Li-Po inductive charging

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webtop
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Li-Po inductive charging

Post by webtop » Sun Feb 02, 2014 10:50 pm

Hey guys,

I am building some micro drones as part of a mobile wireless sensor network and need some advice on power options. I am planning on using something like 3.7v Li-Po batteries for each device, and would like the devices to be able to self-charge from an induction station. Is this feasible, and can anyone give me any links/advice on such?

My aim is to have each device understand when it needs to charge, locate the nearest station to it, land on a pad and charge itself.

Are the SFE Li-Po options able to do this, do I need specialist chargers, and do I need to isolate the battery from the circuit during recharge?

Thanks for any and all help.
Paul

jremington
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Re: Li-Po inductive charging

Post by jremington » Mon Feb 03, 2014 5:04 pm

LiPo batteries require a special charge protocol and also need to be protected from over discharge or they will quickly be destroyed. They can be charged in place, but you must have a switch or some other way to isolate the battery, so that the charge controller is not misled.

You can buy inexpensive battery protection PCBs here: http://www.all-battery.com/protectivepc ... packs.aspx

MichaelN
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Re: Li-Po inductive charging

Post by MichaelN » Thu Feb 06, 2014 9:15 am

jremington wrote:...They can be charged in place, but you must have a switch or some other way to isolate the battery, so that the charge controller is not misled.
I don't think this is really necessary, especially if the drone isn't itself using much power during charging.

jremington
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Re: Li-Po inductive charging

Post by jremington » Thu Feb 06, 2014 1:57 pm

especially if the drone isn't itself using much power
How will the charger know that?

waltr
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Re: Li-Po inductive charging

Post by waltr » Thu Feb 06, 2014 2:44 pm

If the circuit being power draws much less than the charger can supply then the only issue is that in the 'constant current' charge state the current is less and this state will last longer than if the circuit is not powered. The second charge state is started when the battery Voltage reaches a fixed point (typically 4.2V for a LiPo). The Voltage is then held constant until the current drops to a preset point. If the circuit draws more than this current then the charger will not know the battery is finished charging. If you use a charger chip where this current finish preset is settable (with a resistor or some other method) then the charger will stop charging or indicate a finished charge.
There are many devices that can run while the battery (LiPo) is being charged, the Apple Ipod is one I know of.

jremington
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Re: Li-Po inductive charging

Post by jremington » Thu Feb 06, 2014 4:42 pm

There are many devices that can run while the battery (LiPo) is being charged, the Apple Ipod is one I know of.
Certainly, and the manufacturers have entire teams of highly qualified engineers that are creating an optimal design for a product, which they know inside and out, to be safe and reliable. For a hobbyist forum I believe that is best to give good advice, rather than suggest an approach that is possible -- if you know what you are doing.

For this reason, I recommend that LiPo or Li-ion batteries be charged out of circuit, with a charge controller that is designed for the task. It is also wise to include in the target device a battery protection circuit that prevents over discharge and destruction of the battery.

MichaelN
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Re: Li-Po inductive charging

Post by MichaelN » Fri Feb 07, 2014 5:01 am

jremington wrote:For a hobbyist forum I believe that is best to give good advice, rather than suggest an approach that is possible -- if you know what you are doing.

For this reason, I recommend that LiPo or Li-ion batteries be charged out of circuit, with a charge controller that is designed for the task.
I think you're being overly cautious. As per Walt's explanation, the consequences of running the circuit while charging shouldn't be an issue.
jremington wrote:It is also wise to include in the target device a battery protection circuit that prevents over discharge and destruction of the battery.
By all means, include a circuit that disconnects the load to prevent over-discharge, but that's another matter than charging while the load is connected.

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