Carefree battery charging?

A modern NiMh charger charges rapidly to 80% charge and then finishes off slowly and maintains a trickle to counter any autodischarge

http://www.google.ca/search?hl=en&q=%22modern+NiMH%22+%2Bchargers

The question, specifically, was whether the cells would discharge if the charger is disconnected (by the timer) from mains power (so that no trickle charging could be maintained even if the charger has the capability). The reason, by the weigh (and this will be of interest to MASU as well), for my decision to use the timer was that I previously noticed that a charger left powered too long will cause the cells to become quite hot--but no discernible degradation of cells as yet, fortunately. My reading on the subject informs that it is the heat that damages the cells; so that--accidental overheating by charging too long--is what the timer was intended to prevent. Thank you for your information about NiMH charging--which is also in agreement with my readings on the topic.

The simple answer is it depends how much money you spent on the charger in the first place.

The majority of battery chargers are pile of crap and should never be sold. Having said that there are good ones out there. A good battery charger should be able to not damage or overcharge a battery if left charging indefinably and should not discharge the battery if the power is removed. I however suspect you have little faith in the charger you are using since you have it on a timer.

You can check to see if the battery can discharge through the charger with a multi meter set to measure OHMS or DIODE. With the charger powered off and battery removed check the resistance across the battery terminals of the charger in both directions.

With a good charger you should see an open circuit both ways. If this is the case then the battery will not discharge through the charger.

With a not so good charger you may see an open circuit one way and short the other. This type of charger will also not discharge the battery back through the charger.

An el-cheapo charger will give you a resistive load that may or may not vary depending on the polarity of the meter leads. This type of charger will discharge the battery at a rate inversely proportional to the resistance you see.

Considering the cost of rechargeable batteries it amazes me that people skimp on chargers. My opinion is to spend the money and purchase a good charger that will not damage the batteries in any way. You will find it excellent investment in much less time than you think. One wrecked set of batteries is all it usually takes to convince people.

masu,

Thank you for a very astute and perceptive reply; and your suspicion--see also my reply to aurizon regarding battery heating--about my reasons for using a timer is also probably correct--which I will soon know by using your resistance check procedure, for which I also thank you. I hate to have to admit being among those "amazements" you speak of, but fortunately I have not had to incur any expensive (i.e., near-term-cell-failure) lessons as yet. (I was able to have one 4-cell battery replaced by the seller when one cell failed; but there would be no way to know if the failure of that one cell in a battery of four was a result of charging or simply a mfg defect which I just happened to purchase.) My reason--such as it was--for not having selected a more expensive charger (instead of two less expensive chargers) were that: (1) one charger came, along with cells, in a camera accessory kit which also included case, tripod, and lens wipes; (2) the other, Duracell charger was neither most expensive nor least expensive for use with the Duracell cells purchased (as a second battery) for the camera; (3) I am not a "rabid" photograph taker, so it seemed that having a backup battery and modestly priced chargers--along with applying some attentativeness--would be sufficient to meet my needs for both download power as well as camera power when needed. So far this has worked out adequately...as the cells reach the one year mark in age with no significant reduction in apparent capacity. Anyway, thanks again for the reply...and I'll let you know how the charger tests work out.

It depends on the charger. I bought a spare battery for my cellphone along with a desktop charger. It allows me to charge the battery without it being in the phone.

On the first day I used it, my wife unplugged it after the light turned from red to green (meaning it was fully charged) and left the battery in it. I was a little upset when I got home and discovered it. I showed her the effect by plugging it in again. The light showed red (meaning the battery had discharged).

Ergo, remove the battery when charging is done.

Vulcan,

Thanks for the reply; and your anecdote about your cellphone battery, along with the other posts, gives good assurance that my question was not ill-conceived. Your approach to maintaining ready phone power availability seems like a good one, especially (for some) in situations where there is no wired phone to fall back on. Coincidentally, what part of the Phillipines? Tagalog? Ilocano? Other? Just curious, as I am espoused to a stateside Filipina.

The reason why I bought a spare battery and desktop charger is two fold:

1. Having a spare, charged battery assures me that I my phone is always available. Okay for everyday use but inadequate if I plan to go to a place with no electricity for several days.
2. There is a tendency to recharge before the battery fully drains (the cellphone automatically switches off) to assure continued operation. This reduces the life of the battery. Having a spare allows me to recharge when it is fully drained. My batteries are more than a year old and still get three to four days each.

What part of the Philippines? Well, I was born in Bataan, lived in various parts of Metro Manila for most of my life, and presently living in Cagayan de Oro City in Mindanao (past 12 years). I am Tagalog.

You may find it interesting that I have only ever come across two half descent battery chargers in my entire life. One was a desktop charger for mobile phone batteries and the other was for charging NiCd batteries for model racing cars. Both chargers would discharge the battery fully before recharging and used a pulse width modulation method of regulating the charging process.

I was about to say that I had never seen a lead acid charger that was even mildly descent but I remembered one that was used in an Uninterruptible Power Supply (UPS) that I worked on some years back that was very good. The only problem is that that charger required considerable set up and maintenance and cost $50,000. Apart from that the only way to properly charge PbAcid batteries of any form is to build the charger yourself. I guess that makes it three half descent battery chargers in a lifetime, which is still pretty woeful considering I am an electrical, computer and control systems engineer.

I am currently researching methods of supplying power to the instrumentation in a glider and have found absolutely nothing world wide that even comes close to doing what I want. As a result I am building the system from scratch myself. My current thinking is that there is going to be a PIC microcontroller looking after each battery and how it is charged and discharged. This sounds like a hell of a lot of overkill but by doing it this way you can keep a history of each battery in great detail and it gives you an almost unlimited amount of control over how the battery is both charged and discharged. In fact this is what is done with LiIon battery packs. Poor charging or discharging techniques can be explosively unpleasant with LiIon batteries

The way you described how your charger lets the cells get very hot if left on charge would indicate to me that the charger doesn't have a trickle charge system but just keeps pumping away at the battery. I suspect you will find that it consists of a bridge rectifier and resistor and that's all. If that's the case when you test it with the meter you will find it open circuit both ways, or close to it, but will get a momentary short as you connect the meter. If you can open the charger up without damaging it, or voiding any warranty it might be worth having a look. From your description though it's not sounding good.

NO NO All the chargers; how simple it may be [a single Diode rectifier in the out-put of a transformer secondary] will not allow backward current which may cause discharge of cell(s)

Excuse me, Haajee, not all chargers are equal.

Also, diodes do have leakage current in the reverse direction. This leakage is very small, minuscule, if you want, but it's there. I know because my electronics instructor demonstrated it to our class.

Ergo, a battery will discharge if left in the charger.

reverse leakage in a silicon rectifier diode is in the nanoamps = trivial. self discharge of the battery is a far larger loss factor, and many batteries can self discharge in 30-90 days or so.

http://www.corrosion-doctors.org/Batteries/self-compare.htm

Yes, that's why I said "minuscule". Trivial maybe, but still a drain on the battery. My old NiCd charger manual says not to leave the battery in the charger even when you unplug it though the circuit is only a transformer, diode and resistor (Wasu's going to have a fit!).

But you're right, about self-discharge. I forgot about that.

Aaaaaaaaaaaaahhhhhhhh that not a battery charger it's a BATTERY DESTROYER!!!!!!

See!!! I knew it!!!

Actually, I got a couple of good years out of it. I changed the values of the resistor and transformer so that it starts out at normal charge and, as the voltage in the battery rose, the charging current dropped until it reached the trickle charge level. Took much longer to charge but reduced the damaged batteries to zero.

Stopped using NiCds after reading about the ban.

Always happy to oblige a fellow CR4 engineers. Such a polite request could never go unanswered.

Since you changed the resistor and transformer it only left the diode, did you think of replacing the silicon diode with a germanium diode to reduce the forward bias voltage drop. It reminds me of a bloke I once knew that claimed he had had an axe all his life. He had replaced the handle seven times and the head three but he had had it for as long as he could remember.

I did think of changing the diode but I didn't have any germanium types in my box of assorted collections. Anyway, the forward bias voltage drop didn't matter much to me at the time.

I guess I shouldn't have been surprised to see that the charger was all of three parts (not including the cord and the casing). 'Didn't even have an LED for power indication. It was a real cheapo. The NiCds got real warm during charging which prompted the modification.

I replaced it later with a model that automatically switched to trickle charge when it reached full. After going through several batteries, I added a discharging circuit that automatically switched to charging when the voltage dropped below a certain level. The batteries lasted longer but I eventually switched to NiMH batteries. Much better than the NiCds.

QUOTE:

"He had replaced the handle seven times and the head three but he had had it for as long as he could remember."

Seems as if I have ran into a relative of his somewhere. Different type of thinking, that IS for sure! It did bring a chuckle though. Thanks!

Ken