simple battery charger Q&A

We have a discussion going on over in the Garmin newsgroup concerning USB style chargers.... like for phones, iPods, GPS, etc

Someone mentioned that a charger/supply with too much current could fry the device..... most of us jumped on that and said the device would only draw what it required. BUT - what about stand alone battery chargers that limit current to 10% of the MAH ?

I thought I had the concepts down, but then decided I was confused between two topics:

1 - charging circuit for standalone batteries 2 - USB 5v power supply for AC or car adapter

Also - if the internal rechargeable batteries in all these units are just connected to the outside world in "parallel" like jump starting a car battery... then what does the "load" actually look like to the charger (ohm's law) ?

And back to my confusion - does a "simple" stand alone battery charger work different than a power supply in this case ? ie - jumping a car battery with another seems to work fine with no "current limiting" ?

Here is the thread I copied from the Garmin newsgroup. Thanks for any and all clarifications.

-------------------

OK..... sitting here thinking about the issue as it pertains to charging batteries as commomly discussed here on the flashlight forum -

formatting link

The OP might be totally correct about the issue of amps/current, because there are TWO totally different scenarios at work here.

- Charging a battery and needing some "current limiting" capability

- Powering a device that acts as a normal load to the charger/supply

The standalone battery, like a AA or car battery, has a pretty low internal resistance itself. In fact, that's a difference between charging and using AA's that are nicd, nimh, or using alkaline vs lithium in a flashlight. A flashlight using a AA battery can draw as much current as it can and the internal resistance of the battery is the limiting factor.

So - the charging question also has to look at the battery as the "load", and with the internal resistance very small, it almost looks like a dead short.

Therefore, the charger is normally responsible for any "current limiting". The normal charging rates (ma) are usually described as say 10% of "battery capacity", with anything higher being "faster" but also causing the battery to heat up... Too much current or "C" and the battery gets very hot and could be cooked & destroyed. If the "charger" is more of a "supply" then the issue of current limiting becomes very important, and in fact - could FRY the battery if the current is not limited somewhere or somehow.

We don't know if there is something inside the GPS to limit the charging current, or it totally depends on the external USB charger ->>> THAT is the issue -

There MUST be some form of current limiting.... else disaster could indeed happen.