Comment by dragontamer

Comment by dragontamer 2 days 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 2 days 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.

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dragontamer 2 days 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.
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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).
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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).

Reply View | 3 replies
- amelius 2 days ago
  
  Ok, but do you agree that in this day and age of cheap electronics, it is really not worth it to even consider all this and it is better to just make a charger that doesn't overcharge and doesn't come with extra instructions?
  
  Reply View | 1 reply
  
  dragontamer 2 days ago
  
  It doesn't really make sense to buy a $20 charger when you only have $16 or so worth of AA cells (ie: a 16 pack from Amazon Basics). The $10 charger makes a lot more sense.
  As for the knowledge problem: that's easily solved. I already told you what to do so you know how to work the $10 charger now. If you still want the $20 charger that's fine, it's your money.
  
  Reply View | 0 replies
- BeetleB 2 days ago
  
  I suppose there are lots of different chargers. The one I had made both charging lights turn off when one turned green. Maybe it was overcharging but still showing as red till both were charged?
  
  Reply View | 0 replies

amelius 2 days 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.

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dragontamer 2 days 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.

Reply View | 4 replies
- D13Fd 2 days ago
  
  I'm not sure that's true in practice. Chargers are often shared in a household and you may not have control over people putting things back on the charger. My preference is to have something that charges more quickly, rather than a bunch of spare batteries.
  Plus if you leave batteries on the overcharge-style charger for long periods of time, I'm pretty sure they just dissipate the extra charge as heat, and that charge is applied indefinitely. They essentially turn into little electric space heaters, sending that energy down the drain.
  
  Reply View | 3 replies
  
  dragontamer 2 days ago
  
  Don't leave them in the charger! You precharge and then store for a year+. Modern NiMHs only need topping off once per year or less.
  So you have two piles. Good charged NiMHs and uncharged depleted NiMHs. While(uncharged.size() >= 2) put them in charger.
  Then once per year, do a refresh charge on all the NiMHs. Easy-peasy.
  
  Reply View | 2 replies