Nicad vs Nimh battery in phone

Just had to replace the batteries in a pair of Vtech phones (model 20-2481). Old were nicad. New are Nimh. Differences? Best way to charge, how often?
Figure some of the kind folks in here will have knowledge.
TIA
LB
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NiCad can develop memory, should be deep discharged at time before charging.
Nimh can be topped of without any memory effects. Lat longer, generally more cycles. Often more power in the same sized cell.
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Edwin Pawlowski wrote:

Hi, But charging circuits are different. You can't charge NiMh in NiCad charger or vice versa. Tony
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Depends. Some chargers can handle both. Older ones probably not.
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Depends in a couple of different ways. Some modern chargers can charge both and have a switch to select which is in use. I haven't seen anything like that on a portable phone. There may be an auto-detect mechanism - my cell phone can use NiCd, NiMH or Li-Ion and figures it out itself.
Second way - if the charger is a _slow_ charger, either NiCd or NiMH can be charged. But slow chargers tend to be dumb chargers and the risk of overcharge is there. Also, I can't remember what guidelines quantify the difference between fast and slow - 100 mA? less? more?.
Mike
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Yes.
No. You can do it; you just have to monitor current. Of course, it is much easier and more practical to stick with separate chargers or to purchase one "smart" charger which you can quickly connect into any battery cradle. Heck, you can charge just about any cells or battery packs up to 12 volts with a toy train transformer & a capacitor if you know what you are doing.
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charging.
Nimh are preferred for applications such as cell phones because they have higher capacity. However, the NiCad have higher current capability and are preferred for applications such as portable tools.
NICad memory has been proved to be somewhat a myth. It exists only under very controlled conditions such as repeately discharging to a specific state of charge. And, it the situation is reversible by one heavy discharge and charge cycle. It is best to not discharge any battery any more than necessary. Normal use will result in discharge to different levels of charge. Many experts will tell you regardless of what the manufacturer says avoid total discharge. I agree. personal experience using NiCad batteries in an application where they lasted for more than 30 years with proper care, never being allowed to discharge below 40% state of charge and never over charged (that's the tough part to do for most users).
For extensive detail do a Google search for "NiCad Memory" Myth
A good one: http://vancouver-webpages.com/pub/peter/nicad.faq
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--


"HRL" < snipped-for-privacy@xxx.net> wrote in message
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On Sat, 09 Oct 2004 18:25:13 GMT, snipped-for-privacy@notmine.com wrote:

Probably more than you want to know here http://www.rcbatteryclinic.com /
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Many chargers will do both. NiMH hold more electricity.
From what I can remember, NiCd can be cycled more times (up to 1,000) and NiMH only go for 400 or so.
They have essentially the same operating voltage, 1.2 volts per cell.
Use the old charger. Eat chocolate. Smile at pretty women. Be happy.
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Christopher A. Young
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Stormin Mormon wrote:

I am:-))
Thanks.
BTW the Nimh batteries were the std Radio shack replacement for the batteries that came with the phone.
LB
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Ni-Cd batteries most definitely DO display a memory effect over time if maintained improperly-- it is no myth. Airlines and industrial facilities invest huge amounts of money in charging and maintenance equipment to prevent this very thing from happening. NiCD cells can get pretty expensive in larger sizes and proper maintenance equipment saves them $$$ in the long run. Proper maintentance and charging/discharging can virtually eliminate this phenomenon. Though for avg. consumer purposes, the biggest practical decrease in cell life is due to overcharging of the cells.
NiMh and NiCd batteries display similar performance characteristics (i.e. current delivery and total capacity) however, NiMh batteries generally can accept fewer total charging cycles (though this characteristic is improving over time with advances in manufacturing technology). Some of the advantages of NiMH are that they are lighter and more environmentally friendly (no cadmium). They will eventually replace NiCD in my opinion for general consumer use. Most likely they will eventually replace NiCD in industrial applications (starting jet engines, etc) when the technology catches up with NiCD and gets more refined.
Charging the two types of batteries is similar, but definitely not the same. The safest and cheapest way to charge BOTH types of batteries is at a current rating of 10% of total capacity, and a voltage of at least 1.4V (at 20 deg C) for no more than 15 hours, after which dropping the current to 5% of total capacity to maintain the charge without damaging the battery. Example: a 100mAh cell would need to be charged at 10mA for no more than 15 hours, then the charger (usually by a timer) would disengage the charging cycle or reduce the current to 5mA to maintain the charge so as to not generate appreciable temps on the cell.
You can't really damage a NiCD or NiMH battery by overcharging as long as you keep the current 10% of total capacity or lower. When you overcharge a battery, new chemical reactions occur to absorb the excess current, which in most cases generates heat that leads to the cell damage (in extreme cases venting of the battery-- basically vaporizing the internals of the cell). All modern batteries have an oxygen recycling catalyst integrated with them to handle overcharging, but it is usually only can keep up with rates of 10% total capacity or less.
The problem with leaving the charging current at 10% for longer than 15 hours or so is that the battery will heat up, and the heat will eventually decrease the total life of the cell.
Probably your phone charger will by default deliver 5% total capacity at 1.4V/cell or so. This should keep your cells in good working order (albeit charging them a little slow-- it will probably take 18-20+ hours to get a full charge from a completely discharged state.)
Fast charging NiCd and NiMH batteries is similar but they need different methods of detecting end of charge in order to prevent overcharging. A voltage rise followed by a drop (called minus delta V detection or -dV) can be reliably used to detect a NiCD end of charge (in most practical cases), while with NiMH the cell must be temperature monitored (dT or +dT detection) which requires a temperature sensor on each cell (usually a more expensive gadget in a charger). A good benchmark for determining end of NiMH charge for dT/dt is a temp increase of 1 to 2 deg C per minute.
If you try to charge a NiMH battery with a -dV detection circut only, you will most likely either undercharge or overcharge the battery. This is due to the fact that NiMH batteries' characteristics (voltage depressions, etc) are much more erratic and subject to wide variations depending on cell temperature. For this reason, when many people stick a NiMH battery in a NiCD charger, they normally either fry the battery or consistently undercharge the cell-- both of which will result in poor percieved performance.
You can charge a NiCd battery with a dT NiMH charger and get reliable charge termination in just about all cases, but you can't charge a NiMH battery reliably with most NiCd chargers (-dV). There are some high-end NiCD chargers that use dT detection and can be used for NiMH, but if you didn't pay at least $500 for the charger when you bought it, it most likely will not display this characteristic.
Hope this helps.
snipped-for-privacy@notmine.com wrote in message

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Now that's an answer.

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wrote:

Yes, that was a very good answer. Particularly since I just bought a few Nimh AA cells at the local PC fair so that I can use them in a light for my bike. I also bought a cheapo charger that claims to work for both Nicad and Nimh. Having charged the batteries for two days I just tried them out and they lasted one and one half commutes. That translates to 90 minutes. Not good.
But now I see that the charger is a POS and I should use a better charger since they're probably being undercharged. I do have a better one that has separate settings for Nimh and Nicad, as well as standard and high capacity. I'll try that one and see if they perform better.
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Having charged the batteries for two days I

Don't give up on the dumb charger and nimh's yet- it takes a couple of charge/discharge cycles to exercise a nimh to it's rated capacity . Pat
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