Ni-cd or Li-Ion

[La Poste]

Is there a big difference in performance between cordless drills that use Ni-Cd and those that use LI-Ion?

And what about NiMH?

I don't really know that much about the subject.

Thanks.

 

[Dan23584]

With Li-Ion the batteries tend to charge faster and last a little longer, the main advantage though it they are smaller and lighter.

Pricier though.

 

[Dan23584]

Li-ion vs Ni-cad power tools - YouTube

 

[Octopus]

What ever you decide on, buy the drill complete with 2 batteries - they are very expensive to buy as a spare part.

 

[Mr.Si]

My dewalt combi has 2x 18v packs which are of the NiMH flavour but one is not charging properly and the other loses power pretty quickly.

Get L-ion batteries - they are a much better battery technology than those with nickel in them.

Sent from my Samsung Galaxy S II via Tapatalk

 

[Richard Burns]

Nickel cadmium batteries contain the very toxic metal cadmium and should be being phased out now, but still seem to be about in great numbers.

NiCd came first and were notorious for the memory effect where they fail to fully charge once they are charged from part way. However they are reasonable and will work well. the memory effect is apparently re settable to some extent by completely discharging under low load and then recharging.
NiMH (Nickel metal hydride) replaced NiCd and work in a slightly better manner (less memory effect) and have greater longevity.
Li-ion (lithium ion) are the next version and have little memory effect and have better discharge characteristics and longevity then Ni-MH, however they are still significantly more expensive. However I would say the advantages outweigh the costs.

Just my thoughts not backed up by anything except hearsay!

 

[DNS1]

I've not got huge experience with batteries in power tools, but have had loads of laptops over the years.

The Ni-Cads were frankly rubbish (my first laptop, a 1995 Compaq)

The Ni-MHs in my 2002 Compaq and 2005 Asus were much better, but began to flag a little over time

The Li-ions in my 2008 Samsung are first class and still going strong, not noticed any drop in performance at all.

 

[MarkieSparkie]

As a former battery application and development engineer, I am probably in a better position than most members to provide some authoritative information on this subject. A lot of myth and misinformation surrounds Nicad and NiMh cells/batteries in particular, much of it fuelled by the internet. Please bear with me while I find time to write it later today.

 

[MarkieSparkie]

[FONT=&amp]All about Nicads. Part 1.[/FONT]
​

[FONT=&amp]How does a[/FONT][FONT=&amp]n [/FONT][FONT=&amp]electro-chemical[/FONT][FONT=&amp]cell work?[/FONT]
[FONT=&amp]First, a bit of nomenclature. A cell is a single electrochemical[/FONT][FONT=&amp] device [/FONT][FONT=&amp]with a single anode and a single cathode. A battery is a collection [/FONT][FONT=&amp]of cells, usually connected in series to obtain a higher terminal[/FONT][FONT=&amp] voltage.[/FONT]
[FONT=&amp]Batteries, whether they are primary (use once) or secondary (rechargeable) are devices which convert chemical energy into electrical energy. In the [/FONT][FONT=&amp]case of the latter, they can take electrical energy and store it as chemical energy for later use.[/FONT]
[FONT=&amp]The key to electro-chemistry are the processes of oxidation and [/FONT][FONT=&amp]reduction.[/FONT][FONT=&amp] Remember the mnemonic "LEO (the lion) goes GER (grr??)" [/FONT][FONT=&amp]—[/FONT][FONT=&amp] Lose[/FONT][FONT=&amp] Electrons[/FONT][FONT=&amp]– [/FONT][FONT=&amp]Oxidation, Gain Electrons - Reduction. When one oxidises a material, [/FONT][FONT=&amp]it gives up electrons it becomes more positively charged, or [/FONT][FONT=&amp]enters a higher oxidation state. Likewise, when one reduces a [/FONT][FONT=&amp]material,[/FONT][FONT=&amp] one is adding electrons to it and either making it negatively charged [/FONT][FONT=&amp]or reducing its oxidation state.[/FONT]
[FONT=&amp]Now, one can make a cell using two materials, say A and B and[/FONT]
[FONT=&amp]immersing[/FONT][FONT=&amp] them in a solution which can conduct ions, called an electrolyte.[/FONT][FONT=&amp] (An [/FONT][FONT=&amp]ion is a charged atom or radical of a molecule capable of [/FONT][FONT=&amp]transferring[/FONT][FONT=&amp] electrical charge). Now, let us say that material A is easily [/FONT][FONT=&amp]oxidised [/FONT][FONT=&amp]—[/FONT][FONT=&amp]it likes to lose electrons, while B is a material that likes to [/FONT][FONT=&amp]be reduced. When these two materials are immersed in an electrolyte, and a circuit is completed from A to B, A is oxidised and electrons [/FONT][FONT=&amp]are released to flow to the circuit. After performing electrical[/FONT][FONT=&amp] work, the electrons flow into B, where B is reduced. The circuit from B to [/FONT][FONT=&amp]A is completed by the flow of ions in the electrolyte. A secondary[/FONT][FONT=&amp] cell can be reversed by forcing electrons into A, and reducing the [/FONT][FONT=&amp]oxidised[/FONT][FONT=&amp] A to regain unoxidised A for use again.[/FONT]
[FONT=&amp]This, of course, is an over simplified view, as only certain[/FONT][FONT=&amp] combinations[/FONT][FONT=&amp] of materials and electrolytes provide useful and practical[/FONT][FONT=&amp] batteries.[/FONT]
[FONT=&amp]Oh, one more bit of nomenclature:- The cathode is where reduction[/FONT]
[FONT=&amp]takes place,[/FONT][FONT=&amp]and the anode is where oxidation takes place. So, in a battery which [/FONT][FONT=&amp]is producing current, the positive terminal is the cathode, and the [/FONT][FONT=&amp]negative [/FONT][FONT=&amp]terminal is the anode.[/FONT]

[FONT=&amp]NiCd system:[/FONT]
[FONT=&amp]When the cell is fully charged:[/FONT][FONT=&amp]- [/FONT][FONT=&amp]The cathode is composed of Nickelic Hydroxide.[/FONT][FONT=&amp] Now, nickel is one of those elements that has multiple oxidation states [/FONT][FONT=&amp]— [/FONT][FONT=&amp]it can lose a different number of electrons per atom, depending on how hard[/FONT][FONT=&amp] it is coerced. Nickel is usually found with oxidation states of 0 (free metal), +2, +3 and +4. The +2 state is referred with a -ous suffix,[/FONT][FONT=&amp]while the +3 and +4 states are referred with a -ic suffix. So, nickelic hydroxide is really NiOOH (the nickel has a charge of + 3) or [/FONT][FONT=&amp]Ni(OH) (the nickel has a charge of +4)[/FONT]
[FONT=&amp]The anode is composed of free cadmium metal (zero oxidation).[/FONT][FONT=&amp] The electrolyte is usually a solution of potassium hydroxide (KOH).[/FONT]
[FONT=&amp]When one connects a load to the cell, as explained earlier, the anode[/FONT]
[FONT=&amp]is [/FONT][FONT=&amp]oxidised and the cathode is reduced. Electrons leave the anode where[/FONT][FONT=&amp] the cadmium is oxidized and forms Cd(OH), plus 2 free electrons.[/FONT][FONT=&amp] These two electrons go to the cathode where they reduce the nickelic hydroxide to form nickelous hydroxide or Ni(OH) (where the nickel has [/FONT][FONT=&amp]a charge of +2)[/FONT]
[FONT=&amp]This reaction can take place until the materials are exhausted. In[/FONT]
[FONT=&amp]theory,[/FONT][FONT=&amp] cells are manufactured so that both anode and cathode are spent at [/FONT][FONT=&amp]roughly equal rates.[/FONT]

 

[MarkieSparkie]

All about Nicads. Part 2.​

[FONT=&amp]What happens when cells are charged?[/FONT]
[FONT=&amp]Well, in a nutshell, the inverse of the discharge. To charge,[/FONT][FONT=&amp]one is forcing current back into the cell (opposite of discharge [/FONT][FONT=&amp]current).[/FONT]
[FONT=&amp]Here, electrons are being taken out of the positive terminal, and forced[/FONT][FONT=&amp] into the negative terminal. This means that the material at the [/FONT][FONT=&amp]positive [/FONT][FONT=&amp]terminal is being oxidized (hence is now the anode [/FONT][FONT=&amp]—[/FONT][FONT=&amp] confusing, eh?) [/FONT][FONT=&amp]and[/FONT][FONT=&amp] material at the negative terminal is being reduced (now the cathode).[/FONT]
[FONT=&amp]In the NiCd system, the cadmium hydroxide is being reconverted into[/FONT]
[FONT=&amp]cadmium,[/FONT][FONT=&amp]and the nickelous hydroxide is being reconverted to nickelic[/FONT]
[FONT=&amp]hydroxide.[/FONT][FONT=&amp]Note that the electrolyte in both charge and discharge is a means to [/FONT][FONT=&amp]move [/FONT][FONT=&amp]the hydroxyl (OH-) ions around. Unlike the lead-acid system, the [/FONT][FONT=&amp]electrolyte [/FONT][FONT=&amp]really doesn't change in composition too much between the charged and [/FONT][FONT=&amp]discharged state.[/FONT]
[FONT=&amp]The easy part of charging is reconverting the spent material on the plates to the charged condition. The hard part is knowing when to stop. Let us take a moment to think about what happens when we overcharge the battery. Once all the nickelous hydroxide is converted into nickelic hydroxide, and in theory all the cadmium hydroxide is converted into cadmium, the charging current has to go somewhere. As the energy of the charging current cannot go into more chemical energy, it goes into splitting water (water is still the major constituent of the electrolyte). Just like the age old chemistry experiment of splitting water into hydrogen and oxygen, a fully charged NiCd cell does the same thing. You are forcing oxidation at the positive terminal and reduction at the negative.[/FONT]
[FONT=&amp]When one oxidises water (actually the OH-) ion, one produces oxygen. Likewise, at the negative terminal (now the cathode), one produces hydrogen.[/FONT][FONT=&amp] This of course is bad. Oxygen + hydrogen = BOOM. Cell[/FONT][FONT=&amp] manufacturers,[/FONT][FONT=&amp] or at least their lawyers, frown on this from happening. So, they [/FONT][FONT=&amp]cheat.[/FONT]
[FONT=&amp]During manufacture, they deliberately oversize the negative plate,[/FONT]
[FONT=&amp]And[/FONT][FONT=&amp] they partially discharge it. That is, they put a fully charged[/FONT][FONT=&amp] Positive [/FONT][FONT=&amp]plate, but put a slightly discharged, but bigger plate of cadmium in.[/FONT][FONT=&amp] The amount of free cadmium in the oversized plate is matched to[/FONT][FONT=&amp] discharge [/FONT][FONT=&amp]in step with the amount of nickelic hydroxide provided in the [/FONT][FONT=&amp]positive[/FONT][FONT=&amp] plate.[/FONT]
[FONT=&amp]Now consider what happens as full charge is achieved. Oxidation of[/FONT]
[FONT=&amp]water starts at the anode, but since the cathode is oversized, and[/FONT][FONT=&amp] has[/FONT][FONT=&amp] excess hydroxide, the current continues to produce cadmium metal[/FONT][FONT=&amp] instead [/FONT][FONT=&amp]of hydrogen. At the same time, the separator (the material used to[/FONT][FONT=&amp] prevent [/FONT][FONT=&amp]the plates from shorting) is designed to allow oxygen gas to diffuse [/FONT][FONT=&amp]through, from the positive to the negative plate. The free oxygen then [/FONT][FONT=&amp]oxidises [/FONT][FONT=&amp]the cadmium metal to form more cadmium hydroxide to prevent hydrogen from being formed. Voila [/FONT][FONT=&amp]—[/FONT][FONT=&amp] a safe battery.[/FONT]

[FONT=&amp]It looks like batteries are well protected then?[/FONT]
[FONT=&amp]Not so fast ... this scheme will work only as long as the[/FONT][FONT=&amp] over charging [/FONT][FONT=&amp]current is limited to a value such that the rate of oxygen liberation [/FONT][FONT=&amp]at [/FONT][FONT=&amp]the anode is less than or equal to the rate of diffusion across the [/FONT][FONT=&amp]separator.[/FONT]
[FONT=&amp]If the overcharging current is too high, excess oxygen is produced at[/FONT]
[FONT=&amp]the anode, and since not enough oxygen can diffuse across to make up [/FONT][FONT=&amp]for [/FONT][FONT=&amp]the reduction at the cathode, the excess cadmium hydroxide is used[/FONT][FONT=&amp] up.[/FONT][FONT=&amp] Then, hydrogen is formed. This leads to a dangerous situation, due [/FONT][FONT=&amp]to both[/FONT][FONT=&amp] fire and overpressure. Cells are designed to vent when this [/FONT][FONT=&amp]condition[/FONT][FONT=&amp] occurs, releasing the excess hydrogen and oxygen to the air before [/FONT][FONT=&amp]really bad things happen. While this may keep one's cells from[/FONT][FONT=&amp] blowing up, [/FONT][FONT=&amp]it does damage them, since one is losing material from one's cell.[/FONT][FONT=&amp] As one [/FONT][FONT=&amp]loses water, it upsets the chemical balance inside the cell [/FONT][FONT=&amp]—[/FONT][FONT=&amp] lose[/FONT][FONT=&amp] enough [/FONT][FONT=&amp]water, and it stops working.[/FONT][FONT=&amp] Another problem is that the process of generating oxygen, and[/FONT][FONT=&amp] recombining [/FONT][FONT=&amp]it at the cathode generates heat. With a moderate amount of current,[/FONT][FONT=&amp]the cell temperature can rise considerably, to 50 or 60[/FONT]°[FONT=&amp]C.[/FONT][FONT=&amp] If after charging, the batteries are hot, then you have excessively overcharged [/FONT][FONT=&amp]them [/FONT][FONT=&amp]—[/FONT][FONT=&amp] slap yourself on your wrist.[/FONT]

 

[MarkieSparkie]

All about Nicads. Part 3.​

[FONT=&amp]Fact 1:-[/FONT][FONT=&amp] Charge control is the key to battery management.[/FONT]
[FONT=&amp]More batteries are destroyed or damaged by bad charging techniques[/FONT]
[FONT=&amp]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.[/FONT]

[FONT=&amp]Fact 2:- Heat is the enemy of batteries.[/FONT]
[FONT=&amp]A NiCd stored, used, or charged under high temperature conditions[/FONT]
[FONT=&amp]will die [/FONT][FONT=&amp]an early death. Heat causes the separator to weaken, the seals to[/FONT][FONT=&amp] weaken,[/FONT][FONT=&amp]and greatly accelerates changes in the plate material, some of which[/FONT][FONT=&amp] may [/FONT][FONT=&amp]cause[/FONT][FONT=&amp] "memory effect".[/FONT]
[FONT=&amp]So even though the cells may not vent, the heat by-product is wearing[/FONT][FONT=&amp] down the cells. Specifically, hydrolysis or degradation of the[/FONT]
[FONT=&amp]separator [/FONT][FONT=&amp]material, usually polyamide, is greatly accelerated at high[/FONT]
[FONT=&amp]temperatures. [/FONT][FONT=&amp]This leads to premature cell failure (see later in text).[/FONT]

[FONT=&amp]What are battery manufacturers doing to prevent damage from overcharging?[/FONT]
[FONT=&amp]The demand for rapid charging has lead to a great [/FONT][FONT=&amp]increase [/FONT][FONT=&amp]in overcharging abuse. Most all NiCd cells can be rapid charged.[/FONT][FONT=&amp]The[/FONT][FONT=&amp] trick is to stop charging when it is fully charged. The so called[/FONT][FONT=&amp] "rapid[/FONT][FONT=&amp]-[/FONT][FONT=&amp]charge" type of cells just incorporate protection against[/FONT][FONT=&amp] overcharging at [/FONT][FONT=&amp]high currents. Most often, this is done with activated carbon inserted[/FONT][FONT=&amp] in the cell to promote the collection of oxygen and to deliver it to[/FONT][FONT=&amp] the [/FONT][FONT=&amp]cathode for recombination. By increasing the rate of oxygen [/FONT][FONT=&amp]transport,[/FONT][FONT=&amp]one is increasing the ability of the cell to resist venting. Note [/FONT][FONT=&amp]however,[/FONT][FONT=&amp] that heat is still generated.[/FONT][FONT=&amp] The price one pays for this is reduced capacity. Everything takes[/FONT][FONT=&amp] space in [/FONT][FONT=&amp]the cell, and space for carbon means less space for active material.[/FONT][FONT=&amp] Also, there have been some indications that carbon can cause the cadmium [/FONT][FONT=&amp]metal to corrode, possibly leading to a shorter life.[/FONT]

[FONT=&amp]What about high-temperature batteries?[/FONT]
[FONT=&amp]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 [/FONT][FONT=&amp]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 [/FONT][FONT=&amp]—[/FONT][FONT=&amp] learn to take care of the cells to avoid overheating them.[/FONT]

[FONT=&amp]What about those super-high capacity cells?[/FONT]
[FONT=&amp]The manufacturers are in a numbers game. It used to be that[/FONT]
[FONT=&amp]AA cells were 450 mAh. Then came 500, then 600 mAh. Now, 2000 mAh and beyond is possible. [/FONT]

[FONT=&amp]How is this achieved? [/FONT]
[FONT=&amp]Well, the highest capacity [/FONT][FONT=&amp]cells use foamy or spongy backing material for their plates. This [/FONT][FONT=&amp]allows [/FONT][FONT=&amp]packing more active material into the plates, but the cost is higher[/FONT][FONT=&amp] resistance. Recall that one of the great virtues of NiCds is their[/FONT][FONT=&amp] low internal resistance [/FONT][FONT=&amp]—[/FONT][FONT=&amp] this allows large discharge currents for starting motors, for example.[/FONT]
[FONT=&amp]So far, sintered plate cells are the best for low internal resistance.[/FONT][FONT=&amp] For most low power consume [/FONT][FONT=&amp]applications, the internal resistance isn't an issue [/FONT][FONT=&amp]—[/FONT][FONT=&amp] for high[/FONT][FONT=&amp] power [/FONT][FONT=&amp]applications [/FONT][FONT=&amp](e.g. more than 10A of current), it is a concern.[/FONT]

 

[MarkieSparkie]

All about Nicads. Part 4.​

[FONT=&amp]Does the memory effect exist?[/FONT]
[FONT=&amp]Yes, but it’s not the big issue most seem to believe.[/FONT][FONT=&amp] First of all, the term memory effect is quite unscientific. People tend to attribute any failure of a NiCd to memory effect.[/FONT]
[FONT=&amp]The first cases of true memory effect were seen in spacecraft batteries subjected to[/FONT][FONT=&amp] a repeated discharge/charge cycle that was a fixed percentage of their total capacity (due to the earth's shadow). After many cycles, when called upon to provide the full capacity, the battery failed to do so. Since we aren't in space, the above is not really relevant as your battery/cells won't experience it[/FONT].
[FONT=&amp]I have only seen one true memory effect, this was an MOD nickel-cadmium application in which the cells were repeatedly discharged to 25% [/FONT][FONT=&amp]± 1% [/FONT][FONT=&amp]of their available capacity by computer control, then recharged to 100% capacity without intentional overcharge. This long term, repetitive regime, with no provision for intentional overcharge, resulted in loss of capacity beyond the 25% discharge point. From this it is easy to see how the “memory” phenomenon, whereby NiCd batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity, grew in to the “bogeyman” of the uninformed. In this case it was easily eliminated by simply reprogramming the computer to allow for a period of intentional overcharging. In fact, true memory effect is always a completely reversible condition; even in those rare cases where in cannot be avoided, it can be easily erased. Note:- This was not solved by the intentional full discharge to 0V, the destructive method peddled by the self-styled “memory effect” guru’s to clear the problem.[/FONT]

[FONT=&amp]Fact 3:- True memory effect cannot exist if any one of the following conditions is true[/FONT]
[FONT=&amp]1) [/FONT][FONT=&amp]Batteries achieve a full intentional overcharge period.[/FONT]
[FONT=&amp]2) [/FONT][FONT=&amp]Discharge is not exactly the same each cycle ± 2-3%[/FONT]
[FONT=&amp]3) [/FONT][FONT=&amp]Discharge is to less than 1.0 V/cell[/FONT]
[FONT=&amp]From the above it is easy to see that true memory effect is a rare event only seen under extreme controlled conditions.[/FONT]

[FONT=&amp]However, a phenomenon exists where the discharge voltage for a given load is lower than it should be, can give the appearance of a lowered capacity, in reality the capacity has not changed at all, the cell has just experienced a simple voltage depression. Unfortunately, it is common for this to be wrongly called “memory effect”. The correct term is Voltage Depression.[/FONT]

[FONT=&amp]Let us look at various causes of voltage depression.[/FONT]
[FONT=&amp]Voltage depression can be attributed to changes in the negative or cadmium plate. Recall that charging involves converting Cd(OH) to Cd metal.[/FONT][FONT=&amp]Ordinarily, and under moderate charging currents, the cadmium that is deposited is micro-crystalline (i.e. very small crystals). Now, metallurgical thermodynamics states that grain boundaries (boundaries between the crystals) are high energy regions, and given time, the tendency of metals is for the grains to coalesce and form larger crystals.[/FONT][FONT=&amp] This is bad for the battery since it makes the cadmium harder to dissolve [/FONT][FONT=&amp]during high current discharge, and leads to high internal resistance and voltage depression.[/FONT]
[FONT=&amp]The way to avoid voltage depression is to avoid forming large crystal [/FONT][FONT=&amp]cadmium. Very slow charging is bad, as slow growth aids large[/FONT][FONT=&amp] crystal growth (recall growing copper sulphate crystals at school). High temperatures are[/FONT][FONT=&amp] bad, since the nucleation and growth of crystals is exponentially[/FONT][FONT=&amp] driven by temperature. The problem is that given time, one will [/FONT][FONT=&amp]get growth of cadmium crystals, and thus, one needs to reform the[/FONT][FONT=&amp] material.[/FONT]
[FONT=&amp]Partial cycling of the cells means that the material deep within the[/FONT]
[FONT=&amp]plate never gets reformed. This leads to a growth of the crystals.[/FONT]
[FONT=&amp]By a proper execution of a discharge/charge cycle, one destroys the[/FONT]
[FONT=&amp]large[/FONT][FONT=&amp] crystal cadmium and replace it with a micro-crystalline form best [/FONT][FONT=&amp]for discharge.[/FONT]
[FONT=&amp]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 individual separated cells, one risks reversing the cells of the battery and that kills them. Perhaps once in a while, use the pack until it is 90% discharged, or to a cell voltage of 1.0 V under light load. Here, about 95% of the cells capacity is used, and for all intents and purposes, it is discharged. At this point, recharge it properly, and that's it.[/FONT]
[FONT=&amp]The more common "memory effect" isn't memory at all, but voltage[/FONT][FONT=&amp] depression caused by unintentional overcharging. Positive plate electro-chemistry[/FONT][FONT=&amp] is very complicated, but overcharging changes the crystal structure[/FONT][FONT=&amp] of the nickelic hydroxide from beta-nickelic hydroxide to gamma-[/FONT][FONT=&amp]nickelic[/FONT][FONT=&amp] hydroxide. The electrochemical potential of the gamma form is [/FONT][FONT=&amp]about 40 to 50 mV less than the beta form. This results in[/FONT][FONT=&amp] a lower discharge voltage. In a six cell (7.2v) pack, this means[/FONT][FONT=&amp] a loss of 300 mV. Don't overcharge! Leaving cells on a[/FONT][FONT=&amp] trickle charger encourages formation of gamma nickelic hydroxide.[/FONT][FONT=&amp] Therefore expect the cells to discharge at a lower voltage.[/FONT]

 

[MarkieSparkie]

All about Nicads. Part 5.​

[FONT=&amp]Summary:[/FONT]
[FONT=&amp]1) DON'T deliberately discharge the batteries to avoid memory.[/FONT]
[FONT=&amp]2) DO let the cells discharge to 1.0 V/cell on occasion through normal[/FONT]
[FONT=&amp]use.[/FONT]
[FONT=&amp]3) DON'T leave the cells on trickle charge for long times, unless[/FONT]
[FONT=&amp]voltage depression can be tolerated.[/FONT]
[FONT=&amp]4) DO protect the cells from high temperature both in charging and storage.[/FONT]
[FONT=&amp]5) DON'T unintentionally overcharge the cells. Use a good charging technique. [/FONT]
[FONT=&amp]6) DO choose cells wisely. Sponge/foam plates will not tolerate high[/FONT]
[FONT=&amp]charge/discharge currents as well as a sintered plate.[/FONT]

[FONT=&amp]I need to "properly" charge cells. How do I do that?[/FONT]
[FONT=&amp]There are many methods of charging. The oldest standard method is the trickle or 15 hour method. This involves using a current of about 50 mA (for AA[/FONT][FONT=&amp] cells) and leaving them on charge for 15 hours. At this current [/FONT][FONT=&amp]level,[/FONT][FONT=&amp] oxygen diffusion is more than enough to take care of the excess [/FONT][FONT=&amp]current [/FONT][FONT=&amp]once full charge is achieved. Of course, one runs the risk of voltage[/FONT][FONT=&amp] depression due to unintentional overcharge.[/FONT]
[FONT=&amp]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 integrated circuits out there built specifically to look for this. The Maxim MAX712 and 713 integrated circuits are ones that come most readily to mind. 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.[/FONT]
[FONT=&amp]Another cheap way is to measure the cell temperature. The cell[/FONT][FONT=&amp] temperature[/FONT][FONT=&amp] will rise steeply as full charge is reached. When the cell [/FONT][FONT=&amp]temperature[/FONT][FONT=&amp] rises to 10[/FONT]°[FONT=&amp]C or so above ambient, stop charging, or go into [/FONT][FONT=&amp]trickle mode.[/FONT]
[FONT=&amp]Whatever method one chooses, a fail safe timer is a requirement with[/FONT]
[FONT=&amp]high [/FONT]charge currents. Don't charge for more than double the cell capacity [FONT=&amp](i.e. for a 800 mAh cell, no more [/FONT][FONT=&amp]than 1600 mAh of charge)[/FONT][FONT=&amp], because if you do you will definitely be in to unintentional overcharging and all the consequential harmful effects[/FONT][FONT=&amp]. Most of the major power tool manufacturers use a combination of all these charging techniques in their battery chargers.[/FONT]

[FONT=&amp]I've seen terminology like "Cn" where n is a number. What is this?[/FONT]
[FONT=&amp]This is a method of expressing current as a fraction of the Ah[/FONT]
[FONT=&amp]rating[/FONT][FONT=&amp] of a cell. Simply, a 100 mA current means much higher charge or discharge to a small AAA cell [/FONT][FONT=&amp]than to a large D cell. So, rather than use absolute units of[/FONT][FONT=&amp] amperes, [/FONT][FONT=&amp]cell manufacturers often use fractions of cell capacity, or C. A typical 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.[/FONT]

[FONT=&amp]What's the right charge current?[/FONT]
[FONT=&amp]If using an unregulated charger [/FONT][FONT=&amp]—[/FONT][FONT=&amp] one that doesn't [/FONT][FONT=&amp]do any detection of full charge, then one must restrict your charge[/FONT][FONT=&amp] current to the overcharge capacity of your cell. All NiCd cells I[/FONT][FONT=&amp] have[/FONT][FONT=&amp] seen can handle C/10 (approx. 50 mA for AA cell) indefinitely without[/FONT][FONT=&amp] venting. This is not to say that one won't get voltage depression, but[/FONT][FONT=&amp] rather that one won't destroy the cell(s).[/FONT][FONT=&amp] If one wants to get a bit more aggressive, a C/3 charge will recharge[/FONT][FONT=&amp] the [/FONT][FONT=&amp]cells in about 4 hours, and at this rate, most cells will handle a [/FONT][FONT=&amp]bit[/FONT][FONT=&amp] of overcharge without too much trouble. That is, if one catches the [/FONT][FONT=&amp]cells within an hour of full charge, things should be OK. No [/FONT][FONT=&amp]unintentional overcharge [/FONT][FONT=&amp]is best of course.[/FONT]
[FONT=&amp]Only with automatic means of full charge detection should one use[/FONT]
[FONT=&amp]charge[/FONT][FONT=&amp] currents above C/2. At this current level and above, many cells can [/FONT][FONT=&amp]be [/FONT][FONT=&amp]easily damaged by overcharging. Those that have oxygen absorbers may [/FONT][FONT=&amp]not vent, but will still get quite hot.[/FONT]
[FONT=&amp]With a good charge control circuit, charge currents in excess of C[/FONT][FONT=&amp] have [/FONT][FONT=&amp]been used [/FONT][FONT=&amp]—[/FONT][FONT=&amp] the problem here becomes reduced charge efficiency and [/FONT][FONT=&amp]internal heating from resistive losses. Unless one is in a great hurry,[/FONT][FONT=&amp] avoid rates greater than C.[/FONT]

[FONT=&amp]What is cell reversal, and why is it so bad?[/FONT]
[FONT=&amp]In a battery, not all cells are created equal. One will be[/FONT][FONT=&amp] weaker[/FONT][FONT=&amp] than the others. As the battery is discharged, the weakest cell[/FONT][FONT=&amp] will use[/FONT][FONT=&amp] up all its active material. Now, as discharge continues,[/FONT][FONT=&amp]the current through the dead cell is becomes a charging current, except that it is reversed. So, now reduction is occurring at the[/FONT][FONT=&amp] positive [/FONT][FONT=&amp]terminal. As there is no more nickelic hydroxide, it reduces the [/FONT][FONT=&amp]water,[/FONT][FONT=&amp]and produces hydrogen. Cell pressure builds, and it vents. The cell [/FONT][FONT=&amp]has lost water and the life of the cell has been shortened, this is the big danger of battery cycling to prevent memory. Invariably,[/FONT][FONT=&amp]unless one is very careful, one ends up reversing a cell. It does [/FONT][FONT=&amp]much[/FONT][FONT=&amp] more harm than the cycling does good. Also, keep in mind that cells[/FONT][FONT=&amp] to [/FONT][FONT=&amp]have a finite life. Each cycle is a bit of life.[/FONT]

[FONT=&amp]Fact 4[/FONT][FONT=&amp]:- Take care of your cells, and they will take care of you.[/FONT]
[FONT=&amp]I have a set of cells from 1981 (>30 years old) that are still working. Sintered [/FONT][FONT=&amp]plate,[/FONT][FONT=&amp] 450 mAh old technology [/FONT][FONT=&amp]—[/FONT][FONT=&amp] originally trickle charged, now rapid [/FONT][FONT=&amp]charged[/FONT][FONT=&amp] many times using a modified delta-V electro-plating technique.[/FONT]

 

[MarkieSparkie]

All about Nicads. Part 6​

[FONT=&amp]My battery/cells have died. I was kind to them, what happened?[/FONT]
[FONT=&amp]All good things must come to an end. No matter how well one[/FONT]
[FONT=&amp]takes care[/FONT][FONT=&amp] of the cells, they will eventually die. There are two main reasons [/FONT][FONT=&amp]cells [/FONT][FONT=&amp]fail, other than abuse. One is separator failure, and the other is[/FONT][FONT=&amp] degradation of the active material. The first is far more common, [/FONT][FONT=&amp]and [/FONT][FONT=&amp]the result is a shorted cell. Every time a cell is charged, the[/FONT][FONT=&amp] active[/FONT][FONT=&amp] material is re-deposited on the plates. Ideally, this occurs[/FONT][FONT=&amp] uniformly [/FONT][FONT=&amp]across the surface of the plate.[/FONT][FONT=&amp] However, in reality, there will be bumps and valleys. When[/FONT][FONT=&amp] there are bumps on both the positive and negative plates are[/FONT][FONT=&amp] adjacent, [/FONT][FONT=&amp]separated only by the separator, the resistance between those[/FONT][FONT=&amp] two points is slightly less than in other regions of the cell.[/FONT]
[FONT=&amp]So, the current density there rises. This means that more[/FONT][FONT=&amp] material is deposited there, contributing to even more "bumpiness". [/FONT][FONT=&amp]In reality, needles called dendrites form, and given time,[/FONT][FONT=&amp] they can force themselves through the separator to short the cell.[/FONT][FONT=&amp] A cell that appears to self-discharge in a couple of days has[/FONT][FONT=&amp] dendrite[/FONT][FONT=&amp] problems, and will soon completely short out. Plan to replace the[/FONT][FONT=&amp] cell.[/FONT][FONT=&amp] Degradation of the active plate material is just a normal ageing[/FONT][FONT=&amp] process[/FONT][FONT=&amp] of cycling. Both of these mechanisms are very good reasons to avoid[/FONT][FONT=&amp] cycling the cells after each use. Cells should live to about 1000[/FONT][FONT=&amp] cycles [/FONT][FONT=&amp]if treated properly. Anything over that is a bonus.[/FONT]


[FONT=&amp]I've seen people talk about zapping batteries and cells to revive them on You-Tube.[/FONT]
[FONT=&amp]Does this quick fix work?[/FONT]
[FONT=&amp]When cells short due to dendrites, the piece of material that is actually shorting the cell is very thin. So, by forcing a huge impulse of current into the cell, one can vaporize the dendrite - sort of blowing a fuse.[/FONT][FONT=&amp] This works, and can revive an otherwise shorted cell. However, it is[/FONT][FONT=&amp] a stopgap measure at best. First, the fact that one dendrite has [/FONT][FONT=&amp]formed [/FONT][FONT=&amp]means that another is not too far behind. Second, the material that [/FONT][FONT=&amp]was vaporized has now permeated the separator material, forming a [/FONT][FONT=&amp]resistor that shorts the plates. The cell may no longer be shorted,[/FONT][FONT=&amp] but will still have a poor charge retention.[/FONT][FONT=&amp] Besides, unless done properly, this can be dangerous as large[/FONT][FONT=&amp] currents [/FONT][FONT=&amp]are necessary. If attempted on a battery, the more cells in the battery,[/FONT] the higher the high current source voltage will need to be, with all the consequential dangers.

[FONT=&amp]What about self-discharge?[/FONT]
[FONT=&amp]NiCds have a bad habit of discharging when you just leave[/FONT][FONT=&amp] them.[/FONT][FONT=&amp] Fortunately you can recharge them. The NiCd cells discharge about[/FONT][FONT=&amp] one percent a day, maybe a bit less for the modern versions. Expect them to be mostly flat[/FONT][FONT=&amp] after[/FONT][FONT=&amp] 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[/FONT][FONT=&amp] making oxygen, not making up for self-discharge. If you want to make[/FONT][FONT=&amp] something to keep your cells from self-discharging, make a 1 to 2 mA[/FONT][FONT=&amp] current[/FONT][FONT=&amp] source. That should more than overcome self-discharge.[/FONT]

 

[SPARTYKUS]

Thanks Markie, need a day off (or on tod) to read though!