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I'm new to electronics and I'm trying to build a charging circuit for NiMH batteries. There are four batteries in total. Two are in series, and the other two are in series. Then these are connected in parallel. I need to be able to switch them all to series when I plug in the USB for charging. I have an idea and please tell me if it will work. If not could you give me a way, thanks.

schematic

simulate this circuit – Schematic created using CircuitLab


The batteries i'm using are the Eneloop xx which are 1.2v so the USB would work because they add up to 4.8v. I was doing research about charging nimh batteries and found out that it is best to charge them in series and not parallel. So if this wont work than could you help me figure out what will switch it to series?

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  • \$\begingroup\$ Sorry Funkyguy. I'm new to this site I didn't know. \$\endgroup\$
    – Trevor
    Commented Aug 1, 2014 at 17:28

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Your circuit doesn't do what you want it to - both batteries are permanently wired in parallel, as is the charger. The diode does nothing.

You probably don't want to charge them in series from USB anyway, because you would need at least 5.6V (1.4V/cell x 4 cells) but you only have 5.0V available. Charging in parallel can be done efficiently with a switch-mode current regulator, or just a resistor to limit current (and perhaps a blocking diode to prevent discharging into the USB port when not powered) if you don't mind a lower charge rate.

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  • \$\begingroup\$ So in series and then parallel like I have, the cells will discharge at the same rate? \$\endgroup\$
    – Trevor
    Commented Aug 1, 2014 at 17:17
  • \$\begingroup\$ Yes - provided that they have the same internal resistance and state of charge (which is why you should not mix new and old, charged and discharged, or different make/model cells). \$\endgroup\$ Commented Aug 2, 2014 at 19:24
  • \$\begingroup\$ So charging them In parallel would I need a little more than 1.2v or a little more than 4.8v? \$\endgroup\$
    – Trevor
    Commented Aug 2, 2014 at 19:27
  • \$\begingroup\$ If they are all in parallel then you need a bit more than 1.2V (up to 1.6V if fast charging). If the cells are in individual holders then you should either charge them in series or have a separate charging circuit for each cell, because the battery contacts cannot be guaranteed to have equal resistance. If the battery is to be charged in the device then it may be easier (and more reliable) to have them permanently wired together to form a battery pack (with welded or soldered connections). \$\endgroup\$ Commented Aug 2, 2014 at 20:39
  • \$\begingroup\$ So if I charged them individually I would need 1.6v for each cell or 1.6v for all 4 cells? \$\endgroup\$
    – Trevor
    Commented Aug 2, 2014 at 22:30

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