P
ps56k
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 -
http://www.candlepowerforums.com/vb/forumdisplay.php?f=9
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.
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 -
http://www.candlepowerforums.com/vb/forumdisplay.php?f=9
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.
