[NSRCA-discussion] NiMH Question

[Ed Alt]

Mike:
Unless the batteries were put away in a fairly low state of charge or unless they were connected to some accessory that slowly drained them, they should not have self discharged so much as to damage themselves just over winter.  Were they connected to any type of voltage regulator / failsafe switch setup for example?  Some, not all, exhibit enough OFF leakage current to completely drain a pack that size within a couple of months.  Others could drain it a fair amount over several months.  Depending on brand/model of a failsafe type regulator, you could see anywhere from 30 or so micro amps (uA) to 7.5mA flow between battery and regulator (NOT the receiver side) when the regulator is off.  Something like a Jaccio Smartswitch performs pretty well over extended OFF times, but multi-months could result in a noticeable drain, combined with the packs own self discharge.  I would say that if you had the batteries disconnected and left fully charged, they should have been OK and if not, they are suspect.  Odd coincidence they would both drain so completely though.

Before tossing them, it might be a good idea to see if something in the charging circuit is to blame.  Assuming that the cutoff levels are set right, a poor connection between the battery and the voltage sensing circuitry in the charger could "trick" it into an early cutoff during the discharge portion of the cycling. There is some resistance in every wire and connection point and whenever current is flowing in either direction (charge/discharge), there is some voltage drop across that path.  Ideally, there would be no drop for the most accurate sensing of the actual battery voltage, but for example, assume some dirty/worn connector contacts and some long charger leads and you wind up with a combined resistance of 1 ohm.  At the discharge rate of 650 mA, the voltage drop along that path is 0.65V, so assuming a 4 cell pack, you "lost" about 0.163V per cell of cutoff voltage, i.e., the discharge would have ended early.  The higher the discharge rate, the worse this gets - at 1A, the 1 ohm assumed lead/connector resistance would drop 1V, hence .25V / cell in a 4 cell pack.  The fact that it seems to have improved some when you lowered the cutoff voltage from 1.1 to 1.0 / cell at the same time that you increased the discharge rate could make sense.  For my example of 1 ohm total R in the leads,  0.25V / cell - 0.163V / cell yields 0.087V.  Subtract that from the 0.1V delta in the cutoff voltage you set and you effectively had the discharger cycle seeing 0.013 voltage lower per cell, on average.  

You should change only one factor at a time to get a better picture of what's going on. If you keep the discharge rate at .65A and try it again at 1.0V / cell cutoff, you should expect even better than you saw at the 1A discharge rate.  BTW, other factors vary - higher discharge rates also affect the real output of the pack. Also don't forget that there's also internal cell resistance that adds into this, just as the wire/connector resistance does. Some NiMh cells have a fairly high internal resistance and won't perform well on higher discharge rate capacity checks.  Net-net:  The closer you run the disharge cycle to the average current use of the pack in flight, the more accurate your results.  I wouldn't go any higher than 250 mA if you want to closely approximate what happens on a per flight basis in a typical pattern bird.  The shorter the charger leads and the better the contact is in the connectors, the more accurate the results as well.  BTW, IF you do have the high resistance problem in the charger leads and IF you are also charging at a fairly high rate, you are also losing something in perceived capacity.  Early cutoff can happen here as well.

I hope this helps.
Ed
  ----- Original Message ----- 
  From: Michael Cohen 
  To: NSRCA Mailing List 
  Sent: Thursday, February 23, 2006 9:48 PM
  Subject: [NSRCA-discussion] NiMH Question


  I am cycling my batteries and getting ready for the flying season.  On my initial 5 cycle run at my NiMH batteries, both my 1650 mAh packs cycled very low.  One pack that I have not used since end of November only cycled up to 850 mAh and the other pack I have not used since end of September only cycled up to 1000 mAh.  My charger is set to 5 cycles, charge and discharge are set to .65 amps with a cutoff voltage at 1.1 Volts per cell.  I am now trying 1.00 amp charge/discharge with a 1.0 volt per cell cutoff and it is looking better, but I have some questions:

  1)  Did I damage the batteries by letting them self-discharge too low?  I know NiMH batteries have a higher self discharge rate than NiCD batteries, but I am not aware letting the NiMH batteries run down could cause damage.

  2)  Is this a normal behavior?  Maybe NiMH just need a little more juice at the beginning of the season.

  3)  Is the cold garage a factor?

  Thoughts?

  Mike Cohen


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