Turnigy - Lithium Polymer
aka : LiPo
This year, 2010, I finally got into Lipo,
!!! Warning Warning Warning !!!
Ok, a word of caution before i start, when i first got into electrics and decided that lead acid was a dead end, i researched long and hard about " LiPo " .. if you search on google for RC ( Remote Control ) forums and look at the warnings given about these batteries you will find alot of horror stories of fires etc.. All batteries have the potential to cause problems if you don't care for them properly, however, lipo is even more sensitive to this and must be well understood and respected.....
Lipo is not for the careless, lazy or the absent minded, or for those who just want to plug it in and go to bed. They should always be supervised while charging and charged in a safe area.... away from flammable objects....
That being said, current generation Lipo has improved over the years and are relatively safe to use IF YOU EDUCATE YOURSELF ABOUT HOW TO CARE FOR THEM !!!!!...
They are powerfull, and capable of storing the most energy in the smallest lightest package on the market right now. For this reason, they are highly attractive when building a high performance or long range bike without weighing more than a buick.
Mid April 2010, i took the plunge and ordered my first shipment of packs from www.hobbyking.com
( First shipment arrived fairly well packaged, could use a bit more cardboard but it all arrived intact )
Along with the batteries i ordered a Turnigy 4 x 6 charger, It has 4 chargers built in one box that can charge up to 6 cells per pack. There are RC chargers that can do 10, up to 12 cells in series but they cost a fair bit more money.. more on that later.
Plugged it in for the first time, oh, important note, most RC chargers do not plug into regular house plugs, they need to be powered by a 12v source, reason for this is that they are usually used out in the field and can charge LiPo packs from a big 12v lead acid battery, like from your car ! ..
Along with my shipment, i got 4 HobbyKing.com balancers. These little devices do a few interesting things, they display the voltage of up to 6 cells at a time, and as you can see below one of the cells in this pack is 0v !!!!!.. not good !
If you look at the buttons on the unit, you can see a " Balance " button, what this does is drain the higer voltage cells down to the lowest one to equalize the voltage of all the cells in the pack. It does this using Resistors on a circuit board, and they are meant for smaller packs in rc toys, so it's a slow process with large capacity ebike packs, but still gets the job done if you use them strictly as ballancers, or when charging very slowly as they canot burn off alot of energy very quicly.. so you could easily be charging faster than the unit can keep things in check..
In order to determine if the 0v is due to a bad cell or a bad connection, and to satisfy my curiosity, i opened up the shell to have a look.
These cells are made of foil pouches, and the tabs are then soldered together on top of a circuit board for support..
At each cell connection, there is a balance wire, this allows you to read the voltage of each cell in the pack externally.
Unfortunately, no loose wire here, that cell is really 0v..
I put that pack to the side, and got to work on the next 4 packs..
Above, using 4 HK balancers, i can monitor the voltage of all 24 cells at the same time, plugged the 4 " packs " of 6 cells each in series for 100v and drained it using my Light Bulb Discharger in order to monitor the voltage under load.. These 4 packs were the best matched of the 8 packs in this shiment.
After making a few mistakes, woops, i decided to color code my wires, getting 4 pairs of wires to the right place without crossing anything was vital.
After fully charging and balancing every cell in every pack, i cycled them 3 times and determined that they are all healthy.
Below, i got 4.3ah from the 5ah cells and still at 3.66v per cell ( testing 24 cells in series ), so no doubt they are good for a full 5ah if i was going to drain them flat, something you should avoid doing with lipo.. or all battery packs for that matter.
These 4 packs went to the Chaos !! and gave me a significant weight saving. !
72v 20ah LiMn = 38.6 lbs
100v 5ah LiPo = 7.4 lbs
Now, yes it's lighter, but also 1.4 kwh vs 0.5 kwh of capacity, so 33% of my previous capacity but still enough to get me from home to work and back, 13 kms at full throttle with power to spare.. also, since it's 100v vs 72v i got more speed !
As of this writing, Dec 11 2010, 60 cycles and 700 kms ( 158ah )
Next it was time to deal with that 0v cell, i could have claimed warranty from Hoby King for this pack, but that requires shipping it to back to China, the cost of shipping would be just as much as just buying a new pack, so not worth it.
I built an Anderson Powerpole pigtail to tap into the JST connector from the pack.
With a bucket of sand and the garden hose on standby, i headed outsite.
One of the things to be aware of about lipo, aparently, is that if discharged below 3.0v, they can explode upon recharging ! Many reports of RC ( Remote Control ) users of cells puffing like a balloon and rupturing.. so i was ready for it with camera in hand.
However, having done alot of research before i even purchased these, and having had the experience with other lithium chemistries that self-discharged slowly over a long time period below the spec low voltage, vs being draing at high power while in use...
slowly, with a 2 amp charger, i recharged the cell a few seconds at a time, on, off, on, off, until it got to 3.0v and would remain there when the charger was removed, checked for any signs of heat, no puffing, i allows the 2 amp charger to bring the cell to 3.9v and keep it there for a while before going to 4.2v and testing it for capacity....
Amazingly, the cell is ok, as i write this i walked over to the pack in question that has been used a few dozen times on my BMX and my leaf blower, all ok so far !
More pack testing, these are the 4 out of 16 packs i have that have bad or less than perfect cells, still usefull for things like this :
Now, about the Turnigy 4x6 charger, after a short while using it, a few things to note :
- The fan is on all the time, and on high..
- Max 2 amps when charging 6 cells, kinda lame for an RC charger
- All those wires are a recipe for disaster even when color coded.. specially since the display does not line up with the output wires and with wires on both sides of the unit it gets confusing easy.. and no room for that with lipo.
- The outputs are not isolated.... i blew up 3 of the channels trying to charge single cells withing a pack in series and zap...
Then i got one of these :
Much much better, more expensive, i paid 180 $ for it from hobbyking, but it is one awsome unit.
You can power it from anything from a 12v car battery to a 24v power supply, can charge at up to 30 amps !!!!! and 10 cells !
Each 5ah cell can be charged at up to 10 amps, but when you parallel packs for 10ah, 15ah, 20ah, etc... you can charge with more amps !
Full featured, you can also plug a USB cable and wire it to your computer to record discharge graphs, it will charge a seperate pack from the pack you are dicharging as a load.. pretty cool ! i have not used this yet, but i will soon and report back.
This is another very usefull gadget / tool , it will display voltage of up to 8 cells at once, and will beep to sound the alarm if any cell goes over/under adjustable limits. There are 2 types, the S and the M series, one of them can output to a USB cable into your computer voltage data, the other canot. They are very accurate, more so than the HK balancer.
So as you can see above, there are various ways to charge and monitor lipo cells while you charge..
One problem however, is that not very many RC toys run more than 36v ( 10 cells ), but many powerfull Ebikes run 48v +..
So what do you do ?
Well, there are many ways to deal with it, depending on your needs and wants, budget and know how.. I'll try to outline a few here, don't take this as the end-all be-all of lipo charging, these are only some of the things i've tried.. and passing it along for education...
My Norco Chaos !! for example, runs at 100v, so that means i have 24 cells in series !!!
Above, notice the layers of PVC plastic between each p ack, and the nylon filament tape, i actually compressed the packs together with a wood clamp ( gently ) and with non-streching filament tape secured the packs together tightly, this prevents the packs from expanding and contracting while charge/discharging, also keeps the packs solidly in place while off-roading and jumping curbs on the norco !
Below, this is a MeanWell power supply, I found 4 of these on ebay, for like 15$ each, they are not exactly chargers but they can be used for the purpose..
This is a : SP-480-24 .. up to 480w at 24v.. so 20 amps !
Now, a little bit about " C " rate, and various battery stuff: Battery Information ( page in progress )
My 100v battery pack is only 5ah ( 5000 mah ), and the specs say that for these cells i should not charge at any more than " 2C " .. so for a 5ah, 2C = 10 amps !
The problem here is that since this MW power supply is not tecnically a " charger " .. the Max Amps are not adjustable, The voltate IS ( to a small degree ) , but not the amps !
When you power up the power supply with nothing else connected to them, you can measure the DC voltage at the V+ and V- terminals, on the far left there is a " +v adj" screw that you can turn up or down to set the maximum voltage the unit will output.
What will happen is that anytime you connect the power supply to something, it will output as many amps as needed to get the output to the set voltage, up to 20 amps ( the unit's maximum ) ... .. Once the voltage goes up and up and up... and reaches the setpoint, it will reduce power ( amps ) until it is no longer outputs any power at all and just maintains the set voltage.
Confused yet ? .. Keep reading..
My pack is made of 24 cells, so 24 x 4.20v per cell = 100.8v
This would be 100% fully charged, with all cells at exactly 4.20v
It's a widely accepted fact that charging to slightly lower than full voltage extends the life of lithium batteries. Not only that, 24 cells in a pack like this will not always be perfectly balanced.. cell voltages will drift apart over time and the power supplies have no clue other than the total pack voltage.
Let's say there is one bad cell or more in the pack, and at 100.8v total, one cell could be a 3.50v while some of the others could be well over 4.20v causing a dangerous situation..
LiPo Cells SHOULD NEVER BE ALLOWED TO GO OVER 4.20V !!!
So with that in mind, i adjusted them all for a goal of 4.17v per cell, 4.17 x 24 = 100 v ( leaving a 0.8v buffer )
The meanwell 24v ( 2 units below ) were set to 25.02v
The 3rd supply, a 48v 4 amps Max unit was set to 50.04v
First time hooking all this up, i headed outside again, with the bucket of sand and the garden hose...
When in doubt, don't do it in the house !!!
Geek moment, warning, and pay attention !!!!....
When you connect power supplies in series, just like batteries, the total voltage is the sum of all the supplies, BUT, they are all limited by the lowest of the bunch, so the Meanwell units can do up to 20 amps, the top PS ( power supply ) is limited to 4 amps.. so the 4 amp supply limits the whole thing to 4 amps max.
This is exactly what i wanted because my 5ah pack canot be charged at 20 amps, that would be double it's maximum, but at 4 amps i'm well within the limits..
100v x 4 amps = 400w total charging power !
" if " i doubled my batery pack capacity, for 10ah, i could charge at up to 20 amps ( 2C ) for :
100v x 20 amps = 2000w !!!... but this would be too much for your average house outlet and the breakers would pop.... so 400w it is !
Above, as you can see, i have 3.98 amps, and the voltage of the empty pack charging is currently at 92.5v and rising slowly, once the battery reaches 100.0v the Amps start to drop off slowly down to 0 and the pack ends up fully charged !
A lesson i learned along the way is that the metal bodies of the power supplies need to be seperated, i used pvc plastic pieces, otherwise they short out ( something to do with not being isolated ) I don't fully understand the details, but i know the 48v needs to be seperated from the MW units..! lol )
Why did i go thru all this trouble ?
The convenience of being able to simply connect one cable to the pack ! otherwise i have to seperate the 4 packs, charge them one at a time, then re-connect the packs in series, this can lead to making a mistake resulting in a really bad day... or forgetting a pack and over-discharging it on the next ride.
" Battery Management System "
See this : Headway BMS problems
These things are meant to prevent you from doing harm to your battery pack and provide safety while charging, the biggest problem i've had with them is the fact that most of them also have a " short circuit " or " Maximum Amps " limit that tends to get in the way of having fun.. And kicking in when nothing is really wrong..
There are high-power BMS's available out there, but be ready to pay a fair bit of cash for one !
In the works for quite some time is this :
The current generation of boards are an improvement over this older version : V 2.x on my chopper
Me, well, i enjoy watching my celllog and related screen tools, keeping an eye on capacity and being near by when i'm charging.. I will likely fit the chaos with a BMS next year , 2011, but for 2010 i went without.
Below, is my original RC charger, Thunder Power brand, it is a good charger, but the software is a bit too picky, it sometimes refuses to charge if a pack is too low ( but not over-discharged ) .. and it does not directly connect to the ballance leads, it requires an external ballancer unit.
Parallel packs !
Are you sitting comfortably ?
Ok, so far i've covered series connections, remember :
6S = 6 cells
5S = 5 cells
2S = 2 cells
Now to complicate things a bit.. " P " = Parallel.. !
6S 1P = 6 cells
6S 2P = 12 cells
5S 4P = 20 cells
Connecting cells in parallel does not increase voltage, it only increases capacity.. or mah.. or ah..
The cells i'm using are 5ah, or 5000mah, same thing.. 5 amp hour means the cell can provide 5 amps for one hour.
But, what if i want to run for 2 hours ?... i need bouble the capacity !
There are 2 ways to connect packs in parallel.
1- At cell level
2- At pack level
Above, is a wire harness i had to make that combines the packs in parallel at cell level.. at first glance it looks like it should all go up in a ball of fire but if you can picture this, what is happening is that cell1 from each pack, and cell2 from each pack, etc are all being forced into having the same voltage, so that as far as the charger is concerned , 4 x 5ah cells in P is just like a single 20ah cell.
An example of 4 packs in parallel, the connections can be ilustrated in different ways but to show how 4 cells in parallel act like 20ah, and then in series those blocks of 4 equal 16v ( 4 x 4.0v )
And Method 2:
So, same voltage, same capacity, the difference is that in method 1 if a single cell in a group of 4 goes bad for any reason, it takes the other 3 cells in it's group along with it.
In method 2 if a single cell goes bad, it will force the other strings to equalize voltage with the sum of the remaining cells in the string wih the dud...
Why and/or how a bad cell happens ? Well, either from mechanical damage, or internal shorts, this usually happens early in a pack's life or late as it wears out, the weakest cell will always be the pack's limiting factor. It's important to monitor the packs on a regular basis and find bad cells before they ruin a pack...
Pros and cons for both, but method 1 is prefered when using smaller cells.
If you were building a large EV, option 2 might be better in that you could monitor each cell individually with a complex BMS.. when working with car sized packs into the 200ah capacity, each cell is expensive enough to justify the expensive BMS, but on smaller ebike packs, like in my case, I don't even use a BMS, method 1 works best to simplify charging with an RC charger.
The RC bike, for example, runs on 48v, 12 cells in Series..
I purchased 6S packs for this, assembled into packs of 6S 1P from HK, i was able to test them individually at first, making sure they all match, then, making sure that all the cells are at the same voltage, build 2 blocks of 4 packs each in parallel for 20ah
Each 4pack below is 24v ( 6S ) 20ah ( 4P )
When i charge the RC bike i use the Icharger ( above ) and charge each 4 pack block as 6S ( 6 cells ), seperately, at 10 amps.
Technically, being 20ah, i could charge this block at up to 40 amps for a 30 minute empty/full charge.. that would be 800w, compare that to the average ebike
Lithium Polymer ( lithium cobalt ) cells are fully charged at 4.20v per cell, operate at or above 3.7v for most of the cycle, and are completely empty at 3.0v
Alkaline ( standard AA cells ) = 1.5v per cell
Nimh/Nicad = 1.2v per cell
Sealed Lead Acid ( SLA ) = 2v per cell
LiFePo4 = 3.2v per cell
Li Cobalt/Manganese ( lipo ) = 4.2v per cell
So as you can see above, it takes a different amount of cells in order to make a pack.
Alot of things to consider when deciding on what battery to use for your projects,
Up to this point i have been using lithium but other chemistries like manganese ( LiMn ) and iron phosphate ( LiFePo4 ) , the Turnigy packs are Lithium Cobalt ( LiCo. The reason for this was primarily safety.