Battery pack design?

[mesiodens]

Hi all,

I need help putting together a battery pack and charger set up for a medical laser. The device is pretty small but currently has no internal power source. I want to couple it with a battery pack to make it portable.

I am a dentist, a hobbyist, and an obsessive DIY'er, but not an electrical engineer. So I thought somebody on this forum might be able to help me determine what components I need to put together a battery pack.

The laser runs on 9 volts DC and draws 5.5 amps. I need it to have enough power to run for 2 hours, be roughly 3x4x1.5 inches, and be easily portable, hence the thought of lithium ion batteries.

Any and all contributions would be appreciated!

 

[itsmoked]

I'd suggest you look to R/C battery pack suppliers. They can supply everything to do with recharging and pack format. Their packs also have sturdy packaging you want and ease of swapping-out etc., etc. Then you need an interposing little board to regulate the appropriate pack's voltage or current to what your laser specifically needs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mesiodens]

Thanks itsmoked, I hadn't thought of that. I'll look for some R/C parts vendors.

Also, I'm not really conversant in figuring the needed capacity, so please excuse my ignorance. I figure the battery pack should be about 11,500mAh. Does that sound right?

Thanks!

 

[IRstuff]

What's the instantaneous requirement? Some lasers, particularly pulsed ones, have variable current demands.

You stated 5.5A for 2 hrs, which is indeed 11Ah, but generally, you need to have a regulator, so you should assume a worst-case of 80% efficiency, and allow another 80% for battery variability and end-of-life. This would put you at 17.2Ah

LiPO batteries potentially offer a reasonable solution:

http://www.ebay.com/itm/MAXAMPS-LiP...Control_Parts_Accessories&hash=item43c959d73e

is probably a bit of an overkill, but you can look around for other choices. Note that these batteries probably exceed your dimensional requirements.

TTFN
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[VEBill]

Everyone will be happier if you stay away from the least possible capacity. Unless your "2 hours" already contains a margin, then you should add a healthy margin in the capacity.

 

[itsmoked]

Picking batteries comes with a LOT of trade-offs. You need to think deeply about them and how they affect your system's usage.

For instance, if the LiPO battery suggested above is the right size but still too heavy, (since it's almost 5 pounds), meaning your laser must be 'cord powered' to it then this may mean the battery's weight has less importance after all. Switching to a 22AHr lead-acid (LA) battery would change the price from $630 down to $50. Weight goes up to 13 lbs. You cannot ship large LiPO batteries on passenger aircraft. Does this affect you? They typically have to be shipped as hazardous materials. Sealed LA batteries can be passenger plane shipped. LA batteries can also be purchased anywhere so if another country was involved you could skip shipping all together and pick one up locally. Chargers for LA batteries cost 1/4 what LiPO chargers do. Easy solar charging of LA batteries is also available.

Lots to think about.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mesiodens]

Thanks to everyone for the input.
IRstuff: I have no idea what the instantaneous requirement would be. However, when the laser device is in use it’s powered up for about 15 minutes total, and of that the actual laser emission can vary from 30 seconds to 2 or 3 minutes total. After that the device would go back to the charger until needed for another patient.

Size is a consideration. If it gets too big or heavy I’d rather go wires and 110 AC. So I was hoping to stay within 3-4 lbs if possible. But $$$ is a real consideration. I was hoping to keep this project under $200. The LIPO would break the bank.


VEBLL: I think the above addresses your question too. I thought 2 hours would be a sufficient buffer.

Itsmoked: You’re right. As I learn about all this, it seems there will be some tradeoffs.

 

[mesiodens]

Continuation of previous post...

The link above is to a photo of the laser. It's small, light, and portable. So a lead acid or LIPO battery may be too big or heavy. That's why I was interested in LI batteries.

Of course the more I learn the more I don't know. Like how to I get a 9 volt pack of batteries. I can't find that voltage in any available pack of LI's. Do I need a PCB and a dc/dc converter? If so which ones do I use? Ahhhh! My poor head is splitting!!!

Thanks to all for the help so far!!

 

[itsmoked]

Strikes me as a product that would possibly work better with much smaller LiPO batteries good for only a few minutes and sit on a charger/supply. That way it's always recharging whenever it's set on a counter. If the user expects to use it for an extended period they have to leave it "docked" on the charger. Otherwise, power it from the wall and avoid all the maintenance issues, charging issues, shipping issues, and general reliability problems brought on with ANY batteries. Using any batteries is going to add the cost of charging them.

Any batteries are going to have end-of-life issues which aggravate users and sully product reputations.

I try to keep batteries out of products I design unless they're actually a fundamental functional requirement like a flashlight would require. I would not include them just because it would be "more convenient" if you see what I mean.

Your product would demand the user continually plug-in to recharge which perhaps is less convenient than; paying less, having no O-o-L problems, and never having a "dead battery".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Another option, given the most recent information is NiMH C or D cells.

http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=ni mh d cell&_fscr=1

The D cells are specified for 10 Ah, and 6 of them is 9 V-ish. Note that in any case, your recharge time will be significantly longer than the use time, so that's something to consider.

Or, you could go for a smaller LiPO:

http://www.ebay.com/itm/FLOUREON-11...Control_Parts_Accessories&hash=item417e11dafb

This one would certainly do the job for most short usages, and LiPOs can allow for rapid charging.

Or lithium batteries:

http://www.ebay.com/itm/1x-3-7V-600...US_Rechargeable_Batteries&hash=item2592d66c07

3 of these provides 11.1 V like the LiPO, but the weight appears to be only about 1/4 lb, which seems to be the lightest option. 6 Ah seems to be pretty reasonable.

TTFN
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[VEBill]

"Do I need a ... dc/dc converter?"

Almost certainly.

If this can be an integrated design, then you might be able to use a laser diode current driver that can run directly from the battery pack. But more likely you'll need a DC-DC converter to provide the specified voltage to the laser subassembly.

eBay has plenty on offer, but not for serious medical use (obviously). But you can tour what's available and what the boards look like.

 

[GregLocock]

There would be no weight or cost advantage in a lower voltage pack+DC-DC converter compared with a 9V pack of the same energy storage with the same battery chemistry.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[VEBill]

I don't think you can hit 9.0 volts with 'N' x Li-ion cells. The pack voltage would pass through 9.0 volts on the way by, but one wouldn't even be able to call it 9 volt nominal. 11.1 volts nominal maybe?

My assumption was of a higher voltage pack, converted down to a stable 9.0 volts. If you go the other way the current goes up. Either would work, choose your poison.

 

[IRstuff]

Yep, 11.1 was one choice, the other would be 18.5, would have provided the original requirement with about 6 Ah batteries.

TTFN
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