Is this power adapter ok to use?

[Aarong81]

I have a model airplane with a 7.2 volt battery and I just made a couple new battery packs with much more capacity rating. But now it takes alot longer to charge and I'm shopping for another power adapter so I can charge at least two at a time. My new battery packs are AAA cells and are 7.2 volt and 1800 mAh Ni-MH. The original power adapter has an output of 7.2 volt and 300 mA but my shopping for another adapter has led me to advertizements for adapters that say they charge 6v, 7.2v and 9v batteries and the adapter is actually a 12v 400mAh.
IS IT SAFE TO CHARGE 7.2 BATTERY PACKS WITH A 12v ADAPTER?????

And if so, do I need to do any special calculations for charging time to figure in the extra voltage? I come up with 7.2 hours when charging a 1800 mAh Ni-Mh 7.2v battery with a 7.2v 300mAh adapter. But the extra voltage of a 12v adapter, if its ok, would seem to multiply the amperage so I'm confused if it requires a special calculation to figure charge time.

 

[Aarong81]

This is the ad I was referring to...http://www.batteryjunction.com/12v300.html It just seems taboo to charge a 7.2v battery with 12v.

 

[Harald Kapp]

The ad says "discontinued" and gives no useful information on the adapter.
Connecting a 12 C source directly to a 7.2 V battery will result in a high current. What happens then depends on the construction of the voltage source:
- a high power source will deliver a high current and the battery will be destroyed
- a low power source will limit the current and the output voltage will break down.
The latter is equivalent to simple NiCd chargers that use a high voltage plus a series resistor for current limiting. You'll have to take care of the charging time yourself in order not to overcharge the battery.

Since the original adapter is no longer available, here's a replacement http://www.batteryjunction.com/unsmch.html offered by the same dealer. It says in the description that this charger has an automatic voltage detection. It also says that there are some safety features built into this adapter.

Harald

Harald

 

[mechtronics]

have you tried it at all? voltage sometimes doesnt necessarily matter, all depends on the current.

 

[Aarong81]

I have not tried using a higher voltage charger yet, but I need to correct my voltage in the packs I am using. They are made of 7 AAA cells and total 8.4 volts 1800 mAh NiMH. I've charged them with the same charger I have for the lesser capacity battery packs but it seems the new packs will not charge all the way. They even drain quicker than the old smaller cap bat packs. Does it make any sense that a high mAh rated battery will not charge completely with a low current charger? The charger is 8.4v 300mA and the new batteries are 8.4v 1800mA. This doesn't seem unusual to me, that would take 7.2 hrs to charge and seems like a healthy time frame to charge. I only charged these for about 5 hours before testing them and I expected them to be at least 3/4 charged. They drained in less than half the time of the really small capacity 600mAh packs when they are full charge. Even at 3/4 charge I would expect alot longer run time because of the higher mAh rating. Do I need more amps to charge these new packs? Or is a 5 hr charge time just expecting too much on a first time charge?

On a side note, does anything look to be a problem with the new battery pack, soldering, wiring, ect?

 

[CocaCola]

Where did you get that new pack? It's generally bad practice to solder to batteries, and generally only seen on DIY hobbyist projects, any 'legit' battery company should be tack welding the terminals/tabs... Also the lack of any type of insulation against shorts aka heat shrink or other casing is disturbing... This lack of 'professionalism' in assembly would also lead me to believe that the new cells could very well be 'cheap' import cells and thus the printed mAh rating is well, lets just say pick a number from a hat that looks good and run with it value... Especially when they are claiming 1800mAh out of a AAA sized NiMH when most reputable name brand cell manufactures max out at about 800-1000 mAh for AAA sized cells... Yes, you might be able to get AAA sized cells in the 1800 mAh range (unlikely), but they would be costly and few and far between offerings and I would question that they are properly rated even then, anything over 1000 mAh is likely fake...

** Edit never mind I see you made the pack, the question still begs where did you get the cells? And did you apply excessive or long heat when soldering? I'm willing to bet those new cells are at best 800 mAh, probably 600-700 mAH realistically, since I just glanced on Ebay and see them being sold by numerous less than reputable sellers...

 

[BobK]

You cannot charge NIMH batteries just by connecting them to a voltage source. Read here. Then buy a proper charger for them.

http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Bob

 

[(*steve*)]

My question (related to Bob's above) is "What are the original batteries?"

It looks like 4 cells. I can't imagine they're tiny lead acid cells(!) but maybe LiPo in series/parallel? 8.4 volts sounds right for that.

 

[Aarong81]

Yes it was constructed cheaply by myself, I used a low temp solder to connect the cells so they wouldn't be damaged from over heating.

The link provided says
"A NiMH charger can charge NiCd, but not the other way around. The original NiCd chargers would overcharge NiMH.

To charge NiMH with a NiCd charger, estimate the time and disconnect the charger manually. Do not leave NiMH on charge longer than needed (full charge detection may not work and the trickle charge current is too high for NiMH)."

So NiCd chargers will not work but when using NiCd chargers estimate the time and disconnect the charger manually.

Many NiMH battery packs are sold with an AC adapter like the one I am using as it was sold together, is that considered a NiCd charger? The only part that made sense on that website is that NiMH batteries don't slow charge well and charge better with a fast charger. I think maybe I am using too little current to charge a large capacity NiMH pack, almost considered a trickle charge.

What would constitute a fast charge, 1 to 2 hours?

 

[Aarong81]

My question (related to Bob's above) is "What are the original batteries?"

It looks like 4 cells. I can't imagine they're tiny lead acid cells(!) but maybe LiPo in series/parallel? 8.4 volts sounds right for that.

The original batteries are pictured in yellow and are 7 AAA cells at 600mAh. They came with the charger pictured, an 8.4v 300mAh AC adapter. I'm thinking that charger is too low current to charge 1800mAh NiMH battery since NiMH's don't slow charge well.

 

[CocaCola]

I'm thinking that charger is too low current to charge 1800mAh NiMH battery since NiMH's don't slow charge well.

I'm thinking more likely you were sold a big load of BS from the manufacture and they are far from the 1800 mAh rating, more like probably 600-700 mAh...

 

[Aarong81]

I'm thinking more likely you were sold a big load of BS from the manufacture and they are far from the 1800 mAh rating, more like probably 600-700 mAh...

I think so too, with the exception of what you speculate mine might actually be. LOL They are probably less than that.

I just searched for the highest capacity AAA's and bought some.....cheaply. No harm, lesson learned. Maybe I'll make a 7 AAA cell 1000mAh NiMH pack, at least those exist.

The only thing certain is that 1800 mAh NiMH AAA's are fake.

Thanks for the replies!
This thread has nowhere else to go unless someone wants to focus on my soldering job. Hopefully I'll not check back in to see how ugly it gets!

 

[(*steve*)]

NiMH's don't slow charge well.

They don't?

Have you tried measuring the voltage across each cell in your pack after charging them? Don't have anything else connected.

Do it, then post the results.

It would be interesting to see the same from the old pack too if you can (or total voltage if you can't)

 

[Aarong81]

No clue really, Bob posted a link http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride that says...
"It is difficult, if not impossible, to slow-charge a NiMH battery."
"Nickel-based batteries are best fast charged; a lingering slow charge causes memory."

 

[BobK]

I built my own NIMH charger to use with two drill-drivers that I had. They came with what was essetially a trickle charger, whch killed the batteries in about 1 year of light use. I learned about charging NIMH from sites like the one I posted above. To charge them correctly you need to detect a small drop in the terminal voltage.

Bob

 

[(*steve*)]

I clearly didn't post my reply yesterday...

I understand that issue. The problem is not the slow charge, but the determination of end of charge.

However my other questions would be interesting to get an answer to.