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Comment by amelius

9 hours ago

Speaking of which, I really hate those chargers that force you to use two batteries instead of one. I get that it is cheaper to design it that way, but come on.

12 comments

amelius

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dragontamer  8 hours ago

NiMH chemistry allows for safe overcharging though. If the chemistry allows for it, why not take advantage of it and have cheaper chargers?

The downside is that "save overcharging" only works at very low charging rates. That's why the double-charger designs all have 10+ hour charge times (mine actually has a 20-hour charge time).

But in practice? Its cheaper to buy 4 extra AA NiMH batteries to keep charged rather than upgrade to the faster chargers. So just keep some spares topped off and you should be fine.

  • BeetleB  1 hour ago

    I think what he is referring to is this:

    If one battery is 50%, and the other at 70%, and you put both in, one will end up at 80% and the other at 100%. When one is full, those cheap chargers stop charging the pair.

    • dragontamer  17 minutes ago

      That's not how it works at all.

      When one is 80% and the other is 100%, the full one enters a state of over charge, reaching 105% or even 110% charge. This is safe.

      The H2 reaction then rapidly speeds up, leaking energy in the form of heat. The full battery heats up from overcharge but is otherwise safe.

      You end up with both batteries at 100% and maybe 110%, and a day or two later the 110% overcharge settles down to 100% by leaking out.

      -------

      So you waste a bit of power but as long as the trickle charge is safe and as long as the overcharge is only for a few dozen hours or so, it's fine. In the very long term (if you keep doing this) the NiMH could get damaged. But if we are talking about a once-per-yeqr top off charge, then it's fine.

      The problem is like I said before: the safe rate of overcharge is low. This means that these chargers must charge slowly, maybe 10 hours or longer.

      Any faster risks blowing through the NiMH innate ability to take an overcharge and convert it into heat. (This results in a forceful vent, a 'pop' sound that permanently damages the NiMH as the H2 gas escapes the safety hatch).

      ------

      Note that these super cheap chargers are simply a glorified 10+ hour timer. They don't even check the state of the AA cells.

      So if you stick a 50% full battery in, it will charge the battery to 150%, most likely. (But safely, as the NiMH just leaks out the excess energy as heat, as I said earlier).

  • amelius  6 hours ago

    Sure, but those NiMH have the same form factor by necessity, so it is confusing.

    I would be surprised if the extra costs for the electronics for charging an extra cell are more than a few dollars.

    • dragontamer  6 hours ago

      NiMH cells are like $1 each. Seriously, Amazon.com is quoting me $18 for 16 cells, AA Amazon Basics NiMH.

      As I said earlier: it's likely a better strategy to buy 4 extra cells and keep them pre charged / topped off, rather than spending a few dollars on a better charger.

      2 replies →

alnwlsn  7 hours ago

Speaking of which, I have yet to see a consumer battery charger that isn't incredibly cheap feeling. Even ones for charging different chemistries or that let you set current rates are a mess of low-quality molding and cryptic button presses.

  • jasomill  6 hours ago

    I've never had a problem with the basic 4-cell chargers that Panasonic includes with their Eneloop starter kits, other than the mildly annoying fact that they cover more than a single plug on a power strip.

    Build quality seems fine, and I've been using them for decades without a single failure, so I've never seen any reason to even investigate alternatives.

    • jasona123  1 hour ago

      Panasonic does have one that charges over USB (micro-USB input though) so you can use an old 5V Apple brick and only take up one spot. It also supports USB out via a full-size USB-A port. Picked one up since I liked the idea of being able to use AAs in a pinch for recharging.