Here's are some battery/battery charging questions. The scene: A Hobbico MKII charger, a 12v (actually 12.2, to be exact) power supply, a 500mAH 7 cell "8.4v" nicad pack and a 950mAH 3 cell "11.1v" Li-Ion pack. Here are the questions:
Why is it a "8.4v" nicad pack when 9.5v or more (no load) is what it measures when close to fully charged?
Along the same lines, 11.1v for a Li-Ion pack is close to discharged (from what I've seen in these pages)? Isn't 12.6v (no load) what I should expect from 3 fully charged Li-Ion cells?
When I charged my "8.4v" pack (at 500mAH) I was monitoring it with my DVM and it was up to 11v when I shut it down manually. Is this normal? What kind of terminal voltage should I expect at peak charge?
Assuming 12.6v is fully charged for a 3 cell Li-Ion, will I be able to fully charge this pack with my charger on a 12v power supply? I can only get about 12v (no load) after charging (and shutting it off manually). Again, what kind of terminal voltage should I expect just before it shuts off (hopefully by itself someday).
NiCd cells have a nominal voltage of 1.2 volts/cell, meaning the nominal voltage for a 7-cell pack is 8.4. I couldn't find the charge cutoff voltage for the Hobbico MKII, but the Hobbico Accu-Cycle Plus uses a charge cutoff voltage of 1.74, which would yield a charge cutoff voltage of 12.18 volts for a 7-cell pack (7 x 1.74 = 12.18 unless my calculator is busted)..
A lower volts/cell cutoff figure would lower the actual cutoff voltage. Presuming your figures were taken at peak, it would appear the number for your charger is 1.35, unless the pack wasn't fully charged. 1.35 does seem sorta low.
See previous answer.
Dunno from Li-Ion packs, haven't used 'em, but . . . if you want to charge a battery you need a higher supply voltage than the nominal battery voltage. A Li-Ion (or NiCd or NiMh) pack of "X" voltage requires "X+Y" source voltage to charge, where X is the nominal pack voltage and Y is the percentage increase necessary to achieve the desired charge rate. Haven't the foggiest notion of the value of Y for Li-Ion/Li-Poly or NiMh.
Some chargers have a "source voltage boost" circuit to allow charging of near-12-volt packs from a 12v source. One way to identify such chargers is the number of cells they'll charge.
The Hobbico Quick Field Charger Mk II can only handle 8 cells, and it has no 12v boost circuitry.
The Radio South Pro Charger Mk II (and the Quad Pro Charger) have the capacity to charge up to 10 cells, and there is a 12v boost circuit. See
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One of the variables concerning chargers is the output regulation. NiCd batteries are normally charged with a constant-current regulator, meaning that the voltage varies over the duration of the charge (it climbs almost constantly) to keep the current at a constant value.
Other types of batteries, e.g. lead-acid, use a constant _voltage_ regulator, meaning that the voltage is constant and the current varies over the duration of the charge (it goes down as the battery gets fat).
As I said, I dunno from Li-Ion or Li-Poly, but the charger characteristics are dependent on the battery chemistry, which is why you shouldn't mix charger types and battery types unless the charger is capable of properly charging the specific chemistry of the battery at hand.
As for available voltage from a pack which has "rested" after charging, that too is entirely dependent on the battery chemistry.
NiCd packs will drop fairly quickly to near-nominal pack voltage, then remain at nominal voltage for the duration of their capacity, then drop again when discharged; the change in discharge at exhaustion is called a "knee", and that's sort of what it looks like on a graph.
Other types have a different sort of discharge curve, which in turn is why you need to be aware of the discharge characteristics of the particular battery chemistry.
For example, if NiCds are considered the standard for discharge curves, then NiMh are considered "rapid" in that once they reach exhaustion the available current drops much more quickly than NiCd types; the available current drops rather dramatically. NiMh batteries got a bad rap early on because many modelers treated NiMh packs like NiCd packs, and got bit because of the rapid voltage drop exhibited by NiMh chemistry compared to NiCd chemistry at exhaustion. A lot of folks thought they had enough remaining capacity for "just one more flight", and that proved not to be the case.
Also, NiMh pack exhibit a steady voltage decline whereas NiCd packs tend to maintain the voltage at a much more constant value over the discharge period, which further confused the unwary user.
Clear, huh ?
I'm sure His Redness will step in any time now and straighten out the mess I've made . . .
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