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Reducing usb amps for charging small lithium batteries.

Metaltrades01

Dec 17, 2020
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Hi there.
I have a basic understanding of electronics, and have made up 18650 battery packs etc.

I would like to charge a samsung s5 and bicycle lights, etc (which are powered by internal 300 to 500 mAh lithium ion batteries), from a 12v gsm lead acid battery.

The obvious choice (i would think, And trying to keep all this as simple as possible... ) is a 12v car adaptor with multiple (4) outlets from eBay etc.

My issue is that the adaptor output is around 2.4 amps for each outlet, which would fry/shorten the life of a 300 mAh lithium ion battery. (And cook the 100 mAh battery in my head phones).

I have searched the net/banggood etc for a module (usb to usb 1amp out), without success.

Maybe there is a better way...

Any advice or sugestions are appreciated. Thanks.

And the secondary question is does it need a fuse/s.

Cheers.
 

Harald Kapp

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Nov 17, 2011
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My issue is that the adaptor output is around 2.4 amps for each outlet
A common misunderstanding on your side: The output of the adaptor is capable of up to 2.4 A. This doesn't mean it will forcibly drive 2.4 A into any load you attach. The load (smartphone, lights etc.) that you attach to the 5 V output of the adaptor needs to have an internal charge controller anyway to be able to charge the internal battery (~3.6 V if LiIon) from a 5 V source. This controller will take care of correct current regulation.
And the secondary question is does it need a fuse/s.
Always a good idea. I suggest you use a fuse on the 12 V side. Fuses on the 5 V side are possible but the currents will vary a lot so finding a suitable fuse for all loads will be a challenge.
 

Metaltrades01

Dec 17, 2020
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Oh of coarse. Thanks for that.
I feel like it was a dumb question, but it was driven by...I have had my Samsung S5 on a 10W charger and the Phone battery got pretty warm.
 

Audioguru

Sep 24, 2016
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To avoid an explosion and fire, a Li-Ion battery cell must be charged from a proper charger circuit.
Cell phones have a proper charger circuit inside. 5V from USB or an adapter feeds the charger circuit, not the battery cell.
 

Metaltrades01

Dec 17, 2020
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Dec 17, 2020
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Thanks Audioguru, yes that is right i understand. My original question was wrong, but I thinking... 2+ amps into a 0.1amp battery charging circuit in a pair of earbuds was going to cause issues.
 

Audioguru

Sep 24, 2016
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Local hobby stores sell radio controlled drones, airplanes, cars and boats that all use 100mAh lithium batteries and sell proper chargers for them. Banggood, ebay, Ali and Amazon sell cheap Chinese copies that might not be safe to use.
 
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