligence Response
Direct answer to the question
Your Gardena 25.2 V charger Model 04025-25.640.00 is not entering normal charge mode. A battery voltage of 24 V is not too low for this system; it should normally begin charging.
The repeating red → green → double-red sequence is best treated as a fault/retry indication. The most likely causes are:
Do not try to force-charge the battery directly unless you have proper lithium-ion charging equipment and experience.
Detailed problem analysis
1. Your voltage readings are consistent with a 7-series Li-ion pack
A Gardena “25.2 V” lithium pack is very likely a 7S lithium-ion battery, meaning seven cell groups in series.
Your stated full-charge voltage:
[ 28.7\text{ V} / 7 = 4.10\text{ V per cell} ]
That is reasonable for a conservative Li-ion charger. Many 7S Li-ion packs charge to 29.4 V if charged to 4.20 V/cell, but some garden-tool packs deliberately stop around 28.7 V to improve battery life.
Your present battery voltage:
[ 24.0\text{ V} / 7 \approx 3.43\text{ V per cell} ]
That is a normal partially discharged state. It is not deeply discharged. A healthy charger and healthy battery should normally accept charge from this voltage.
So the problem is probably not simply low battery voltage.
2. Meaning of the red / green / double-red cycling
Without the exact Gardena service manual for that charger, I would not claim the LED code with absolute certainty. However, electrically, that cyclic behavior usually means:
For lithium-ion chargers, the usual safety checks include:
Because your battery is at 24 V, the charger should not be rejecting it for undervoltage. That points more toward temperature-sense, BMS, connector, or charger failure.
Practical diagnostic procedure
Step 1: Check LED behavior with no battery connected
Disconnect the battery completely and power the charger from the mains.
Observe the LED.
If the charger faults even with no battery, the battery is probably not the primary problem.
Step 2: Measure charger output voltage with no battery
Using a multimeter set to DC volts, measure the charger output connector carefully.
Expected approximate value:
Possible results:
Some smart chargers do not output full voltage until they detect a battery, so this test is useful but not always conclusive.
Step 3: Inspect and clean the contacts
This is one of the most common faults in garden equipment because of moisture, grass residue, fertilizer dust, and corrosion.
With the charger unplugged from the mains:
Clean using:
Do not use WD-40 as a primary contact cleaner for this application.
After cleaning, reconnect the battery firmly and test again.
Step 4: Check the battery temperature-sensor path
Many lithium tool batteries have at least three connections:
The charger may refuse to charge if it cannot see the battery temperature sensor.
A common NTC thermistor value is:
The exact value depends on Gardena’s design, but the key point is this:
With the battery disconnected from the charger, measure resistance between the likely T/NTC terminal and battery negative.
Possible results:
For an NTC sensor, resistance normally decreases as temperature rises.
If the charger cannot read the NTC correctly, it may interpret the battery as too cold, too hot, or unsafe, and refuse to charge.
Step 5: Measure battery voltage while connected to the charger
Connect the battery to the charger and place the multimeter across the battery positive and negative terminals.
Watch the voltage during the LED sequence.
If voltage does not rise after several minutes, it is not charging.
Most likely faults, ranked
1. Dirty or poor connector contact
This is the first thing to eliminate.
A poor contact on the main positive/negative terminals may cause voltage drop. A poor contact on the temperature-sense terminal may make the charger reject the battery.
Recommended action:
2. Open or incorrect NTC temperature-sensor signal
This is a very plausible cause for a charger that detects the battery voltage but refuses to charge.
Symptoms:
Recommended test:
If the reading is open-circuit, the battery pack likely has an internal sensor or wiring fault.
3. Battery BMS protection or cell imbalance
Even if the whole battery measures 24 V, one internal cell group could be much lower than the others.
For a 7S pack, a healthy pack near 24 V should have roughly:
[ 3.4\text{ V per cell group} ]
If one group is, for example, 2.0 V, while others are higher, the total pack voltage may still look acceptable, but the BMS may prevent charging.
Only check individual cell groups if you are competent working on lithium-ion packs. There is fire and short-circuit risk.
Typical cell-group checks:
4. Charger cable or connector wire fracture
If the charger cable has been bent repeatedly, one conductor may be broken internally.
This is especially relevant if the charger uses a three-wire output: positive, negative, and sense/NTC.
Test:
If the LED behavior changes when flexing the cable, suspect a broken conductor.
5. Internal charger failure
The charger may have an internal switch-mode power supply fault. Common causes include:
Typical symptoms:
If the charger is faulty, replacement is usually more practical than repair unless you are comfortable repairing mains-powered SMPS circuits.
Safety notes
This is a lithium-ion system. Avoid the following:
If replacing the charger, match:
Practical guidelines
I would diagnose it in this order:
The fastest way to separate the fault is:
Brief summary
Your battery at 24 V is not too flat; it should charge. The red/green/double-red repeating pattern indicates the charger is detecting a fault and restarting. The most likely causes are a bad contact, failed NTC temperature-sensor circuit, battery BMS/cell imbalance, or charger internal failure.
Start with contact cleaning and simple multimeter tests. If the charger shows the same fault pattern with no battery or has unstable output voltage, replace or repair the charger. If the charger looks normal with no load but faults only with this battery, investigate the battery’s NTC/sense line and internal cell balance.