Is there any problem with leaving the Ryobi 18v batteries in
the charger for days/weeks/months after the green light
comes on? In the past I have removed them soon after the
green light comes on, indicating fully charged; it later
appears that after days or weeks much of the charge has been
Former slayer of dragons; practice now limited to sacred
What I have noticed is my 2 ryobi chargers overcharge my pack going
by the temp method of charge, a nicad battery is fully charged just
when temp increases so I take out my pack when its done by the green
light, my sony and sanyo charger dont do this to ryobis extreme of
heating a pack, I dont know if it continues to charge or at what
voltage if it does, but a warm pack of near 80f is full to
overcharged, and I dont know how much of the heat is the charger
itself but what I see is it overcharges packs all the time. A 1.2v
nicad is discharged at 1.2v and fully charged at 1.33v or so, you
would need to check voltage and temp to figure out if ryobi is keeping
it overcharged or cycling, or nothing at all since its supposed to be
a smart charger. Overcharging cooks a battery into shorter life,
overdischarging is not good either. My ryobi packs dont hold a charge
either so I dont think they are top grade Sanyo or Panasonic cells,
but you dont pay top price either. Companies make big money selling
replacement bateries so I dont trust chargers to be designed for
longest battery life.
A nicad is approximately 80% charged when the temperature starts to
increase; this is also about when the voltage starts to decrease. In
contrast, NiMH don't start to heat until about 100% charge, at which
time their voltage flattens or decreases very, very slightly. And
it's normal or the temperature to rise 20F.
Voltage isn't accurate enough to indicate full charge, and 1.33V is
too low. Instead chargers use rate of voltage change (or even the
rate of rate of change), temperature, or rate of temperature change.
Charging to just 80% of capacity will make NiMH and nicad batteries
last longer, but I doubt that's done except for ultra-high reliability
equipment (space) or hybrid cars. Some chargers follow the full
charge with a lower current top-off charge that can add 10% more
NiCd batteries have an inherent self-discharge rate,something like 5% per
Some chargers may slowly discharge a pack if left in.
I suspect the cheaper brands may be that way.
Perhaps the Ryobi website has a FAQ?
How long does it take for the self-discharge to discharge them when
you are unhappy. After a couple of weeks, they will lose enuf that it
will be noticeable if they are NiCAds. You should give them a little
jolt every couple of weeks if you want them to not disappoint when you
really need them if they are NiCads. Nickel metal hydrides are not as
bad, LI cells are the best.
I don't know how long it takes. I only make use of these
cordless tools on a random basis. When something breaks
that requires drilling or cutting I put one of the batteries
into the tool and go to work. I can only get about 15
minutes of sawing or drilling before there is a noticeable
weakness. Then I put the battery in use on the charger, use
the spare until it gives out, and sometimes I've finished
the project by then; if not, I take a break until the green
light comes on showing fully charged. Then I can saw and
drill much longer - I don't know because I've never reached
that maximum usage level with a freshly charged battery.
Thanks for your comments.
with your type of intermittent usage,you need tools that use the Lithium
ion batteries. They retain something like 80% charge after 6 months.
I would also select tools that come with a 1 hour charger.
My old Makita 6095 uses 9.6V sticks,and they charge in a hour,so I recharge
them before starting a job.Makita sells some cordless tools with lithium
packs,but they are not inexpensive.Black& decker sells some lower cost Li-
ISTR that Ryobi chargers take 3-5 hrs or even overnight.
Maybe they sell Li-ion packs for your present tool,but you'd likely need a
It shouldn't hurt, provided it's not done it regularly. Most
batteries fail when a cell overcharges and grows a crystalline whisker
long enough to short adjacent plates. Some people apply high current
at 12-100V DC to that cell alone to vaporize the whisker, but it
usually grows back in a day or week. Chargers are designed to sense
full charge and either shut down completely or go into maintenance
mode where slow low current pulses are sent out. The pulsation is
intended to cause whiskers to break up before they grow too long, but
it doesn't seem entirely successful.
Sometimes a cell doesn't short but merely goes into reverse polarity,
and that can be cured by applying low current directly to that cell
for a minute and then immediately charging the whole battery
normally. This can be done with an ordinary 1.5V alkaline cell
connected to the affected nicad through a 10 ohm resistor (+ terminal
to + terminal; tie negative terminals directly together)
It's normal for batteries to warm up when they reach full charge.
Nicads start warming at approximately 80% full charge, NiMHs at about
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