B&B Electronics Parallel Printer Card PIOC Manuale Utente Scaricare il pdf (Pagina 18)

For smaller capacity cells, it’s probably a good idea to stick to 1C charge rates, but as the

cell capacity gets larger, the cell can handle higher rates.

For another, there is some evidence to indicate that a higher charge rate will result in

better performance on the track. Racers routinely charge these cells at 5A (5000mA) and

better. One racer likes to charge his high capacity NimH cells at 10A (better than a 3C

charge rate) because he claims that it improves the performance!

Peak Detect

Notice the little voltage “hump” at the 1C point in the graph above, followed by the drop

in voltage. This is called “peaking” and this is the method used by many chargers,

including the T35-GFX, to determine when the charge is finished. When you set the peak

detect voltage in the T35-GFX, you are telling the T35-GFX to terminate the charge

when the voltage drops back from the peak, or maximum voltage seen during the charge

process, by the peak detect amount. If you use the T35-GFX graphing feature to look at

the charge curve, you will be able to see this hump for yourself.

The actual peak rate of change and the magnitude of peak negative dropback voltage are

highly dependent on the charge current. At charge rates of, say, 0.5C and below, the peak

negative dropback voltage may be too small to serve as a reliable indicator of charge

completion.

A typical value for peak detect for a NimH pack is 0.007 to 0.010 V/cell, or 0.04 to 0.06

volts for a 6 cell pack. The value for a NiCd cell will be somewhat higher, typically 0.008

to 0.013 V/cell, as the NiCd cell exhibits a larger peak negative dropback voltage than a

NimH. CEI recommends using the lowest value required to reach a temp of 130° to 140°

False Peaking

As you can see from the graph, the voltage at the start of the charge cycle will jump up

some amount as current is applied. Then, the voltage will “level out” for a while, or

slowly rise. It is at this point that NimH (and NiCd) packs often exhibit a characteristic

called “false peaking”. The pack voltage will actually drop, even though the pack is not

yet near full charge. This effect can be magnified if “dead shorting” or deep discharging

the battery. If you are not aware of this, you may think your packs are fully charged, but

in reality, they are only partially charged. If your pack appears to have peaked too soon, a

quick test is to feel the pack to see if it is warm. If it is not good and warm, that means

that the pack false-peaked and is not fully charged. If the T35-GFX indicates something

less than the 1C mAHr rating has been delivered to the pack, it is almost a certainty that

the pack has false peaked.

False peaking can be overcome in the T35-GFX by using the “Long Lockout” feature to

cause the T35-GFX to ignore voltage drop across the pack at the beginning of the charge

cycle. It will automatically re-enable its peak detect monitoring after the time for false

peak rejection has expired. This has proven to be quite reliable as a method to avoid false

peaking. It eliminates the hassle of having to restart the charge cycle because the pack

false peaked.

Cell Heating

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B&B Electronics Parallel Printer Card PIOC Manuale Utente Pagina 18