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Lithium batteries for embedded systems

I'm building something that's approximately a laptop, with a total number of units of about 6, maybe one or two more. Laptops need batteries, as big (Wh) and as small (cm^3, kg) as possible, and this is no different. It's worse, actually, as I can't optimise the power consumption like a laptop can.

For this reason, lead acid, NiMH and NiCd are right out, and lithium batteries of some sort are probably the way to go. Using raw lithium cells is not feasible, both for safety and time-consuming-ness of building and testing a charger circuit. Using a real laptop battery is awkward because they tend to want to talk to a charge manager inside the laptop, and that's not documented. And of course the battery and charging/balancing circuit must be small.

So far the best solution I've come up with is laptop secondary batteries - you charge them from 12V and they emit 5/9/12/19V to suit your laptop/phone/whatever, and all the management of the lithium cells is within the pack. There are dozens of kinds on Amazon. Currently I'm using a 40Wh unit.

Which is all fine... except they're too clever for their own good. For example, some switch on if they detect a plug inserted into the output - not good if the output wire is permanently connected. Others try ramping the output voltage until they detect current being taken. And none of this is documented, you have to guess what's going on inside from the behaviour.

The other thing that's needed is some kind of power sensing so the 'laptop' knows when to shut down, rather than just having the power whipped away from it. But many of these have awkward battery status readouts - press the button and watch the LCD. Don't really want to automate that with a solenoid and a pile of photodiodes.

I'm sure this kind of simple voltage-in/voltage+status-out power pack must exist somewhere in the embedded world but I've failed to find any. Any suggestions of where to look?

Thanks Theo

Reply to
Theo Markettos
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Model airplanes.

Reply to
Paul Rubin

Me thinks that you should be looking at something that is already nearly like a laptop. For just 6 units your time would be way more profitably spent adapting something like one of the latest Ultra Book convertables.

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Michael Karas 
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Reply to
Michael Karas

There are many safety issues with LiPo batteries that have been dealt with in laptops with lots and lots of fancy circuitry.

In the model airplane world they're dealt with by an education campaign that sums up to "you're stupid if you charge your batteries where they'll burn down your house".

Model airplane LiPos can and will burst into flame -- we had a member in our club who burnt down his shop several years ago with one. Recently, our club president nearly burnt his house down with one. (Ironically, it was because he was trying not to stink up his concrete-floor garage. He put the swelling and warming battery on a rug and headed toward the back yard through his house -- before he was out the back door, the battery had burnt through the rug. He got it into a metal bucket or a pot or something, and out the door -- but you get the picture).

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

Power tools? The chargers are at least separate from the packs, but I'm not sure if there'll be hooks to work out the timely shutdown issue. LiPos do have a fairly clear voltage ramp as they discharge, unlike NiCd cells, so you may just be able to monitor voltage and go from there, particularly if your device presents a constant drain.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Seconded. The Dynon glass panel avionics in my airplane use pretty much standard Lion R/C battery packs for backup power. They work fine and already have standardized monitoring connections brought out.

Reply to
Jim Stewart

They tend to use LiPO, which is 'as much power as possible, we don't care if it sets fire'. I'm not sure keeping a bucket of sand handy at all times is going to be appreciated, especially inside airliners...

Theo

Reply to
Theo Markettos

There are enough issues with freigthing any Lithium batteries they are NOT allowed by air freight full stop. However many of crew and passengers will be carrying Lithium batteries in an active configuration possibly also charging on airline power outlets.

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Paul Carpenter          | paul@pcserviceselectronics.co.uk 
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Reply to
Paul

I don't think it's the LiPo part that's the real issue, so much as the total lack of protection devices.

Granted, when one flies model airplanes one is also smacking one's battery packs into the ground at a high rate of speed, which isn't a common use case for other bits of kit.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Yes. Power tools use LiFePO4 (Lithium Iron Phosphate) which has lower power density than LiPo, but is much safer and manageable. You can buy cells and chargers at Hobby King.

Model airplane LiPo'sare optimised for minimum weight and high current, regardless of lifetime. Others have noted the safety issues.

Clifford Heath.

Reply to
Clifford Heath

Another idea occurs to me: the Sony InfoLithium NP-F970 and similar camcorder batteries. These are conveniently brick shaped, come in various capacities up to around 7800 mAH * 7.4 volts, and have a well supported external charging and monitoring ecosystem. Laptop batteries are much weirder by comparison.

Model airplane and power tool batteries are designed for high discharge current at the cost of energy density. That may or may not be the right thing for your application.

Reply to
Paul Rubin

Thanks, that does look interesting, particularly things like this:

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Though it's rather an old 'standard' and future battery availability could be an issue.

It seems there's also the Smart Battery System, but that's just a monitoring protocol (over SMBus) rather than an electrical standard.

My other thought was to use a supercap as a mini-UPS in conjunction with a phone/laptop extension battery - you don't get software battery charge indication (though that is available by pushing the button), but you do get time to tidy up after the main battery has failed.

Theo

Reply to
Theo Markettos

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