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Old Jul 31, 2012, 11:31 PM
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Nimh charging

When is a nimh rx pack done charging? I read when its warm, when the voltage starts to drop...What's the right answer?
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Old Jul 31, 2012, 11:33 PM
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Both. These happen concurrently. When a Nixx cell reaches full charge, it dissipates the charge energy as heat and also exhibits a voltage depression as a result.
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Old Jul 31, 2012, 11:38 PM
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Originally Posted by mrforsyth View Post
Both. These happen concurrently. When a Nixx cell reaches full charge, it dissipates the charge energy as heat and also exhibits a voltage depression as a result.
So do i basically have to watch the voltage drop? Its at 7.23 right now. How drastic is the drop?
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Old Jul 31, 2012, 11:56 PM
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The size of the drop is a function of the charge rate, as higher charge rates develop a more rapid increase in temperature, resulting in a more pronounced drop in voltage. Most automatic chargers look for this drop, but would need a charge rate of about 1/2C or higher to detect it reliably. By the time you see it with a voltmeter, I think you'd be well on the way to permanent damage, because we're talking millivolts, here. If, however, you charge at the rate recommended for a periodic forming charge, 1/10C, you just let it charge for about 15 hours, as that rate won't overheat the pack.
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Old Aug 01, 2012, 12:03 AM
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Originally Posted by Wintr View Post
The size of the drop is a function of the charge rate, as higher charge rates develop a more rapid increase in temperature, resulting in a more pronounced drop in voltage. Most automatic chargers look for this drop, but would need a charge rate of about 1/2C or higher to detect it reliably. By the time you see it with a voltmeter, I think you'd be well on the way to permanent damage, because we're talking millivolts, here. If, however, you charge at the rate recommended for a periodic forming charge, 1/10C, you just let it charge for about 15 hours, as that rate won't overheat the pack.
I appreciate the explanation but I didn't register any of that. Electricity is not my strong suit. Im really just looking to know when the battery is done charging and when it needs to be charged again
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Old Aug 01, 2012, 12:14 AM
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I have a better understanding if lipos. Is there a way my icharger can let me know when its done charging?
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Old Aug 01, 2012, 02:13 AM
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Its at 7.23v? (as/of two and one-half hours ago)?
Something is wrong, stop the charge now!
I assume your NiMh receiver pack contains four cells.
iCharger instructions
According to page 6, NiMh high is 1.6v and low is 0.85 x4 = high 6.4v
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Old Aug 01, 2012, 03:04 AM
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Originally Posted by rick.benjamin View Post
Its at 7.23v? (as/of two and one-half hours ago)?
Something is wrong, stop the charge now!
I assume your NiMh receiver pack contains four cells.
iCharger instructions
According to page 6, NiMh high is 1.6v and low is 0.85 x4 = high 6.4v
Its a 5 cell. 2700 mah pack
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Old Aug 01, 2012, 03:07 AM
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Originally Posted by BrightCap232 View Post
I appreciate the explanation but I didn't register any of that. Electricity is not my strong suit. Im really just looking to know when the battery is done charging and when it needs to be charged again
Its actually simple maths ....

take the capacity of your NiMH pack ie 2700mAh

take the charge rate of your power source ie 100mA

(I'm just using example numbers for charger...)

2700 / 100 = 27hrs ..... the theoretical time to charge your pack from full discharged state. But this is not what happens in practice. There are resistances and drops that reduce the effective charge and increase the time required. It is usual to multiply the theoretical charge time by 1.4 to arrive at full charge state ... this is when using a sensible rate of charge not exceeding 1/6th of pack capacity. At higher rates - you reduce that factor arriving at 5 - 10% extra at high rates to prevent overstressing the pack. In fact it is not good to keep charging packs at high rates ... it is better for conventional NiXX technology packs to be charged at 1/10th rate for long life.

So taking our example of the 2700mAh pack charged at 100mA ... you would allow 27 x 1.4 hours for charge from full discharge. That is 37.8 hours.
At the rate being 1/27th ... anything from 1/10th and lower rate can be left quite safely charging well over time without damage to pack. But do not leave indefinitely unless a real low rate. So if you only discharge partly to an unknown level ... as long as rate is 1/10th or lower - stick it on for full time ... you'll be fine.

Therefore do the maths ... it's so easy and you'll start to charge up all sorts of packs without thinking too much !

nb : The same maths applies to NiCD (generally not available now due to environmental controls on Cadmium) and LiIon ..... I know there will be others who say they are not same ... but based on a Technician advice from a Wortld Leading battery manufacturer that I know - he said just charge same way and it's not actually necessary to have a dedicated charger - they are similar enough to take it at sensible charge rates.

I have a Pro-Tech charger for NiXX .. it detects the Delta peak point - the point that voltage drops a tiny bit at full charge and reverts to very low trickle rate. It really is worth getting an auto detect charger ... they are not expensive now.
I also have an old Futaba M charger that is used to keep my Tx's and Rx's topped up ... I use 2300mAh NiMh packs and the M charger puts out 50mA. That is so low that I can leave my gear plugged in literally indefinitely without worry.

Nigel
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Old Aug 01, 2012, 08:45 PM
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Originally Posted by BrightCap232 View Post
I have a better understanding if lipos. Is there a way my icharger can let me know when its done charging?
If you have this charger: http://www.hobbyking.com/hobbyking/s...e_Charger.html, it has a setting for NiMH batteries, so should have an auto-detect for charge complete, and should make the same noises it does for LiPo batteries.

You 'form' a new NiMH pack by discharging fully (4.25V for a 5 cell pack), then charging for about 15 hours at the 1/10C rate, or 270mA (0.27A) into a 2700mAH battery; 0.3A is close enough. Once that is done, you can charge at 1.3A (~1/2C) to 2.7A (1C) using the auto feature of the charger. Just repeat a forming charge every dozen or so charges, just to keep the battery in good shape. Yes, charging at higher than the 1/10C rate will reduce the life some.
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Old Aug 01, 2012, 09:03 PM
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Originally Posted by Wintr View Post
If you have this charger: http://www.hobbyking.com/hobbyking/s...e_Charger.html, it has a setting for NiMH batteries, so should have an auto-detect for charge complete, and should make the same noises it does for LiPo batteries. Thoughts?

You 'form' a new NiMH pack by discharging fully (4.25V for a 5 cell pack), then charging for about 15 hours at the 1/10C rate, or 270mA (0.27A) into a 2700mAH battery; 0.3A is close enough. Once that is done, you can charge at 1.3A (~1/2C) to 2.7A (1C) using the auto feature of the charger. Just repeat a forming charge every dozen or so charges, just to keep the battery in good shape. Yes, charging at higher than the 1/10C rate will reduce the life some.
Thanks. That all makes sense now. I'm contemplating going another route now. Smart-fly super regulator and two 2S 1550 mAh lipos. Ive been thinking of a redundant battery setup and like the smart fly units
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Old Aug 01, 2012, 11:57 PM
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So, this is for a receiver? Probably a good idea to go with a separate regulator and LiPo over NiMH, if the regulator can handle the servo current. NiMH cells aren't noted for their ability to provide the surges of current that a few active servos can demand.
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Old Aug 02, 2012, 12:07 AM
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Originally Posted by Wintr View Post
So, this is for a receiver? Probably a good idea to go with a separate regulator and LiPo over NiMH, if the regulator can handle the servo current. NiMH cells aren't noted for their ability to provide the surges of current that a few active servos can demand.
Yea for the rx. The smart-fly super regulator and (2) 2S 1550 mAh lipos is what i'm looking at. That setup seems right to me and also keeps my CG where I want it (according to the weights of everything) I have high torque analog servos. 6 total servos.
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