MPM
Member
Registered: Jun 2000
Location: Sunnyvale, CA
Posts: 216 |
 NiMH do NOT have a memory problem
quote:
Originally posted by scsiboy
Nickel Metal Hydride (NiMH) batteries also suffer from memory problems. I use L-ion whenever possible, including on my mobile phone, because it has no memory issues. [/B]
This incorrect: NiMH batteries do NOT have a "memory" problem.
What many people refer to as the "memory" problem in NiCd batteries is technically called the "voltage depression effect". What happens is that the voltage of the NiCd battery insead of following a very flat slope down, suddenly drops a bit - and then flattens out again, as the battery is dischared. It is this sudden drop that causes the low battery circuitry in many devices to think that the battery is just about dead - but it is really not.
This is caused by the crystaline structure of the cadmium in the battery. When the battery is recharged the cadmium is plated back out onto the cathode. This plating process results in a very fine grained structure to the cadimum. But over time these small crystals merge into much larger crystals. These larger crystals do not react as well as the fine grained ones do.
Now if a NiCd is repeatedly and only partly discharged to say, 75% of full, and then recharged, only some of the cadmium is being reacted and reformed into fine grained crystals. But durring these partial cycles, the cadmium deeper in the cathode plate has formed large grained crystals. So now, when the battery is discharged beyond that large/small grained crystal boundary, the voltage takes that sudden drop, and your camcorder/PDA/cellphone thinks "LOW BATTERY" and tells the user that.
But in my example, the battery would only be 25% discharged! This is what leads people to think that there is a "memory" effect in NiCd batteries. A simple full discharge and then full recharge will cure this. That's why it is recommended to do a full discharge/recharge cycle on NiCds every now and then.
NiMH batteries do not contain any cadmium so therefore do not have the voltage depression effect and therefore do not appear to the consumer to have the "memory" effect.
See http://www.repairfaq.org/ELE/F_NiCd_Battery.html for more info about NiCd batteries.
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10-30-2000 08:47 PM
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