Wall Wart / Battery Charger Question

i have misplaced the charger for my dog training colar and transmitter. i opened the transmitter and there it says 7.2V nmh on the battery pack. what specs on a wall wart would work as the replacement charger ? i found a 7.8 volt one on evilbay for less than $12, where the factory supplier sez upward of $40 for the direct replacement which is quite a bit higher priced than i expected to pay.
is this similar to talking about car chargers where a 12 volt battery needs 13+ volts to charge ? ?

yes sir, you will need a slightly higher voltage than battery voltage to charge the batteries.
7.8v will charge them, you may wish to see if there is a amp or micro amp rating on the charger you wish to purchase.
the new charger may charge just fine but it's charge rate may differ from the OEM equipment.
that's not necessarily a bad thing, it may just take a few hours more or less to charge your battery pack than the the OEM dependent on capacity of the new equipment

The fully charge voltage for Nimh and nicd cells is 1.45v / cell. You have a 6 cell pack so you will need about 9 volts minimum. However it does depend on how your unit is set up to charge. If you are charging it from an unregulated supply you should limit the charging current to around 30 ma. for a 10 to 12 hour charge.

David

Some electronic devices have internal charge management systems. My Remington shaver has two AA size NiMH batteries with a 600 mah capacity. The wall wart charger has a rating of 12 v. dc @ 650 ma. It can recharge in less than an hour. The 4 cell NiMH charger for my Olympus digital camera has an output rating of 1.2 volts @ 490ma. It takes around 10 -12 hours to charge the 1450 mah batteries.

Transformer type wall warts are notoriously bad in stating the output voltage. Some give the open circuit voltage and some give the voltage under load. Many times, the voltage is purposely set higher to account for voltage drops due to circuit resistance. Switching type wall warts are usually more stable. They can oftentimes be identified by their smaller size and lower weight and possibly the acceptance of a wide range of input voltages (e.g., 90 -250 volt a.c.).

NiMH batteries can accept a high charge current and correspondingly short charge time but high current chargers usually have some sort of temperature management to prevent overheating.

Bob

Also continuing on from Bob's post. Some of these adapters are also designed to RUN the product while it is charging, hence have a much higher current rating than required for just battery charging.

Unless the product boasts quick charging most adapters will be fairly low current devices.

David

thanks guys, for the info. even tho i am still electronically challenged, i believe i understood what y'all are saying. .

Check with Batteries and Bulbs Plus, they have all kinds of DC stuff

Any 12 volt charger will work as long as you limit the current. I use a volt-ohm meter
on 500 milliamps scale and add a series resistance of some kind. A small 12 volt
bulb will work or a wire rheostat is even better as you can control the current. Fifty
to 100 milliamps would be a good current to start with as it would not overheat the batteries
and would drop off somewhat over time.