Need suggestions for a portable battery for a 250 W DC motor

[tin7tin]

Hi everyone! I need to find a portable power supply system for a specific motor and I don't know how to go about it since I have no experience in this area. My motor is a brushed DC motor from Maxon, specifically RE65 (model number 388987): voltage = 48V ; Nominal power: 250W. I want to use the motor in its "short-duration operation" mode, where I extract more power from it over a short duration of operation.

My requirements for the battery system are:
Power: 300W
Voltage: 48V
Portable
Preferably small and light

Most batteries I have researched are of the AC type. and the DC ones have very low voltage and/or power rating. Please suggest the type of battery I should research and also some companies that may supply the battery-or at least where to look. Please also suggest If I should prefer a different voltage too. The power however cannot be less than 300W. I have attached the product catalogue page for reference. Thank you!

 

[IRstuff]

Why is it that the second most important parameter for this type of problem not provided, i.e., your "short duration?"

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[VEBill]

This sounds like ("Preferably small and light") a typical 'drone' project. Pure guess, please disregard if incorrect. Point being, eBay is awash with Li-Poly batteries and motor controllers for these sorts of (drone?) projects, although your application might exceed the typical norm. But the apparent commonality to drones might help with your searching.

Keep in mind that it might be easier to switch motors at any point in the design.

 

[tin7tin]

Sorry about that; Short duration ranges from 15s to 6s. This is not fixed , since my final load on the mechanical system is not fixed

And it isn't a drone project. But, this pathway might be helpful for me to find the battery that I need!

 

[IRstuff]

Once, twice, how many total uses before recharging? 300 W * 15 s = 1250 mWhr from whence you can determine the best combination of voltage and current capacity. I concur with VE1BLL's comments about LiPO batteries. The ones specifically intended for Airsoft applications might be most applicable, since they have specs like 30x capacity, i.e., a 5600 mAhr LiPO could dump ~170A for EXTREMELY short durations, which would be overkill for you. A 7 or 8 cell LiPO could support a 24V motor, which would only be drawing 12A or so, that would only be ~2x capacity for a 5600 mAhr battery

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[VEBill]

The 48 volt requirement of your proposed motor will prove to be an annoyance. I don't think you'll want to use a DC-DC converter for this sort of power level. Nor 16 batteries... As mentioned, be open to different motors if possible.

Worth mentioning, you'll need to include safety features to protect equipment and people. Those powerful batteries can bite.

 

[itsmoked]

I agree with VE1BLL to the point that I believe your project will fail based on this inappropriate motor voltage. A 48V motor brings issues, probably bigger than the rest of your design challenges.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[tin7tin]

Thanks for your inputs itsmoked, VE1BLL, and IRstuff. I can and probably will change the motor to a 24V one. I always thought that the higher current would be an issue too, hence I decided to go with the 48V one.

My project is basically a lifting device, so it operates only for the short duration of lifting from H = 0 to 1000mm in around 15s. So cycle - wise , 300W is used per cycle. And right thereafter the device will not be used for a considerably large amount of time.

I was also looking at batteries used in electric bikes- they have adequate protection already and many of the batteries fall within my power and voltage range. Would this be a good area to look at, or is this also "overkill" ?

 

[VEBill]

e-bike batteries perhaps (?) wouldn't be in the same 'insane' peak current range capabilities as the cells and batteries intended for drones and R/C vehicle applications. They'll also be bigger and heavier.

You'll see some very heavy gauge wiring being used, because providing a higher supply voltage is even more of an inconvenience than high current. It should be easy to work out the optimum voltage (and thus current, and thus necessary battery solution); just try a few options. It's likely to be a lower voltage that matches available Li-poly packs, or several packs in series.

 

[tin7tin]

VE1BLL, thanks for your suggestions. One of my major concerns is that as motor voltage goes lower, the nominal rpm and consequently peak rpm will reduce. I am already compromising and reducing required value of motor output rpm for my system. But it looks like because of the battery I may have to reformulate my system parameters. I will explore all options.

 

[IRstuff]

You could use lower current capacity multiplier batteries; you just need to make sure that the minimum requirements are met. For example, a typical auto battery might be 100 Ahr, and still produce 300 A for cranking for a few minutes. Likewise, you can look to a similar battery that's not so big, but delivers your current requirements with margin. How long is "a considerably large amount of time?" If too long, you need to consider self discharge of the battery. As for degraded performance, you simply need to make sure your capacity accounts for losses. You also did not specify how many times this battery has to be able to do this without recharging.


If it's too long, I would suggest conventional alkaline D-cell type batteries. While D-cells are not specifically rated in terms of large discharge currents, they can do it. A typical AA battery can dump 10A for short durations, ~5 to 10 minutes. This is how most RC cars operate, and why they need to recharged so soon.

This sort of problem is all about margins, which is why a more complete set of requirements is mandatory.

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[itsmoked]

If your device has to be handheld then you're probably stuck with Li-Ion batteries. Then, as VE1BLL suggests, they come in ~3.6V only. Stacking them together gives voltages like 10.8, 14.4, 18.0, 21.6, and 25.2V.
Two of those are typical packs for 12V and 24V applications and that's the 14.4V and 25.2V packs respectively.
The most commonly found Li-Ion battery is the 18650 which supplies about 2600mAhrs. You can draw about 2C from them possibly more for a short time.

If not hand held I'd be reaching for Lead Acid batteries as they can dish out a lot of current for a short while, are probably 1/5th the cost, and chargers are simple and cheap. No protection and charge balancing circuits are needed as they are for Li-Ion applications.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Huh, interesting about the voltages, since LiPOs are in multiples of 3.7V. This battery theoretically will handle at least 12 operations. But Li batteries have higher self discharge rates than alkaline:

http://www.manoonpong.com/Other/main_page=page_2.pdf

As long as your capacity is sized for the idle time discharge, then you should be good to go. As an example, you could get:

http://www.ebay.com/itm/MaxAMPS-110...41?pt=LH_DefaultDomain_0&hash=item35eb0d7ac1,

which could theoretically stand idle for a 18 months and still provide at least 12 operations.

I used extremely conservative numbers:
> 40A*15s
> 50% derate capacity for high current
> 5%/month self discharge

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