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Charging questions for NiMH - 3/14/2008 3:23:05 AM   
theJoseph


 

Posts: 9
Joined: 1/13/2008
From: , FL, USA
Status: offline 		if I have a 6V 300mah nimh battery what would be a good mA rate to trickle charge it?

and does the milli-Ampage need to change if I use a battery with a different mAh rating.. or does it stay the same?

thanks
       Post #: 1

RE: Charging questions for NiMH - 3/14/2008 5:33:02 AM   
Sir Raleigh


 

Posts: 281
Joined: 4/7/2007
From: Raleigh, NC, USA
Status: offline 		Trickle charge is usually considered about 10/C or 10% of your batteries capacity. So, a 30mAh charger would be a good start. However, I would not leave any charger on the batteries for over 14-16 hours. You really aren't supposed to trickle charge NiMH batteries over a long period of time.

As for other batteries, it would depend on the capacity of the battery. The larger the battery the longer it will take to charge using that same charger.

< Message edited by Sir Raleigh -- 3/14/2008 9:57:42 PM >

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(in reply to theJoseph)
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RE: Charging questions for NiMH - 3/14/2008 1:03:46 PM   
Leo L


 

Posts: 728
Joined: 10/12/2005
From: Flushing, NY, USA
Status: offline 		There is no good reason to trickle charge your NiMh battery. Charge it at 1C (0.3 Amps for your 300mah battery) using a peak detect charger. The battery can be used anytime within the next 12 hours. If more than 12 hours have elapsed since it was charged, put it on the charger to "Top It Off" before your flight. DO NOT TOP OFF A LiPo!

(in reply to Sir Raleigh)
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RE: Charging questions for NiMH - 3/14/2008 6:35:10 PM   
Swift427



Posts: 482
Joined: 3/15/2007
From: Minneapolis, MN, USA
Status: offline
quote:

ORIGINAL: Leo L
There is no good reason to trickle charge your NiMh battery.


He may be referring to some instructions that suggest a new NiMH should be trickle charged for its intial preconditioning for up to 15 hours before being put into use. At least this is/was the thinking at BatteryUniversity a year ago when I started down/up the learning curve. However, all NiMH manufactures may now be taking the extra time needed to do the preconditioning before shipment. Or is preconditioning for 15 hours at 0.1C really unnecessary? Maybe a precondition charge at .3C for a few hours is more then adequate.

As much as NiMH chemistry allows for peak charging I still charge them at 0.75C with my Dynamite Peak Vision charger. In other words a 1000mAh (1.0 amp) NiMH would be charged at .75amps instead of a 1.00amp(1.0C) setting as it is still charging with a NiMH algorithum. The Vision Peak charger is expensive, but I like it's versatility. Whether you use an expensive charger or a cheapy I would recommend that you monitior your NiMH as it nears the end of its charge cycle. By this I mean that as soon as it feels warmer then your hand it is fully charged.

Because NiMH slowly discharge when sitting idle (even for a couple hours) it is a good idea to top them off by charging them for 3-to-5 minutes until they again feel warmer than your hand just before flying. Also, I believe BatteryUniversity recommends resting a NiMH for 1 hour before

The whole thinking/rush to peak charge as quick as possible by many RC enthusiasts may not be necessary. I believe BatteryUniversity recommends resting a used NiMH for 1 hour before recharging. Does any one have an opinion as to: (1) How much the life of a NiMH is extended by resting for 1 hour as opposed to recharging a used NiMH as soon as it no longer feels warmer than your hand (in 5 minutes); and (2) Charging a NiMH at .75C instead of 1.0C to extend the life and performance out of a NiMH.



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RE: Charging questions for NiMH - 5/23/2008 4:37:50 AM   
Dranac


 

Posts: 3
Joined: 12/26/2007
From: Roundup, MT, USA
Status: offline 		It's a good idea to overnight charge all NiMH, NiCad and lead acid batteries before putting them into service. The purpose is to balance out any cells that are undercharged relative to others in the pack. On a deep initial discharge with nickel batteries, any cells that were carrying a partial charge could be depleted so low before low cutoff voltage of the pack is reached that they go into reversal and will not take a charge. This is ESPECIALLY true with nickel packs made from lots of cells. A single cell depleting early will NOT trip the low voltage cutoff if a half dozen or more cells are still going strong. Forward current through a depleted nickel battery REVERSES the cell and it will will no longer accept a charge.

This happened to me with a new 8-cell NiMH pack recently. I put it on a fast charger with peak detect and all cells but one came up to full charge quickly and caused the charger to go into trickle charge. I should have left it on trickle charge for several hours to bring any lazy cells up to full charge. I immediately put it in my Nexstar and used it until the low voltage cutoff circuitry caused surging of the motor. On recharging, one cell did not take a charge and it measured as a 7-cell pack relative to the other 8-cell pack.

I saved the day by putting the battery pack in my freezer and getting it cold. The reversed cell decided to take a charge and it has been doing well ever since.

If you reverse a cell on a new nickel battery pack by not trickle charging and balancing the cells before a deep discharge, give the "cold soak" treatment to the pack and try charging again. You just might save it. If on an old nickel pack, very unlikely to save it. I doubt that a LiPo pack could be saved if you neglected to balance charge it and then totally discharged one cell. Also, LiPo packs will not tolerate overcharging even if at a trickle rate.

Do NOT use a NiCad charger on NiMH cells, but the reverse is acceptable. Smart chargers are highly advised for NiMH. NiCad likes C/10 rate for 16 hours just fine.

As a former biomedical engineering technician, I bought rechargeable batteries of different types by the cases and replaced many failed battery packs and had to deal with numerous types of battery charging circuitry. Lead acid batteries were the worst for failing, as failure to keep them fully charged lead to rapid plate sulfation and loss of capacity. The device would get used then stuck back somewhere without a recharge or the battery would self discharge then rapidly sulfate. I pulled 6-year old nickel battries out of service that were still doing a good job. 10-year old NiCad batteries that had never been put into service (never charged) were as good as new batteries when I pulled them out of storage. Try THAT with a LiPo or lead acid battery. Lithium primary cells are the only other cell as tough as nickel cells, but with an energy density several times as high. LiPo batteries have a very short shelf life from manufacture to putting into service. Buy those from a shop that has fast turnover of inventory. Once you DO initially charge a nickel battery, USE IT HARD, because they like to be used. Just don't overheat them or shove forward current through them once depleted. If you take it out of service and fail to keep it charged, a nickel battery will die much sooner than had you used it.

Nickel and Cadmium are toxic to humans. Throw out any nickel battery that has a leak. A couple of new AA NiMH batteries leaked while overnight charging, the smell was strong when I got out of bed, and I was sick for the rest of the day. Charge them away from people if charging overnight.



< Message edited by Dranac -- 5/23/2008 5:28:21 AM >

(in reply to Swift427)
       Post #: 5

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