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> VHFHomepage > HeatWhat are battery manufacturers doing to prevent damage from overcharging?
How about those high-temp batteries?
What about those super-high capacity cells?
I need to “properly” charge cells. How do I do that?
I’ve seen terminology like “C/n” where n is a number. What it gives?
What’s the right charge current?
You
mentioned cell reversal. What is that, and why is it so bad?
(see below)
Q: What are battery manufacturers doing to prevent damage from overcharging?
A: Quite a lot. The demand for rapid charging has lead to a great increase in overcharging abuse. Most all NiCd cells can be rapid charged. The trick is to stop charging when it is fully charged. The so called “rapid charge” type of cells just incorporate protection against overcharging at high currents. Most often, this is done with activated carbon inserted in the cell to promote the collection of oxygen and to deliver it to the cathode for recombination. By increasing the rate of oxygen transport, one is increasing the ability of the cell to resist venting. Note however, that heat is still generated. The price one pays for this is reduced capacity. Everything takes space in the cell, and space for carbon means less space for active material. Also, there have been some indications that carbon can cause the cadmium metal to corrode, possibly leading to a shorter life.
Q: How about those high-temp batteries?
A:
There are ways to make NiCd cells more resistant to the damaging effects of
heat. Mainly, using polypropylene separators and changing the electrolyte to
sodium hydroxide makes the cells more durable under high temperatures. However,
the cost is higher, and the internal resistance is raised, making high current
discharge more difficult. Unless one knows that cells will be used at high
temperatures, don’t bother—learn to take care of the cells to avoid
overheating them.
Q:
What about those super-high capacity cells?
A:
Yes, the manufacturers are in a numbers game. It used to be that AA cells were
450 mAh. Then came 500, then 600 mAh. Now, 700, 800 and even 900 mAh cells are
available. Next year, Sanyo will introduce a 950 mAh cell. OK, so what’s going
on? Well, the highest capacity cells use foamy or spongy backing material for
their plates. This allows packing more active material into the plates, but the
cost is higher resistance. Recall that one of the great virtues of NiCds is
their low internal resistance—this allows large discharge currents for
transmitting, for example. So far, the highest capacity sintered plate (best for
low resistance) cell I have seen is the Sanyo KR-800 cell, rated at 800 mAh. The
Panasonic 900 mAh cell is of the foam type, and may work for a specific
application, but expect higher resistance. I also suspect (but am not sure) that
the Millennium cells are also foam type. For most consumer applications, the
internal resistance isn’t an issue—for high power transmitting (e.g. more
than 1A of current), it can be a concern.
Q: I need to “properly” charge cells. How do I do that?
A: 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. If one ends up putting in more than double the charge capacity of the cell, then something is wrong. 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).
Q:
I’ve seen terminology like “C/n” where n is a number. What it gives?
A:
This is a method of expressing current as a fraction of the Ah rating of a cell.
Simply, a 100 mA current means much more to a small N cell than to a large D
cell. So, rather than use absolute units of amperes, cell manufacturers often
use fractions of cell capacity, or C. A typical good AA cell has a capacity of
700 mAh, so C = 700 mA. A current of C/10 is therefore 70 mA, while C/2 is 350
mA.
Q:
What’s the right charge current?
A: Depends. 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 NiCd 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. With a good charge control circuit, charge currents in excess of C have been used—the problem here becomes reduced charge efficiency and internal heating from ohmic losses. Unless one is in a great hurry, avoid rates greater than C.
Q: You mentioned cell reversal. What is that, and why is it so bad?
A: In a battery, not all cells are created equal. One will be weaker than the others. So, as the battery is discharged, the weakest cell will use up all its active material. Now, as discharge continues, the current through the dead cell is becomes a charging current, except that it is reversed. So, now reduction is occurring at the positive terminal. As there is no more nickelic hydroxide, it reduces the water, and produces hydrogen. Cell pressure builds, and it vents. The cell has lost water and the life of the cell has been shortened This is the big danger of battery cycling to prevent memory. Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells to have a finite life. Each cycle is a bit of life.
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