How do you know when a battery is fully charged? - RC Groups
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Apr 07, 2004, 11:18 PM
Registered User

How do you know when a battery is fully charged?

I have a T-Hawk from Toytronics and have their NiMH battery packs. I am currently using a Wattage PF-12 charger to charge the battery packs. Every time I charge the battery, the battery charger stops after about 20 minutes and goes in to trickle mode. However, I know from flight experience that if I donít perform another charging cycle with the battery Iíll only have about 5 minutes or so of usable power. I usually get 12-15 minutes if I charge the battery twice. I am also using a voltmeter to check the battery voltage and use it to gauge the level of power in the battery. A ďdeadĒ battery, a battery run down to the point where my BEC kicks in, measures about 8.4V. A pack thatís only been through the charge cycle once usually reads about 9.6V while a pack thatís been through two charge cycles usually measures about 10.3-10.5V. The packs Iím using are 2/3A size 900 mAh 8.4V NiMH with mini tamiya connectors. I have the PF-12 set to 1100 mAh for charging purposes. Please help with the following questions:

1)What voltage should my pack read when fully charged? Is voltage a good determination of charge?

2)How is the 1.2V per cell established? Is that at full charge, discharge, or something else?

3)By doing 2 charge cycles on a pack, am I damaging the batteries? I assume not since the battery packs do not get warm until towards the end of the second charge. Total charge time for two cycles is also about 45 minutes.

4)Is my PF-12 false peaking at the end of the first charge or am I doing something wrong here?

5)At the end of a flight, is it best to run the pack down until the BEC cuts off the motor to help the batteryís longevity?

Thanks for your help!
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Apr 08, 2004, 02:28 AM
Our Daddy and Heli Junkie
Fred Bronk's Avatar
Well, this really belongs in the battery forum which is where I will send it when I finish

1) about 1.5 volts a cell right off the charger.

2)that is the rated nominal voltage of nimh/nicads under normal use.

3)I doubt you are damaging them since they don't get to hot till the end of a charge, as they should. About 45 minute is right for a nimh pack that is discharged when charged at 1.5C

4) Maybe, but the pack could be bad too. Be sure it is set to 1.5C (900ma x 1.5 = 1350ma) or about 1300-1400 ma in nimh mode.

5)NO, but a full cycle won't hurt every now and then.

6) the tamiya plugs are junk, they could be the probelm too.

7) the BEC never cuts off the battery, it is the LVC (low voltage cutoff) that cuts out the power to the motor only allowing the BEC to still power the radio. The BEC always works

8) if you have a 7 cell pack (8.4v) it should fully charge to about 12 volts. fully discharged should be about 6v, but will recover after a few minutes.

9) use your meter on the battery leads as you charge and see what the voltage is and if it looks about like what I have posted.

By the way, I like the PF-12 and now have the PF-dual on my bench
Apr 08, 2004, 10:52 AM
Registered User
Originally posted by Fred Bronk
Well, this really belongs in the battery forum which is where I will send it when I finish

4) Maybe, but the pack could be bad too. Be sure it is set to 1.5C (900ma x 1.5 = 1350ma) or about 1300-1400 ma in nimh mode.

It's interesting that you say to charge the battery at 1.5C when the PF-12's instructions specifically state that NiMH batteries should be charged at no higher then 1.25C.

6) the tamiya plugs are junk, they could be the probelm too.

Perhaps but I don't have a soldering iron yet. In the future I will.

Thanks for the advice!
Apr 08, 2004, 01:05 PM
Registered User
CADman's Avatar
When I was using NiMH batteries I always gave them a "break-in"
charge because of the following advice from the NiMH FAQ's on the Radical RC website:

"...If you take a new pack and put it on a Delta Peak or Peak detect charger or any other kind of NiMH rated fast charger it will not be filled up. I don't care what the instructions say. I'm sure they say to never charge a new un-conditioned battery on them. If they do not tell you not to charge new packs on them then they are wrong. The voltage curve is measured and calculated by these chargers in order to predict the peak. The voltage curve is not normal on a brand new pack. It needs one slow conditioning charge before the curve becomes stable. Try as you may, you can't fill up a new battery on one of these chargers. After it's first C/10 charge, it will work fine.
We've had many customers write us saying "I've cycled this pack 3 times now and it has not come up anywhere near where it's rating is." I remind them to put one slow overnight charge on the pack, they always write back and say "You were right! It cycles perfect now." So, we've been there many times. Don't suffer the frustration of trying to fast charge a new pack if you want the best out of it. After it's initial break in charge you can fast charge all you like..."

It will be difficult to do this on the PF-12 since it times out after 90 minutes I think. If you don't know somebody with a variable rate trickle charger, I guess you can set the PF-12 at .9 amps and every hour stop and restart the charge process. Not practical, I know, but you only need to do it once. I used the PF-12 for a year and a half on my 720 8-cell NiMH Switchback packs and it always peaked like it should.

Just a thought...
Apr 11, 2004, 08:09 AM
Repair Specialist
Ken-Ohki's Avatar
Your pack is full when the voltage starts to drop backwards wile charging. Monitor your pack with a meter, and you will soon know your packs true charge levels.

The voltage will drop some after you remove it from the charger,, this is normal.

In my experience. a 1.2 volt rechargable tops off around 1.45 volts when fully charged, then drops to about 1.39 after it has been off the charger for a few minutes. SO a 8 cell pack would be around 11.6 fresh off the charger, and drop to a around 11.12 after a few min off the charger.

These are my results, Yours may varie.

Apr 11, 2004, 08:25 AM
Registered User
Andy W's Avatar
Originally Posted by Ken-Ohki
These are my results, Yours may varie.
They will vary considerably - by cell chemistry, capacity, even model, charge rate, etc.
Apr 11, 2004, 02:54 PM
Sunspot's Avatar

T Hawk 900 mah nimh

My battries also false peak for my grandsons Hawk. I just restart my Triton and press on...It will then get up to full. (If everything else is ok)
My flying buddies replaced the junk plugs with Ultra deans and have NO problems.....This eliminated all but the junk plugs...I am going to change mine out today....I will let you know...

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