Noob LiPo C question.... and others

[teddy_kgb]

I've been researching LiPo, been reading many posts here, but still have a few questions. I've searched for answers but could not find them so I apologize if this has been asked before.

I understand what the C number refers to but don't understand all the implications. With a 48V 500W motor, if I push more amps (higher C) then the motor will deliver more power (Volts x Amps = Watts), kind of like opening up the throttle and supply more fuel to your gas engine, right? But is the motor rated to only handle a certain amount of amps and above that it could burn out? Basically I'm trying to figure out what C rating from my battery pack would be ideal for my motor. Are there other advantages to have a higher C rating other than giving your motor more power when you need it? How does the motor controller fit into this equation?

Also, the higher amp discharge will drain the amp hours quicker but does it also reduce the voltage of the pack, drop the voltage down? I read I need to be concerned about the voltage dipping below 3.2 or so.

I was reading that it is bad to fully charge your battery pack to 4.2V, especially if you leave it on the charger. Do they not make a charger that you can set to charge to 3.9V (for example), and then it shuts off the charging? It would be a real problem for me if I have to monitor the charging that closely, I would like to just set the pack on the charger outside and leave it.

Lastly, I was looking on hobbyking at the different packs, is there an advantage to buying several smaller packs and series them together or.....? I see people refer to 5s or 6s packs like there is some advantage to a certain configuration. Is there much difference to getting 3 4s 14.8v packs as apposed to 2 6s 22.2v packs?

Thanks.

 

[miuan]

I understand what the C number refers to but don't understand all the implications. With a 48V 500W motor, if I push more amps (higher C) then the motor will deliver more power (Volts x Amps = Watts), kind of like opening up the throttle and supply more fuel to your gas engine, right? But is the motor rated to only handle a certain amount of amps and above that it could burn out? Basically I'm trying to figure out what C rating from my battery pack would be ideal for my motor. Are there other advantages to have a higher C rating other than giving your motor more power when you need it? How does the motor controller fit into this equation?

First off, you don't PUSH power from the battery. The motor and controller combo pulls amps from battery depending on max. current set by the controller, and your throtte position. As you already understand the C rating, you can see the battery can provide much more amps than you'll ever need with off the shelf ebike kits. So you can settle with 20C Turnigy stuff which is very solid and better that Zippy. I use the 4S hardpacks which are the least expensive at $ per Wh and the hard cases are pretty durable too .

Also, the higher amp discharge will drain the amp hours quicker but does it also reduce the voltage of the pack, drop the voltage down? I read I need to be concerned about the voltage dipping below 3.2 or so.

The voltage will drop as much as 0.1V per cell at 2-3C discharge rate. A healthy cut off voltage is 3.7V at no load state, or 3.6V loaded. This will ensure you won't get any cell too low even if the pack is a bit unbalanced. Below 3.65V the cells are almost empty and voltage drops dangerously fast. Below 3V is where serious damage may occur.

I was reading that it is bad to fully charge your battery pack to 4.2V, especially if you leave it on the charger. Do they not make a charger that you can set to charge to 3.9V (for example), and then it shuts off the charging? It would be a real problem for me if I have to monitor the charging that closely, I would like to just set the pack on the charger outside and leave it.

You will learn how long it takes to charge to a certain voltage, so you can just set up a timer and shut down the charger manually when you prefer. You can also monitor the voltage while charging, so you don't go to 4.2V all the time... I usually stop at 4.1 or 4.15V which exteds battery life.

Most bulk chargers have adjustable voltage, but it involves removing the cover off and messing with a screwdriver, so this is only good for setting the general cutoff voltage, like 4.1 or so.

Lastly, I was looking on hobbyking at the different packs, is there an advantage to buying several smaller packs and series them together or.....? I see people refer to 5s or 6s packs like there is some advantage to a certain configuration. Is there much difference to getting 3 4s 14.8v packs as apposed to 2 6s 22.2v packs?

You want the least interconnections, so the bigger the single brick the better. You also want to parallel the packs at cell level. These parallel connectors can be found at epbuddy, I also bought some on ebay.

Bulk chargers can be had here:
http://www.greenbikekit.com/index.php/battery-charger/360w.html

You will need a balancer of some kind. I use these with much success:
http://www.ebay.com/itm/200775947686

And a voltmeter for charge and discharge (bar mount) monitoring:
http://www.ebay.com/itm/160902083777

While shopping at hobbyking, get a pack of spare HXT 4mm plugs with red protectors. You will need them to connect the battery to controller.
If you are lazy at soldering, there are also premade Y-plugs to parallel 2 battery strings into one, so you can run 10Ah using 5Ah cells
http://www.hobbyking.com/hobbyking/store/__18604__HXT4mm_Battery_Harness_14AWG_for_2_Packs_in_Parallel.html

If you have a 48V controller, the max. cell count you can use is 15.

 

[DAND214]

And a voltmeter for charge and discharge (bar mount) monitoring:
http://www.ebay.com/itm/160902083777

What are you goig to do with a 30v meter?

Dan

 

[Philistine]

The higher the C rate the better, because that means you can draw more amps (and also charge with higher amps), without damaging or reducing the life of the battery. You can think of the C rate is telling you how much amps you can theoretically pull from the battery without damaging it. The reason it is quoted as a "C" rate, as opposed to just telling you the allowable amps, is that it depends on the size of the overrall pack, ie, the more packs in parallel you have (ie the more AH available in the pack) then the more amps you will be able to pull. This is because theoretically the C rate, multiplied by the AH tells you what the supposed possible Amps you can pull. eg, if you had a 20C 10ah pack, then you could theoretically pull 200 amps, of course, in reality you wouldn't want to continuously pull 200amps from a 20C 10ah pack, but that is the idea. Basically you want the highest C rating that you can afford (although you really only need over 30C lipo if you are running big current, or small packs) - personally I go for 30C - I have noticed mates with 20C packs do tend to go out of balance a bit more.

You don't need to worry about voltage sag, it is simply an annoyance (because you lose speed) - the more AH you have on board, the less voltage sag you will see. Like a woman's chesticles, sag increases with age unforunately, but as long as you have your LVC on your controller set low enough, it will never be an issue. When people say you want to be careful about not going below 3.2V, they are referring to resting voltage, not voltage under load. The issue with lipo, is that most often people run it without a BMS, so whereas with Lifepo4, a noob can run their battery dead flat, the BMS will trip out when the battery is flat, and no harm will be done. However, Lipo will let you drain it as flat as you like, but once you go into the danger zone, and then you recharge that is when the fireworks can begin. So it is important to have something like a Cycle Analyst to monitor your voltage, to give you a rough idea of where your overrall pack voltage and AH used is at. Personally, it is my opinion that if you make sure you have enough AH for your task, and target to not go over 70% used (so if your commute is 10AH, then you should have say 15AH on board), then you will never have a problem. The Lipo fires almost always involve Noobs, and people slapping together a 5AH pack, and then pushing it to its limits, then recharging. I have lipo packs ranging from 12S 40AH to 20S 20AH, and I use them everyday and I probably balance charge once a year or so. Having said that I check all my packs carefully before building a pack, then balance them tightly, parrallel the balance wires, and I almost never go deeper than 70% discharged. On the rare occassions I have gone to 90% deep etc, the cells go out of balance and the trouble starts. This is why I am saying you should not chince out on the AH.

In terms of bulk chargers (depending on the voltage and size/AH of pack you intend to build), I would look at at the BMS Battery chargers, as they have bigger Wattage chargers available than the link above, personally I can't stand anything weaker than a 900W charger myself, but I build decent size packs with good C rates. With a BMS battery charger you can actually set the cutout voltage when you order it (so, for example, although 12S is technically 4.2v*12= 50.4V, I just ordered mine to cut at 50V (being 4.16v per cell), but you can choose (and even later adjust) the cutout voltage. If ordering a BMS battery charger you should also indicate it is for Lipo, not Lifepo4, and they will have it set so that it just cuts dead off when reaching its voltage (as opposed to float charging).

Take the time to read the lipo noob thread, because as I said, the trouble always starts with Lipo with people not taking the time to understand the basics. In my opinion Lipo is my personal choice of battery of all choices currently, but I will only help mates build packs if they are willing to be lectured on the basics and if I am confident they are the type of people to obey the rules.

 

[dogman dan]

I'll try to write a simple answer for once. Hard to get very specific till you tell us what you want to do, with what.

Most of the typical direct drive motors are designed for about 500-800 watts continous. And they come with controllers in the kit that put out about 20 amps, sometimes 30. Depending on the voltage, a 20 amp controller will supply between 800 and 1200w peak. Under most conditions, riding on paved road grades less than 7%, you'll never smoke a motor.

Supplying that amount of power, your battery needs to be about 15 ah in size, if it's the less expensive 2c grade stuff. 15 ah times 2c = 30 amps max. But mostly the motor will draw less than 20 amps so there is enough extra c rate to prevent a shortage later, when the battery is weaker and old. If you have a better c rate, then a smaller pack than 15 ah can be used.

In general, 20 mph takes 400-600w, 30 mph 800-1000w. The bigger the motor and higher amp the controller, the more watts are used to get to speed. So a big motor and big controller can really punish a battery.


But we of course don't stay with that stock kit. We'll take the same cheap motor and run 3 times the rated 800w, or more. At that point, it's not a continuous use situation anymore, it's just a matter of how hot is the motor getting today, and how quick do we need to stop before smoke pours from the motor today. The weather, and how you are riding matters, so it's becoming standard to put a thermometer inside the motor if you are going to push the wattage to the limit.

 

[teddy_kgb]

Thanks everyone.

My specific details: The bike/trike is a catrike speed trike. I have not purchased the motor/controller batteries yet but plan on a Bafang 48V 500W rear 20" wheel with the standard controller that comes with the kit from greenbikekit.com. My commute is about 10 miles and I have some hills so I was thinking a 10Ah pack would be big enough? I can charge at the end of each 10 miles if necessary. I would like to cruise at around 25 MPH maybe 30 with pedal assist. Would 4 of these packs be good:
http://www.hobbyking.com/hobbyking/store/__9518__Turnigy_5000mAh_6S_30C_Lipo_Pack.html
2 in series and then 2 paralleled to give me 44.4 V at 10 Ah. Does it matter that they are only 44.4 volts on a 48 volt motor?
Would these packs be worth the upgrade?:
http://www.hobbyking.com/hobbyking/store/__11937__Turnigy_nano_tech_5000mah_6S_25_50C_Lipo_Pack.html
A bit higher C rating and the nano-tech (which I understand gives a longer life cycle)?

So, as you drain the battery from 10Ah to 3Ah (for example) the voltage will also be drained from 4.2 to something lower, like around 3.7 ? My limited theoretical knowledge is that as a battery drains, the Amp hours will reduce but the voltage should stay the same. No?

Thanks. And I am trying to learn by reading here, I have read all the noob threads but I still have these questions.

 

[Philistine]

So, as you drain the battery from 10Ah to 3Ah (for example) the voltage will also be drained from 4.2 to something lower, like around 3.7 ?

Yes, there are two things going on with voltage, there is 'voltage sag' which is the degree to which the voltage will drop under load, and that will vary from nothing to a lot, depending on the load (ie the demand on the battery by the controller/motor) and also on the C rate of the battery and the amount of AH in the pack. The higher the C rate and AH in the pack, the less the voltage sag. Secondly, over the discharge of the battery, the resting voltage will also drop, that is what people mean when they say don't let the resting voltage go below XYZ, they mean the resting voltage. As you use AH the voltage will drop. It is not as simple as just monitoring AH used (although for a long time that will be fine as a measure) because as the pack ages, it will lose AH, and hence you will see the voltage dropping faster (ie after less AH used) than it did when new. This is why you should at least monitor two things, overrall pack voltage, and also AH used. Personally that is all I use (I use no cell level monitoring, other than the fact that I have the cells paralleled, and I can check them with a Cellog every now and then. But if you are monitoring pack voltage and AH used constantly (eg through a cycle analyst), then in my opinion you are fine. I have once had a cell fail (it failed due to my fault, due to a bad balance wire connection being soldered on, but because I had parralleled the cells, the whole line of cells dropped to the floor), but I instantly noticed the problem because my pack voltage started dropping way too early, so I caught it.

I keep hammering it, but just make sure you carry as much battery as you can afford (in terms of dollars and weight), because that is always where the problems start for people, trying to underbudget on AH.

 

[Ykick]

10Ah "rated" is probably around 9Ah real world if observing conservative voltages. That should be fine for your 10 miles using that combo but it won't leave you with much of a safety buffer. And deep discharges will add that much more stress and subsequent balancing to your pack.

What if you encounter strong headwinds or forced to detour? Maybe forget something and forced to turn around? Colder weather doesn't help either. And finally, no battery in the world increases capacity as it ages so it's only gonna get worse as the miles add up.

I'd suggest 15Ah rated capacity for your stated parameters.

 

[Philistine]

I agree with all that Ykick said, and I would say do it this way, work out how many AH you are going to actually need/use (let us say it was 10-12AH), then I would say you should have at least 15AH on board, and preferably 20AH if you can. I personally target 50% depth of discharge before recharging, and at worst 70%, and I have only gone over 70% on very rare occasions. By doing that, I only balance charge once a year, and blind/bulk charge everyday, and apart from my own screwups learning what the hell I was doing, I have never had to replace a single Lipo pack after 2 years of ebiking with lipo everyday.

As I keep saying, all the Lipo problems I have ever seen were people pushing its limits. People always quote the energy density/lightness of lipo as its drawcard, but to me that is just for extreme applications (eg race bikes etc..). For reliable ebiking, its benefit is that you can make totally modular packs that you can plug and play packs in and out of, and build packs in a modular way, but that benefit comes with being responsible enough to recognise the actual capacity limitations and obey them. Building a Lifepo4 pack is a production in itself, Lipo involves a soldering iron or a crimper, and then just applying lego skills, but people get complacent and presume that they can take their lipo packs to their full AH rating, and that is when the tears and the fires and the insurance adjuster man come into play....

 

[DAND214]

Thanks everyone.

My specific details: The bike/trike is a catrike speed trike. I have not purchased the motor/controller batteries yet but plan on a Bafang 48V 500W rear 20" wheel with the standard controller that comes with the kit from greenbikekit.com. My commute is about 10 miles and I have some hills so I was thinking a 10Ah pack would be big enough? I can charge at the end of each 10 miles if necessary. I would like to cruise at around 25 MPH maybe 30 with pedal assist. Would 4 of these packs be good:
http://www.hobbyking.com/hobbyking/store/__9518__Turnigy_5000mAh_6S_30C_Lipo_Pack.html
2 in series and then 2 paralleled to give me 44.4 V at 10 Ah. Does it matter that they are only 44.4 volts on a 48 volt motor?
Would these packs be worth the upgrade?:
http://www.hobbyking.com/hobbyking/store/__11937__Turnigy_nano_tech_5000mah_6S_25_50C_Lipo_Pack.html
A bit higher C rating and the nano-tech (which I understand gives a longer life cycle)?

So, as you drain the battery from 10Ah to 3Ah (for example) the voltage will also be drained from 4.2 to something lower, like around 3.7 ? My limited theoretical knowledge is that as a battery drains, the Amp hours will reduce but the voltage should stay the same. No?

Thanks. And I am trying to learn by reading here, I have read all the noob threads but I still have these questions.

I don't think you will see 25/30 out of 44v nominal setup on a 20" wheel.
you are also better with more AH cap than c rate since you won't bother those cells with that motor.

Myself I like the tringy 20c packs for less money you can get the bigger AH. Been 2 years running them 15s4p with 8t Mac and 9C 1206 hbs. 45 amp controller on a 26" wheel. balance maybe once evey 6 months. Run 50% dod with a few almost 90% on long rides like 50 miles.

if you are staying with the 30c packs buy the Nano=teck for just a couple bucks more. 15Ah is a better choice if not 20. Bigger is better in this case.

Have you looked at the simulator http://www.ebikes.ca/simulator/
to see what you are going to get out of that setup?
On the 36v500w BPM you might get just over 20mph. get the 332rpm one or it will be real slow.

My wifes POS has a 8fun 36v350w 26" wheel with a bigeer controller and 44v Lipo and it tops out at 22mph

Dan

 

[teddy_kgb]

Thanks for the info all. And that simulator is pretty cool, i'm gonna play with it some more later.

And I'm going to get a 15Ah pack. But regarding the voltage, if my motor is 48V 500W if I had a pack that was say 66V for example with that just be wasted voltage. If the motor is 48V will any more than that not get through to the motor because it is limited by the controller?

Thanks again.

 

[Philistine]

if my motor is 48V 500W if I had a pack that was say 66V for example with that just be wasted voltage. If the motor is 48V will any more than that not get through to the motor because it is limited by the controller?

Motors are not set to or limited to a particular voltage, that is just how some people "sell" them. What is limited (potentially) is the controller (depending on the caps in the controller), so if a controller is quoted as a "48v controller" and you threw 100V at it, it might blow. But a quality controller set up for high voltage can take anything from 36v through to 100v+ (over 100V, there are issues like the type of Mosfets in the controller). So the extra voltage will never be "wasted" it is just that if your controller is not able to take the higher voltage it could blow. As long as your controller is able to take the voltage, then you can throw any voltage you like at any motor and it will go faster, it is just that if it is not designed or modded for high wattage, you might cook the motor quickly.

 

[dogman dan]

A few more basics you might find helpfull. Using 36v batteries, a good rule of thumb for guessing range is 1 ah per mile. With 48v , .75 ah per mile. This rule of thumb does allow for about 10-15 % of the battery capacity to not be used. The rule is for 20-25 mph travel, so faster takes more power. It is assumed you pedal some, but never stop using the motor.

So a 44v lipo pack, 12s , should get you the 10 miles with little problem at speeds up to 25 mph.

Most "48v" controllers toleate up to 63v fully charged battery voltage. With lipo 15s is the very limit, and some like 14s for insurance. 14s charges to 59v. 12s is very popular since two 6s packs or three 4s packs makes a 12s bundle. 12s charges to 50v. 48v lead charges to 55-56v.

 

[teddy_kgb]

Before reading your post I started thinking maybe I should do 15s with 3 5s 2p which would give me 55.5 V and 10ah, with 55v can I get away with 10ah? This would be cheaper than going 44v 15ah.

 

[dogman dan]

What you actually get can vary a ton. You might have hills or headwinds, you might tuck or ride upright with an open coat a flapping. Then there is the speed, which always heavily affects total range.

But let's speculate some and do the math, based on a speed of about 23 mph average.

It's good to undercharge lipo slightly, so many charge to 4.1v rather than 4.2v. Then it's equally good to stop by the time you reach 3.7 or 3.65v per cell. Resting voltage, it will be a bit less under load, which is still ok.

So call the usable capacity of that 10 ah pack 8 ah. 8 ah divided by .75 ah per mile, per the rule of thumb for range when using 48v, is just about 10 miles. The rule of thumb also includes some reserve capacity, because your capacity will be less in winter, and you will need more if a storm gives you a big headwind.

So a 15s 10 ah lipo pack would just be capable of your needs when fairly new. As it ages you might need more, but you could easily make it a 15 ah later when funds are avaliable.

By slowing down 5 mph, you could easily make 15 miles range when you need it. If you cruise at 30 mph, you'd need the extra capacity of a 15 ah pack.

I don't recall if it got mentioned yet, but to charge to 4.1v, you set the RC charger to lilo, rather than lipo. You can discharge to 3.5v when you just must use it, but stopping sooner helps the cells keep from getting as unbalanced. The extra money to get 15 ah would very likely pay for itself in a longer lifespan for the entire pack.

But the real reason to buy 15 ah, is you simply are not going to be ready to stop riding after just 10 miles. 15 ah would get you up to 20 miles at 25 mph. (4.2v to 3.5v) Possibly more.