# Charging one battery using another



## RipplesOfLife (Jul 26, 2007)

Does this really work well? (when there's no other way to charge your batteries)

I went cycling today, and when it was night time, I turned on my backlight and nothing. And I don't remember it being dim the last time used it. But I may have left it on.

When I got home, I checked the light with other batteries and it worked.

Anyways, I put a fully charged battery parallel to the dead battery and connected + to + and - to - for about 5 seconds. And seemed to have charged the battery up. (well, at least it got the light working)

Is there any real harm in doing the above? Both batteries are AAAs but the one is of a higher capacitance. Actually the fully charged one was a NiMH and the dead one was a NiCad.

Now logically, if the fully charged battery was of 1000 mAh and the dead battery was 500. Then if the dead battery was giving enough time to fully charge up, the result would be that both batteries will end up with 500 mAh in them?

I think to make it a bit safer would be to put a resistor in parallel(?) as well. And of course, best not to try this with a lithium battery.


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## half-watt (Jul 26, 2007)

RipplesOfLife said:


> Does this really work well? (when there's no other way to charge your batteries)
> 
> I went cycling today, and when it was night time, I turned on my backlight and nothing. And I don't remember it being dim the last time used it. But I may have left it on.
> 
> ...





i'm not going to tell you what to do. i'd like to give you something to think about, even though it's not Li-ion or even primary Li batts.


without commenting upon any other aspect of your approach, as far as the final conclusion drawn, think "heat losses". think small. i think you'll understand. 


also, do you know the charge rates (it will vary) that are occurring during the process? you might want to give that some thought too. it relates to a number of issues, but does touch on the point of "heat".


since i've never read anything on this, nor conducted firsthand experiments in this fashion, i'll leave more practical advice to someone else.


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## greenLED (Jul 26, 2007)

Half-watt, I'm no EE here. No amount of thinking will help me reach an answer. I only know that "reverse charging" can go 

Could you please elaborate on your answer?


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## half-watt (Jul 26, 2007)

greenLED said:


> Half-watt, I'm no EE here. No amount of thinking will help me reach an answer. I only know that "reverse charging" can go
> 
> Could you please elaborate on your answer?




first, aren't all rechargeable batteries reversed charged? not sure? take a voltmeter and check the output of your charger and then note the direction that a cell ought to be inserted to charge it. The key is that the charger (hopefully) controls the charging process. Let's not get into "hole" flow versus electron flow.


HOWEVER,...

i'm *NOT* suggesting that he do it. 

i'm trying to get him to think for himself on the ramifications of doing it. 

hence, the little clues i thought that i was dropping as to some very low level ramifications of attempting to do what he is suggesting ("heat" for instance). 

i was aiming, perhaps too subtly and NOT too clearly, at teaching a man to fish instead of giving him a fish.

also, from a purely theorectical standpoint (not that this was the most important aspect, but it can form a foundation for future application to other situations when one understands the underlying principles) wanted to teach that we don't live in a perfect world. there are physical principles that don't permit perfect transforms. essentially, as far as this subject goes, some energy is converted to heat - at least in my universe.

now, he did say that he wasn't going to do it with Li or Li-ion based battery chemistry. however, i've seen an old D-cell swell from heat when i was younger and vomit out some of its innards.

Perhaps i should have been more to the point - just as you were.


ok. i'm with you Green-LED (love that alias and avatar by the way) not a good idea. don't do it.


Or, better, as Dr. Ian Malcolm (the geeky mathematician,...sorry Dr. M...Chaostician, in black from Jurassic Park) said something to the effect of, "Just because you can do something, doesn't mean that you ought to do something" (in that case it was genetically engineering dinosaurs - maybe it was more like "You were so busy creating dinosaurs, you never stopped to ask if you should").


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## legtu (Jul 26, 2007)

here's an informative thread. it's mainly about li-ion's in parallel but the principles should also apply to any other types of batts.


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## monkeyboy (Jul 26, 2007)

I don't see any problems with charging this way but obviously you will never reach full charge.

BTW; + to + and - to - is forward charging not reverse.

reverse charging is when you have one dead cell and one full cell and connect + to - and + to - (with a resistance load). The dead cell charges backwards. This is effectively like putting a cell into the charger the wrong way.


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## half-watt (Jul 26, 2007)

legtu said:


> here's an informative thread. it's mainly about li-ion's in parallel but the principles should also apply to any other types of batts.




unless i've totally misunderstood OP, he's putting them in series - backwards from each other.


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## half-watt (Jul 26, 2007)

monkeyboy said:


> I don't see any problems with charging this way but obviously you will never reach full charge.
> 
> BTW; + to + and - to - is forward charging not reverse.
> 
> reverse charging is when you have one dead cell and one full cell and connect + to - and + to - (with a resistance load). The dead cell charges backwards. This is effectively like putting a cell into the charger the wrong way.




forwards? backwards? though i adopted, for the sake of a response, another Poster's terminology, i do need some tutoring on proper terminology.

never liked that particular terminology, but then we need a standard to communicate by don't we.


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## legtu (Jul 26, 2007)

half-watt said:


> unless i've totally misunderstood OP, he's putting them in series - backwards from each other.



the OP said... 



RipplesOfLife said:


> ...
> I put a fully charged battery parallel to the dead battery and connected + to + and - to - for about 5 seconds.
> ...



based from that, the connection should look something like this:
(+) --- (+) 
(-) --- (-)

for 'reverse-charging' to occur, the connection would have to be like this:
(+) --- (-) 
(-) --- (+)


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## greenLED (Jul 26, 2007)

Thanks for the link, legtu.


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## Empath (Jul 26, 2007)

The stronger cell will discharge into the weaker, and "charge" it, but you won't have two fully charged cells.

Manufacturer recommendations used to strongly advise against placing NiMH or NiCad cells in parallel for use and/or charging. I don't see it as often, now days. The concern was that a damaged or shorted cell could cause a good cell in parallel to overheat, rupture or even explode. It's not that using parallel cells or charging in parallel is inefficient, impractical or inferior in application. It's because of safety.


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## Handlobraesing (Jul 26, 2007)

RC people do it all the time using a 12v powered hobby charger powered by 12v lead acid battery to charge NiMH/NiCd battery for their helicopter/plane.


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## jayflash (Jul 26, 2007)

To quickly get a few mA into a discharged cell you should be OK. If using a D cell to charge an AA cell I'd check the temperature of the smaller cell periodically and limit the charge time.


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## RipplesOfLife (Jul 27, 2007)

Well, after like 30 minutes the battery went dead again. And that's without turning on the light.

A lot of you have said, the dead battery may not be able to fully charge. That is true depending on the capacities involved.

If both batteries are of the same capacity, the end result will most likely be that both batteries will have a little less than half capacity. So two 1000 mAh AAAs will have about 500 mAh each.

Now if the fully charged battery is of a higher capacity than the dead battery, the dead battery can become fully charged. The higher capacity should be at the most 2x the lower capacity.

And course if the fully charged battery is of a lower capacity than the dead battery, the dead battery can only gain half of the charge of the charged battery.

But in all cases, the amount of current travelling between the two batteries may be harmful to them, and that's where you may get your heat and explosions. Which of course prevents you from fully charging the dead battery. But I think the newer 15 minute quick charge batteries may be able to handle it.


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## legtu (Jul 27, 2007)

it's not just about capacity. in order to be able to 'push' a certain amount of current into a discharged battery, you also need voltage potential. 

a fully-charged NiMH is around 1.4v and li-ion's @ 4.2v. 

using alkalines to 'charge' a NiMH will cause the alks voltage to sag quickly due to the large amount of current involved. the NiMH batt's voltage would then quickly rise due to the current surge and eventually, both of their voltage will equalize and you'll have 0 current between the two. this will most likely happen waaay before the NiMH batt is even close to maybe getting a half charge.

the same applies with using a fully or freshly charged NiMH except for the massive current surge involved. the 'charging' batt's voltage will eventually sag, the voltage of the battery being charged will quickly rise and then 0 current.

now, using 2 batteries in series should be able to fully charge a depleted battery but you'll definitely need something to limit current or .


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## SilverFox (Jul 28, 2007)

Hello RipplesOfLife,

As Empath has pointed out, there are some safety issues, but if both cells are healthy, it usually does not hurt anything.

As far as charging a dead cell with a fully charged cell, good luck with NiMh and NiCd. This will work with lead acid and Li-Ion, but not with NiMh and NiCd cells.

You need around a 0.5 volt potential to charge NiMh and NiCd cells. When you parallel two cells, their voltage will equalize, but not their state of charge. I will take that back, a little. The eventually will equalize, but it takes days. Tests have been done paralleling two packs, one discharge with one fully charged, and after 24 hours less than a 10% transfer of capacity had taken place. The voltage of both packs were about equal, but as soon as you applied a load, the discharged pack quickly dived.

In an emergency I would take two NiMh cells in series and parallel a discharged cell with those two. You would have to carefully monitor the heat of the cell you are trying to charge, but with this method you would at least have enough potential to charge. By the way, this is not recommended at all, but if you have an area that is tolerant to the ramifications of venting cells and take the proper safety precautions against fumes and spraying chemicals, it can be an informative experiment.

Tom


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## Handlobraesing (Jul 29, 2007)

SilverFox said:


> Hello RipplesOfLife,
> 
> As Empath has pointed out, there are some safety issues, but if both cells are healthy, it usually does not hurt anything.
> 
> ...



No reason you can't use a resistor...


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