# reducing charger current

Discussion in 'General Electronics Discussion' started by stickman, Apr 20, 2012.

1. ### stickman

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Apr 20, 2012
I have a little AC/DC LiPo charger that (according to the sticker) outputs 4.6V 800mA. I'd like to try reducing that current to ~ 250-300mA. Would that just be a resistor on an output lead? If so, what size?
Thanks SO much for help.

2. ### CocaCola

3,635
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Apr 7, 2012
Most LiPo chargers are 'smart' meaning the get feedback from the battery as they charge, so trying to hack them without also hacking the feedback 'brains' IMO is not a good route to take...

3. ### Harald KappModeratorModerator

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Nov 17, 2011
I don' t know how the charger will react if a part of the charging current is detracted from the battery.
In principle you would connect a resistor in parallel to the battery. Not in series.
The resistor is calculated as R=V/I where V= voltage of the LiPo, I=current not used for charging.

V= 4.6 V
I=880 mA - 300 mA = 500 mA (this way 300 mA are available for the battery, 500 mA are waste heat in the resistor.
-> R=9.2 Ohm
Beware of the power requirements: The resistor will dissipate P=V*I = 2.3 W. You need a power resistor.

And don't forget the LiPo balancer.

Harald

4. ### stickman

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Apr 20, 2012

In this case, I don't think this charger is any 'smarter' than a cell phone charger. It actually looks like a cell phone charger. The batteries I'm charging with it have something referred to as a 'protection circuit' which, I believe, replaces the need for a brain on the charger (?)
I dunno. I was just hoping to reduce the current to lengthen charge times to prolong the batteries' lives.
Thanks again, you guys.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Depending on how smart the charger is, with a resistor in parallel it may reduce the charge current to a value lower than that through the parallel resistor.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No, it means that if things go *really* wrong, you're unlikely to have a fire.

And that's "unlikely", not "won't"

If the protection ever kicks in, you should have stopped charging/discharging quite some time ago.

7. ### stickman

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Apr 20, 2012
So, being that I'm far less smart than whatever the smartness of the charger, maybe I should just leave well enough alone and find a better charger.
Yet another chapter in my 'Foilbles of a Cheapskate'.
Thanks a lot for the help, all!

8. ### Harald KappModeratorModerator

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Nov 17, 2011
If the battery is "smart", then probably the charge current is regulated by the battery's smart circuit, not the power supply. In that case the power supply's label may just be the rating, meaning max. current=800mA. This doesn't mean the battery draws 800 mA during charge. Have you measured the actual charge current?
In any way I wouldn't mess with the "smart" circuit within the battery (unless you are very shure what you do) because charging a LiPo in the wron way may result in explosive destruction.

Harald

9. ### stickman

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Apr 20, 2012
Oh, heck no! This was a naive assumption that I might be able to drop in a resistor on the charger. I'm not about to monkey around with the battery.
Very good safety advice. Thanks.

10. ### stickman

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Apr 20, 2012
And that's what I thought that circuitry was partially for on the battery- the battery will charge as fast as it's capable of, as long as the charger was up to it. I don't have an accurate way to measure. I thought I could limit the charger output for a slow charge. At this point, I'm scrapping the idea in favor of safety. I wouldn't know what I was looking at if I opened the charger (to assess it's 'smartness') and am now doubting the capabilities of the battery circuit.

Last edited: Apr 20, 2012
11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
On the other hand, 800mA may be a perfectly acceptable charge current.

What is the rated capacity of the battery. If it's lower than 800mAh then I'd be concerned. However charging at 1C is often within spec for LiPo cells.

12. ### stickman

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Apr 20, 2012
Batteries are 240mAh. Yes, that's why I wanted to reduce the current. This charger came as a 'quick charger' for these batteries and has worked fine, as such. I would have liked to continue to use the darn thing with a lower current output to avoid having to buy another charger and/or more batteries when they fail pre-maturely.