Building a battery-powered charger: feasible project for a beginner?

[Rhythmdvl]

I have a device powered by a user-inaccessible lithium ion rechargeable battery (2600 mAh capacity). It normally charges in a couple hours by a plug-in DC adapter (it says 9v 1.3a), but we’re taking it camping and won’t be around either a power outlet or a car (a car will get us there, so carrying a few pounds of alkaline batteries is feasible).

I’d like to put together a device similar to a Coleman CPX portable electronics charger. It takes four D cell batteries and has a USB output to charge cell phones and the like. It won’t work for this, as the device in question does not have a USB port and needs more power than the Coleman provides. So I’m hoping to cobble together something like it, but that takes more /different batteries and will plug into the charging base (the charger and charging base are separate, so if necessary (e.g. it’s not an easily-found connector) I can buy a second charger and cannibalize its base or plug).

I hope this is a sensible question—and that I provided all relevant information.

Thanks for any help or direction!

 

[(*steve*)]

Since the device required 9.1V or thereabouts, a portable charger capable of 5V is unlikely to help.

 

[Rhythmdvl]

Since the device required 9.1V or thereabouts, a portable charger capable of 5V is unlikely to help.

Certainly! I meant to be clearer--I linked to the Coleman as an example of the type of gadget I was looking to put together. The Coleman has neither the right connector nor the right amount of power.

I'm hoping it's fairly straightforward to build something that has the right amount of power (i.e. more or larger batteries) and will be able to connect to my device.

 

[(*steve*)]

Do you have a car handy?

A small switchmode regulator (DC-DC converter) set for a 9V output would be ideal.

SOmething like this.

 

[Rhythmdvl]

Do you have a car handy?

A small switchmode regulator (DC-DC converter) set for a 9V output would be ideal.

We'll be taking a car to the camping area, so a bit of weight won't be a problem--but the car won't be available for charging. In searching for something off the shelf I've found battery packs that are capable of jump-starting a car, but I'd rather not lug something so large and heavy to the campsite. Plus, who wants to share the tent with such an enormous item?

 

[(*steve*)]

How about 6 alkaline D cells?

They'll provide about 9V and should last a while.

 

[KrisBlueNZ]

How about 6 alkaline D cells? They'll provide about 9V and should last a while.

I think that's what Mr. Rhythmdvl was suggesting.

Whether it's workable or not comes down to the total amount of energy needed. Your charger is rated for 9V and 1.3A so let's assume that you need to provide a 9V supply and that the rechargeable device will draw 1.3A. You say the device charges in about two hours. I guess you'll want to recharge it several times.

Digikey (a useful source of data sheets) list several D-sized alkaline cells.
Energizer alkaline D cell: http://www.digikey.com/product-detail/en/EN95/N105-ND/704820
Panasonic alkaline D cell: http://www.digikey.com/product-detail/en/LR20XWA/BB/P643-ND/2043734

A quick look at the graphs in the data sheets for those two cells shows that even at only 1A load, the service life is less than one hour (assuming the terminal voltage needs to be at least 1.2V). However, if you can reduce the load current on the cell, its service lifetime increases quite dramatically. For example, using three cells in parallel, the current per cell would be 1.3A / 3 which is 433 mA. At that current, a cell will last about 8 hours (Energizer and Panasonic), which would give you four charge cycles.

Unfortunately, this would require three cells connected in parallel, duplicated six times, to give you your 9V, i.e. 18 cells! At about 150g per cell, you would be carrying a 2.7 kg battery! And the cells would cost about USD 30. Is that feasible for you? If not, there are other battery chemistries with a better energy-to-weight ratio, but they're significantly more expensive.

These figures are not exact; I'm just doing an initial feasibility check. The service life figures assume a terminal voltage of 1.2V but six cells at 1.2V each is only 7.2V and you need 9V. So in practice you might only get three charge cycles out of them.

So is it feasible to use Alkalines? Borderline, I think. What do you think?

 

[Rhythmdvl]

Thanks for helping me understand why this isn’t so straightforward.


I have no idea if it makes a difference, but I mentioned charging time only to relate what I know about the charger (I’m ignorant to the point of knowing what’s relevant). If it doesn’t matter overall, we’re not in a hurry or need to duplicate the speed of plugging it into a wall—we can charge it overnight.

You mentioned “other battery chemistries with a better energy-to-weight ratio.” Does that mean other D Cell batteries or would changing to a motorcycle/lawn tractor/UPS/other battery make a difference?

Oh, we’d also be happy with just one recharge (short camping trip).

 

[eKretz]

Steve's idea of a switching reg is a good one. I'd use Li-ion batteries like 18650's. They have good energy density in a small package. You can cannibalize them pretty easily these days from power drill battery packs or from old laptop batteries. Use google to get a charger for them and you're good to go. I'd use them individually rather than build a pack, that way they don't get out of whack to each other voltage-wise. Alternatively you could purchase a ready-made pack with protection circuitry. You could use an SLA battery, but a Li-ion will give you the same capacity at less than 1/3 the weight and a lot less bulk.

 

[(*steve*)]

Thanks for helping me understand why this isn’t so straightforward.

Thanks for realising it -- seriously!

I have no idea if it makes a difference, but I mentioned charging time only to relate what I know about the charger (I’m ignorant to the point of knowing what’s relevant). If it doesn’t matter overall, we’re not in a hurry or need to duplicate the speed of plugging it into a wall—we can charge it overnight.

Except in cases where the device under charge can detect the type of charger and draw current accordingly, the charge current is pretty much set and you need to be able to provide said current, so the charge time will not differ substantially.

Oh, we’d also be happy with just one recharge (short camping trip).

A spare battery for the device may be a simpler solution.

 

[eKretz]

I have a device powered by a user-inaccessible lithium ion rechargeable battery (2600 mAh capacity). It normally charges in a couple hours by a plug-in DC adapter (it says 9v 1.3a), but we’re taking it camping and won’t be around either a power outlet or a car (a car will get us there, so carrying a few pounds of alkaline batteries is feasible).

I’d like to put together a device similar to a Coleman CPX portable electronics charger. It takes four D cell batteries and has a USB output to charge cell phones and the like. It won’t work for this, as the device in question does not have a USB port and needs more power than the Coleman provides. So I’m hoping to cobble together something like it, but that takes more /different batteries and will plug into the charging base (the charger and charging base are separate, so if necessary (e.g. it’s not an easily-found connector) I can buy a second charger and cannibalize its base or plug).

I hope this is a sensible question—and that I provided all relevant information.

Thanks for any help or direction!

Steve, you must have forgot the highlighted part of the OP.

Here's a quick comparison of similar power capacity SLA and Li-ion batteries. The SLA is a 12V (13.8V fully charged) 7200mAh capacity and the Li-ion pack would be 12.6V fully charged at 6600mAh capacity. The SLA weighs about 4.5 pounds and the Li-ion pack under a pound.

 

[KrisBlueNZ]

As Steve says, the charge current (and therefore the charge time) is determined by the charger inside the device. You can't slow down the charging process by reducing the amount of current you provide; if you try to do that, the device will think the adapter is faulty, and will stop charging.

As eKretz says, your best bet is probably a stack of lithium-ion cells. I'm not sure whether you will need to regulate them though; it depends on the characteristics of the device. Can you tell us what it is? I'm guessing it's a GPS receiver, right? Can you give us the model number and the approximate age please. Also, do you know the voltage of the internal lithium-ion pack?

It seems a bit silly to be charging one lithium-ion cell from a lithium-ion pack, but it's probably your best option.

 

[Rhythmdvl]

Sorry for the delay in responding—we took a long weekend and are busy playing catch-up with our clients.

I’m amazed at the size comparison between SLA and Li-ion batteries. Until now I’d assumed that Li-ions’ were common and useful because of their rechargeability, not their power density (if that’s the right term).

So it sounds as if connecting a set of batteries to the charging base is impractical. It seems as if the next best thing (or rather, the only next thing) is to go with a portable powerpack. May I ask a couple followup questions (and please let me know if I should move these to a new thread)?

First, are there simple calculations for determining which option is the best? That is, bells and whistles aside, is there a relatively straightforward calculation that provides something like power unit per dollar? For example, this, this and this (with an external battery) all seem to do what’s necessary, but for widely different costs. I guess I’m asking what the key stats are.

Second, isn’t this overkill? These are all designed to jumpstart a car and power a television. Am I missing smaller versions of the same thing, or is the market for that so small that this is the only way to go? Also, am I going in the right direction? If I’m not mistaken, these packs take DC power from their batteries, convert it to AC power for the outlet, then the DC brick re-converts it to DC to charge the device. Am I losing non-negligible amounts of heat energy to the conversion processes?

Lastly, is there any way to calculate ‘refilling’ capacity? I’m sorry I’m making up terms that probably grate on your ears, but I don’t know what it’s called. Basically, from the stats of the device and the stats of the portable power packs, is it possible to determine how many recharges I could get?

The device is, admittedly, purely recreational and self-indulgent. It’s a Pax Ploom loose leaf vaporizer. The tech specs came from a chat with their customer service: a lithium-ion rechargeable battery (2600 mAh capacity) that charges from a 9v 1.3a DC adapter.

Again, thanks for your help and insight into all this!

Cheers,

Rhythm

 

[eKretz]

Yeah, those are not going to be super efficient use of power, but they will work. I made a battery powered charger for my iPhone the other night when we had a nasty storm that took out power for the night. Still had the wifi router, (battery backup) but my iPhone was dead. Luckily I had some Li-ion 18650s fully charged and sitting. I just used 2 in series to get me up a little over 8 volts and ran them into a 7805 reg, then divided that using resistors to get Vcc, D+, and D- voltages to the correct pins with my USB adapter. I mounted my 7805 on a spare heat sink and I was good to go! And yes, the energy density of Li-ion batteries is really pretty awesome. Much better than just about anything else at the moment I think.

You could do something similar with a Li-ion pack and it will be a heck of a lot less bulky than the items you linked. I think you're talking 15+ pounds for those babies, and the last one you'd need to lug around a battery, because that one doesn't come with a battery, just alligator clips to hook up to one. A Li-ion pack like the one I posted a pic of will get you quite a few charges of your battery.

 

[eKretz]

Re-reading your post, I see that I missed a few points. Your device is probably powered by a single 18650 Li-ion battery @2600mAh. The cells in my pic are 2200mAh each, they are older batteries. There are devices used as power packs for recharging mobile devices, but they are often specific to certain devices and almost universally charge at 5V over USB. You can make your own pretty easily if all you need is 2 wires for charging. It would probably be a good idea to use a 12V pack or SLA and regulate it to 9V like your adapter just to be safe.

As far as determining charges, an easy way if you have a multimeter is to check the amperage that your device charges at and time how long it takes to charge from nearly fully dead. Then you can just use battery capacity to determine about how many charges you'd get. So if it takes 2 hours at 1A to charge, you know you need 2000mAh capacity to get one charge, so if you had a 6600mAh pack you'd get 3 full charges.