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Old 03-14-09 | 04:23 PM
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CaptCarrot
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Joined: Feb 2009
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From: Dorset, SW England, United Kingdom

Bikes: Heavily modded Cannondale Hooligan 1 (2009) and an upgraded Raleigh Max Zero-G

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.
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