Designing a custom battery pack

[Batarray]

Hi folks, i am requesting some help in regards to a battery pack that im designing. Im a mechanical engineer by trade, but dip into the electrical field alot in my personal time, tings from RC cars to high performance drag racing Li-po powered golf carts are just a few of the things that keep me busy after work. Having been exposed to the the new lithum battery world however just hobby related; Has help greatly in the quest that our company has taken in designing a custom pack for one of our large customers. However i have begun to hit a road block with the design and management wants to start over again. So i need your insite and help with the design. And what would be "Safe" to use in a retail/commercial enviroment.

So heres the deal, one of our customers was looking for a pack that can fit into the base of one of their units that will power a led panel for a full working day (8am-8pm) approx 10hours. We have gotten a custom made pack thats around 9000+mAh, that outputs 14.4v (i have a regulator that brings it down to 12v, and increases run time about 45minutes) to run a LED panel that pulls about 800ma~1A (applications can vary) used in a store display unit where there is no electricity. Because this is a low production unt (less then 200 packs), the tooling charges for the plastic case and other misc custom components have blown the costs for this simple device out of the water. Its in the upwards of 650$ a unit. We got a prototype pack made without the finished plastic case, and it works great for our use, but it required the need for a custom charger, and injected molded case, because of that the costs skyrocketed.

We have gone back to the drawing board, and looking at what we can use out there thats already in existance. Pre made laptop battery packs were the first thing that came to my mind even before we decided to go down the route of a custom pack, but capacity and storage limitations quickly became evident. (average max 6600mAh, ~7.4v) I have yet to find something in the 8-9KmAh range with 12+v of output. That can be easily charged, and not burn the store down at night when charging.

The one alternative i was wondering if anyone on here could help would be is there any possibility that i could series any of these laptop battery packs to give me the 12+v i need without any expensive control boards or such. And also make a simple plug in charger for it. These perferably need to be off the shelf components. Any custom dock bays and such we can make here on site. Laptop batteries are cheap and a whole system would be in the 150$

Ive also began looking down the route of specialty high capacity battery packs used in the film industry such as camcorder batteries and such. But most of the panels we will be needing to run will require 2 batteries in series.

Any help would be greatly appreciated.

And to make it even tougher, we have a size limitation. Were hoping to not exceed 5" x7" x3.5"tall. Which our prototype pack fit perfectly.

 

[IRstuff]

Paralleling is generally less desirable, since there's the risk that the weakest battery(ies) will suck power from the stronger one(s). You can stack them in series until you get sufficient watt-hours (~120 Wh) you can make it work, assuming your 12V regulator is a switcher. So, if you have 10 V batteries at, say 8 Ah, then two of them produces ~160 Wh, which should be sufficient for your application. You'll also need to make sure your switcher is sufficiently efficient for your application.

this is too long, apparently:

http://www.laptopbattery.net/laptop...-Battery=484172-001&from=68628:2668:HP:LAPTOP

TTFN
faq731-376

 

[Batarray]

We have a bit of room for an increase in length.

The regulator i have now is a stepdown regulator. Dealing with the standard lithium configuration of around 14.4v, it would run the leds a bit too bright. So a stepdown regulator was needed. However like you mentioned, i never really thought about using a step up regulator, because i was concerned about efficency. But paralleing 2 laptop packs that are less then 12v. would probably work well. How efficent are these step up regulators, 85%+?

Thanks

 

[IRstuff]

With two batteries in series, it would still be a step-down, from ~20V down to whatever you're looking for.

Maybe 80% would be a more reasonable number:

http://datasheets.maxim-ic.com/en/ds/MAX15020.pdf

page 5 has a graph of efficiency vs. current and input voltage

TTFN
faq731-376

 

[Batarray]

My apologies, i thought you ment putting the 2 packs in parallel. but i understand why you advise against it.

I found a 7.4v pack thats 12000mAh. Putting those 2 in series, and a stepdown regulator would be a good alternative to what we have now. Let me talk to the guys if they want to explore this path again. What limited us lastime going down this route was the charger if i recall (about a year ago). I only need to find a good cost effective standalone charger for the batteries. Laptops charge the battery internaly.

Camcorder batteries though have many options for multiple charging stations and such. But i cant seem to find anything over 6600mAh. I will have to dig deeper. If i could find a 8500mAh pack thats 7.4v that would be great, 2 of those and im off to the races again. Plus finding a dual bank charger would be easy as can be.

I am even tempted to try out the 7.2v RC car packs in series, however they also are limited by capacity (max, 6800mAh), and placing 4 packs in a units base really cuts into our size limitations, but (14.4v @ 13600mAh) is nice. But theres safety concerns with charging RC packs.


Appreicate the help.

 

[IRstuff]

"Laptops charge the battery internaly"

Actually, not completely true, since my laptop has a charger/adapter that weights about 3 lbs. In fact, when I plug in the charger for my previous laptop from the same manufacturer, the laptop complains about the charger being underpowered.

You're dumping ~1A for 12 hr, and you probably only have 10 hr to recharge, so you need to be dumping ~1,5A into this puppy for a reasonable charging time margin. Nonetheless, an iPad charger is only about 1 cubic inch, and does 2A at 5V, and you're only looking for ~1.5A at ~16V, so seems like it should only be about 4 cubic inches or so.

TTFN
faq731-376

 

[itsmoked]

Are you sure lead acid won't work? You could shave a fortune off that price and they come in cubic shapes.

Outstanding chargers are cheap.

Your packaging could consist of just a cap that hides the terminals and a fuse if desired.

Lead acids are safer. And, since no one is carrying these things over a mountain the specific weight shouldn't be an issue for that A-Hr rating.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Batarray]

Management initaly ruled out SLA's because of the size, but now they seem to not have much of a choice anymore. Theres lots of electronic stores my way, ill have to go and check out what they have size wize for testing and go on from there. Thanks

With the terminals the way they are, a nice charging dock could be made, plus with SLA's being packaged the way they are (no exposed cells like a li-po), it sloves our case packaging delema. As you mentioned, a small protective cap or such to reduce the risk of shorting the terminals would be all it would need.