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Battery question - two 7.2 V packs to get 14.4 V



 
 
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  #1  
Old November 23rd 05, 09:07 PM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Hello

Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.

Thank you




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  #2  
Old November 23rd 05, 09:18 PM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Sharp wrote:

Hello

Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.

Thank you


Even better is charge each cell separately, though that's not possible if it's already
assembled.

The failure mode of multi-cell batteries is, if you run them until the weakest cell
is dead (the rest generating voltage still), the strong cells reverse-charge the
weak ones as you use it, ruining those cells.

So it's best if all cells are equally (ie fully) charged, so they all die pretty much
at once and don't ruin each other. (Or you recharge the thing before the weakest cell
is near dead)

You can't guarantee charging all the cells equally unless you fully charge them
individually ; lacking that, you do the best you can.


--
Ron Hardin


On the internet, nobody knows you're a jerk.
  #3  
Old November 23rd 05, 09:58 PM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V


"Ron Hardin" wrote: Even better is charge each cell separately, though
that's not possible if it's already assembled. (clip)
^^^^^^^^^^^^^^^
I can see how that could happen if you threw a handful of cells with
different ages, brands, states of charge, etc into a circuit. There would
likely be a weakest one or two that would be "dragging their feet" while the
others were still pulling. This is undoubtedly why instructions always say
to replace the cells at the same time. But on a light, the output drops off
a lot when the voltge gets low, whether this is due to uniform discharge of
the cells or one very weak one. You would not go on running the light under
those conditions.

The fact that multi-cell batteries are universally used to power lights and
tools attests to the fact that this is a rare failure mode.

I see no reason not to charge two batteries separately and then run them in
series, provided they match the characteristics of the charger. If you had
a heavy enough charger, you could even charge them in parallel and discharge
them in series.


  #4  
Old November 23rd 05, 10:18 PM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Leo Lichtman wrote:

"Ron Hardin" wrote: Even better is charge each cell separately, though
that's not possible if it's already assembled. (clip)
^^^^^^^^^^^^^^^
I can see how that could happen if you threw a handful of cells with
different ages, brands, states of charge, etc into a circuit. There would
likely be a weakest one or two that would be "dragging their feet" while the
others were still pulling. This is undoubtedly why instructions always say
to replace the cells at the same time. But on a light, the output drops off
a lot when the voltge gets low, whether this is due to uniform discharge of
the cells or one very weak one. You would not go on running the light under
those conditions.


I've reduced a brand new 9v NiMH battery to 7.5v by letting the device stay
turned on accidentally for a week. The dead cell never came back, it's just
a short now. (The battery continues to work as a 7.5v battery however.)

Those are certainly matched cells; but the stronger ones got the weak one nevertheless.
There's always one that dies first, and it gets ganged up on.

--
Ron Hardin


On the internet, nobody knows you're a jerk.
  #5  
Old November 23rd 05, 10:27 PM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Leo Lichtman wrote:

The fact that multi-cell batteries are universally used to power lights and tools attests to the fact that this is a rare failure mode.


I'm with Ron on this one. Even when a battery pack is assembled with
all brand new, seemingly identical cells, there's a good chance that at
least one cell will not be as robust as the others.

The further you run the pack down, the greater the chance of "reverse
charging" the weakest cell. Or a "whisker" can grow, shorting out a
cell. The whisker can sometimes be "zapped" with a jolt of higher
voltage, but the cell will never be quite the same. I've had many NiCd
packs and it's not unusual to see failures of this type.

Art Harris, N2AH

  #6  
Old November 24th 05, 01:03 AM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

I am an Electrical Engineer with about 25 years experience. The simple
answer is yes it will be fine. All the problems being discussed could happen
if the packs are used individually or in series.


  #7  
Old November 24th 05, 01:25 AM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Thank you guys for your quick answers !


"Larry" wrote in message
...
I am an Electrical Engineer with about 25 years experience. The simple
answer is yes it will be fine. All the problems being discussed could
happen if the packs are used individually or in series.



  #8  
Old November 24th 05, 01:45 AM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

You guys seem to be arguing against Sharp's idea using "backwards logic."
He wants to discharge two battery packs in series, and charge them
separately. Ron suggested charging the cells separately, in order to avoid
a failure mode that remotely COULD occur during discharge. Ron cited his
experience of accidentally discharging a battery pack to exhaustion and
destroying one of the cells.

Can we agree that this was an accidental abuse of the battery pack, and has
nothing to do with the manner in which the charging was done? I will stick
to my point, that if you are running a light from these batteries, you are
going to recharge them when the light goes dim, unless you have the same
lapse that Ron describes, and walk away from the bike leaving the lights on.
And, if you *did* do that, it would not have any relation to whether you had
charged a lot of cells individually, or in two factory-wired battery packs.

Art makes the point that in any battery (group of cells) one cell is likely
to be weaker than the others. Again, this will be true whether or not the
cells are in battery packs or separate. If you run the pack to exhaustion,
the weakest cell is likely to be "reverse charged." So don't do that.

The only remaining point is: by charging the cells individually, are you
more certain that they will all be equal when put back in service? I don't
see how. If fact, charging them individually increases the likelihood that
each will get a slightly different current, and for a somewhat different
time. The best way to get them all alike would be to charge them all in
series--that way they all get an identical charging current for an identical
time.

So, Sharp, I say, "Go for it."


  #9  
Old November 24th 05, 02:45 AM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

On Wed, 23 Nov 2005 21:07:39 +0100, "Sharp" wrote:

Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.


Certainly you can do that. No reason whatsoever not to.

Jasper
  #10  
Old November 24th 05, 04:52 AM posted to rec.bicycles.tech
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Default Battery question - two 7.2 V packs to get 14.4 V

Jasper Janssen wrote:
On Wed, 23 Nov 2005 21:07:39 +0100, "Sharp" wrote:


Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.



Certainly you can do that. No reason whatsoever not to.

Jasper


I use one of those chargers and they work really well. I've convinced
several other people to buy them also. I've made my own battery packs
with 10 cells each. One is 2.5 amp hour and the other is 4 amp hour. I'm
running twin 5 watt Luxeons. I have the same setup on my road bike and
my mountain bike and share the batterys.

Marty
 




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