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Old Apr 14, 2010, 09:09 AM
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Originally Posted by Von Trippenhopf View Post
Can't help you with the voltage drop on liFePO4 however if they are being trickle charged and subject to a drain at the same time they might never reach max volts.
No, they are not being charged at the same time as they are in use - there is a switch that flips the batteries between connection to the circuit OR connection to the charger.

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I don't know enough about liFePO4 to comment on whether it's possible or advisable to connect them permanently to a charger.
I guess this depends on the charger - if it automatically stops charging when the batteries are full?

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It almost sounds like you'd be better off leaving the cells out of the design.

Which specific characteristics are you refering to ?
Lifetime, ability to hold voltage output up to depletion, low internal impedance (probably low noise) - high current delivery (although not specifically needed here)

I guess all I'm looking for is a low current single-cell LifePO4 charger that turns off when finished, either off-the-shelf or DIY.

I know there are ICs from Microchip (MCP73123) that will do this job but they are DFN package & not easy to DIY solder. The eval board is too expensive - has anybody got a board or knows of a board that uses these chips?
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Old Apr 14, 2010, 01:50 PM
Prof. Aeronautical Inventor
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Quote:
Originally Posted by jkeny View Post
I guess this depends on the charger - if it automatically stops charging when the batteries are full?
Because lithium based cells are sensitive to overcharge, I doubt you'll find any charger for them that doesn't cut off when the cell is fully charged.

If you have the cells hardwired to a charge circuit, when you turn the device off the charger will come on and charge the cells.

However, at some point the charge is likely to come to an end the charge circuit will still be attached to the cells.

This is likely to lead to some discharge through the connected circuit.

I know of many occasions where people have left lipos connected (sometimes overnight) and at the end of the charge they have discharged to the point of no return.

Maybe if you provided some more info as to what you are hoping to achieve other people would be in a better position to be able to offer advice.

As it is, it just looks like you are making life more difficult for yourself.

It's likely there are many ways to achieve whatever it is that you are attempting.

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Old Apr 14, 2010, 02:30 PM
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Originally Posted by Von Trippenhopf View Post
Because lithium based cells are sensitive to overcharge, I doubt you'll find any charger for them that doesn't cut off when the cell is fully charged.

If you have the cells hardwired to a charge circuit, when you turn the device off the charger will come on and charge the cells.

However, at some point the charge is likely to come to an end the charge circuit will still be attached to the cells.

This is likely to lead to some discharge through the connected circuit.

I know of many occasions where people have left lipos connected (sometimes overnight) and at the end of the charge they have discharged to the point of no return.
Yes, this is what I want to avoid & I see some LiFePO4 chargers saying that they can't be left permanently connected - Voltphreaks single cell charger one for instance "Note, these chargers cannot be left permanently connected to battery, as chargers will slowly drain the battery."

Quote:
Maybe if you provided some more info as to what you are hoping to achieve other people would be in a better position to be able to offer advice.

As it is, it just looks like you are making life more difficult for yourself.

It's likely there are many ways to achieve whatever it is that you are attempting.

I've probably given all the info but here goes again:
I have a USB audio device powered by two LiFePO4 batteries - these power different parts of the circuit. I want to have a situation where when I'm not running the unit from the batteries they are being charged from a charger. I don't want to have to plug in & out chargers but simply a plug & forget solution. When the unit is switched off, the switch connects the 2 batteries in parallel to a charger socket into which I was hoping to have a charger permanently connected. I don't need battery balancing, I don't need lcd, I don't need any options just a dumb, "smart" charger I don't need any expense <$20.

Thanks for your time & help
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Old Apr 14, 2010, 02:37 PM
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Originally Posted by jkeny View Post
I've probably given all the info but here goes again:
I have a USB audio device powered by two LiFePO4 batteries - these power different parts of the circuit. I want to have a situation where when I'm not running the unit from the batteries they are being charged from a charger. I don't want to have to plug in & out chargers but simply a plug & forget solution. When the unit is switched off, the switch connects the 2 batteries in parallel to a charger socket into which I was hoping to have a charger permanently connected. I don't need battery balancing, I don't need lcd, I don't need any options just a dumb, "smart" charger I don't need any expense <$20.

Thanks for your time & help
I still can't see why you 'need' LiFe cells - it's merely complicating your life.

Use Nimh and a trickle charger of 1/10C or preferably less.

You can then leave the charger permanently connected and in the event of a power down - if the cells did discharge - they wouldn't be harmed by the experience.

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Old Apr 14, 2010, 02:43 PM
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Originally Posted by Von Trippenhopf View Post
I still can't see why you 'need' LiFe cells - it's merely complicating your life.

Use Nimh and a trickle charger of 1/10C or preferably less.

You can then leave the charger permanently connected and in the event of a power down - if the cells did discharge - they wouldn't be harmed by the experience.

I'm committed to the LiFePO4 cells - I'm using 2300mAH A123 cells so at the drain current that some of these chargers drop to, would it not take a long time to discharge them below the minimum of 2.4V? Is it being over-cautious to worry about this discharging?

Edit: Oh & remember they're paralleled so really 4,600mAh - even longer

So is there a cheap single cell LifePO4 charger around? Voltphreaks one is a maybe, Tenergy one is going out of stock shortly - any others?
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Old Apr 14, 2010, 03:21 PM
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Originally Posted by jkeny View Post
I'm committed to the LiFePO4 cells - I'm using 2300mAH A123 cells so at the drain current that some of these chargers drop to, would it not take a long time to discharge them below the minimum of 2.4V? Is it being over-cautious to worry about this discharging?
Any discharge is wear & tear on the cells - they can only be charged and discharged a certain number of times.

The rate of discharge through the charge circuit would depend on the circuit itself - I have to say it's likely that it wouldn't be recommended by the manufacturer of the charger or of the cells.

There are likely other safety implications, so my best advice to you would be to start a fresh thread with a new title and very clearly state in the title that you want advice on LiFePO4 cells 2300mAH A123.

As you pointed out they have "ability to hold voltage output up to depletion" and "high current delivery" so it's very unlikely the voltage would drop to the level you require until they were fairly discharged.

Personally I suspect you are on a hiding to nothing, but might learn something of interest as you go.

It looks like the wrong technology in the wrong application to me, so as I said, I'd advise you to seek specific advice.



I hope it works out for you, whatever you try.

Edit: just an afterthought - A diode would drop the voltage by maybe 0.5v and stop the cells discharging through the circuit
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Old Apr 14, 2010, 03:25 PM
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Thanks for your time Von!
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Old Apr 15, 2010, 12:53 AM
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...so my best advice to you would be to start a fresh thread with a new title
+1. Let's leave this thread to its original purpose, please start one of your own for your project, perhaps in the DIY sub-forum where you will find lots of clever folk that could help.
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Old Apr 15, 2010, 03:26 AM
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Thanks guys, done! but not in the DIY forum, doh!
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Old Apr 20, 2010, 09:47 AM
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Thanks Eric, nice link!
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Old Apr 22, 2010, 01:40 AM
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dksjpof 'odjf;slkjdfps']plkmj '
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Old Apr 27, 2010, 09:47 AM
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Are they supposed to be different?

So I bought 2 Zippy Flightmax batteries from HobbyKing. Both 3S 11.1V 2200mAh but they are not the same size (picture below). Will the difference in size have any influence on the performance of the lipo's?
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Old Apr 27, 2010, 10:21 AM
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That's funny, I've never seen that in the same shipment. I'd have to test them to find out the difference, but this definitely shows the random, changing nature of these packs. Thanks for sharing, and let us know how they hold up/perform.

Also, can you share the weights on these two packs?

Cheers,
David
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Old Apr 27, 2010, 11:30 AM
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It looks like one may be a 20C and the other a 25C, any indication of the discharge capacity ?
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Old Apr 27, 2010, 11:50 AM
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They are both labeled 25C. Besides weight you might add measurements of len., width and height. I'm guessing one is mislabeled and really is higher or lower C as
rszanti suggests.
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