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Hi tiktak,
when you have to discharge NiMh cells and need to know what's e.g. residual charge
or for how many % have you discharged them you can look at discharge curves.
There you see something xC means x-times capacity discharging current (constant)...
then can find out voltage for a particular level.
Now come back to the problem: you need 80% discharge and in few (20-30) minutes.
Suppose you use a 10ohm resistor 30W (big and with metallic heatsink)... you get
about for 10cells 15v, then:
15/10 = 1.5Ampere
power dissipated is about 22W, so resistor become hot... and maybe you need additional heat
sink.
You can use also e.g. 2x 20ohm 20W each resistors in parallel, same thing, runs cooler.
Or also an old car lamp at 12V... find one that need 1.2 Amper (15w): 10ohm and lot of watts
OK now you know which current you have discarging cells:
with 10ohm resistor -> 1.5 A
with lamp -> 1.5 A the same
OK , now 1.5A/C = 1.5/0.700 = 2.14
You're discharging at 2.14C
Now look at the picture you'll have each cell deliver something like 1.08V at 80% discharge
with 2C rate, so you have to espect a bit less voltage, we don't need exact values, say it's
1.06V for cell.
Of course, discharge current isn't constant... but is a good estimate.
OK now you know what to measure, during discharge to know what's going on and if cells are at
80% discharge : look at voltage for the pack 10, 10*1.06 = 10.6V
When voltage of the pack drops under 10.6v with load attached you actually have discharged more
than 80%, so then you can start charging them.
Easy.
So you have just to put the resistor(s) or lamp I've indicated directly on the pack output, and watch at measured voltage accross
load using multimeter. That's it.
If use lamp you have also a visual indication of discharge by intensity of light.
Kind regards,
Max
when you have to discharge NiMh cells and need to know what's e.g. residual charge
or for how many % have you discharged them you can look at discharge curves.
There you see something xC means x-times capacity discharging current (constant)...
then can find out voltage for a particular level.
Now come back to the problem: you need 80% discharge and in few (20-30) minutes.
Suppose you use a 10ohm resistor 30W (big and with metallic heatsink)... you get
about for 10cells 15v, then:
15/10 = 1.5Ampere
power dissipated is about 22W, so resistor become hot... and maybe you need additional heat
sink.
You can use also e.g. 2x 20ohm 20W each resistors in parallel, same thing, runs cooler.
Or also an old car lamp at 12V... find one that need 1.2 Amper (15w): 10ohm and lot of watts
OK now you know which current you have discarging cells:
with 10ohm resistor -> 1.5 A
with lamp -> 1.5 A the same
OK , now 1.5A/C = 1.5/0.700 = 2.14
You're discharging at 2.14C
Now look at the picture you'll have each cell deliver something like 1.08V at 80% discharge
with 2C rate, so you have to espect a bit less voltage, we don't need exact values, say it's
1.06V for cell.
Of course, discharge current isn't constant... but is a good estimate.
OK now you know what to measure, during discharge to know what's going on and if cells are at
80% discharge : look at voltage for the pack 10, 10*1.06 = 10.6V
When voltage of the pack drops under 10.6v with load attached you actually have discharged more
than 80%, so then you can start charging them.
Easy.
So you have just to put the resistor(s) or lamp I've indicated directly on the pack output, and watch at measured voltage accross
load using multimeter. That's it.
If use lamp you have also a visual indication of discharge by intensity of light.
Kind regards,
Max
it sound (target or battery checker at switchon) and so your current battery last is around 15-16hours with fully charged 700mAh batteries.
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