prasad said:
Hello there,
I understand that one of the common methods for termination of fast
charge is the delta V technique. Kindly clarify the following :
1. Is the voltage to be monitored while charging current is flowing or after
switching it off briefly?
2. If the latter, how soon ( or how much later ) should the voltage be
measured after the current is switched off?
This is a complex issue.
If you measure it with the current on, resistive effects in the path
come into play.
IF you turn the current off, the voltage starts to drop, so you must
measure at EXACTLY the same delay every time.
I once built a charger that used a 555 as the timing and voltage
detector. The voltage dropped for the entire off time, but since the
off time was constant, the thing worked great.
I tried to duplicate the operation using the PC parallel port for
control. This FAILED miserably because the sample time latency was not
constant. I leave the current on.
3. How often should the voltage be monitered if the charging rate is around
0.5C?
This is another tradeoff area. First consider how much current you
want. If the current is too low, the zero slope may not happen. Too
much and temperature effects cloud the issue. I've been using C as the
charging current with reasonable success.
I monitor the current every minute with the current left on for the
reason above. I terminate after five consecutive readings of the same
value Or at the first lower value.
Here are some of the reasons why.
Noise is an issue if you try to measure small changes.
My voltage measurement is just the voltage readback from a programmable
supply. It has only 10mV resolution. It really should be better, but
since I'm charging at least 4 cells most of the time, it works ok.
For series cells, the zero slope happens at different times for each
cell. In aggregate, it smears out.
The problem gets much worse for NiCd cause the pack may never go
negative. Most of my charging is for NiCd.
The 5 minute compromise seems to work for me.
4. What should be the optimum maximum voltage for nimh cells?
You're gonna find this varies all over the map depending on the age and
abuse of the cells. I use 1.6V/cell as an overvoltage termination, but
sometimes, even that is not high enough for OLD computer packs.
Don't forget to compensate for the resistance in the test setup.
My wire and circuit breaker are 0.3 ohm total. Charging one cell at
4A, the error term is almost as large as the cell voltage.
5. If zero delta is used, should the charging be stopped immediately on
finding that two consecutive readings are the same , or should we wait for a
few consecutive readings?
Depends on how far apart the readings, see above.
Don't forget to terminate on time. Once you've put 1.2X C into the
pack, you might as well quit. Don't forget to terminate on temperature.
If you start with a full pack or if something goes wrong, you want to
shut it down before it explodes or catches fire.
Cell life will be better if you're conservative.
mike
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