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I have a rechargeable battery pack 4*AA of 1.2V NiMh type, which when measured fully charged (individually by a smart amp regulated charger) and without load shows some 6V. The load will be a PIC microcontroller drawing almost nothing almost all day long except for some 5-10 minutes few times a day activating a 5V relay. To not change batteries too often I am considering to connect a 5V switching power supply from an old phone charger. It shows 5V both without and with load. Charger would only supply power and charge batteries when the relay is activated, thus hopefully prolonging battery life and not drawing unnecessary load when nothing needs it.

First worry is to not fry the chip, which can tolerate up to 5.8V, but the batteries giving out 6V.

  1. Is that safe enough to just go with it and instead not have the batteries charged so high - charge them with say the stable 5V from the phone charger?
  2. Will the batteries tolerate such below nominal charging setup continuously or get damaged or charged over the 5V up to the 6V?
  3. Do I need to lower the voltage down to 5V from batteries to chip with some diode?
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    \$\begingroup\$ why do you need the batteries if you can use the phone charger? 6V on a 5.8V pic is not ok anyway. \$\endgroup\$
    – Vladimir Cravero
    Commented Mar 29, 2016 at 12:41
  • \$\begingroup\$ So the phone charger does not use any power 95% of time, when nothing happens. Just maximum autonomy with less power used. And some green thinking :) \$\endgroup\$
    – uldics
    Commented Mar 29, 2016 at 13:18
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    \$\begingroup\$ Green thinking? Either I completely misunderstood your setup, or you misunderstood thermodynamics. \$\endgroup\$
    – Vladimir Cravero
    Commented Mar 29, 2016 at 14:30
  • \$\begingroup\$ Seconding Vladimir's comment, because your answer didn't clarify: Why can't you just use the phone charger all the time? \$\endgroup\$
    – uint128_t
    Commented Mar 29, 2016 at 14:51
  • \$\begingroup\$ Yes, could be I am making this unnecessarily complex. Thought the charger would only be supplied when activated, thus not staying connected all day long. The only all day long activity would just be the PIC supplied from batteries. \$\endgroup\$
    – uldics
    Commented Mar 30, 2016 at 5:07

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fully charging the batteries will obviously fry your chip. don't ever charge them somewhere else.

in your setup, the batteries will never exceed 1V (*5), which is ok for your chip. 1V (*5) is the cut off voltage for NimH cells. They will always be almost empty, so a short battery life and fast capacity losses of your cells.

Can you add another cell and use converter? A linear converter would probably fit your needs, a step-down converter helps with a longer battery life

Edit: you can still use your phone charger with a switch, either manually controlled or a simple relais to switch to external supply voltage. Charging batteries is more complicated and I suggest using an external charger, you probaly own anyway.

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  • \$\begingroup\$ A step down converter after the batteries doesn't eliminate the battery charging necessity (install and forget). And as I understand it would draw more than the chip, as its efficiency is 80-90%. Or I probably did not understand your idea. \$\endgroup\$
    – uldics
    Commented Mar 29, 2016 at 13:15
  • \$\begingroup\$ Charging is going to be harder, the step down converter allows for using fully charged batteries. The additional cell accounts for the voltage the converter drops. \$\endgroup\$
    – Christian
    Commented Mar 29, 2016 at 13:22
  • \$\begingroup\$ OK. But then I never will have fully charged batteries, as the supply is 5V - and one more battery to feed. \$\endgroup\$
    – uldics
    Commented Mar 29, 2016 at 13:27
  • \$\begingroup\$ How fixed are you on the 5V power supply? If you would agree to a 12V supply and a step down converter, you could use one of these battery charging chips, like here: ti.com/product/BQ24401 \$\endgroup\$
    – Christian
    Commented Mar 29, 2016 at 14:08
  • \$\begingroup\$ Yes, that sounds a nice option. Just the complexity increasing. Probably best to just ditch the batteries alltogether. \$\endgroup\$
    – uldics
    Commented Mar 30, 2016 at 5:10

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