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'[EE]: Making batteries idiot-proof ... questions'
NOTHING is idiot-proof, as idiots are cunning and very resourceful,
MUCH more so than design engineers..
But I have discovered that building a 12V NiMH pack, and installing
a Maxim one-wire thermometer is about as good as it gets. You use the
thermometer to monitor both charging AND battery failure, in that a bad
cell heats up when being charged AND being used. You use a simple
PIC to monitor battery voltage, so if it overheats before topping off (15V),
the battery pack is defective.
Don't fiddle with NiCADs, the technology is considered by MOST US states to
be too toxic to be sold anymore.
On Fri, Jul 4, 2008 at 5:29 AM, Cedric Chang <nope9.com> wrote: cc
|Do not disable a pack and ruin a consumer's investment unless there is a way to
'reset' it, since there are always unforeseen things. The protection is to
limit your liability for the most part. You cannot know every field situation
For instance, during an emergency, it may be possible to locate a replacement
cell rather than a whole pack, that MAY be going bad due to one cell crossing
an arbitrary temperature point. It could also be that it was charging in the
sun and was close to max temp to begin with. An unresettable pack would be dead
by the pic than... this is very valid in third world situations where
replacements of anything are hard to get, and an 8 cell pack may 'fix' 8
failing packs if designed right, vs leaving the team or people only one good
pack from the mfg.
For resource, usage, and cost and supply reasons, consider the following if
using a pic with the ability to kill the pack:
- only shut the pack down at the temp. extreme, not permanently kill it
- allow cells to be replaced
- if your pic identifies a particular cell, flash it to the user via an led,
indicating which cell
- compare to starting ambient temp to better tell why the temp rise
- when 'getting close' to high temps, cut back on current and manage the temp,
not just shut it off.
You are at the mercy of cell manufacture, and I have seen so many packs for
things go bad for one cell (even identical AA's treated identically) that get
years back by changing one cell, that it's an extreme waste. The practice with
Li-ion packs to kill them outright is just as bad. Make it a hurdle to get to
the reset, so it won't be done casually, but consider the consumer and possible
extreme situations. If anything, you will accomplish the goal for the idiot
consumer, while differentiating your product for those that are in tougher
situations. It will be the product one invests in, rather than a throw-away.
I'm sure there's green points in there too... ;)
Bob Axtell wrote:
> -----Original Message-----
> From: mit.edu On Behalf Of Bob Axtell piclist-bounces
> Sent: Monday, July 07, 2008 2:19 PM
> Don't fiddle with NiCADs, the technology is considered by MOST US
> states to be too toxic to be sold anymore.
I have not seen any US State banning NiCad's, do you have a reference?
AFAICT, they are still widely available which is a good thing because other
than lead-acid no other rechargeable is suitable and reasonably priced for
use below freezing. Even the EU RoHS standards allow NiCad battery usage.
As with all battery types there are rules for disposal in all the States but
I have seen no evidence of any attempt at an outright ban.
> --Bob A
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