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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> Charging For Dummies
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Charging For Dummies - 9/4/2003 2:17:04 PM   
adam_one


 

Posts: 399
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		[QUOTE]So. Two compeating hypotisis - 40% or 60% ? I'm happy to go with a contant curent charge on my pack for 10 hours as outlined by adam_one. [/QUOTE]

Let's say that 40% more energy should be regarded as an average figure since the accurate value is rather difficult to determine as battery manufacturers differ

One may say that the batteries are more sensitive to overcharge when high charging currents are used (above 1C).
According to PowerStream the NiCds charging efficiency is 83% when fast charge is used, which means the need of about 20% more energy.
However, they also state that the numbers are just to give an idea, since there are differences between manufacturers.

< Message edited by adam_one -- Sep 4 2003 1:13PM >

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Charging For Dummies - 9/5/2003 8:58:49 AM   
marvintm



Posts: 77
Joined: 9/3/2003
From: Cleveland, OH,
Status: offline 		A silly idea struck me on difference between NiMH and NiCd. Size matters....

Just for fun, I put two * fully charged * batteries on C/10 and watched them most of the day. They were:

- 700 ma AA NiCd
- 1850 ma AA NiMH

8 hours into it, the NiMH is getting warm. The NiCd is not.
14 hours into it, the NiMH is really warm. The NiCd is ~ body temp. (I know, how warm) No measurements were taken.

These are physically identical AA size packs but the NiMH has 2.6 times more current input at the C/10 rate. Niether fully charged pak has anywhere to put the power but too dissipate it as heat. With the same surface area to dissipate the heat, it stands to reason that the NiMH will get warmer, faster.

IMO all the critical charge issues surrounding them must be driven by this simple issue of high density and heat dissipation.

Anyways.......Sascha,

You're talkin NiCd now on your last post including the powerstream quote. The original post was regarding NiMH specifically. In my experience abusing NiCD paks, I don't think you don't need to be painfully specific on the overnight rate with a timer.

For use with your regulator, Red used to mention in seminars modifying a lamp timer for a one-shot 16 hour "forget proof" timer. This would work nice.

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Charging For Dummies - 9/5/2003 10:52:22 AM   
Sascha



Posts: 5
Joined: 9/1/2003
From: RichmondVictoria, AUSTRALIA
Status: offline 		marvintm, please accept my appologies for hijacking a NiMH discussion

I appreciate the test that you ran. food for thought! I think I'm just about ready to have a crack at getting my wall-wart into a battery charging mood.

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Charging For Dummies - 9/5/2003 12:48:19 PM   
adam_one


 

Posts: 399
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		Marvintm:

That was an interesting observation.
Is true that the NiMHs batteries get warm during most part of the charging process while NiCds only get warm when they reach the fully charged status, about 45deg. Celsius or 10deg. above the ambient temp.
Thats because the active material inside the NiMHs batteries has less space to expand than the NiCds.
The NiMHs have also greater internal resistance, which means that they cant deliver as much instantaneous current as the NiCds even if the NiMHs have higher energy density (capacity/volume) is not recommended to charge them at higher rates than 1C.
The NiMHs have also 50% higher self-discharge rate.
However, the NiMH are more environment friendly.

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Charging For Dummies - 9/5/2003 3:01:57 PM   
Red Scholefield



Posts: 4980
Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline 		[QUOTE]Originally posted by adam_one
Marvintm:

That was an interesting observation.
Is true that the NiMHs batteries get warm during most part of the charging process while NiCds only get warm when they reach the fully charged status, about 45deg. Celsius or 10deg. above the ambient temp.


[COLOR=red]The reason that you see different temperature profiles is very basic. One system is exothermic during the charge process and the other endothermic.[/COLOR]

Thats because the active material inside the NiMHs batteries has less space to expand than the NiCds.

[COLOR=red]Where did you get this piece of information? There is no "expansion" of the active material in either scenario.[/COLOR]

The NiMHs have also greater internal resistance, which means that they cant deliver as much instantaneous current as the NiCds even if the NiMHs have higher energy density (capacity/volume) is not recommended to charge them at higher rates than 1C.

[COLOR=red]The internal resistance plays a significant role in the discharge voltage profile, but has little to do with the charging.[/COLOR]

The NiMHs have also 50% higher self-discharge rate.
However, the NiMH are more environment friendly. [/QUOTE]

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Charging For Dummies - 9/5/2003 7:28:23 PM   
adam_one


 

Posts: 399
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		[QUOTE]The reason that you see different temperature profiles is very basic. One system is exothermic during the charge process and the other endothermic. [/QUOTE]

The fact that one system is exothermic and the other may be endothermic has already been observed by Marvintm.
But considering that "exothermic" means temperature rise, we may also say that the both systems rise the temperature during the charging process.
Anyhow, you still didn't explain the reason why it is so.
[QUOTE]Where did you get this piece of information? There is no "expansion" of the active material in either scenario. [/QUOTE]
I tried to keep my "explanation" as simple as possible hence the word "material".
During NiMH's charging and discharging processes there is an electrochemical reaction where hydrogen gas is formed and moved between the cell's electrodes through a porous separator. This leads to an expansion inside the cell, which may cause a too high pressure that must be released through a vent.
A gas is a "matter" and if I'm translating it correct the word material derives from "matter".

< Message edited by adam_one -- 9/8/2003 6:30:43 AM >

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RE: Charging For Dummies - 9/24/2003 12:30:46 AM   
Goku



Posts: 491
Joined: 3/13/2003
From: Boise, ID, USA
Status: offline 		I've been lurking here and have found several of those "battery charging mysteries" in life are not quite as mysterious anymore. (I'm an airplane guy and am just getting into electric cars).

I have a question.... It seems to me that one of our biggest enemies is heat, right?

So does that make it bad to recharge a hot pack that just got used up in my car? if so, can I throw it in the freezer, or in a cooler to cool it faster? or is the drastic change intemperature bad?

I have a cordless drill whose charger won't charge it if the pack is hot (it indicates full charge), so I stick the charger (with battery in it) in the freezer (yeah- power cord is sticking through the closed door ) and as it cools it down it starts charging...

Can this same principle be aplpied to my RC car batteries?

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RE: Charging For Dummies - 9/24/2003 1:29:28 AM   
marvintm



Posts: 77
Joined: 9/3/2003
From: Cleveland, OH,
Status: offline 		Your one cold dude!
Your batteries would be called.... Ice Packs.

Yes high heat is a bad thing on NiCd, worse on NiMH.

Most cordless tools nowadays have a third connection that is used for a
temperature sensor built into the pack. If it is reading above the expected temp, it wont charge anymore. You have an active safety doing its job.

Don't know how cold temps effect charge.... ??

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RE: Charging For Dummies - 9/25/2003 8:52:06 AM   
adam_one


 

Posts: 399
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline
quote:

So does that make it bad to recharge a hot pack that just got used up in my car? if so, can I throw it in the freezer, or in a cooler to cool it faster? or is the drastic change intemperature bad?


I guess that drastic temperature changes are often no good for most of devices…
Keeping batteries in a refrigerator (not in a freezer) will reduce their self-discharge rate, but it is recommended to use and charge them at ambient temperature, (above 0 deg. C).
Also prolonged working temperatures above about 40 deg. C will reduce the batteries' life.
By having your batteries in the freezer while charging them will “deceive” your charger temperature protection, which may result in overcharging.

< Message edited by adam_one -- 9/25/2003 9:54:27 AM >

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RE: Charging For Dummies - 9/25/2003 3:15:59 PM   
Goku



Posts: 491
Joined: 3/13/2003
From: Boise, ID, USA
Status: offline 		Yeah, That makes sense....

Thanks!

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RE: Charging For Dummies - 9/27/2003 9:56:01 PM   
basmntdweller


 

Posts: 1731
Joined: 1/3/2002
From: Indianapolis, IN, USA
Status: offline 		In reading through this thread, one of the statements is leading me to believe that charging at a C/10 rate overnight could still be damaging to a partially discharged NiMH pack. I have been wanting to slow charge my planes and helis from my truck battery so I don't have to load and unload every day. I'm also getting the impression my packs would be better maintained by charging them with my Hobbico mkII at C/2 rate. Am I correct? I have four NiMH rx packs from 1450mah to 2000mah and two tx 1650mah NiMH packs. If the Hobbico MKII is the way to go, I could easily rig up three of them to handle all charging without having to unload. That would be the ideal situation. Does the Hobbico MKII fall back to any kind of trickle after it peaks?
Thanks,,, bamsntdweller


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RE: Charging For Dummies - 11/16/2003 8:28:17 AM   
frfrank


 

Posts: 1
Joined: 11/16/2003
From: Broomfield, CO, USA
Status: offline 		Hello, I have learned much about charging amperage here, what about charging VOLTAGE? Specifically, what is the maximum voltage a 7.2v NiMh 1600 mAh pack can be charged with for overnight charging. I can't seem to find a simple wall wart that will outputs 9v and 160 ma. But I have found one that puts out 12v and 150mA...
Thanks for any help.

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RE: Charging For Dummies - 11/16/2003 4:23:49 PM   
Flying Geezer



Posts: 1049
Joined: 4/12/2003
From: Bowling Green, KY, USA
Status: offline 		Just an observation: Beginners start with a 500 mAh tx and 500 mAh rx pack charged by a wall wart. Works fine, don't forget to charge, don't worry about putting a full charge on a partially depleted pack. Nicads and nimh are pretty tough, as compared to the newer(very volatile) chemistries now emerging.

When a pilot becomes somewhat proficient, and is ready to invest in bigger planes, bigger engines, bigger packs, etc., it's time to invest in a modern charging system. Smart chargers have proven to be reliable and safe. They take the guess work out of charging. Charging at 1C has very little effect on the overall life of a battery pack, assuming you are not going to try to squeeze 10-15 years out of a pack. As Red pointed out, nimh packs get pretty warm at peak. Its the nature of the beast and it's not a problem.

There are smart chargers available from about $50 up, Up, UP, so there is no reason for a serious pilot not to own a smart charger. No offense to the c/10 purists.

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RE: Charging For Dummies - 11/16/2003 4:53:09 PM