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锂聚合物电池

2018-4-22 10:50| 发布者: dymodel| 查看: 375| 评论: 0|原作者: firpo

摘要: 锂聚合物电池 在许多方面,我很感激我回到了有限赛车在镍基电池的主导地位的尾部来。 镍金属氢化物(“镍氢电池”)是一个死的技术。 现代镍氢电池是非常坦率地垃圾和我来说,我很高兴看到他们回来。 各种锂基细 ...
锂聚合物电池

在许多方面,我很感激我回到了有限赛车在镍基电池的主导地位的尾部来。 镍金属氢化物(“镍氢电池”)是一个死的技术。 现代镍氢电池是非常坦率地垃圾和我来说,我很高兴看到他们回来。 各种锂基细胞,取代了镍氢蚀现在在很多方面他们说,现在没有任何逻辑论证,可以使用镍氢电池在电动赛艇作出。

现代镍氢电池是脆弱的,喜怒无常的走兽,明显降低在你的面前时,开始使用的容量和性能都落客从他们第一次充电快速循环。 事实上,有些人看着他们nimhs后失去只是在板凳上几个周期的能力大量的资金。

镍氢电池是一种剥离的电动工具行业。 今天没有新的电动工具使用nimhs。 由于与nimhs质量问题,电动工具制造商已经抛弃了他们,并提出集体锂基电池。 电动工具使用锂基电池重量更轻,比他们的前辈更强大的镍氢电池。

In many ways, I'm thankful that my return to FE racing has come at the tail end of the dominance of nickel-based cells. Nickel-metal-hydride ("nimh") is a dead technology. Modern nimh cells are quite frankly rubbish and I for one am glad to see the back of them. The various lithium-based cells that are now displacing nimh eclipse them in so many ways that there is now no logical argument that can be made for using nimh in electric racing boats.
Modern nimh cells are fragile, temperamental beasts that visibly degrade in front of you when you start using them, with both capacity and performance dropping off rapidly from the first charge cycle. Indeed, some people have watched their nimhs lose significant amounts of capacity after just a few cycles on the bench.
Nimh cells are a spin-off from the power tool industry. No new power tools today use nimhs. Because of quality issues with nimhs, the power tool manufacturers have abandoned them and moved en masse to lithium-based cells. Power tools utilising lithium-based cells are lighter and much more powerful than their nimh predecessors.
Pushed Too Far
The problem of reduced lifespan is made worse when nimhs are conditioned to produce higher power outputs, a process known as "pushing". People who compete at the very top levels of Fast Electric racing in the European and World Championships have literally got to the point of "one cell pack per race". They will use a new pushed cell pack once for an important race, then discard it, as it will never again produce the same power output.
Obviously, this is a ridiculous state of affairs, especially at £14 per cell. Hopefully, with the adoption of lithium-based cells by Naviga at the 2008 Praesidium, we have seen an end to this expensive insanity.

Lithium-based cells will, in theory, prove to be a great leveller in competition. To be successful with nimh cells meant having access to the very best cells available. There are stories of racers buying 20,000 or so cells per year, selecting the top 10% for their own use then selling on the rest. Obviously, this skews things in favour of those racers with the resources to buy huge quantities of cells and the equipment and time to analyse and match them. For the "average" racer (you and me), we simply don't have the time, money, equipment or inclination to do this, so must make do with what's available off the shelf. Lithium changes all this. 
Lithium cells do not exhibit the huge variance in capacity or performance that nimh cells do. That's not to say lithium cells are all identical; they're not. Also, because lithium cells require regular balancing (ideally every charge), they maintain a much closer "match" than nimhs, which can vary wildly after a few cycles despite supposedly being matched by the vendor (at least, that's what the little label might claim!). 
Lithium cells deliver huge capacity and have very low internal resistance. Internal resistance determines how much the output voltage is depressed under load and how hot the cell will become during charging and discharge. Nimhs get hot (or very hot) under charge/discharge, because of their relatively high internal resistance. Lithium cells barely get warm. 
The Great Capacity Lie
There are many different nimh cells available, with some frankly scandalous claims made about capacity. The label on a cell may claim "4200", but in practice you are extremely unlikely to ever see 4200 mah of usable capacity. The numbers printed on the label usually bear very little relationship to the actual performance of the cell. 
Given the blatant lies printed on nimh cell labels regarding capacity, I'm surprised cell suppliers have not been prosecuted here in the UK under our "Sale Of Goods" Act. Were a car company to market a car as having a capacity of five people, but you could only actually carry three, the car company would be in a lot of trouble. 
On the day I'm writing this (August 2008), the packs I bought 5 months ago for the 2008 season have had less than 20 cycles each and have lost between 25% and 35% of their original capacity, with a number of cells failing completely. Having vowed to buy no more nimhs, my racing this year is almost done, with three meetings left to go. 
The Good Old Days
If one compares today's nimh cells with Sanyo's 1700 and 2000 SCRC nicad cells of the mid 1990s, the contrast is striking. The old Sanyo nicads were near-indestructible and would stand the severest of abuse. Charged at 7 amps on the Simprop NC Manager's "burp" charging cycle, 2000 SCRCs would take between 2200 and 2400 mah of capacity and return over 2000 mah under discharge, coming out of the boat so hot that the glue on the velcro would melt and the heat shrink would split every run. 
I got so fed up replacing heat shrink on my cell packs that I resorted to spraying them matt black with high temperature exhaust paint to try and help them radiate heat during discharge. Needless to say, a couple of coats of paint weren't exactly the best electrical insulator in the world, and I doubt I'd get away with such a practice today! Despite this kind of routine abuse, the old Sanyos lasted remarkably well and I ran the same packs for two seasons. 
Lithium Polymer
There are a number of different cell chemistries that come under the umbrella term "lithium-based". From this point onwards, when reference is made to lithium cells, I'm talking about "lithium polymer", the most widely available cell for FE boats and the type Naviga have now allowed for competition. 
Regardless of chemistry, any cell basically comprises positive and negative electrodes immersed in an electrolyte. The chemical reaction that takes place in the cell produces voltage and current until the reacting compounds are exhausted. A typical analogy when discussing electricity is to compare the electric current flowing through a conductor to the flow of water in a pipe. The voltage is the pressure of the water, the current is the flow rate over time. 
At this point, I could bore you rigid with a lot of facts about lipos, and frankly much of it, while interesting, simply isn't relevant to this article. Instead, if you absolutely must know more, I would suggest reading this article on Wikipedia. 
Lithium polymer ("lipo") cells are very much more robust than nimh, have a lower internal resistance, a much higher energy density and are effectively half the weight of equivalent nimhs. Lipo packs come ready made and ready to use; they don't require all that tedious mucking about with hammerhead soldering irons to make end-to-end soldered packs. Simply cut the pack wires to the required length, solder on some connectors and you're good to go. 
Lithium polymer cells begin to degrade as soon as they are manufactured, in other words, they have a finite "shelf life". You can slow the rate at which your lipo packs degrade and maximise their usable lifespan by storing them correctly when you're not using them. Degradation is dependent on cell charge state and temperature. Store your packs at a low temperature, and no more than 40% charged. You can store lipos in a fridge, but warm them back up to ambient temperature before charging, and definitely do so before discharging. Drawing high current from a cold lipo pack risks damaging the pack and causing it to balloon.

Voltage and "C" Rating
Lipo packs are graded in terms of capacity in milliamp hour, voltage and "C" rating. A pack that is rated at 20C is capable of a sustained current drain without damage of 20 times its capacity. For example, a 20C pack of 5000mah capacity should be capable of delivering a constant 20 x 5 amps, or 100 amps. 
One lipo cell has a nominal voltage of 3.7 volts (compared to 1.2 volts for a nimh) and is considered fully charged at 4.2 volts and fully discharged at 3 volts. We now encounter the initially confusing convention for describing lipo packs. Two lipo cells linked in series to produce 7.4 volts nominal is known as "2S", or 2 Series. Mono and Hydro 1 classes run either 7 nimhs, or 2S lipos. Mono and Hydro 2 run either 12 nimhs or 4S (ie., 4 in series) lipos. 

Lipo cells in series. The maximum voltage of a lipo cell is 4.2 volts. With series packs, the capacity is 1C. 

Lipo packs can be joined in parallel to share the current load between two packs, either to extend run times or to allow the drawing of extremely high currents for very high power applications like speed records attempts. For example, if you have two 4S packs of 5000mah capacity and 14.8 volts nominal, linking these two packs in a parallel configuration would give you a 10,000 mah pack of 14.8 volts nominal with double the C rating. Thus, two 5000mah 30C packs linked in parallel would be capable of a sustained 300 amps (30 x 5 x 2). Two 4S packs in parallel is denoted as "4S2P", or 4 series 2 parallel. Currently, Naviga do not allow paralleled lipo packs for competition, but you're free to use paralleled configurations in non-competition boats. 

Lipo cells in parallel. Joining packs together in parallel doubles the available capacity and the maximum current drain. With parallel packs, the capacity is 2C. 

Buying Lithium Polymer Cells
Lipos are one of those items for which price is absolutely no guide at all to quality. There is a very prominent supplier of lipos here in the UK that advertises heavily and charges top dollar for its products, and the verdict from many FE racers who have bought this brand is that they're pretty poor and definitely not worth the premium prices charged. I've heard of a number of racers who have spent good money on premium brands and who have been seriously disappointed by the performance and longevity of these expensive cells. 
Conversely, there are many racers who have bought cheap lipo packs from ebay and direct from places like Hobby City in Honk Kong at prices less than half of some of the so-called premium brands which have outperformed the more expensive cells in both voltage and capacity. So, buying lipos is somewhat of a lottery. My advice would be to avoid buying expensive premium brands as they simply do not seem to be worth the money. 
Charging Lithium Polymer Cells
Lipo packs should be charged with a charger that is lipo compatible. You must not under any circumstances charge lipos using a nimh or nicad program. Doing so risks at best damage to the pack, at worst, if the cell pack is of an older type (less than 20C) you risk a fire or explosion. 
Lipo packs don't require an initial trickle charge to form them. Simply charge at 1C and limit the current drain for the first few cycles. 
Balancing
The major difference between lipo and nimh is that lipo packs require balancing. Balancing simply means equalising the voltage, and thus the charge state, of all the cells in the pack. Lipo cells are extremely intolerant of any amount of deep discharge. Similarly, lipos should not be subject to any significant overcharging. Overcharging and deep discharge can both cause irreparable cell damage. By ensuring that all the cells in the pack are balanced and at very near the same charge state, you remove the risk of one cell being driven into deep discharge by the other cells near the end of a run, or of one or more cells being overcharged when near the end of a charge. 
All commercially available lipo packs come fitted with a balancing tap, this being a bundle of thin wires terminating in a connector that plugs into the balancer. The balancing tap allows the balancer to measure individual cell voltages and thus balance the pack.

Balancing circuits can be built-in to the charger itself, as in the case of the Schulze 10.36-8 or be a separate unit that can be used with any lipo compatible charger, for example the Schulze LipoProfiBal 8. 
Nimh cells are charged at a varying current. Lipos are charged at a constant current until near then end of a charge when the charger will reduce the current to ensure that full capacity is achieved, without risking overcharging. This means that 80% capacity is reached quickly, with the remaining 20% taking as long again as the current reduces almost to nothing near the end of the charge. 
This is where buying an expensive charger like the Schulze 10.36-8 is an advantage, as these chargers will perform the final charging sequence much faster than a cheaper charger. The balancing circuit in the Schulze can operate at 1 amp; a cheaper charger will be nowhere near this. 

Graphing lipo cells under discharge with Akkusoft using Schulze's nextGen 10.36-8 charger. 

If there is one really, really annoying aspect to lipo cells, it is the childish insistence of all the different brands on using different types of balance lead plug. God knows why they cannot all standardise on a single design and make the paying customer's (ie., the people who keep them in business) life a lot easier, but there we are. If you buy lots of different lipo packs, you will soon end up with a collection of balancer adaptor leads, and some of these things are not cheap. 
Electronic Speed Controllers
Lipo packs can be ruined by deep discharge. For this reason, it is vitally important that you use a lipo-compatible speed controller with a low voltage cut-off feature that will cut the power to the motor when the voltage of the pack drops below a pre-determined limit, usually 3 volts per cell. Because the speed controller is measuring voltage under load, it is often desirable when drawing very high currents to set the cut-off voltage a bit lower at around 2.8 volts per cell to allow for voltage depression under load. When cut-off occurs and the load is removed, pack voltage will rapidly climb back above 3 volts per cell, allowing your boat to be recovered. At moderate current loads, you're probably best sticking with 3 volts per cell as your cut-off limit, as the voltage drop under light current loads is not so pronounced. 
Choosing a cut-off voltage is a compromise between getting all of the available capacity from the cell pack and the lifespan of the pack. If you want the longest lifespan from your lipos, set the cut-off at 3.2 volts per cell, which will pretty much eliminate the possibility of a deep discharge. If you're prepared to trade pack life for performance, then 3 volts or even 2.8 for very high power will let you scrape a bit more out of your lipos. 
Schulze market a device called a "LiPoDiMATIC" that takes this process to the next level. LiPoDiMATIC monitors individual lipo cell voltages and triggers a cut-off when any one cell in a pack reaches 3 volts (or whatever voltage you select). I made the point earlier that lipo cells, good as they are compared to nimhs, are not perfectly identical. Using LiPoDiMATIC or similar systems, you can ensure you get the longest life from a lipo pack by preventing inadvertant deep discharge of any of the cells in a pack. 
The logical progression of the LiPoDiMATIC idea is to integrate it into a speed controller, which Schulze have done with their "Future Value" series of controllers. 
Dos and Don'ts
Do:
Use a charger that has a program specifically for lipos
Use a cell balancer, ideally on every charge
Use high quality connectors
Cool the pack before re-charging
Use a speed controller with a low voltage cut-off
Don't:
Discharge the pack below 3 volts per cell
Short-circuit the pack
Allow the pack to overheat
Charge at more than 1C
Discharge at more than the rated "C" value
Safety
There's a lot of hysterical old nonsense talked about how dangerous lipo cells are. It's true that cells from a few years ago were a bit fragile and, if mistreated, could explosively combust in spectacular fashion. Today's lipos of 20C rating and above are very much more robust and will not explode or combust, even if overcharged, dropped or punctured. However, lipos can be damaged, principally by over-discharging them below 3 volts per cell. If a lipo pack balloons or swells, stop using it immediately and dispose of it in the approved manner. 
Lipos promise the "holy grail" of Fast Electric racing: high speeds and long run times. If you're new to Fast Electrics, do not for one moment consider buying and using nimhs. Follow a few basic rules and lipos will give you excellent performance and longevity; nimhs will not.

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