NiMh CHARGING

[Bluebottle]

I've just aquired some AA 3000 mAh NiMh batteries and put them in a mini charger which connects to the USB port of my laptop after about an hour the batteries got extremely hot, too hot to touch actually, can anyone tell me if this is normal ??

[AlanBeckett]

If they're new why do you need to charge them? They may already be fully charged.
It's normal for a long charge - 14 hours - to use 0.1C, that is 10% of the capacity, so in your case 300mA.
The USB Port can potentially supply 500mA, but that shouldn't do any harm.
When you overcharge them they get very hot, although they should survive OK. Check the terminal voltage. If it's at 1.4 volts or slightly above it's full.
Alan

[Bluebottle]

If they're new why do you need to charge them? They may already be fully charged.
It's normal for a long charge - 14 hours - to use 0.1C, that is 10% of the capacity, so in your case 300mA.
The USB Port can potentially supply 500mA, but that shouldn't do any harm.
When you overcharge them they get very hot, although they should survive OK. Check the terminal voltage. If it's at 1.4 volts or slightly above it's full.
Alan

These batteries came by post, as far as I know you are not allowed to send fully charged batteries through the post and as it happened these batteries were not charged. I think the problem was the mini charger it didn't appear to have a lot of regulation, I've now put them into a wallwart type charger and they seem to be charging nicely with a gradual voltage rise and not overheating ............

[AlanBeckett]

Sorry, but that's simply not true. They are normally assembled in a (chemically) charged condition, and I know of no prohibition against sending them through the Post in any condition. Millions of them are sent daily and I haven't seen any complaints. Cerainly the ones I used in industry effectively arrived in a charged state although we did cycle them before putting them into service just to make sure we could achieve the maximum capacity. I'd be pretty dubious about any that arrived effectively flat. They do self discharge in storage but it takes a very long time.
As I said before the USB Port should only be capable of 500mA and they should stand that OK.
Curious.
Alan

[Skywave]

. . . and as it happened, these batteries were not charged.

And that leads me to ask: How did you determine that they were not charged? Presumably you measured the battery terminal voltage of each one. What was the voltage measurement?

Al.

[rich r]

I've had fully charged lead acid car batteries sent through the post (well ParcelForce). Admittedly in strong cardboard boxes, packed with polystyrene chips and baking powder. So there doesn't seem to be any such restrictions.

[Joe]

As far as I know, you can send batteries "In their retail packaging" through the normal post. Whether something is 'allowed' or not mean little.

I'd always thought that they'd come at least partly charged as they don't like being stored flat.

I'd also agree that with Alan that a standard USB port can't deliver a harmful current. That said, equipment doesn't always read the text-books!

- Joe

[Bluebottle]

I know where I got the "no charged batteries in the post" from it is in the PO regulations that you can't send Mobile phones etc through the post with charged batteries in them whether this is because they are a fire hazard or to stop them operating it doesn't say. The batteries I've just received arrived with a terminal voltage of below 1.2v so I put them on charge for a top up

[AlanBeckett]

That's more sensible. Modern Mobile Phone batteries are Lithium Ion and are definitely a fire hazard in the wrong circumstances, plus they won't want a Phone in a parcel ringing unexpectedly
1.2 volts on a Ni-MH cell is about right for mid charge, depending on how accurate your meter is. Cell manufacturers often quote 0.9v as end of discharge 'cause it makes the capacity look better. However it's hardly worth the trouble as below 1.1 or so the voltage falls like a brick.
http://www.varta-microbattery.com/en/mb_...cal_en.pdf
http://www.varta-microbattery.com/en/mb_...S55115.pdf
http://www.varta-microbattery.com/en/mb_...171_en.pdf

Alan

[rich r]

That makes more sense- a charged battery in isolation is no risk. But a charged battery fitted to an item of equipment could be.