See post #5. Using it right now doing a nice slow 200 mA charge. With 2 AA's in that baby it will do 1800 mA.

I have a 1hr energizer charger that will do 1-2-3 or 4 cells, AA or NiMh. Its cheap and the batteries end up too hot to hold. When I was running 12AA battery packs I used it for a while.. Lots of dead cells now.
I no longer use this charger, figuring cells aren't really that cheap.

A little over a buck a piece for some generic. That's not too bad.

compared with their generic alkaline counterparts?

If you are going to fry Rechargeables you might as well spend out on alks or even lithium, as in the long run it will cost the same.

HOWEVER - invest in a decent charger and soon the rechargeable starts paying for iteslf

I'm not sure I understand your point. Here's what I'm saying. For a little over a buck, I can get a rechargeable nimh that can be used many times over even if I don't take care of it and fry the hell out of it. Much more cost efficient than an alkaline.

We're saying the same thing, right?

maybe - the cheapes (US$) I have seen lithiums for is ~0.75c. Why go to through the hassle of charging if ya gonna fry?

With the lithiums you are guaranteed to have pretty much good cells.

If you are frying - you have no idea how good the cells are and how long they will last. PLUS you have had to charge them (however long that takes). Plus you have spent the electricity to charge them (marginal I know) AND you have had to buy the charger to fry them with.

I If you are going to use rechargeables, then it is worthwhile investing in a decent charger. It doesn't have to charge one at a time, but it does need to be "smart"

Rather than invest in a new charger, is there a simple way I can drain my AA battery so I can charge it properly with my dead one? Can I short circuit it?

We agree. :thumb:

The Cold Heat soldering iron relies on short-circuiting AA batteries across the soldered elements. With the steps you propose you might be on the way to force yourself to invest in (a) new batteries, (b) new charger and (c) maybe even new home, if you leave badly damaged batteries in a dumb fast charger.

Seriously, DO NOT try it. Buy extra batteries. After a while, another one will die.

Buy an inexpensive adjustable current-regulated power supply. Charge at C/10 or less. When the battery starts to feel warm then it is fully charged.

I have the C204F. http://www.users.on.net/~mhains/Table2.html

I believe this to be very bad advice because:

1) If the battery starts to feel warm at C/10 it could already very overcharged (if the battery is partially charged to start with) by the time you check or depending on capacity/internal resistance the battery could get warm 1/2 way through the charging process.

We aren't baking a cake here, you know.

2) Nimh have much poorer overcharge absorption than Nicad and will be damaged if trickle charged at more than 0.05C. The Nimh simply can't absorb the extra power after they reach peak saturation.

3) Slow charges cause crystalline formation (memory).

While these are all bad things - Nimh do not like being trickle charged - I believe cheap fast chargers are more damaging.

If you don't mind having batteries that are always overcharged or undercharged and have greatly shortened life expectancy (hundreds of cycles less), use a cheap charger.

:)

Charging NiMH at C/10 or under is commonly done with good results. I personally do this and have some NiMH battery packs that are 8 years old and still cycle to near capacity.

In my experience a limited overcharge at this rate is not a problem, but they should not be left on charge permanently. One can either charge for a set amount of time (say 14 hours at C/10) and accept that there will be some overcharge or terminate the charge when a small rise in temperature is noticed. The temperature rise is perceptible when charging AA cells at 150 mA or higher. I'm not sure it will be noticeable at lower charge rates. It is probably best to use both criteria, i.e., if no temp rise is felt after 14 hours then stop the charge.

I believe (this is just an opinion) that memory effect is nonexistent in NiMH cells. There is no need to cycle NiMH cells other than to check their capacity.

If you are charging at C/10 and can feel a temperature increase at the end of the charge cycle that doesn't mean it is fully charged, it means it is failing to absorb the overcharge and you aren't doing the battery any favours.

+1 for any Maha

NiMH DO suffer from memory effect (or what people think is memory effect anyway), just not as much as NiCd.

Some useful info here and here

Are they the constant current pulse chargers?

I was looking at them today

They call their tech FLEX negative pulse charging though I don't know what that means or whether their "conditioning" claims are true. I can tell you that in practice I have owned a C401 series for years now and have yet to throw away a battery purchased since. It even made some of the batts fried by a cheap Rayovac useable again, though limited in capacity. It has been worth every penny.

http://en.wikipedia.org/wiki/Battery_charger

RE: negative pulse charging
http://www.rcbatteryclinic.com/burp.html



And some good advice on fast charging
http://www.rcbatteryclinic.com/ (http://www.rcbatteryclinic.com/fastchrg.html)

Most 'pulse' chargers are just using semi-rectified A/C. :)

Unknown, I don't understand the distinction you are making. When a cell is fully charged it will dissipate any further charge energy as heat. This is an indication that the cell is fully charged. At low charge rates the heat is not damaging to the cell. This is why Sanyo and other manufacturer's state a standard timed charge rate at C/10.

On the other hand, cells can be damaged by use of an unreliable peak-detect charger. Because peak detect chargers charge at a relatively high rate, overcharge is not tolerated well. A missed delta will often cause damage.

In my experience, packs always charged at C/10 or less will have a longer life span than packs charged on a peak detect charger. I realize this is contrary to Battery University. I suspect (just my opinion) that Buchmann is more interested in promoting his company than in being completely honest.

The very fact that it is a timed charge is proof that the heat is damaging to the cell - why else have such a solid cut off ? Clearly, Sanyo are aware that to charge for longer WILL damage the cell. It's a low key budget attempt at not overcharging fully discharged cells and makes no allowances for loss of capacity or partly discharged cells.

A Nimh cell can only dissipate so much heat, temperature is not an indiction of state of charge, whereas rate of temperature change is.

So, anyone who believes that checking if a battery is 'warm' (in relation to what?) to ascertain charge state is sorely mistaken.

I said earlier I also believe cheap fast chargers (and even some that are not so cheap) are damaging.

When charging a single Nimh cell I select a C rating and monitor the temperature and delta peak - or rather my charger does. I do not own, use or recommend a Buchmann charger. :)

You seem somewhat confused on some basic facts: Different types of chargers use delta peak detection not just fast chargers. In fact, the delta peak is easier to detect with higher charge rates. A fast charger is not necessarily using peak detect.

Right. A peak detect charger needs to have a relatively high charge rate in order to obtain a discernible peak. Peak charging at C/10 is not generally done because the voltage inflection is too small. Because relatively high rates (compared to C/10) are used, a missed peak can be a problem. Overcharging at C/10 is not a problem. Overcharging at C or C/2 or C/3 IS a problem.

The very fact that your charger has two detection methods should tell you something. If peak detect alone were completely reliable then the temperature sense would not be needed. If overcharging because of a missed peak were harmless then temperature sense would not be needed.

And by your own argument, if the temperature/temp change sensor were reliable by themselves, there would be no need for peak detection.

Anyways, doing some more reading, it would seem that multiple cut-offs are used in universal chargers for various reasons, not least because...

Negative delta V (NDV) Cut-off charge system

This is the most popular method for rapid charging for Nicads.

Batteries are charged at constant current of between 0.5 and 1.0 C rate. The battery voltage rises as charging progresses to a peak when fully charged then subsequently falls. This voltage drop, -delta V, is due to polarisation or oxygen build up inside the cell which starts to occur once the cell is fully charged. At this point the cell enters the overcharge danger zone and the temperature begins to rise rapidly since the chemical changes are complete and the excess electrical energy is converted into heat. The voltage drop occurs regardless of the discharge level or ambient temperature and it can therefore be detected and used to identify the peak and hence to cut off the charger when the battery has reached its full charge or switch to trickle charge.

This method is not suitable for charging currents less than 0.5 C since delta V becomes difficult to detect. False delta V can occur at the start of the charge with excessively discharged cells. This is overcome by using a timer to delay the detection of delta V sufficiently to avoid the problem.

dT/dt Charge system

NiMH batteries do not demonstrate such a pronounced NDV voltage drop when they reach the end of the charging cycle, and so the NDV cut off method is not reliable for ending the NiMH charge. Instead the charger senses the rate of increase of the cell temperature per unit time. When a predetermined rate is reached the rapid charge is stopped and the charge method is switched to trickle charge. This method is more expensive but avoids overcharge and gives longer life. Because extended trickle charging can damage a NiMH battery, the use of a timer to regulate the total charging time is recommended.