How Batteries WorkDifferent types of batteries are frequently used & relied on in video field production. It's important to understand the different types if you are to get maximum usefulness & long life out of them. Here are three common types (lead acid, Nicad & NiMH) & their characteristics. This tutorial is a work in progress & will be added to as time permits.
Nickel Metal Hydride (NiMH) Batteries:NiMH batteries are more expensive than NiCad batteries, but NiMH suffers far less from the memory effect (false bottom effect), so NiMH batteries don't periodically need discharging (conditioning or exercising) before they are recharged, though this might help after 100 cycles, if runtimes become short. NiMH is far less toxic to the environment than NiCad, because it has no cadmium in it. NiMH has a substantially better power to weight ratio than NiCad (energy density is up to 30 or 40% better) (i.e. NiMH is lighter weight per unit of energy). NiMH batteries can take internal heat up to 140 F (60 C). Like NiCads, NiMH uses cells that are nominally 1.2 Volts each, actually higher when fully charged (1.35 - 1.4 Volts unloaded). Hi quality NiMH batteries can last a very long time, though often NiMH batteries typically last about 75% as many cycles as NiCads (typically NiMH recharges 500 - 900 cycles, or more if not fully discharged). More specifically, a properly maintained quality NiMH battery will put out about 70% of it's capacity after 500 full discharge/charges cycles. The higher the energy density (i.e. 9 vs. 7.5 Amps for D size), the less number of charging cycles it will take. Shallow rather than deep discharge cycles are preferred. NiMH batteries are reported to be safer when used in situations where they might be exposed to water or fire. Some small consumer sized AA NiMH batteries (often the higher priced ones) can be charged very fast (under an hour), but they require a special intelligent charger (to avoid overcharging) & fast charging can sometimes shorten the life of the battery because of increased heat build up. A 0.25C or 25% charge rate of about 4 hours charge for an empty NiMH battery is considered a conservative charge rate & a 0.1C or 10% charge is considered a slow charge rate of 13-16 hours. Deep discharging NiMH batteries below 1.0 Volts per cell is not recommended & excessive discharging below 1.0 Volt per cell can cause irreversible damage, especially with several cells in series (can cause the weakest cells to reverse polarity). Self discharge rates for NiMH are about 1.5 - 2 times faster than NiCads. NiMH batteries self discharge faster if stored at high temperatures, so store in a cool place & ideally at about 40% charged (i.e. don't store NiMH batteries for long periods of time in a fully charged condition or a fully empty condition, charge just before use). If stored fully charged at room temperature (20C or 68F) for more than a few days, top up the charge before using to get maximum capacity. Examples, a NiMH battery stored at 0C (32F) for 30 days, will have self discharged to the point of having about 92% of it's capacity left & stored at 20C (68F) for 30 days about 75% of it's capacity left & stored at 40C (104F) for 30 days about 40 - 52% of it's capacity left. A NiMH battery can be discharged at up to 3 times it's mAh rating, although you would only receive about 85% of the rated capacity (i.e. a 2 Amp/hour or 2000 mAh cell can safely discharge up to 6 Amps or 6000 mA), however to maintain long battery life, it is recommended to not discharge a NiMH battery any faster than it's 1C Amp rating (i.e. don't discharge a 2000 mAh cell any faster than 2000 mA). NiMH batteries have a much less distinct full indicator using negative delta Voltage (-dV) than NiCads, so a charger sensitive to this subtle -dV is required when using a fast smart charger and it is a good idea to back up the faster charge cut off with another circuit like a timer, thermistor temperature rise (dT/dt), peak Voltage detect, or zero Voltage rise detect circuit (0 dV), otherwise you could easily overcharge the NiMH batteries & damage them.
Nickel-Cadmium (NiCad) Batteries:Overview: A Nicad (Nickel-cadmium) battery type is often used on video equipment, though this battery type is considered old technology. Nicads are relatively expensive, but they are difficult to damage, require less care, usually last a longer time (more cycles) & can often be discharged or charged at a faster rate than glass matte or gel-cell lead acid batteries. Nicads can develop a "memory" (false bottom effect) whereby they won't accept a full discharge if they are routinely discharged to the same level, then charged. For this reason it is a good idea to discharge them to different levels & to occasionally completely discharge a nicad battery before recharging to "break" the memory (conditioning or exercising). In theory, nicad batteries are not damaged if left in a deep discharge or complete uncharged condition for a long period of time, but in practice I have suspected that this can shorten their life span. Nicads are made up of several capacitor like battery cells wired in series. Each cell is nominally 1.2 Volts (actual unloaded fully charged about 1.25-1.35 Volts), so 10 cells would total nominally 12 Volts (actual unloaded about 12.5-13.5 Volts), 11 cells would total nominally 13.2 Volts (actual unloaded about 13.75-14.85 Volts) & 12 cells would total nominal 14.4 Volts (actual unloaded about 15.0-16.2 Volts). Unlike glass matte & gell-cell batteries which can more easily be manufactured, nicad batteries in 12, 13.2 or 14.4 Volt configurations require much more labour to manufacturer, hence their extra expense. Some high draw video equipment doesn't work too well on glass matte or gel-cells that have an unloaded Voltage of 12.9, but might easily dip below 11 Volts during a heavy draw or at the 50% discharge level, so nicad batteries offer an advantage in that they can be wired with 11 or 12 cells rather than 10, so that the unloaded & loaded Voltages are higher. Additionally, nicads have a low internal resistance which means that they can hold up the high Voltage level right until the battery is nearly drained, whereas glass matte & gell-cel batteries gradually decrease in Voltage as the battery drains (glass matte & gel-cells have a more sloped Voltage decay curve). If your equipment is sensitive to not functioning correctly if Voltage drops low, or shuts itself down prematurely when Voltage reaches the 11- 11.5 Volt range, then nicad batteries may be a better choice than glass matte or gel-cell batteries. Nicads have a medium rate of self-discharge, but in my experience, a nicad will self-discharge much quicker than a glass matte or gell-cell battery, especially in the first few days after a charge, so a nicad will need a top up charge after a month of storage, whereas a glass matte or gel-cell battery often won't. Amp/hour for Amp/hour nicads are bulkier than glass matte or gel-cell batteries because of the cylinder shape of the cells, but both have a similar energy to weight ratio. A different type of intelligent charger is required by a nicad than a glass matte or gel-cell. Intelligent NiCad chargers typically are built for the number of cells in the pack & to sense a Voltage drop or negative delta V as the fully charged cut off point, which is a characteristic of NiCads when they reach a charged condition. NiCads often also have a high temperature thermistor for safety cut off should the charger over charge. Cadmium is highly toxic to the environment if not disposed of properly. Nicads and lights: It often is not a good idea to power 12 Volt camera lights from a Nicad that has 11 cells (nominal 13.2 Volts) or especially a nicad that has 12 cells (nominal 14.4 Volts). At nominal 14.4 Volts (which can actually be 15-16.2 Volts), a 12 Volt light is getting at least 20% higher Voltage than it was designed for, which will dramatically increase the amount of light output, but also dramatically shorten the useable life of the bulb. A rough rule of thumb is that a 10% increase in Voltage gives a 40% increase in light output & a 20% increase in Voltage gives a 96% increase in light output. It's easy to see that when using a 14.4 Volt battery to power a 12 Volt bulb, it is burning almost twice as bright & therefore will burn out much faster. Older 12 Volt bulbs sometimes blow when the surge of a 14.4 Volt battery is applied. When powering 12 Volt lights, it's best to use a 10 cell nicad battery (nominal 12 Volts) or a glass matte or gel-cell battery (nominal 12 Volts). Charge/discharge rates: Nicads have a lower internal impedance (resistance), so they won't stay in a charged condition quite as long as a glass matte or gel-cell battery before they self discharge. Fortunately this lower internal impedance also means that nicad batteries can put out more current (Amperage) over a short period of time, compared to a similarly sized glass matte or gel-cell battery & very high quality nicads take less time to charge than glass matte or gel-cell batteries. A rough rule of thumb is that a high quality nicad battery can accept a charge of up to 80% of it's Amp/hour rating (i.e. take a minimum of 1.25 - 1.5 hours to charge a fully discharged battery). A glass matte or gel-cell lead acid battery accepts a lower discharge/charge rate of about 40% (sometimes higher) of it's Amp/hour rating (i.e. take a minimum of 2.5 - 3.12 hours to charge a fully discharged battery). Nicad batteries tend to get quite warm as they finish charging & this is ok, but a really hot nicad is not ok. Some NiCad batteries have a thermal cut off if the battery gets too hot during charging. Quality charger: A nicad battery requires a charger with a slightly different Voltage charge level than a lead acid battery, especially if the nicad has 11 or 12 cells & the charger requires a different end of charge intelligent method if a fast charger is used. Often nicad batteries have a thermal cut-off inside that feeds back to the charger telling it to stop the charging once the battery has heated up & or a Voltage peaking sensing circuit.
Glass Matte & Gel-Cell Sealed Lead Acid Batteries:Overview: Sealed glass matte or gelled (gel-cell) lead acid battery types are frequently used in video equipment because liquid (wet or flooded) cell lead acid batteries (like car batteries) can leak acid because they are vented (not sealed). Glass matte & gel-cells are inexpensive compared to nicad batteries & they do not have a "memory" problem like nicads. Unfortunately, unlike nicads, lead acid batteries are easily damaged if they are left in a deep discharge condition, or if they are overcharged, so it is important to charge them up immediately after use & to use a quality charger designed for lead acid batteries. 12 Volt glass matte & gel-cell batteries are made up of six cells inside a battery, each of which is nominally about 2.15 Volts for a total of about 12.9 Volts charged, with no electrical load. Glass matte batteries use a very thick paste (thick like candle wax or toothpaste) of sulphuric acid electrolyte between the positive plates (lead dioxide) & negative plates (sponge lead) of the battery, with a porous insulator between plates so they don't short out. Gel-cell batteries are similar except the electrolyte is like a jelly (like Jello) & can therefore be damaged by freezing. The plates in a glass matte or gel-cell are relatively thin compared to wet cell lead acid batteries, so glass matte or gel-cells discharge/charge a bit faster than wet cells. Glass matte or gel-cells have no fluid movement within the battery like wet lead acid & are therefore no-maintenance because they cannot be topped up with electrolyte. For this reason, it is very important to have a high quality charger that doesn't boil off the electrolyte (called gassing), as this would dry out the electrolyte & cause the battery to fail. After several years of useful life, all rechargeable batteries should not be disposed of in regular garbage which goes in a landfill site, but instead you should contact you municipality for the location of a hazardous waste site. Charge/discharge rates: Glass matte, gel-cell & wet lead acid batteries have a high internal impedance (resistance), so they will stay in a charged condition longer than nicad batteries before they self discharge, though glass matte & gell-cell lead acid batteries don't self discharge as quickly as wet lead acid batteries. Unfortunately this high internal impedance also means that lead acid batteries can't put out as much current (Amperage) over a short period of time as a similarly sized nicad battery, plus glass matte or gel-cells can take longer to charge than nicad batteries. A rough rule of thumb is that a wet lead acid battery can only accept a charge of no more than 25% of it's Amp/hour rating up to about the 80% charge level (called the "bulk charge"), then charge more slowly from 80-100% (called the "equalization" or "absorption" or "acceptance" stage)(i.e. take a minimum of 4-5 hours to charge a fully discharged wet cell battery). A glass matte or gel-cell lead acid battery accepts a higher discharge/charge rate than liquid lead acid because of it's thin close together plates & can be charged at an average of up to 40% or more of it's Amp/hour rating up to the 80% charge level, then charge more slowly from 80-100% (i.e. take a minimum of 2.5 - 3.12 hours to charge a fully discharged glass matte or gel-cell battery using an intelligent charger). The charge pattern isn't linear & a lead acid battery will accept more of a charge when it is nearly empty (sometimes at a 60 - 70% rate) & the charger must gradually taper off that current charge or pulse charge as the battery starts to fill up (over 80% full) as well as limit the Voltage, or it will overcharge causing the battery to get warm & gas, which damages the battery. A slightly warm battery during heavy discharge or charging is ok, but a really hot glass matte or gel-cell battery indicates that damage is being done to the battery. The maximum safe temperature to the battery is 125 degree F (52 degrees C). Quality charger & regulator: A lead acid battery requires a charger with a slightly different Voltage charge level & different end of charge methods than a nicad charger. Because Glass matte or gel-cell batteries are so easily damaged by overcharging compared to nicad batteries, the charger must intelligently limit the Voltage level & taper or pulse the current (Amperage) level when topping off of the charge. Voltage levels on a lead acid battery during charge should typically be regulated to be between 13.6 & 13.8 Volts for the float level, sometimes going as high as 14.2 Volts (maximum 14.4 Volts) for pulsing or end of bulk charge stage. Once a gel-cell is almost charged, the "float" level can drop to 13.2 - 13.6 Volts & can be trickle charged or pulse charged with very little current for several hours or days using a quality intelligent charger, without damage. Using a proper intelligent charger, glass matte or gel-cell batteries do not give off a corrosive vapour (no explosive gases). Very few 12 Volt automatic car battery chargers in the 10-12 Amp size do a fine job of charging glass matte or gell-cel lead acid battery belts in the 6-14 Amp/hour size & some do not (they overheat & destroy the battery). The Schumacher (model SF-1275A) 2/12/75 Amp fully automatic car battery charger from Wal-Mart ($78. CDN)(similar to the model 11-1569-6 at Canadian Tire for $89. CDN) is one model which is poorly designed for small batteries (6 Amp/hour), overcharging & destroying the lead acid battery when set on the 12 Amp setting, despite it's claim of "automatic shut off when battery is fully charged". The problem with this model is that it only outputs in the 5-12 Amp range (doesn't taper current down to zero Amps), which can be too much current for small batteries. Remember that 40% of the Amp/hour rating is the ideal textbook maximum a glass matte or gel-cell battery should take, so a 6 Amp/hour glass matte or gel-cell should never be fed more than 2.4 Amps from the charger (these Schumacher & Canadian Tire models feed it a minimum of 5 Amps) & a 14 Amp/hour glass matte or gel-cell should never be fed more than 5.6 Amps. Regardless of the maximum Amp capacity of the charger (i.e. 6, 10, 12 or 15 Amps), try to choose an automatic transformer charger that will automatically taper down the current (Amperage) right to zero, so that it will never feed more current (Amps) than the battery can take. An example of a charger that usually seems to do this quite well is the Schumacher model SF-51A-PE Ship'N Shore 10 Amp fully automatic battery charger for marine & deep cycle batteries. My experiments with this model successfully charge a small 7 Amp/hour lead acid glass matte battery without damaging it, regardless of how long it stays connected for, as long as the switch is in the automatic position. However & have found quality control issues with this model, as some pulse charged lower than the ideal at 13.25 Volts & consequently never quite fully charge the battery. Deep Discharge: As a general rule of thumb, the deeper you discharge a lead acid battery & the longer you leave it in that discharged condition, the more it will shorten the useful life cycles of the battery. Often it is a good idea not to discharge a lead acid battery beyond it's 50% charge level, but if you do deep discharge it beyond 50%, it is all the more important to get it charged up immediately, because sulphation can start to occur when a lead acid battery is left in a discharged condition (especially liquid lead acid) & this will dramatically reduce the useful life of the battery. An example of not discharging a glass matte or gel-cell beyond 50% would be to only use a 100 Watt 12 Volt light continuously for a maximum of 22 minutes using a 12 Amp/hour belt before you put it back on the charger. It is not uncommon to be able to get 4-7 years of life out of a lead acid battery if it is properly taken care of & depending on the number & depth of the discharge/charge cycles. Liquid lead acid batteries are more susceptible to sulphation than glass matte or gel-cell lead acid batteries & for this reason, glass matte or gel-cells are more suited for deep discharging, which is common in video, marine, golf cart & handicap applications. Battery Capacities: Liquid lead acid car batteries are rated in "cold cranking Amps" (which is the number of Amps that can be delivered at 0 degrees F or -17.8 degrees C, to crank an engine for 30 seconds without falling below 7.2 Volts), and rated in "reserve capacity" (which is the number of minutes the battery can deliver 25 Amps at no less than 10.5 Volts at 80 degrees F or 26.7 degrees C). Unfortunately those ratings are meaningless for some other applications such as for video equipment & marine, so glass matte or gel-cell batteries are also rated in "Amp/hours", which is the number of Amps times hours at nominal 12 Volts at 80 degrees F (26.7 degrees C) that the battery can continuously deliver measured over a 20 hour period & going no lower than 10.5 Volts. For example, a 20 Amp/hour gel-cell battery should be able to deliver 1 Amp at 12 Volts (12 Watts) continuously for 20 hours before it is completely discharged. The rating is over 20 hours because glass matte & gel-cells have a high internal resistance. The quicker you discharge the lead acid battery, the less total Amp/hours you will get out of it. For example if you discharged this 20 Amp/hour battery at 2 Amps (24 Watts) you would only get 16.8 Amp/hours or 8.4 hours (84% of rated capacity). Discharged at 4 Amps (48 Watts) you would only get 14 Amp/hours or 3.5 hours (70% of rated capacity). Discharged at 10 Amps (120 Watts) you would only get 10.8 Amp/hours or about 1 hour (54% of rated capacity) before the battery was fully discharged or reached it's minimally acceptable level of 10.5 Volts. For this reason, with video devices & lights it is generally a good idea to have oversized glass matte or gel-cell batteries, than if you were powering the same devices with a nicad battery. A new 12 Amp/hour 12 Volt gel-cell battery belt, can power a 100 Watt camera light for up to 45 minutes of continuous use before it is fully discharged & the light gets too dim (& changes in colour temperature). However, I typically only run the batteries down to 50% by using two or three of these belts when covering an event where I need a camera light on nearly full time, but I switch belts each 15-20 minutes & put the first belt on high charge while using the second battery belt.
IN THE FUTURE I MAY EXPAND THIS TUTORIAL TO INCLUDE MORE INFORMATION ON THE FOLLOWING TYPES OF RECHARGEABLE BATTERIES Lithium Ion (LiON) Batteries3.3 Volts per cell, with higher energy density per unit of volume & weight than other battery types (triple that of NiCad). Does not suffer from memory effect. By Doug Hembruff. |
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