Electricity

2005-06-11
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Disambiguation: “cell phones” (Mobile phones)

This project was started for the HP 200LX (see HP 200LX and Power), as I wanted to learn more about battery life and such.

These notes are old and I still haven’t wrapped my head around all the misinformation out there. On top of that, technologies have changed a lot since this stuff was noted.

Table of Contents [hide]

Keywords ∞

  • shoe generator
  • Piezoelectric generators
  • piezoelectric
  • lithium advancements

  • flexible batteries

Stuff ∞

  • Get a 4 AA NiMH solar recharger
  • Get 4 1600mA NiMH batteries – don’t recharge them inside the palmtop.

  • Time the lifespan of the batteries

Source not recorded:

Battery manufacturers recommend to slow charge a new NICAD battery for 24 hours before use. This initial trickle charge helps to redistribute the electrolyte to remedy dry spots on the separator that may appear when the electrolyte gravitates to the bottom of the cell during long storage. A slow charge also helps to bring all the individual cells within a battery pack up to an equal charge level because each cell may have self-discharged to different capacity levels during storage.

Commonly recommended way to get the maximum performance from your laptop battery. Refresh is the process of allowing your computer battery to drain completely (to do this you will need to disable all power management utilities). Once drained recharge the battery to 100% this will refresh all cells within the battery. Doing this every few months will help extend the life of your portable computer battery

The end-of-discharge voltage of the Li-ion needs to be carefully controlled and must, under no circumstances, go below 2.5V per cell. Protection circuit intrinsic to the Li-ion pack prevent the battery from being discharged below the safe limit. If allowed to self-discharge below 2.5V, unrecoverable capacity loss occurs when stored in that condition for three months or longer. Similar to the NiMH and SLA, more cycles can be obtained by partially, rather than fully discharging the Li-ion battery.

NICAD / NiHM = 1.25V

Nicad ∞

nicad has more of a memory effect

  • first charge = slow charge

faster recharge time

Source not recorded:

Use up the charge in the battery before recharging, preventing failure due to voltage memory. Store the battery in a discharged condition. Allow the battery to cool after use, before recharging.

More batteries are destroyed or damaged by bad charging techniques than all other causes combined. Once a battery reaches full charge, the charging current has to go somewhere — most often, generating heat and gases. Both are bad for batteries.

A NiCd stored, used, or charged under high temperature conditions will die an early death. Heat causes the separator to weaken, the seals to weaken, and greatly accelerates changes in the plate material, some of which cause the dreaded memory effect.

The trade-off for a rapid-chargable battery is reduced capacity

Partial recharge will only rebuild the negative plate, stimulating cadmium large-crystal growth, which reduces lifespan since the crystal cannot be broken down readily when it is large.

This does NOT mean that one needs to cycle one’s battery each time it is used. This does more harm than good, and unless it is done on a per cell basis, one risks reversing the cells and that really kills them. Perhaps once in a while, use the pack until it is 90% discharged, or to a cell voltage of 1.0V under light load. Here, about 95% of the cells capacity is used, and for all intensive purposes, is discharged. At this point, recharge it properly, and that’s it.

Don’t overcharge, it screws up the chemistry of the positive plate… trickle charging will do it as well!

DON’T deliberately discharge the batteries to avoid memory

DO let the cells discharge to 1.0V/cell on occasion through normal use

DON’T leave the cells on trickle charge for long times, unless voltage depression can be tolerated

DO protect the cells from high temperature both in charging and storage

DON’T overcharge the cells. Use a good charging technique

DO choose cells wisely. Sponge/foam plates will not tolerate high charge/discharge currents as well as sintered plate batteries will

There are many methods of charging. One is trickle or the old 15 hour method. This involves using a current of about 50 mA (for AA cells) and leaving them on charge for 15 hours. At this current level, oxygen diffusion is more than enough to take care of the excess current once full charge is achieved. Of course, one runs the risk of voltage depression due to overcharge.

The best method is the so called delta-V method. If one plots the terminal voltage of the cell during a charge with a constant voltage, it will continue to rise slowly as charging progresses. At the point of full charge, the cell voltage will drop in a fairly short time. The amount of drop is small, about 10 mV/cell, but is distinctive. There are circuits out there built specifically to look for this. The Maxim MAX712 and 713 ICs are ones that come to mind now. This method is expensive and tedious, but gives good reproducible results. There is a danger in this though. In a battery with a bad cell this delta – V method may not work, and one may end up destroying all the cells, so one needs to be careful.

Another cheap way is to measure the cell temperature. The cell temperature will rise steeply as full charge is reached. When the cell temperature rises to 10 degrees C or so above ambient, stop charging, or go into trickle mode.

Whatever method one chooses, a failsafe timer is a requirement with high charge currents. Don’t let more than double the cell capacity of charge current flow, just in case. (i.e. for a 800 mAh cell, no more than 1600 mAh of charge).

C/”x” == if it is a 1000mA battery, C/10 = 100Ma, C/2 = 500mA

If using an unregulated charger — one that doesn’t do any detection of full charge, then one must restrict your charge current to the overcharge capacity of your cell. All [ 1 ] cells I have seen can handle C/10 (approx. 50 mA for AA cell) indefinitely without venting. This is not to say that one won’t get voltage depression, but rather that one won’t destroy the cell(s).

If one wants to get a bit more aggressive, a C/3 charge will recharge the cells in about 4 hours, and at this rate, most cells will handle a bit of overcharge without too much trouble. That is, if one catches the cells within an hour of full charge, things should be OK. No overcharge is best of course.

Only with automatic means of full charge detection should one use charge currents above C/2. At this current level and above, many cells can be easily damaged by overcharging. Those that have oxygen absorbers may not vent, but will still get quite hot.

Yes, NiCds have a bad habit of going dead when you just leave them. Fortunately you can recharge them. The current cells discharge about 1 percent a day, maybe a bit less. Expect them to be mostly flat after 3 months. Unfortunately, the so called 15 hour trickle chargers more than make up for self-discharge. In fact most of the current goes to making oxygen, not making up for self-discharge. If you want to make something to keep your cells from self-discharging, make a 1 to 2 mA current source. That should more than overcome self-discharge.

NiMh ∞

nimh has less of a memory effect

shallow recharge cycles (the longevity is directly related to the depth of discharge)

slower recharge time

bleeds off 10% of it’s life in 24 hours.. then 10%/month

inadequate charging time will reduce their lifespan

benefits of keeping them in the fridge or freezer. Keep them above -20f and you’ll be sure not to freeze the electroltye

Lithium-Ion (Li-Ion) rechargeable batteries have the highest energy density among commercial batteries; two times that of Ni-Cd systems and their self-discharge is very low. Li-Ion’s are stable and safe because not metallic lithium is used.

Batteries should always be stored in a cool dry place and never be left in direct sunlight, or in temperatures below 30 degrees(F) and above 100 degrees(F). Batteries should be fully charged before storing for an extended period of time.

A good rule of thumb when determining battery life in relation to temperature is that for every 8.3C (15F) average annual temperature above 25C (77F) – used to determine the lifespan of the battery – , the life of the battery is reduced by 50 percent.

Solar Power ∞

Resources ∞

stuff
on batteries
other stuff

Footnotes


  1. Ni Cd[ ↩ ]
4 comments

4 comments on “Electricity”

  3. “Cell and Power technology” was ported into Power. It’s unfortunate I never really finished this project. I would have gotten back into it had I bought the other model of [[digital cameras |digital camera]] which I was looking at, which uses AA cells. Well.. if I get back into my [[HP 200LX]] I’ll look at this again. When I get that money thing sorted out again I might take a stab at it. That’ll spill into the [[DOS]] stuff again and generally be a good thing. Hopefully also for [[task management]].. since that was a drive for the 200lx. Oh, am I going off onto a tangent again? Hrmph.

    Reply

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