Charger issues, advice please

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I have one of the Quad Pro chargers, there are lots of rebranded versions availabe.

The problem I have is this. When charging, the voltage readout on the screen very quickly ( within a few minutes) goes up to max voltage, 8.4v for a 2 cell lipo. When checked with a volt meter the lipo is what you would expect a flat pack to be 7.5 volts ish.

When the charger starts to read 8.4v the charge amps begins to drop, the amps keep dropping till they get to .1 and eventually will finish charging. But this can typically take well over an hour to put 2000mah into a 5a lipo at 1C sometimes over 2 hours on balance charge.

The charger never over charges the lipo, when lipo is charged it is never over 8.35 volts.

Now I am only using a cheap 10amp power supply, but I get the same problem whether I charge 1 or 3 packs.

Any ideas? Anyone had similar problems?