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Old Oct 12, 2003, 10:47 PM
kindjordan is offline
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how long to charge lipoly?


question for you guys....if im charging a koakm 1500mah pack with 225mah of current, how long will it take to charge the pack to full...sorry im bad at math!...thanks
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Old Oct 12, 2003, 11:09 PM
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1500/225 = 6.67 hours. Add a bit since you won't get 100% efficiency. Subtract a bit since you aren't going to be starting with a completely empty battery.
Old Oct 13, 2003, 01:59 PM
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IF you have a charging supply that continues to deliver 225ma right to the top voltage, the 1555/225=6.67 is right. But the math ain't that easy, I'm afraid.

Because I doubt that anybody will ever sell a charger like that. It would have to boost the voltage above what is safe for the pack to keep the current up as the pack approaches full charge. (Current is proportional to the difference in voltage between charger and pack!) As far as I know it wouldn't hurt the cells to do this, but it would be like approaching a every stoplight at 60 MPH and slamming on the brakes at the very last millisecond. One time the road will be slipperier than you thought, and you'll T-bone that cop whose light just turned green.

A liability lawyers' dream!

You will have a current LIMITED, and voltage regulated, charger. i.e. it controls the voltage, and if the load resistace is low enough, it will LIMIT amperage so as not to exceed a peak value. You will have 225 ma into a fully discharged pack, but it will taper off as the pack charge state (i.e. pack voltage) increases. .

I charge at 1C (in your case 1500 ma current) at the start of charge, where the pack is at 3.0v per cell, the current quickly starts to taper off, and it typically takes several hours to get a pack up to 4.0 volts per cell. At which point the charger is still delivering 4.2 volts (per cell), but the current is down to only 100 ma. It'll take 3-4 more hours to get that last little bit in. Starting at only 225 ma it'll likely be 12+ hours, assuming your charger works as I am suggesting.

Set up an ammeter in series during a charge, and watch what it does. Even the cheapest multi-meter will handle 225 ma. Then set it up as a voltmeter in parallel with the pack, and watch it go thru another charge. When the pack is up to 4.0 volts per cell, it's full enough. Don't bother waiting for that last 0.2 volts if it's time to fly.

Better yet, invest in an Astro Whattmeter. Best gizmo I've ever bought for testing motor systems and managing charging.
Last edited by uscra112; Oct 13, 2003 at 02:13 PM.
Old Oct 13, 2003, 02:46 PM
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Charging a LiPoly at X Amps for Y hours is a recipe for disaster! It's a well-known fact that overcharging LiPolys even a little bit will damage the cells, and overcharging a lot will cause them to catch fire!!!

You can't possibly know the initial state of charge of the pack, so you can't possibly know how long to keep the pack on charge at 225mA before it's "full."

The best course of action is either a voltage-and-current-limited laboratory power supply (expensive) or a purpose-built LiPoly charger.
Old Oct 13, 2003, 02:49 PM
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Good Grief, Charllie Brown. It never occurred to me that he might be using something other than a regulated power-supply! Good catch!
Old Oct 13, 2003, 03:06 PM
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i am using a standard 3 amp 13.8 volt power supply and an fma lipoly 202 charger...i was recomended this setup..and its been charging my battery just fine...taking a while though!
Old Oct 13, 2003, 03:18 PM
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Ah. You're safe, then. Will the 202 let you set it for more current?

You can go as high as 1.5 amps with a 1500 pack.
Old Oct 13, 2003, 04:06 PM
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great!!...thanks for all the advice...i think im gonna sell that charger as it will only do 225mah...i dont think my paience will allow for 6 hours of charging between flights.....thanks again guys
Old Oct 13, 2003, 04:13 PM
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Is your pack 8.4 volts or 12.6?
Old Oct 13, 2003, 05:04 PM
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its a 7.4 volt 1500mah kokam pack
Old Oct 14, 2003, 01:38 PM
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Okay, now that we know you're not trying to do something totally outrageous...

To get a rough idea of how long a pack takes to charge, divide its capacity in mAh by the charge rate in mA (multiply Amps by 1000 to get mA).
Old Oct 14, 2003, 01:40 PM
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thansks so much for the advice...it seems that after one day posting and reading about lipolys i think i know all i need....now all i have to do is sell my lipoly charger that charges my 1500 mah pack at 225mah at the fastest. its taking bout 6 hours....i think i can go up to 1amp without concern...agree?
Old Oct 14, 2003, 04:42 PM
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kindjordan,
You can use 1.5 amps

For Lipo's charge at 1C.

To determine the 1C rate for a pack take the pack mah and move the decimal three places to the left.

Ex:
1500mah = 1.5 amps
1020mah = 1.02 amps
880mah = .88 amps
Round off to the closest charge setting you have available.

Jimmy


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