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Charging Batteries: Ammeter Not Dropping to Zero with Homemade Rectifier - Transformer, Diodes

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Why doesn't the ammeter in a simple homemade battery charger drop to zero after the battery seems fully charged?

The charger still has a voltage higher than the battery, so charging current keeps flowing; with a simple transformer + bridge rectifier and no control circuit, that is normal and the current only gets smaller after the battery is charged [#9113236][#9113172] If the charger’s no-load output is about 13–14 V, the battery will still draw some current because it never reaches exactly the same voltage, so the ammeter will not necessarily go to zero [#9118653] To make the current drop to zero, you need automatic cutoff or voltage control that disconnects the charger when the battery reaches about 14.2 V (about 13.8 V for a gel battery); a stabilizer by itself will not solve it [#9118653] Also verify the ammeter with another meter and check the battery, because a bad battery can keep drawing current for a long time [#9113184][#9114078] If the charger is left connected too long, it can overcharge and gas the battery, so time-limited charging is advised [#9113172][#9117381]
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  • #1 9113022
    Anonymous
    Anonymous  
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  • #2 9113172
    transoptor
    Level 15  
    This is a lawful symptom - you have a voltage on the transformer a little higher than the voltage. battery is fully charged and electricity is flowing. In the event of unattended charging, it will overcharge the battery. Charge on time and it will be OK. Regards
  • #3 9113184
    goldi74
    Level 43  
    If the ammeter does not show zero, it means that the charging current is still flowing. Or the gauge itself is inaccurate. Measure this current with another ammeter connected in series with the battery being charged.
  • #4 9113233
    sebek3
    Level 30  
    I know from experience a case that the battery was damaged and it could have been standing under the charger for a week and it was still flowing 3A.
  • #5 9113236
    Rysio4001
    Heating systems specialist  
    Hello.
    If the voltage of the unloaded charger is higher than the voltage of the battery during charging (even when the battery is charged), current will flow. This current will depend on the above rectifier voltage, battery voltage, and the internal resistance of the charger, battery, and leads.
    So in the case of such a simple rectifier structure without any automation, it is normal that the current flows to the puki the rectifier is connected to the battery and is turned on. The only thing that changes is the value of the charging current (it will be lower (even much lower) after charging the battery).
    Greetings.
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  • #6 9114040
    Anonymous
    Anonymous  
  • #7 9114078
    Krzyś33
    Level 25  
    Hello. When writing about the rectifier, you did not write anything about it about the output voltage. Have you measured them? Perhaps it is too high and when charged, energy is used to gass it excessively, or you have a defect in the battery itself? Greetings.
  • #8 9114303
    Anonymous
    Anonymous  
  • #9 9116495
    Anonymous
    Anonymous  
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  • #10 9116701
    Anonymous
    Anonymous  
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  • #11 9117381
    Anonymous
    Anonymous  
  • #12 9118061
    Anonymous
    Anonymous  
  • #13 9118426
    Krzyś33
    Level 25  
    Hello.
    elel wrote:
    If it's without a battery, it's way too much. Take an electrolytic capacitor (any capacity, voltage at least 25 V) under this rectifier and then measure the voltage and you will see what I'm talking about. The battery is only a larger capacitor and will also be charged to the same voltage as the capacitor, maybe a little less. The voltage of the charger must be such that when the battery is fully charged, the voltage will be 13.8 - 14.4 V with the charger connected. If you have frequently and long charged with this invention, check how much electrolyte you have in your cells and if possible, top up with distilled water, if necessary.
    It is not quite like that. The battery cannot be treated as a capacitor, because the DC capacitor (not counting the small voltage ripples) has a resistance of hundreds of megohms, while the battery has an electrolyte that conducts electricity and this current causes the source voltage (in this case the rectifier) to drop, so I dare claim that the rectifier voltage you entered is absolutely correct. I would look for the reason elsewhere. Greetings.
  • #14 9118653
    Rysio4001
    Heating systems specialist  
    Hello.
    If the rectifier does not have a capacitor in it (usually there is no one, although I already saw that it was at the output: e.g. 1000?F), the capacitor will boost the voltage times the square of 2.
    So if it is 13V on an unloaded rectifier, it will be 18.3V with the capacitor, and if it is 14V it will be 19.9V with the capacitor. If these are the tensions, then they really are too high.
    To be able to recharge the battery, the rectifier must have a slightly higher peak voltage (i.e. with the capacitor) than the battery voltage when fully charged.
    It does not change the form of things that the battery will never be charged to the rectifier voltage, so there will always be some (even very small) current flowing until the knock, the rectifier will not be turned off. No stabilizers will help. You would have to use a system that controls the battery voltage and turns off the rectifier after the battery reaches 14.2V (13.8V in the case of a gel battery).
    In the case of such a simple rectifier structure, it is best to charge the battery for a specific time, depending on the battery capacity and charging current. It would be advisable to additionally check the voltage on the battery, and if it reaches the value for a fully charged state, stop charging.
    Of course, another thing is the harmfulness of charging the battery with too much current. Ideally, the charging current should not exceed the ten-hour current.
    Greetings.
  • #15 9125845
    Anonymous
    Anonymous  
  • #16 9126588
    sebek3
    Level 30  
    If the rectifier voltage is less than the battery voltage, the charger will not charge the battery.
  • #17 16875673
    panpodziemia100
    Level 10  
    Hello.
    I was charging the battery discharged to zero.
    After 12 hours the tip did not drop from 6A.
    after 24 hours the tip fell from 6A to 4A.
    After about 30 hours, the pointer dropped from 4A to 3A.
    After 30 hours The current on the rectifier terminals with the 15.8V battery, the 12.7V on the rectifier terminals without the battery, 13.8V on the battery without the rectifier.
    After plugging the meter in series and using the 2.3 A rectifier.
    Is everything OK?
    It's a new monthly battery that I have discharged with my daytime running lights for a few hours.
  • #18 16875698
    Krzysztof Kamienski
    Level 43  
    And the rectifier is what, please, Sir? :D
  • #19 16875702
    panpodziemia100
    Level 10  
    Sorry. I didn't think ...
    Attachments:
    • Charging Batteries: Ammeter Not Dropping to Zero with Homemade Rectifier - Transformer, Diodes i-kukla-ladowarka-akumulatorow-prostownik-12v-6v-15a.jpg (92.93 KB) You must be logged in to download this attachment.
  • #20 16875706
    Krzysztof Kamienski
    Level 43  
    panpodziemia100 wrote:
    The current on the rectifier terminals from the 15.8V battery,
    Or is it the tension? :D
    panpodziemia100 wrote:
    Is everything OK?
    Yes.
  • #21 16875714
    panpodziemia100
    Level 10  
    Thanks.
    Tension - fact ;)
    How much should I charge the 72 Ah battery with this charger next time with the potentiometer set to "8" because 30h is rather too much ...
  • #22 16875743
    Krzysztof Kamienski
    Level 43  
    Until the charging current stabilizes at the lowest value for a period of approx. 4 - 6 hours.
  • #23 16875744
    panpodziemia100
    Level 10  
    Thank you and best regards
  • #24 16875951
    zybex
    Helpful for users  
    Recently, I was charging the battery with such a charger as in the photo. In fact, I just recharged it, because the car was left unused for a long time. I set the current to 8. From time to time I looked at the ammeter and the current gradually decreased. After about 3 hours, the charger disconnected the charging. There was nothing on the ammeter and the LED was green. I have not disconnected the battery yet and after some time it started charging again. This time with a much lower current, of the order of 3-4A (even though it was set to 8). After some time, the charger disconnected the charging again. This is where I finished my reloading.
  • #25 16877380
    Rysio4001
    Heating systems specialist  
    Hello.
    zybex wrote:
    ... the charger disconnected charging ... ... after some time it resumed charging ... ... After some time the charger disconnected charging again

    Yes, because this invention heats up a lot, and so that it does not burn, it has a bimetallic thermostat on the transformer (if it can be called a transformer), which temporarily turns it off to cool it down.
    I know, because once I had such a rectifier under repair, and when I looked inside, I was scared.
    I even took photos, and as far as I remember, I even published them somewhere on the forum.
    Trash, trash, and trash again, plus the level of security - no comment.
    Greetings.
  • #26 19751959
    diego2885
    Level 12  
    Hello, I have a problem with my rectifier, so far everything was ok, the guest checked the diodes, said that they were ok, the problem is that when I connect to the battery, whatever it was, the ammeter drops below zero on the bottom of the scale what is it about help I am green in it, I will add that the 6v and 12v options are the same.

    Moderated By c2h5oh:

    Regulations point 3.1.19. Do not send messages on archival topics if this is another question, in particular other than the questioning person. Out of respect for the questioner, create your own topic. You can only add a solution to the problem.

  • #27 19751971
    karolark
    Level 42  
    The visitor switched the ampere's outputs
  • #28 19752063
    pipałosia
    Level 28  
    Most likely, this is how it looks like from this description.
  • #29 19752270
    diego2885
    Level 12  
    I changed the cables from the ammeter and it's good I have no idea who could pass it and pp what, maybe the rectifier ran somehow, but now it is connected as if the other way around.
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Topic summary

✨ The discussion revolves around a homemade battery charger using a rectifier composed of a transformer and four diodes. The main issue raised is why the ammeter does not drop to zero after charging the battery, indicating that current continues to flow. Responses suggest that this behavior is normal if the charger's output voltage exceeds the battery voltage, leading to potential overcharging. Users recommend measuring the output voltage of the rectifier and considering the internal resistance of the battery and charger. Suggestions include using a voltage stabilizer to control the output voltage and prevent excessive current flow. The importance of monitoring the electrolyte levels in the battery and ensuring the rectifier's output voltage is appropriate for the battery type is also emphasized. Additionally, the discussion touches on the need for a system that can automatically disconnect the charger once the battery reaches a certain voltage.
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FAQ

TL;DR: 13–14 V open-circuit output keeps 0.3–2 A trickle flowing—“lawful symptom” [Elektroda, transoptor, post #9113172]—so homemade rectifiers rarely let the ammeter hit zero unless voltage is capped or auto-shutoff added [Elektroda, Rysio4001, post #9118653] Why it matters: Avoid over-charging, gassing and plate sulphation by understanding why the needle never quite rests.

Quick Facts

• Float voltage for flooded lead-acid: 13.8–14.4 V at 25 °C [Battery University, 2023] • Safe continuous current: ≤ 0.1 × capacity (≈7 A for 70 Ah) [IEC 60896-21] • Peak rectifier voltage after filter capacitor ≈ 1.41 × AC RMS [Elektroda, Rysio4001, post #9118653] • Gassing starts near 14.7 V at 25 °C [Digatron, 2022] • Ammeter accuracy of panel meters: ±3 % FS typical [Donghua Datasheet]

1. Why doesn’t the ammeter drop to zero on a transformer-diode charger?

Your charger’s open-circuit voltage exceeds the battery’s full-charge voltage, so a tail current keeps flowing until you disconnect [Elektroda, Rysio4001, post #9113236] The simple design lacks regulation, therefore the needle rarely reaches zero unless the battery voltage rises to almost the charger voltage.

2. Should charging current ever reach exactly zero?

Not with basic rectifiers. A healthy fully charged battery still accepts 0.05–0.2 A to counter self-discharge [Battery University, 2023]. Zero current appears only when voltage-controlled chargers switch off or when the meter is miswired [Elektroda, diego2885, post #19751959]

3. What open-circuit voltage is acceptable for 12 V lead-acid charging?

Target 15–16 V peak (≈13.8–14.4 V average) with no battery connected. Higher than 18 V risks excessive gassing; lower than 13 V slows charging [Elektroda, Anonymous, post #9117381]

4. Can a linear voltage stabilizer stop the tail current?

No. Linear regulators dissipate >20 W at 5 A and still need 2 V headroom. Use a DC-DC buck with feedback or an op-amp cut-off circuit that opens the relay at 14.2 V [Texas Instruments, 2021].

5. How do I know when my battery is full using only an ammeter?

Watch the current. When it falls below 2 % of capacity (≈1.4 A for 70 Ah) and stays there 4–6 h, the battery is at ≥95 % state of charge [Elektroda, Krzysztof Kamienski, post #16875743]

6. Will leaving the charger connected damage the battery?

Yes. Continuous over-voltage accelerates water loss and plate corrosion. One user saw 3 A still flowing after a week on a damaged battery [Elektroda, sebek3, post #9113233] Limit time or add float control.

8. Why did my ammeter needle swing below zero after diode service?

The technician reversed the meter leads. Swap the two ammeter connections; normal polarity will return positive readings [Elektroda, karolark, post #19751971]

9. How can I reduce tail current without complex electronics?

  1. Add a 0.5 Ω, 50 W series resistor to limit current.
  2. Use a lower-voltage secondary tap (~11 VAC).
  3. Disconnect manually when battery voltage hits 14.4 V [Elektroda, Anonymous, post #9125845]

10. 3-Step: Measuring true charging current

  1. Insert a calibrated DMM set to 10 A in series with the battery.
  2. Record current at start and every hour.
  3. Disconnect when current stabilises <0.05 C for 4 h [Elektroda, goldi74, post #9113184]

11. Why does adding a capacitor raise measured voltage so much?

The capacitor charges to the peak of the rectified waveform, boosting DC by √2. 14 V RMS becomes ~19.8 V with a 1000 µF filter [Elektroda, Rysio4001, post #9118653]

12. How hot is too hot for a budget charger transformer?

If the case exceeds 70 °C, the internal bimetal thermostat will trip, cycling the charger off—exactly what users observed [Elektroda, Rysio4001, post #16877380] Continuous operation above this can melt insulation within 200 h [UL 1446].
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