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I tried to design a charging circuit based on the TP4056 Li-po charger chip. At a USB voltage of 5 V, the battery charging output (Pin 5 of the TP4056) is 4.3 V. This seems acceptable, but there seems to be no regulation in my circuit as the battery voltage will vary with the input voltage:

VCC   Vbatt
4.50--3.87
4.75--4.12
5.00--4.34
5.25--4.61
5.50--4.82

Can anyone suggest what the problem is? I'm reluctant to plug in my Li-pos until I get safe readings from a multimeter.

Schematic is included below:

enter image description here

IBATT connector is shorted for voltage tests.

I do not want to overcharge my batteries with too high a voltage. The TP40456 doesn't seem to be doing its job properly.

ocrdu
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T-RexMan
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  • What battery are you using? How much voltage does it put out by itself (not connected to the charger)? – Bruce Abbott Sep 05 '20 at 06:14
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    haven't checked the TP4056 datasheet, but these 0.7 V between in- and output might simply be the minimum drop. How are you measuring the voltage if you're not attaching a battery? Obviously, the free-running voltage is not what you'll see when you attach anything that actually loads the charger's output. – Marcus Müller Sep 05 '20 at 07:31
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    unshort ibatt and connet an ammeter there, observe current during the tests. – Jasen Слава Україні Sep 05 '20 at 09:02
  • @BruceAbbott I'm using a 1000mAh single cell li-po battery rated for 4.2V. The input voltage is USB 5V. – T-RexMan Sep 06 '20 at 16:49
  • @MarcusMüller I haven't attached a battery to the BATT terminal yet and just measuring the open-circuit voltage. I believe it should be 4.2V with no load. – T-RexMan Sep 06 '20 at 16:51
  • @Jasen Since I don't want to have exploding batteries, I did not test attach a battery to the terminal yet and have no load. Maybe I will try to simulate a load with some resistors. – T-RexMan Sep 06 '20 at 16:52
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    Why should it be 4.2 V without load? – Marcus Müller Sep 06 '20 at 16:58
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    You said "the **battery** outputs 4.3V" but you didn't connect the battery. Please edit your question to correct this incorrect information. – Bruce Abbott Sep 06 '20 at 21:32
  • @BruceAbbott sorry for the confusion, battery output here means the "BATT" output of the TP4065 (pin5) used to charge the battery. The charge voltage is higher than 4.2V. – T-RexMan Sep 07 '20 at 18:29
  • Does the datasheet state what the open circuit voltage is? Is it out of specification? – winny Sep 07 '20 at 19:32

1 Answers1

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The TP4065 could be detecting there is no battery, or a battery that is out-of-spec for normal charging, and do as it sees fit.

It could also be going into CC-charging mode, and then it's a current source and would not be regulating voltage. Its behaviour in the absence of a battery may well be described in its datasheet, I haven't checked.

Anyway: it will be hard to test properly without a battery connected. Pros use a Battery Simulator; when in doubt, I just take the whole rig outside, use a battery, and stand well back; not as safe, but much cheaper.

ocrdu
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  • Did a few tests and swapped in a TP4056 from a brought charging board. The problem seems to be the actual chip itself, I will try to find another supplier of the same chip. – T-RexMan Sep 15 '20 at 05:12