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Wal Wart Voltage Question - 4/9/2008 2:36:11 AM   
nikg


 

Posts: 423
Joined: 6/24/2007
From: , NC, USA
Status: offline 		I have several wal wart power supplies that I use to slow charge packs. They are rated at 9v and 210, 250, 300, 350 and 500 mA. When I check the no load voltage on them it ranges from 11-16v, generally the higher mA warts have the higher no-load voltage readings. I understand that the ratings are given based on the assumption of a load, so the open voltage is not a true indication of the voltage in a given situation.

Now to the question. I use these to cahrge 6 cell packs, but can they also be used to charge 7 and 8 cell packs (NIMH)? I figure 9v is enough for an average of 1.5v per cell when getting just about fully charged on a 6 cell pack, so that is fine. I figured that I would need a 12v power supply to handle an 8 cell pack applying the same philosophy, but do I? Will the warts that have an open voltage of say 13v and higher be OK for charging 7- and 8-cell packs or should I stick to the ratings instead of the open readings. As I understand it, the supply needs to have a higher voltage than the pack in order to keep current flowing to the pack instead of the other way around. What I'm not sure of is how the voltage is affected by the load. When I attach the 6-cell packs the voltage immediately drops to the voltage of the pack and then starts to rise as the pack gets charged, but the supply voltage is never lower than the pack. If the open voltage is 16v and I connect an 8 cell pack then will it charge to over the 9v rating and eventually to around 11.5-12.5v? Didn't want to try this without some feedback as I do not want to ruing any of my supplies or have something go boom (or more likely fizzle)
       Post #: 1

RE: Wal Wart Voltage Question - 4/9/2008 2:42:44 AM   
guver


 

Posts: 8541
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		I think you got a good handle on it. I agree with you that they can charge the 7 and 8 cell packs (especially the ones that are showing higher voltage)

http://www.rcbatteryclinic.com/ Read the article here "all about wall-wart" chargers. I understand that the wattage remains pretty constant so at the higher voltage battery's rate will be reduced as a result. The voltage will always follow the battery.

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       Post #: 2

RE: Wal Wart Voltage Question - 4/9/2008 5:55:45 AM   
patternwannabee


 

Posts: 241
Joined: 10/8/2002
From: Mars, PA, USA
Status: offline 		nikg,

To be sure, measure your current as you charge. You'll have to hook up your ammeter in series.

I've found that numerous chargers/AC adapters do not deliver their rated specs. I'm still trying to understand how this works. I figure it has to do with the battery pack's internal resistance.

I also found that some chargers will push current through a battery with higher voltage than the charger's rating, while others will not register anything on my ammeter. There were also some chargers that put out zero no-load voltage, but when the battery was connected, registered a current, and the battery voltage rose.

I just bought a bunch of wall warts from Mouser hoping I could use them for overnight charging on my higher capacity NiMHs, but found they're not putting out nearly enough current. Now I've been spending the last couple days collecting AC adapters around the house.

I suppose you should also cycle to check the capacity after charging.

(in reply to guver)
       Post #: 3

RE: Wal Wart Voltage Question - 4/9/2008 6:09:14 AM   
Arceenut


 

Posts: 88
Joined: 11/14/2005
From: Aldergrove, BC, CANADA
Status: offline 		Many chargers use a switching current limiter to charge. the batteries. Some (most?, all?) digital ammeters and voltmeters will give erroneous readings when reading this type of current. A cheap analog needle type of meter will give a more accurate reading of this type of current as the reading will be an average of the on/off voltage and current.

(in reply to patternwannabee)
       Post #: 4

RE: Wal Wart Voltage Question - 4/10/2008 3:26:19 AM   
patternwannabee


 

Posts: 241
Joined: 10/8/2002
From: Mars, PA, USA
Status: offline 		RCNut,

Thanks for the info.

I rechecked my readings with a cheap analog multimeter... trouble is, it was so cheap I'm not sure I trust these figures at all.

However, all the readings made with my digital multimeter were in the same ballpark. All the analog readings were less.

So far, the only charger I've found that delivers the rated current at the rated voltage is the wall wart that came with my transmitter.

Do wall warts generate less current as the voltage rises?

In which case, based on the ratings, how do I choose a wall wart that will produce the appropriate charge rate for my various batteries?

(in reply to Arceenut)
       Post #: 5

RE: Wal Wart Voltage Question - 4/10/2008 4:41:15 AM   
nikg


 

Posts: 423
Joined: 6/24/2007
From: , NC, USA
Status: offline 		patternwanabee---I think your statement about generating less current as voltage rises is dead on. I have checked the current from the wal warts at several points during a charge and have noticed the difference. For instance, my 350mA wart measured 420mA near the beginning of a long charge cycle, but closer to the end it measured 350mA which is what is was rated at for 9v.

I think this may be due to one of two things, or maybe both (or neither---someone please correct me if I'm wrong)
1--The power output "wants" to remain constant, so at lower voltage the current must be higher in order to produce the same power as a higher voltage with lower current. This would explain getting closer to the rated current as the voltage neared 9v(rated voltage).
2--I'm less sure of this one, but as a pack gets charged I think the resistance builds. At first, the cells accept current with little resistance because they are "empty", but as they fill they become more resistant. This in turn lowers the amount of current that can flow from the power source. I like to think of it like blowing up a tire with one of those wimpy little bike tire compressors (that I have and love even though it takes forever) With a flat tire you can hear the pump screaming away as it fills the low pressure space, but as the tire builds PSI you can hear the compressor start to struggle. To me this seems analogous to the battery charging situation.

Again, please correct me if I'm wrong, or affirm that I am on to something.

As far as getting a wart that produces an appropriate charge rate, just get one that is C/10 for the lowest mAh pack you want to use it with. You can always charge that pack for 12 hours and charge the higher capacity packs for longer. At such a low current it should not damage anything if you accidentally leave them overcharging for a short period of time (a few hours)

(in reply to patternwannabee)
       Post #: 6

RE: Wal Wart Voltage Question - 4/10/2008 5:08:01 AM   
Arceenut


 

Posts: 88
Joined: 11/14/2005
From: Aldergrove, BC, CANADA
Status: offline 		I assume you are trying to use a standard "wall wart" . I would not recommend it. The "charger" you got with your radio is designed with an internal resistor to limit the current to the battery. Standard wall warts for powering devices do not have a current limiting resistor and will supply far too much current depending on the battery and wall wart. You can use a standard wall wart if you add a resistor in series with the supply.

(in reply to patternwannabee)
       Post #: 7

RE: Wal Wart Voltage Question - 4/10/2008 3:11:42 PM   
Rodney



Posts: 4653
Joined: 12/8/2001
From: FL
Status: offline 		Most of the wall warts are just a transformer followed by a rectifier (this can be full wave or half wave--usually full wave) with usually an LED buffered by a resistor to limit LED curent to indicate that current is flowing. The current is usually limited only by the internal resistance of the transformer windings although some (very few) have a limiting resistor as well. Yes, the current will decrease some as the battery being charged rises in voltage. Since there is usually no filtering of the rectified wave, the current readings you get on a typical meter will be in error--usually reading higher than actual-- unless you have one that reads "root mean square" values. However, the voltage readings you get with a battery connected will be reasonably accurate. Voltage readings with the battery disconnected will be high as you will be reading peak voltages of an unloaded transformer.

(in reply to Arceenut)
       Post #: 8

RE: Wal Wart Voltage Question - 4/10/2008 8:55:51 PM   
Jazzy



Posts: 1097
Joined: 1/8/2002
From: Erie, PA, USA
Status: offline 		You might be surprised by the DC output of wall warts. I've hooked a couple up to some mid to high-end digitizing o-scopes and the output was very clean and stable. While I haven't tested or taken apart a wide variety of wall warts, the ones that I have seen the insides of contained enough capacitance to keep the output clean at the nameplate rating. Results may vary.
The current readings taken with a multimeter may be closer than you think

_____________________________

Aww $#!^.... I left the )@#& transmitter at home!

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       Post #: 9

RE: Wal Wart Voltage Question - 4/10/2008 10:44:29 PM   
rctrax



Posts: 597
Joined: 5/11/2002
From: Mosinee, WI, USA
Status: offline 		I use a multimeter hooked in seires and check the curent going into the batt.If in the range is of the amount I want it is a satifactory charger for this pack.

(in reply to Jazzy)
       Post #: 10

RE: Wal Wart Voltage Question - 4/10/2008 10:46:17 PM   
J-3



Posts: 191
Joined: 2/5/2002
From: Saint Peters, MO, USA
Status: offline 		You might want to take a look at the CH-96 charger, I use two of these to slow charge my high capacity packs, and they do a good job.
...Brian

http://www.hangtimes.com/chargers.html

(in reply to Jazzy)
       Post #: 11

RE: Wal Wart Voltage Question - 4/11/2008 2:33:25 AM   
nikg


 

Posts: 423
Joined: 6/24/2007
From: , NC, USA
Status: offline 		Arceenut---I have been using standard wal warts used to power devices like phones etc. Will these deliver far too much current as you stated? Do I need a resistor? Am I damaging my packs? Am I using my meter wrong (series for current readings)?

I may be wrong, but I thought that any of these power supplies had a limit even without a resistor. The ratings describe the voltage and current capabilities for what they were designed for (phone), but I am skeptical that hooking one of these to a battery would produce extremely high current. I realize there might be some meter error, but is it really that significant? Again, I doubt a wart of any kind rated at 9v and 350mA would push more than 450mA-500mA into a 6 cell pack.

Can somebody clarify whether any of these "class 2" power supplies is OK to use.

(in reply to J-3)
       Post #: 12

RE: Wal Wart Voltage Question - 4/11/2008 4:26:28 AM   
Arceenut


 

Posts: 88
Joined: 11/14/2005
From: Aldergrove, BC, CANADA
Status: offline 		You are correct that the wall wart (WW) will self limit at some point but as you say it can limit itself to some value(450ma). For one thing this is near 1C rate of charge on a 450 mah battery. It is not a good idea to "fast charge" in this manner unless you have some means to stop charging when the pack is full. Overcharging at this rate will certainly cause harm to the battery. Most WW "chargers" are limited to 1/10 charge rate to protect the battery from damage. Most batteries have protection in the form of gas absorption to accept a small amount of overcharge at a low rate. On the other hand a WW rated at 350 ma will be driven beyond the ratings and overheat/fail damage itself or cause a fire in the worst case. Fast chargers have a "full charge" detection built int them to prevent battery damage, IE peak detection, temperature sensing, voltage sensing or a combination of them. I am assuming of course you are charging NiCads or NiMh batteries. Lithium cells are a completely different ball of wax and a proper charger with the proper voltage detection and shut off mechanism is an absolute must for safety.

(in reply to nikg)
       Post #: 13

RE: Wal Wart Voltage Question - 4/12/2008 3:53:42 AM   
nikg


 

Posts: 423
Joined: 6/24/2007
From: , NC, USA
Status: offline 		Arceenut---Thanks for clearing that up. I am charging sub-c GP3300's, so my warts are about C/10 give or take.

I did connect the 9v 350mA WW to my 8 cell pack tonight. The no load voltage was 16.46v on my meter. I did not take any initial current readings once I connected it to the pack, but the initial voltage of the pack was 10.36v. After about 5 hrs it is up to 11.16v. I checked the current at that point and it is pumping out only 200mA, so the current has dropped below the rated output for the first time since I have used this charger. This is consistent with the idea that to produce the same power the current has dropped in light of the higher voltage.

Doing some quick math it appears that total power output is down. I guess this is due to the fact that the 9V rating has been far exceeded and it is just not able to supply current efficiently at the higher voltage.

Am I running the risk of damaging my WW? It seems to be running cooler than normal because I think it is flowing less current, but will the increased voltage have a negative effect?
Also, is the pack safer from overcharge with this reduced current? I know that as you drop below C/10 there is little chance of damage to cells by overcharge. NIMHs don't like a long lingering slow charge according to the Battery University site, but ensuring that they are full by leaving them on 200mA for several hours extra shouldn't hurt anything, right?

(in reply to Arceenut)
       Post #: 14

RE: Wal Wart Voltage Question - 4/12/2008 4:08:12 AM   
Arceenut


 

Posts: 88
Joined: 11/14/2005
From: Aldergrove, BC, CANADA
Status: offline 		You should be all right from what you describe. As lng as you stay around C/10 and the WW does not heat up.

(in reply to nikg)