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Old 05-18-2019, 05:25 PM
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Question
Charging a Lipo with a Lipo

Hi All Just after some clarification,

I have a few of the Isdt Charger they use an xt60 in and out.

I know i can use a lipo to power the unit to charge another battery type.

BUT i have seen a 3s lipo powering the unit and charging a 4S pack, How is this possible?
I thought voltage in would have to be higher that the out or charging voltage?

My goal is to have a 5amp pack that i can use at the field for charning the od 3s lipo and mainly 2s life packs.

Thank you.
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Last edited by The Hodge; 05-18-2019 at 08:39 PM.
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Old 05-18-2019, 05:51 PM
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Magic.

Buck boost converters
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Old 05-18-2019, 05:58 PM
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So Perhaps it has this Tech built it? The Video i saw was battery strait in and out.

I assume i could charge an 11.1v 3s with the same input voltage?
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Old 05-18-2019, 09:32 PM
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Your DC chargers on 12v input charge 6S batteries. Using a lipo input wouldn’t change this.
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Old 05-18-2019, 09:42 PM
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Quote:
Originally Posted by VinceY View Post
Your DC chargers on 12v input charge 6S batteries. Using a lipo input wouldn’t change this.
Hi Vince, Thanks for that, Ive never charged anything higher than 11.1 3s on a 12v input,.

So an 11.1V 3s can change another 11.1v 3s, And a 11.1 v 3s can do a 14.8 4s? any maybe a 5-6s?

Thank you,
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Old 05-18-2019, 10:00 PM
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Yes, if the charger can do it and the input lipo has enough capacity.
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Old 05-18-2019, 10:14 PM
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Chargers use transformers to step up the input voltage so it’s higher than the voltage of the battery being charged.
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Old 05-18-2019, 10:18 PM
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Quote:
Originally Posted by dshriver View Post
Chargers use transformers to step up the input voltage so itís higher than the voltage of the battery being charged.

Great news, Was not aware they were this smart , Is there a limit, Ie 12v in can only go up to 20v ?
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Old 05-18-2019, 11:25 PM
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The limit is more thermally limited, which typically results in a lower charge rate (in amps).
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Old 05-18-2019, 11:58 PM
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Great thank you for that, Ill get my self a 11.1 v lipo @ 5 Amp to be my charge battery for the 7.4 life packs,

Thanks Again everyone.
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Old 05-19-2019, 04:10 PM
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Quote:
Originally Posted by dshriver View Post
Chargers use transformers to step up the input voltage so itís higher than the voltage of the battery being charged.
Transformers do not work in dc. Every charger I have seen uses a buck boost converter

To dc a transformer is a direct short...
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Old 05-19-2019, 04:14 PM
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Quote:
Originally Posted by The Hodge View Post
Great thank you for that, Ill get my self a 11.1 v lipo @ 5 Amp to be my charge battery for the 7.4 life packs,

Thanks Again everyone.
Best option is always to supply the charger with the same ish voltage that it will be charging. Slightly higher by a few volts is best. The larger the voltage gap the more inefficient it becomes (ie heat).

Also understand power is always equal. Voltage x current = voltage x current

So if input voltage is lower than output your current will be higher.

Charging a 2s from a 3s is a pretty good option
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Old 05-20-2019, 06:06 AM
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Sorry Keith! You're right, transformers are for AC only. I know that I shouldn't, but I tend to call buck and booster converters transformers. Buck converters are a switching circuit that uses diodes to reduce voltage, while a boost converter adds capacitors (or an inductor, or both) to boost voltage.

Back to the original question... you need to power your charger with whatever the rated input voltage is. For example, the PowerLab 8 charger takes 10-32v DC input. It will charge up to an 8s pack (33.6v). At 12v the current draw is going to be very high but it'll work.
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Old 05-23-2019, 04:58 PM
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Thanks for all your help guys, Now i understand
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