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I tried to design a charging circuit based on the TP4056 li-po chip. At a usb voltage of 5V, the battery charging output (Pin 5 of TP4056) 4.3V. 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 too reluctant to plug in my li-pos until I get safe readings from a multimeter. Schematic included below enter image description here

IBATT connector is shorted for voltage tests.

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

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  • \$\begingroup\$ What battery are you using? How much voltage does it put out by itself (not connected to the charger)? \$\endgroup\$ – Bruce Abbott Sep 5 '20 at 6:14
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    \$\begingroup\$ 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. \$\endgroup\$ – Marcus Müller Sep 5 '20 at 7:31
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    \$\begingroup\$ unshort ibatt and connet an ammeter there, observe current during the tests. \$\endgroup\$ – Jasen Sep 5 '20 at 9:02
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    \$\begingroup\$ Why should it be 4.2 V without load? \$\endgroup\$ – Marcus Müller Sep 6 '20 at 16:58
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    \$\begingroup\$ You said "the battery outputs 4.3V" but you didn't connect the battery. Please edit your question to correct this incorrect information. \$\endgroup\$ – Bruce Abbott Sep 6 '20 at 21:32
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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 not 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.

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  • \$\begingroup\$ 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. \$\endgroup\$ – T-RexMan Sep 15 '20 at 5:12

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