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Chargin NIMh 6v 10000mah Battery

Discussion in 'Power Electronics' started by jrote1, Jun 11, 2013.

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  1. jrote1

    jrote1

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    Jun 11, 2013
    I am trying to create a circuit to charge a 6v 10000mah NiMh battery. I am open to any suggestions. I have come accross these chips would any of these work ?
    • MAX713CPE
    • LTC4060
    I currently have a 2A 9v power supply is this enough. I also have some 29302WT and lm2576T voltage regulators can these be used for chrging the battery?

    Also can thebattery be under load while charging ?

    Thanks in advance.
     
  2. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    It appears that the 4060 wont output enough voltage for you, it looks like it has a max of 3.6 (2 cells rather than 3 or 4)

    The 713 should work, but with that supply you arent going to be able to charge very fast, you will probably max out around 1A of charge current which means it will take around 10-12 hours to go from dead to fully charged.

    It can be under load but if you exceed the charge current then you will be drawing from the battery and not charging, it will take a lot longer to charge under load than open.
     
  3. jrote1

    jrote1

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    Jun 11, 2013
    What power supply do you think should be suitable and also the documentation of the 713 says the voltage in should not exceed 5v so would it be able to charge or am I reading the documentation wrong?

    Sorry if I am being a bit stupid have never really worked with charging batteries before.
     
  4. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    Pulling directly from the manual
    Page 6 on the right hand side
     
  5. jrote1

    jrote1

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    Jun 11, 2013
    THank you that sorts my issues would you say a 3A power supply would be enough to charge the battery and run a 1.5A load?

    Thanks
     
  6. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    Ok looking again at the manual I linked, it times out after 4 hours, (4.4 to be specific, 264 minutes) This means that ideally you are going to want AT LEAST 2.5 A to charge the battery.

    With a 1.5A load you are going to need a 5A charger (at a minimum) to charge in 4 hours, this is assuming 80% efficiency of the charging circuit which knocks it down to 4A minus the 1.5A load you get your 2.5A charge current, thought I would look at this as an absolute minimum rating, and I would go with a 7-10A charger.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    I agree with GreenGiant here. The MAX713 data sheet specifies that the minimum fast-charge rate is C/4 so with a 10 Ah battery, you need to charge at at least 2.5A. The input supply also needs to provide the load current, so it needs to be able to supply at least 4A.

    From my experience with NiMH cells, I wouldn't recommend relying on negative-delta-V (aka voltage slope) charge termination unless the fast charge current is at least C/2. In that case you would need a source that can deliver 6.5A.

    No matter what charge rate you use, you NEED to monitor the battery pack temperature; at the end of charging, this can shoot up in a surprisingly short time, and NiMH cells contain a LOT of nasty-smelling magic smoke!

    FYI battery chargers are not normally designed to supply a load while they charge the battery; the MAX713 is an exception, because it has a separate current shunt and is designed to do that. It looks like a good choice for your application. Large, sharp or rapid variations of the load current may affect the charger though; it will work best if the load current is steady.

    The PNP pass transistor will need significant heatsinking. Also, the MAX713's DRV output, which provides the base current to this transistor, is rated for 100 mA maximum current, so your pass transistor will need to have a current gain of at least 25 (assuming 2.5A charge current) or 50 (assuming 5A charge current) at that current; many power transistors will not meet this requirement. A possible workaround would be to use a Darlington transistor, although this will affect the current regulation loop and this possibility is not mentioned in the data sheet.

    Also the series diode, D1, is shown as a 1N4001 in the data sheet; you'll need a diode with a much higher current rating than that. In general I think the MAX713 is designed for smaller batteries, and its designers may have made assumptions that could cause problems at the high currents you're talking about. But it's probably worth a try.
     
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