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I'm wondering if MAX712 would charge 4 Eneloop batteries in series. I don't know if it will charge them safely and if it will charge them the correct way. How much voltage will need to be provided to charge the batteries?

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Summary:

  • There are better newer ICs to use.

  • This IC will damage newer high capacity NiMH cells (see below)

  • Note that the page that you referenced said

    "All versions are Not Recommended for New Designs."


I was going to say that it would be an OK charger IC BUT a look at the MAX712/713 datasheet shows that the IC is now dangerous junk and should not be used to charge modern higher capacity NiMH cells (such as Sanyo Enelooop batteries).

The problem is that the IC is always in only one of two states - fast charge or trickle charge. Whereas older lower capacity NiMH cells could be safely trickle charged, newer higher capacity one MUST NOT be trickle charged. This is because the structures and chemical present in older cells which allows recombination of Hydrogen gas formed at end of charge cycle is not present in modern cells - as the manufacturers have 'stolen' the space to make room for additional active material.

  • I looked at as many manufacturer's recommendations as I could find in recent years and none of them 'allowed' trickle charging in the time honoured sense (For NiCd this might have been as high as C/10 indefinitely). Some manufacturers "allow" a very low post charge trickle charge rate for a very short time or in one case an exceptionally low ongoing rate (C/1000?) but this is not the norm.

    The change from OK to forbidden tends to be capacity based and will vary by manufacturer but, as a very rough guide, an AA cell of <= 1600 mAh capacity when this was "leading edge" still allowed trickle charging. However, now that manufacturers have become accustomed to making cells which are averse to trickle charging, even if they make a low capacity 'budget' cell, there is no certainty that they will re-equip the lower capacity cells with the ability to be trickle charged.

    While it is not 103% certain where Eneloops stand in this matter, it is exceedingly unlikely that trickle charging them is a good idea, and their ultra-low self discharge rates (years) means it is not necessary.

There are better newer ICs to use. Note that the page that you referenced said
"All versions are Not Recommended for New Designs."

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  • \$\begingroup\$ The injunction that "newer higher capacity one MUST NOT be trickle charged." is not borne out by the Panasonic Eneloop site FAQ which states, "Most NiMh rechargeable battery chargers may be used to charge eneloop". \$\endgroup\$ – WhatRoughBeast Aug 2 '14 at 10:21
  • \$\begingroup\$ @WhatRoughBeast - I am not complaining that you or anyone do not know this (why should you?), but in recent years I have investigated in depth what as many major battery manufacturers (or major vendors) that I could find data from say about their NimH cells and especially about NimH charging. I've been involved in the manufacture of products using the better part of 1 million NimH cells in the last few years and have a greater than usual interest in the subject. Given the choice I'd not use NimH :-)..... \$\endgroup\$ – Russell McMahon Aug 2 '14 at 11:09
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    \$\begingroup\$ .... when there is no recombination mechanism Hydrogen is generated once charging is complete and even at low rates it still gets generated and has nowhere to go. Hydrogen diffuses out of the battery even if the overpressure vent is not formally tripped and this represents an irrecoverable loss of water and a step on the path to cell death. || SO you'd hope that the statement "Most NiMh rechargeable battery chargers may be used to charge Eneloop" could be understood to include " .... as NimH they do not provide trickle charging". I use a standard MaHa NimH charger for Eneloops. \$\endgroup\$ – Russell McMahon Aug 2 '14 at 11:17
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    \$\begingroup\$ @WhatRoughBeast Actually (1) If you find Panasonic branded Eneloops they are fakes :-). (2) I was not saying that they are wrong (Panasonic are never wrong :-) - and Sanyo seldom are) BUT rather that they said "use any NimH charger" because they were confident that "any charger" you can now buy will not have trickle charging capability. I am confident [tm] that if you look at what Panasonic say about THEIR NimH that you will find that they will say they should NOT be trickle charged. [I haven't looked recently]. \$\endgroup\$ – Russell McMahon Aug 2 '14 at 11:30
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    \$\begingroup\$ Ack! Sanyo, not Panasonic. Damned insubordinate fingers, typing what I wrote, rather than what I meant. \$\endgroup\$ – WhatRoughBeast Aug 2 '14 at 11:35
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It says in the link it can charge 1 to 16 cells in series, so yes, it can charge 4 in series. It has Up to 1A Charge Current with 45V Peak Input Voltage Range, so depending on the input, the longer it will take to charge the batteries, to a point. It also has safety considerations incorporated where it checks the state of charge and temperature during charge to help prevent would-be safety issues. It regulated the output voltage between 7v to 16v depending on charging requirements and conditions.

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