piclist 2003\09\29\211234a >
Thread: NiMH Recharge in 30 minutes?!??
www.piclist.com/techref/power/batterys.htm?key=nimh
BY : Dave VanHorn email (remove spam text)

At 08:50 PM 9/29/2003 -0400, M. Adam Davis wrote:
>Many rechargable batteries, especially high capacity types, accept a
>very fast charge equal to or greater than 1C.  The main problem with
>charging, aside from battery chemistry and design, is heat.  I'm told,
>but have never verified, that the hotter the cell is, the more energy
>goes into heat production rather than into charging.

I think that's not a causal relationship, but yes, the temperature goes up
as the cell becomes less and less efficient at storing the energy.
In NIMH, you can get into a thermal runaway.

>This is one problem with charging at 1C for an hour - it gets warm after
>the first 20 minutes, so you lose more and more of the charging current
>to heat and you don't get a full charge.  You need to charge at some
>larger value (1.1-1.5C) for the hour to get a full 1C into the cell.

A 1 hour charge gets 80-90% charge into the battery.
Simple chargers stop there.
More complex chargers continue at a lower rate, and pack more energy in,
over an extended period.

>Some charges implement a bulk charge feature to get around the heat
>generation.  Since the batteries have a large thermal mass they charge
>them at up to 10C for a short period of time (until they reach a certian
>temperature) and then lower the charge to 1/2C or lower to complete the
>charge.  Then you can complete the charge within 30-60 minutes.

I've never seen a reputable design go that fast.
End of charge would be hard to detect, and if the cells were near full to

>I do not know whether this damages the cell.  Many people who want fast
>charging also want high capacities, and while a 4.5AH 4/3A cell is not
>cheap, they are often more interested in high current output and fast
>charging than they are in cell health during, for instance, RC races.

True. They also solder the cells together end to end, which damages the
insulator/seals, and plugs the vent holes on some designs, and they claim
this does no damage.
Talk to a battery engineer.. They have a hard time figuring why people do
these things, when they state over and over, very clearly, NOT to do it,
and why.

I can see both sides.. I don't have a problem with expending a battery pack
in a couple cycles, if it means winning that "important" race.
However, the fallacy comes in here, that what these guys do to their
batteries must be "the right thing" and "really good" because they win the
races.
Good for winning races, maybe. Good for batteries, almost certainly not.

A friend of mine was a champion R/C racer some time back.
He did harmless things just to see what he could get people to copy.
One race he ran and won, using some cheap pink rims. Next race, pretty much
everyone's running the same cheap pink rims.....

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