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Snoopy
04-02-2015, 06:46 PM
Hello, what do we have to pay attention to when choosing batteries for high torque motors? I suppose we choose a voltage (around 12V, 24V) first based on the requirement of the motor. Then, get the highest current one available to generate more torque. Am I right? Is it correct that it is OK to choose a battery that supplies more amp than mentioned in the motor and driver. However, don't choose one that supplies more voltage than written on the motor? Should I get LIPO or LiFePo4? Thanks.

jwatte
04-02-2015, 06:53 PM
It is correct that it is OK (and often a good idea) to choose a battery rated for higher current than the motor will draw.
(Note that the motor may temporarily draw double when reversing direction.)
Also note that batteries are typically at a voltage higher than rated when fully charged, and lower than rated when discharged.

LiFePO4 is safer than LiPo (less likely to catch fire from abuse.) It also lasts many more charge cycles (between 3x and 10x as many!) before wearing out.
initial purchase cost is slightly higher for LiFePO4. They are also sometimes harder to find in the size/form factor you want.
LiFePO4 is slightly less efficient in volume/weight compared to watt-hours.

I like LiFePO4 because they are safer and more cost efficient over time. But I also use a bunch of LiPo batteries because of their lower cost and easier availability.

Once you go above 6S or 8S, it's hard to find pre-made battery packs, so if you need very high voltage (>25V, say) you will likely want to either put multiple packs in series, or you will want to build your own pack from bare cells. Also, for very high currents (> 30A or so) the cheap protection/management circuitry doesn't work so well, so you may have to pay more there, too.

Snoopy
04-02-2015, 07:21 PM
Thank jwatte for the expert opinion.

In regard to voltage, sometimes it is hard to find exactly 12 or 24V. I suppose if the motor requires 12V, 11.1V battery is not that bad to generate the desired power. How much higher voltage is OK without damaging the motor or driver in case 12 or 24V is required. For 12V, is 14.8V better than 11.1V? For 24V, is 22.2V OK?

Which LiFeP04 companies do you recommend? I want light weight and high current. I cannot find one that offers 12V with high amp. The best I can find is 6.6V at 1600mAh.

Is it save to put in series the LiFeP04 or LIPO by myself? I heard that LIPO batteries are a bit dangerous.

jwatte
04-03-2015, 01:12 PM
it is hard to find exactly 12 or 24V.

You will not find any battery with a precise voltage, for the reasons related to charging state I indicated above. (A fully charged car battery is above 13V, even though its "rated" at 12V.)

If you need exact voltage, you will need some kind of regulation, which is expensive, bulky, and may have poor efficiency.

That being as it may, as long as you're "in the ballpark" on voltage, it really is the current through the motor that matters, because that turns into heat, as well as the top speed, which turns into wear. A motor run slightly above spec voltage will just last slightly shorter. Many stepper motors are rated for 1-2V, and X Amps, and are driven by 12, 24 or 48 Volts, but with a controller that limits the current. The higher voltage lets the motor get to the limited current sooner, leading to higher effective torque.


The best I can find is 6.6V at 1600mAh

That's not current rating; that's capcity (energy) rating. For these kinds of batteries, you get the current rating by multiplying the "C" rating with the capacity. So, a 10C, 1.6 Ah battery, would have a 16A current capacity.

I use these: http://www.all-battery.com/LiFePo4_Lrg.aspx (in a stack of 4)

Others may use something like these:
http://hobbyking.com/hobbyking/store/__14070__ZIPPY_Flightmax_4200mAh_4S1P_30C_LiFePo4_ Pack.html
http://hobbyking.com/hobbyking/store/__14074__ZIPPY_Flightmax_8400mAh_4S2P_30C_LiFePo4_ Pack.html

Snoopy
04-03-2015, 06:50 PM
Thanks.

By "ballpark", by how much extra voltage is acceptable? For example for 12V motor, shall I get 14.8V rather than 11.1V? I asked a specialist of hobbyking. He wrote: "if the motor requires 12 volts sir you need to get 12 volts
neither will work"!?



In terms of real practical use, how the "C" rating affects the performance of the motors? As far as I know, it has something to do with charging and discharging rate.





I use these: http://www.all-battery.com/LiFePo4_Lrg.aspx (in a stack of 4)

Others may use something like these:
http://hobbyking.com/hobbyking/store/__14070__ZIPPY_Flightmax_4200mAh_4S1P_30C_LiFePo4_ Pack.html
http://hobbyking.com/hobbyking/store/__14074__ZIPPY_Flightmax_8400mAh_4S2P_30C_LiFePo4_ Pack.html

Do you have a photo on how to connect the batteries from all-battery in stock of 4? Are connectors available to connect them in series? I am afraid of fooling around with dangerous batteries.

Are there connectors to connect those hobbyking batteries you mentioned to small stepper motor driving circuit?

jwatte
04-05-2015, 12:26 AM
I asked a specialist of hobbyking. He wrote: "if the motor requires 12 volts sir you need to get 12 volts
neither will work"!?

Then he is not an engineer :-) Motors can be over/under-volted, with resulting higher/lower performance, and change in lifetime. Note that the change is not simply linear with voltage, and it depends a lot on the quality of your motor controller. One that can limit current to the appropriate value for the motor is vastly preferrable.

If you want to get a feel for how motors behave, I suggest you buy a few really cheap 6V hobby motors and a few small motor controllers (like the cheaper ones from Pololu) as well as a good quality lab power supply that lets you vary voltage/current. Then drive them hard and soft, and see how they behave, how warm they get, and when they fail. Great learnings for less than $100!

If you're not comfortable soldering or clamp-connector-ing battery tabs, then buy a ready-made pack, rather than flat cells. There are a number of "lead acid replacement" LiFePo4 batteries on the market in the "12V nominal" range.

If a battery C rating is too low for the current draw of your motor, then the voltage will sag, and the motor will not achieve its rated performance. Also, the battery will likely heat up, and in really hard load could catch on fire! I don't recommend it.

Snoopy
04-06-2015, 06:21 PM
Thanks jwatte. What is the preferred C rating?

jwatte
04-06-2015, 07:53 PM
What is the preferred C rating?

That's like asking "what is the best engine for a car?"

What are your needs? What is your budget?

Snoopy
05-26-2015, 02:08 AM
Can anybody please confirm my understanding? Given two LiFePO4/LIPO batteries with same voltage but different mAh, is the one with higher mAh able to turn the same motor under the same load for a longer period of time? If the C rating of the two batteries are the same, given Power = I*I*R, is the one with larger mAh able to generate higher torque? As far as I recall, somebody told me in the past to use thicker wires as they are able to carry more current to generate larger torque. Is that right? Finally, we use LIPO batteries in phones, computers, etc. Usually there is no problem but sometimes we hear incidents of explosions. Are LIPO really that dangerous if I make a robot arm to handle heavy objects?

jwatte
05-26-2015, 12:25 PM
the one with higher mAh able to turn the same motor under the same load for a longer period of time

Correct.


Power = I*I*R, is the one with larger mAh able to generate higher torque

Batteries don't generate torque; motors do. The higher C rating makes the battery able to be used with higher amp draw motors. If a given motor is under the C rating of both batteries, there isn't much of a difference between the two batteries for that motor, although higher C rating usually means less loss in the battery, and ability to charge faster safely.


use thicker wires

You need to size the wires to the actual (average) power draw of your load. Thinner wires will heat up, and drop voltage, and thus lead to less torque in a given motor. Way too thick wires will weigh too much, cost too much, and be too bulky, but will otherwise work fine.


Are LIPO really that dangerous if I make a robot arm to handle heavy objects?

A correctly manufactured, installed, used, and charged LiPo battery is not dangerous as long as you shield it from physical damage.
Be sure to not over-charge it, and not over-discharge it, and not let anything hit it (shield it in a hard box,) buy it from a reputable source, and charge it with a balancing charger, and you'll likely be fine.

Snoopy
05-26-2015, 07:27 PM
Thank you very much jwatte.



A correctly manufactured, installed, used, and charged LiPo battery is not dangerous as long as you shield it from physical damage.
Be sure to not over-charge it, and not over-discharge it, and not let anything hit it (shield it in a hard box,) buy it from a reputable source, and charge it with a balancing charger, and you'll likely be fine.

What do you mean by not "over-charge" and not "over-discharge"? Can I control these two? I think when the battery is fully charged, the LED in the charger changes color? Not sure about discharging...

Does vibration lead to dangers such as explosion or fire from the LIPO? I am thinking of a wheeled robot moving in rugged terrain.

tician
05-26-2015, 07:48 PM
Yes, soft pack LiPo cells without mechanical and electrical protection from damage (which is most hobby batteries) can be made to die a fiery death by many simple methods of abuse.

Attempt to charge LiPo with incorrect charger or incorrect charge profile: fire.
Puncture a LiPo pack: fire.
Short out a LiPo pack: fire, if the wires do not quickly burn out or desolder themselves from the pack.
Overheat a LiPo pack: fire.
Over-charge above 4.2V per cell: fire.
Float charge at 4.2V per cell: decreased life, possible fire.
Over-discharge LiPo below 3.2V per cell: permanent damage to the cell, possible fire during later use and/or charging. If any cell envelope starts bloating, never use that cell/pack again.

Simple rules: get a LiPo balance charger from a reputable source, stick the pack in a sturdy (preferably padded) plastic case, and always use a battery monitor (http://www.trossenrobotics.com/p/1-8S-lipo-battery-voltage-tester-monitor.aspx) when in the robot and listen for it beeping.

Snoopy
05-26-2015, 08:20 PM
Thanks tician.

Is shaking/vibrating/shocking OK if I put it in a wheeled mobile robot moving in rugged surfaces.

Zenta
05-27-2015, 06:29 AM
Is shaking/vibrating/shocking OK if I put it in a wheeled mobile robot moving in rugged surfaces.
Yes. As long as the battery pack are secured and protected. It's recommended to use a hard-case LiPo to prevent accidentally puncturing.

In addition to what Tician mention it is also recommended to store the LiPo at 50% energy. Do not store a fully charged LiPo for a long time. It's not dangerous, but it reduces lifespan.

Overall, a LiPo isn't dangerous and will never burst into flames as long as it is used, charged, discharged, protected in a correct way.

Oh, another thing. I've mentioned this before, always use a main fuse close to the battery connector. In case of a shortcut its better to blow a fuse or the whole robot. :p

Snoopy
06-14-2015, 06:30 PM
Hello, I am looking for battery/batteries to drive two 7.2V DC motors (each motor consumes a current of 1.2A when there is no load, stall current is 18A, the averaged expected motor current is 5A) via a motor driver shield (https://www.pololu.com/product/2507). The desired running time is about 10-20 minutes. The longer the better but I also need to take the weight of the battery (ideally below 350g, the lighter the better) into consideration. Which product do you recommend? What do you think of the following products? Thanks.

http://www.hobbyking.com/hobbyking/store/__24306__Turnigy_nano_tech_A_SPEC_4200mah_2S_65_13 0C_Hardcase_Shorty_Lipo_Pack.html

http://www.hobbyking.com/hobbyking/store/__24305__Turnigy_nano_tech_A_SPEC_6200mah_2S_65_13 0C_Hardcase_Lipo_Pack.html

http://www.hobbyking.com/hobbyking/store/__24310__Turnigy_nano_tech_A_SPEC_4500mah_3S_65_13 0C_Lipo_Boat_Pack.html

http://www.hobbyking.com/hobbyking/store/__59159__Turnigy_nano_tech_3300mah_3S_25_50C_Lipo_ Pack_AR_Warehouse_.html

http://www.hobbyking.com/hobbyking/store/__32691__Turnigy_nano_tech_5000mah_3S_35_70C_Lipo_ Pack_EU_Warehouse_.html

http://www.hobbyking.com/hobbyking/store/__59305__Turnigy_nano_tech_4000mah_3S_25_50C_Lipo_ Pack_AR_Warehouse_.html

http://www.hobbyking.com/hobbyking/store/__20761__Turnigy_nano_tech_4000mah_3S_45_90C_Lipo_ Pack_US_Warehouse_.html


(http://www.hobbyking.com/hobbyking/store/__20761__Turnigy_nano_tech_4000mah_3S_45_90C_Lipo_ Pack_US_Warehouse_.html)

r3n33
06-14-2015, 08:56 PM
Hello, I am looking for battery/batteries to drive two 7.2V DC motors (each motor consumes a current of 1.2A when there is no load, stall current is 18A, the averaged expected motor current is 5A) via a motor driver shield (https://www.pololu.com/product/2507). The desired running time is about 10-20 minutes. The longer the better but I also need to take the weight of the battery (ideally below 350g, the lighter the better) into consideration. Which product do you recommend? What do you think of the following products? Thanks.

(http://www.hobbyking.com/hobbyking/store/__20761__Turnigy_nano_tech_4000mah_3S_45_90C_Lipo_ Pack_US_Warehouse_.html)

Hmm. The first two links are for a 2S (7.4V battery) and will likely work fine with your 7.2V motors. The others are 3S (11.1V battery) and when fully charged (about 12.6V) they will likely damage or shorten the life of your motors unless you have a voltage regulator that you didn't mention. All of the battery packs are the same brand so there isn't much to compare aside from voltage, capacity, discharge rating. If you are only expecting to pull 10A (5A average x2) from the motors the discharge rating on any of these packs seems irrelevant. Now to power these motors for 20 minutes @ 10A equates to 3.33Ah (or 3,300mah) which means they will all satisfy your capacity requirement.

Snoopy
06-14-2015, 11:57 PM
Hmm. The first two links are for a 2S (7.4V battery) and will likely work fine with your 7.2V motors. The others are 3S (11.1V battery) and when fully charged (about 12.6V) they will likely damage or shorten the life of your motors unless you have a voltage regulator that you didn't mention. All of the battery packs are the same brand so there isn't much to compare aside from voltage, capacity, discharge rating. If you are only expecting to pull 10A (5A average x2) from the motors the discharge rating on any of these packs seems irrelevant. Now to power these motors for 20 minutes @ 10A equates to 3.33Ah (or 3,300mah) which means they will all satisfy your capacity requirement.


Thanks r3n33 for your expert opinion. So, I just choose a battery that can offer over 3.33Ah with the size and weight I prefer? How do I connect such battery to the Pololu Dual VNH5019 Motor Shield for Arduino?

In this case, is it worth to get the 3S ones for higher than advertised torque?

I chose Turnigy due to availability. How is this brand compared with others? Any other brand worth looking? I read postings that the Turnigy batteries got swollen easily.

In the hobby king website, they are listed as LIPO but in other site, they are listed as LiFePO4. I am confused.

r3n33
06-15-2015, 10:15 AM
Thanks r3n33 for your expert opinion. So, I just choose a battery that can offer over 3.33Ah with the size and weight I prefer? How do I connect such battery to the Pololu Dual VNH5019 Motor Shield for Arduino?


The wiring information for connecting your battery power to the motor shield is on the product page provided by Pololu. I would propose you could connect a 2S battery directly to the motor power +/- inputs.



In this case, is it worth to get the 3S ones for higher than advertised torque?


No. You should choose a motor that produces the correct amount of torque and provide it with the recommended voltage.



I chose Turnigy due to availability. How is this brand compared with others? Any other brand worth looking? I read postings that the Turnigy batteries got swollen easily.

In the hobby king website, they are listed as LIPO but in other site, they are listed as LiFePO4. I am confused.

I've never used this brand so I cannot compare. All of the batteries you provided links to are listed as LiPo.

tician
06-15-2015, 11:10 AM
Turnigy is one brand that produces multiple battery chemistries (LiPo, LiFePO4, NiMH, etc.). I have not yet had a bad experience with their batteries, but there will almost always be some 'not great'/dud cells that sneak past quality control. Since those motors are unlikely to get anywhere near the peak current levels of the large brushless motors used in planes, it decreases the likelihood of damaging the cells from over-current/over-heating. Just never let the LiPo drop below 3.2V per cell or charge above 4.2V per cell.

jwatte
06-15-2015, 11:14 AM
How do I connect such battery to the Pololu Dual VNH5019 Motor Shield for Arduino?


You find a connector that matches the one on the battery, and ends up in pig tails (stripped wires) on the other end.
Or, like I do when buying from HobbyPartz or HobbyKing: cut the default connector, and solder a female Deans T connector to the wires.
This requires care, for a few reasons:
1) The wires and the Deans are very thick, so they take a lot of heat to actually melt the solder.
2) You don't want to over-do it so you melt the nylon connector housing, or even worse, transmit too much heat to the battery.
3) If you cut and strip both wires at the same time, they can easily touch each other, and short the battery. If you're unlucky, this contact welds, at which point you likely have a battery fire on your hands.
4) Don't forget to thread the heat shrink onto the wire before soldering. Then securely wrap the first connector you solder before you strip the second wire, to reduce risk of shorts.

Snoopy
06-15-2015, 07:01 PM
Just never let the LiPo drop below 3.2V per cell or charge above 4.2V per cell.

Thanks. May I ask how and why? I thought charger would charge at the appropriate voltage automatically.

Snoopy
06-15-2015, 07:02 PM
Thanks jwatte for the info.

tician
06-15-2015, 07:06 PM
If you buy a decent LiPo balancing charger, then you should have no problem charging. The cheap battery monitors/alarms mentioned earlier will help protect against over discharging.

jwatte
06-16-2015, 02:25 PM
May I ask how and why?

If you use an appropriate charger, and use it correctly, then you are unlikely to have a problem with over-charging.
The world is full of people who don't necessarily follow those suggestions at all times, though. And chargers that fail.

Snoopy
06-17-2015, 07:33 AM
Thanks