Is there any harm in leaving rechargeable batteries plugged in over long periods of time? Like remote control toys?

Newer Lithium Ion charging systems tend to charge the battery to maximum charge, and then turn off.  My two-way radio has this charging system.  unfortunately the drawback to this for me is that I cannot use the charging base to power the radio.  The first few nights I had it, I'd drop it into the charging stand in the evening, and in the morning, it would wake me up with the "low battery" chirp, and I would have to lift it out and drop it back in to restart the charge cycle.

 

Lead Acid battery chargers (gar, golf cart batteries) come in three varieties.  the big ones the mechanics and home hobbyists use are a combination bulk/float charger.  they charge the battery rapidly for the first 75-90% of its capacity, and then drop to a slower rate to "top off" the battery.  The second is a "trickle" charger, which charges at the slower rate from the start.  these are usually much cheaper than the first model, but they are slow.  The third style is commonly called a "maintenance charger"  This one may be one of the two previous styles, but it adds a control system to prevent accidental overcharging, and often a secondary system that sends a high voltage pulse through the battery which cleans deposits off of the lead electrodes. (these build up during normal use, and reduce the battery-s ability to deliver power)  This is the only one of the three types it is advisable to leave connected to a lead-acid battery for extended periods of time.  if the other two types are left connected, there is a possibility the battery will be "overcharged" which results in heat build-up, and the electrolyte solution can evaporate out of the cells, which, trust me, is very bad for the battery.

 

Nickel Cadmium, and Metal Hydride batteries sometimes come with continuous chargers, like the trickle chargers for lead acid batteries, and sometimes with fully automatic chargers, like newer Li-Ion systems.  the easiest way to make an educated guess is if there is a light that turns on, or off, or changes color when the charge is complete.  These chargers are usually safe to leave a battery in when it is not in use. 

 

 

OVERALL RULE: to cut through the explanation in simple terms: when you charge a battery, it produces heat.  If you leave a battery on a charger for more than 12 hours, and then feel it, it should not feel warm to the touch.  If it does, the charger is still trying to add charge to the battery.  overcharging ANY battery shortens its life span.

There are several types of rechargeable batteries.  RC toys typically use Nickel Metal Hydride batteries (NiMHs). The chargers for those batteries are "smart chargers," meaning that there is a circuit to monitor the charging current. This prevents the batteries from being overcharged, so they can be left in the charger indefinitely.

Likewise, Lithium Ion batteries have smart-charging systems, but they are built into the batteries themselves. They can be left on charge indefinitely.  Laptop computers and cell phones run on LiIon batteries.  I suspect that some toys do, but I haven't bought any like that for my daughter yet.

Nickel-Cadmium batteries, or NiCds, need to be completely discharged before recharging.  An article that will give you more information than you could ever want to know is here:

http://www.rcbatteryclinic.com/seminar.htm

The problem with NiCds is called the memory effect, a tendency for a battery to supply electric current for only so long as it has done between recharges. Thus if you have a NiCd battery that could run your toy for four hours, and you routinely recharge it after running the toy for only two hours, it will start to run the toy for only two hours before depleting itself and wanting to be recharged.

Look at the toy to see if it runs on NiCds or MiMHs.  If it's NiMHs, you have no worries. If it's running on NiCds, let them discharge completely before recharging, and remove them from the charger after they are fully charged.

No, there's no harm in leaving rechargable batteries plugged in.  If the batteries are in their chargers, the charger automatically turns itself off after the batteries have reached their full capacity.

If they're left in devices for an extended period without use, they're not dangerous either.  The worst that might happen (and I've seen this especially in electronics that people use infrequently) is that the batteries will lose their charge.  A simple stint in their charger should fix that right away though.

Most modern rechargers have a feedback circuit that can detect when charging is complete and stop charging. Batteries can be overcharged and damaged by incorrect or damaged chargers. If the charger is modern and built for the specific batteries being used, then there should be no harm.

 

More information at the link below. I hope this helps.