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Help needed with Lipo charging

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Steve Fisk18/10/2015 17:45:52
20 forum posts
3 photos

right so I'm abit nervous with charging my lipo , it's a 2c 350mah 7.4 , is it right to select 2 cells charge at 0.3amps 7.4 , says the charge finishing at 8.4 , how do I know when the batterys ready to charge again ?

Dave Hopkin18/10/2015 17:53:58
3672 forum posts
294 photos

8.4v sounds spot on the money Steve, for a 2S Lipo you are correct to charge at 2 Cells, though the max current is determined by the C rating x the Capacity in Amps so 2c x .3 = 0,6A max charge rate though charging at a lower rate will give a better life span to the Lipo as your not stressing it

You should recharge when the cell voltage drops to 6.4 (thats with a bit of safety margin built in) though it will be no harm to recharge it when you have used a bit (Lipos don't have a "memory" like Nicads do)

Phil Green18/10/2015 17:56:30
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1515 forum posts
308 photos

Your charging settings are ok, I would suggest balance charging though if your charger has this facility.

What are you using it in Steve? If its a transmitter you'll have to check the voltage manually until you get used to how it performs, very few transmitters have a lipo setting for the meter or display.

If its for an electric powered model, then the speed controller monitors the voltage and will warn you that the battery is low. ESCs do this in various ways, some lower the motor rpm, some pulse the motor, some just cut the motor,

If its for use in an IC model as a receiver and servo flight-pack, then presumably you have some sort of voltage regulator or BEC. Unfortunately these dont generally warn you at all so you would need to check the voltage after each flight and build up an idea of how long it lasts.

Bear in mind that you dont have to wait for it to be flat before charging it - a lipo will come to no harm if its completely full and you try to charge it.

Cheers
Phil

Steve Fisk18/10/2015 18:16:07
20 forum posts
3 photos

How long will it take to charge , I have a balance charger , it's in a model plane , what can I buy to see what state the battery is in ? I have a multimeter but don't know how to use it for volts ? Really grateful for the quick replys

andyh18/10/2015 18:35:37
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421 forum posts
22 photos

charging at 1C will take roughly 1 hour X the %age used / 100. so probably 40-45mins. it may take a little longer if you balance charge & the charger has to top up an individual cell

with a multimeter, put the -ve on the black contact on the JST connector & the +ve on the one next to it. that will give you the voltage of the first cell. move the +ve to the next contact & you'll get the voltage of cell 1 & cell 2 together, & so on for more cells/S's

you can get battery testers for under £20 what will give you the total, & per-cell voltages/%ages which make life a bit easier:

**LINK**

Kevin Wilson18/10/2015 18:40:19
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383 forum posts
13 photos

I always balance charge at 1C so for 2S 350mAh I set a 0.3A charge.
From flat it should take about an hour.

If you set your multimeter to DC Volts on the 20v range (or as low as you can above the expected 8.4v)
Plug the red lead into the volts socket and the black lead into the common (or negative, -ve) socket.

Put the red probe on the battery red connection, and the black probe on the black connection.

With a 350mAh it will probably be a red jst connector so you should see a couple of silver patches in the side of the plug that you can get your multimeter probes onto.

Going forward there are many voltmeters that you can plug into the white balance connector which makes the job easy.

As stated above you should get 4.2v on a fully charged Li-Po cell
So your 2s should measure 8.4v
Flat is subject to opinion, personaly I dont run mine below 3.4 per cell, some people say 3.8 is flat.

Bottom line, and everyone seems to agree that below 3v per cell damages the cell.

So for a 2s 6v is absolute danger zone, 6.6v is flat and 8.4v is charged.

I hope that has helped.

Dave Hopkin18/10/2015 18:51:13
3672 forum posts
294 photos

A battery checker and a watt meter are pretty essential tools for electric flight andyh poster a link to the type of checker most folk have, but here is the same thing much cheaper... **LINK**

Phil Green18/10/2015 18:54:17
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1515 forum posts
308 photos

lipo_cap_vs_v.jpg

Steve Fisk18/10/2015 19:26:00
20 forum posts
3 photos

Cheers for your help everyone, the battery is charged now and no fires ! I'm going to run the prop for abit and monitor how the volts change

bert baker18/10/2015 19:57:03
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1423 forum posts
303 photos

And if you flatten it too much it may never charge up again.

Dave Hopkin18/10/2015 21:00:54
3672 forum posts
294 photos
Posted by bert baker on 18/10/2015 19:57:03:

And if you flatten it too much it may never charge up again.

If it gets down to 3.0V its permanently goosed, I try not to go below 3.4v so experiment and take readings every so often then you'll get a idea of how long it can stay up for - in the air it will draw a bit less power as the prop unloads in the airflow anyway

either take to prop OFF or make sure its restrained in a safe manner you can get to the Lipo without arguing with the prop (and if its in a shed be prepared for a snowstorm of dust!)

Allan Bennett18/10/2015 21:04:46
1540 forum posts
39 photos
Posted by Steve Fisk on 18/10/2015 17:45:52:

right so I'm abit nervous with charging my lipo , it's a 2c 350mah 7.4 , is it right to select 2 cells charge at 0.3amps 7.4 , says the charge finishing at 8.4 , how do I know when the batterys ready to charge again ?

Plenty of good info from the others.

One thing I would check though, is your battery really 2C? The usual convention is that a 2-cell LiPo is called "2S" which means two cells in series; the reason for this is you can also connect them in parallel, in which case it would be a "2P" pack (same voltage as a single cell, but double the mAh capacity). The "P" is not often used these days because it's easier to make a single large cell instead of strapping two smaller ones together, but it was quite common in the early days of LiPo.

The "C" number, which should be quite prominent on the front of the pack allows you to calculate how many amps the pack should be able to deliver -- just multiply the pack's Ah capacity (that's mAh divided by 1000) by the C number and you get the theoretical maximum amps. Most packs these days are rated at least 10C, and often as high as 60C.

In smaller type, probably on the back of the pack, there may be another C number for charging. If not, you should assume that you charge at 1C (350mA, or 0.35A for the pack in question), as others have already said. But many modern batteries can take 2C, or even more, charge rate.

Simon Chaddock18/10/2015 21:08:01
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5438 forum posts
2850 photos

Steve

A 250mAh 2s LiPo is not the most expensive thing in the world so I would not worry too much.

The simple rule I use for all my LiPo is that if it has actually been flown (no matter for how long) then it is recharged before the next flight.

The speed controller (ESC) should stop you completely discharging the battery but if you do fly for so long that the battery low voltage cut off (LVC) is activated (no motor power but you will still have control) then just don't fly for so long next time! wink 2

Repeatedly flying to the LVC point will not do the LiPo much good.

With a bit of practise you soon get to recognise the slight drop in motor power before the LVC actually cuts in.

Steve Fisk18/10/2015 21:13:54
20 forum posts
3 photos

image.jpgIt was mainly the worry about it exploding when charging

Allan Bennett19/10/2015 08:53:05
1540 forum posts
39 photos

Exploding during charging is rare. Despite the number of videos you may see on the internet, just put it into context of the number of LiPos being used around the world.

Having said that, it's wise to take precautions: Always balance-charge is one, and for that it's comforting to have a charger with a screen that displays its status, rather than the simple go/no-go lights on some of the cheaper chargers. Another precaution is to charge with the battery in a fireproof container, and to never charge it unattended. Also don't charge it close to material that can easily catch fire if it does go off.

Bob Cotsford19/10/2015 09:03:15
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7963 forum posts
439 photos

Most chargers these days will ask you to verify the battery voltage, eg 2S, 3S LiPo or LiFe. This is because it's not unknown for someone to select a 3S charge for a 2S pack by mistake - this is where the fire risk comes in as the charger merrily overcharges the pack by a huge amount.

Just get into the habit of checking the charger settings before hitting the GO button and as Allan said, NEVER leave a charging battery unattended.

 

ps - what you have there is a 25C discharge pack, charge it at 1C, ie 350 mA rate or thereabouts.  400mA is unlikely to hurt the battery but 300 will more than likely give it a longer life.

Edited By Bob Cotsford on 19/10/2015 09:06:28

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