| > same charcteristic that my 1200 Mah pack does.On the 1200 when the
> electronic timer is set for 20 min @3.6A near the 20 minute mark
> the charge current begind to drop,at this point I am assuming the
> pack is completely charged on any more time would damage the pack
> The instructions on the charger said a 1700 pack requires 2 15 minute
> charges at 3.6A,however at the end of this the 1700 pack did not
> drop the charge current and I thought it was not fully charged.
> On the next next chatge I set it at 4A for 2x15min and the pack
> got really hot,I quickly stuck it in the freezer to cool it down.
Hey, lets look at this calmly.
A battery is just like a fuel tank. It has a rated capacity, which
you can consider to be how much gas it holds. When full (charged
completely), a 1200 MaH battery can supply 1.200 Amps for 1
Hour before it is discharged. If you draw 2.4 Amps, it lasts 1/2
hour. My model planes typically draw about 15 Amps when running,
this means that the battery lasts 1.2/15 hours, or about .08
hours, or 4.8 minutes.
When you try and recharge (or fill) a discharged (or empty)
battery, you must put back into it what you have removed. If the
battery is fully discharged, and you supplied it with 1.2 Amps of
charging current, it would be recharged in 1 Hour. The cells we
use in our planes and cars are Fast Charge type cells. This means
that they can withstand the heat generated as a result of higher
charging currents without damage, as well as some amount of
over-charge. This lets us charge a 1.2 AmpHour battery in 1/3 of
an hour by supplying it with 3.6 Amps (1.2 * 3), whereas normal charge
rate cells, like the ones in your radio control, charge at 1/10 or
less of their rated capacity. Your charger says use 3.6 Amps for
20 minutes on a 1.2 AH pack. You supply the pack 3.6 Amps for 1/3
of an hour, or 1.2 AHrs, and end up with a fully charged battery.
If you apply the same line of reasoning to a 1700 MaH pack, you
will see that for the same full charge in 20 minutes, a 1700Mah
cell needs 5.100 Amps (thats 3 * 1700). If your charger can't
supply 5.1 Amps, you could drop to 3.4 Amps for 30 minutes for a
full charge. 3.6 Amps for 30 Minutes actually gave your pack a
1800 MaH charge. Fortunately, most of the Sanyo cells can take a
charge as much as 10 or 15 percent above their rated capacity with
ease. I typically charge 1200 MaH packs for 25 minutes at 3.5
amps, which is a 1400 Mah charge, with no problems.
BUT ! 4 amps for 30 minutes is a 2000 MaH charge, this is almost
20 percent over the rated capacity of the cell, and if it didn't
damage the cell, it came awful close to doing so. As to whether
you did damaged them or not, you can only see by discharging them
at a controlled rate and timing how long they last (if you
discharge them a 1.7 Amps, they should deliver close to 1.7 Amps
for 1 hour). Alternatively, you could remove the heatshrink on
the pack and see if any of the cells "vented", in which case you
might see some white crystalline deposits around the positive
terminals of the cells.
Sanyo Batteries are excellent, though, and in all likelyhood you
did them little or no harm. In the future, when you are unsure
about a packs charging characteristics, it is a good idea to
babysit the pack when you charge it, touching it every minute or
so, towards the last 10 minutes of charge, to see if it is getting
real hot (of course, the best solution is to buy a Peak detecting
charger, or thermal cutoff charger, which will monitor the pack
for you automatically).
Randy.
|
| > Randy,would you know why the charge current begins to drop near
> the end of chatge on the 1200 pack?
As the pack reaches full charge, its internal resistance to the
charge volatge and current goes up quite quickly, both the current
and the voltage across the pack starts to drop. This higher
internal resistance is why the pack also starts to heat. The
current drops off because the cell just can't draw as much
anymore, kind of like blowing up a balloon. When the balloon is
empty, you can blow quite hard and fast (with a lot of AMPS !)
relatively easily. As the balloon gets to the limits of the
rubber's ability to expand, it starts to get REAL hard to blow
into it anymore. If you keep blowing, though, you might manage to
squeeze a bit more air into the sucker, but like-as-not, your more
likely to have it blow up in your face.
Delta Peak chargers monitor the rate of change of the voltage
across the pack, and turn off the charge when they detect the
slope of this "curve" going from positive to negative.
Thermal cutoff chargers detect the same "fully charged" condition
by monitoring the pack temperature, stopping the charge when they
see the rate of change of temperature climb quickly.
Randy.
RE the question about fuses, use a 20 Amp.
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