How to Select a Driver

[JamisonM]

I'm hoping to modify a 4D maglite into a quad cree light in the coming weeks and so far, I think I've got most of the supplies I'll need to do so. I've got my heatsink (PXR-19De), reflectors (McR-19XR) and emitters (Cree XR-E), but I'm having a hard time picking out the driver. One of my biggest problems is understanding the math that goes on when selecting a driver. I'll tell you now that most of it is gibberish, though I can understand bits of it. So guys, can you guys help me figure out what I need to know to select a driver?

[DaFABRICATA]

Buy a Shark from the shoppe.

It has a potentiameter on it so that you can ajust the current running to the LEDs. It will run up to 1amp and i think there is a mod you can do to it to run it up to 2amps. It will also accept input voltages up to 20volts.

I have one that runs my triple seoul modded KT4 and KL4 surefire heads and I love em!

[JamisonM]

Buy a Shark from the shoppe.

It has a potentiameter on it so that you can ajust the current running to the LEDs. It will run up to 1amp and i think there is a mod you can do to it to run it up to 2amps. It will also accept input voltages up to 20volts.

I have one that runs my triple seoul modded KT4 surefire head and I love it.

The shark is one I"m looking at. I'm also looking at the nFlex from TaskLED. One thing I want to do with this project is power the Cree LEDs at their max while in spec. I'll also be using 4D cells if at all possible. Sill, I don't understand the math that goes into picking out a driver. That's what I'm trying to understand.

[OhMyGosh]

I would like to help but I don't really understand 'problem with math'. What exactly are you confused about?

[JamisonM]

I would like to help but I don't really understand 'problem with math'. What exactly are you confused about?

I want to know what I need to know to select a driver. There's got to be some kind of reasoning that goes on behind that selecting a driver. I'm hoping to run 4 Cree XR-E's on 4 D cells at 1amp. Will that be possible? If not why? I want to try and understand that "why".

[yellow]

Select a driver:

setup:
4 Crees in Series = 4 * 3,7 V = 14,8 V

what battery?
4 D --> 5,2 Volt
--> boost driver

Shark
--> can work up to 4 Amps

need:
3 times more than the input voltage gives --> ~ 3 A
--> possible

I would vote for an additional Remora.
That way You have a very easy way to incorporate three light levels (low-med-high) with almost no more work.
(and I do this with my current mod)

[Doh!Nut]

To add some words to the numbers above, if your required voltage accross the LED Vf is more than your supply voltage then you need a BOOST driver.
The fatman is another example of a boost driver

If the supply voltage is more than the Vf then you can use a buck driver like the n-flex, or Downboy

As nothing comes for free Power in = Power out (minus efficiency of convertor)

A buck driver works by taking a higher voltage and a low currrent from the battery to give a lower voltage and a higher current at the LED.

A boost driver takes a Low voltage and a high current from the battery to give a lower current and higher voltage at the LED.

The bigger gap between the battery voltage and the Vf of the LED gives a greater variation between the current drawn from the battery and the current through the LED.

The maths above shows that you require three times the voltage at the LED so the battery will be drained of three times the current (plus a bit for efficency)

[JamisonM]

Thanks yellow and Doh!Nut. Doh!Nut, can a presume that the supply voltage is what coming from the batteries before it hits the driver and VF is what comes out of the driver? So, for my setup, the LEDs will require 14.8v total. I'm only supplying them with 6v from the 4 D cells. So that goes through the driver where the high current of the batteries is lessened the low voltage of the batteries is increased so the LEDs can be driven at 1amp each. One thing I still don't get though, is why is three times the voltage needed for the LEDs? BTW guys, I think I'm starting to understand this if slowly.

[waTom]

That's a great topic! I've a 4D next to me and wanted to do exactly the same thing!

Do I have to insulate the lower surface of the emitters for mounting them on the heatsink? I think I've read something about it but can't remember

[yellow]

the voltage of the cells is three times less than the voltage the leds need.
So to receive the desired current, the circuit draws three times more than needed from the cells and changes them to the cover the voltage gap.

to receive the 1 A, it will draw about 3 A,
works with the Shark which could do this till a draw of 4 A and works with the cells, that should be able to give these 3 A.
... figures are very rough ...


waTom: entire different topic.
Better to try to learn this by reading in here.
in short: if Cree led, not needed. If using an anodized aluminium sheet as heatsink, not needed. If using emitters (Cree/SSC) mounted on Star's, not needed. Only needed with SSC bare emitters.

[JamisonM]

the voltage of the cells is three times less than the voltage the leds need.
So to receive the desired current, the circuit draws three times more than needed from the cells and changes them to the cover the voltage gap.

to receive the 1 A, it will draw about 3 A,
works with the Shark which could do this till a draw of 4 A and works with the cells, that should be able to give these 3 A.
... figures are very rough ...

I think I see, but is it really 3. Wouldn't it be around 2.something? Are you rounding up just to be on the safe side? Wouldn't 18v be 3 times the voltage of the 4 D cells?

[OhMyGosh]

To run 4 XR-E LED's in series you need about 14v. The volts (Vf) of an led varies quite a bit so I would pick a driver rated for at least 16v. You want 1 amp through the LEDs (I would lower that a little to 0.8 amp unless you have really good heatsinking or only plan to use it for 1 minute at a time).

The driver needs to put out 1amp x 14volts = 14 watts.

For a good driver with 82% efficiency you would need:
14 x 1.2 = 17 watts from the batteries.

If you assume 4.0 volts from the battery when nearly dead:

17 / 4.0 = 4.25 amps

The driver needs to be able to handle this much input current or you need to add another cell.

One point I have to emphasize, those LED's are going to make a lot of heat. You are going to need to put a lot of thought into where that heat goes. 14w is as much as a small soldering iron. An incandescent light gets rid of heat by infrared so it is less of a problem. You have to get the heat from an LED with a heatsink. This is the #1 problem with multiple high power LED's.
Good Luck!

[JamisonM]

To run 4 XR-E LED's in series you need about 14v. The volts (Vf) of an led varies quite a bit so I would pick a driver rated for at least 16v. You want 1 amp through the LEDs (I would lower that a little to 0.8 amp unless you have really good heatsinking or only plan to use it for 1 minute at a time).

The driver needs to put out 1amp x 14volts = 14 watts.

For a good driver with 82% efficiency you would need:
14 x 1.2 = 17 watts from the batteries.

If you assume 4.0 volts from the battery when nearly dead:

17 / 4.0 = 4.25 amps

The driver needs to be able to handle this much input current or you need to add another cell.

One point I have to emphasize, those LED's are going to make a lot of heat. You are going to need to put a lot of thought into where that heat goes. 14w is as much as a small soldering iron. An incandescent light gets rid of heat by infrared so it is less of a problem. You have to get the heat from an LED with a heatsink. This is the #1 problem with multiple high power LED's.
Good Luck!

Thanks OhMyGosh. Looks I'll have to up the voltage coming from the batteries. I still have a few question though.
When you say the driver will need to be able to handle 4.25amps or else I'll need to add another cell, do you mean in order for the light to actually function; meaning there just isn't enough voltage left in the batteries that the driver can use?

As for as efficiency goes, what kind of impact would the voltage levels of the batteries coming closer the voltage need by the LEDs have? I'm guessing, the closer you got, the better? What about the further you got (having excess) from the LEDs requirements? In both cases, the more you have the less draw from batteries and the longer you'd stay in regulation?

[Cydonia]

You will have to use NiMh (or NiCd) D cells to handle the current loads that 4 Cree @ 1A + driver inefficiencies and circuit resistance will be pulling. You said 6v earlier... that implies alkalines

I would drive each Cree at about 400ma to get good LED efficiency with maximum performance ratio. Less heat, more light, less current required, and doable on alkalines

I've really been thinking of doing a multi Cree using that same heat sink from the sandwich shoppe myself...hmmm

[OhMyGosh]

When you say the driver will need to be able to handle 4.25amps or else I'll need to add another cell, do you mean in order for the light to actually function; meaning there just isn't enough voltage left in the batteries that the driver can use?

The driver will take Power in watts. Watts = volts x amps.
If you raise the volts (add another battery cell) you will lower the amps.
The driver will probably have a limit in how many amps it can take in. To lower the amps, you raise the volts. I just looked at the Sandwich shoppe and it says 4 amp input current max for the shark.

The 4.0 volts for the battery is for when they are nearly dead. As the batteries run down the voltage falls, so the amps has to rise to get the same number of watts.

Yes bringing the input voltage closer to the output makes it a bit more efficient but it is not a big difference unless you are running very low voltage (1 cell lights).

14 watts is pushing the limit for these small boards. I think you would be happier and would have a much more reliable light if you lowered the current from 1000 to 750mA. It would only be ~15% less bright. You could not tell the difference unless you compared side by side.

[wintermute]

If you're using 4 x emitters - you really don't need 1000mA. I agree with OhMyGosh, go 750mA.

Edit, that sounds kind of weird doesn't it..."I agree with OhMyGosh"

[JamisonM]

You will have to use NiMh (or NiCd) D cells to handle the current loads that 4 Cree @ 1A + driver inefficiencies and circuit resistance will be pulling. You said 6v earlier... that implies alkalines

I would drive each Cree at about 400ma to get good LED efficiency with maximum performance ratio. Less heat, more light, less current required, and doable on alkalines

I've really been thinking of doing a multi Cree using that same heat sink from the sandwich shoppe myself...hmmm

I figured that alkalines, or at least the few I was thinking of using, wouldn't work well. I'm guessing I'll have to make due with something that puts out more volts per cell, but with less mAh. I guess that works out in the end though, but can't explain it. I'd like to hear more about it though.

[wintermute]

I figured that alkalines, or at least the few I was thinking of using, wouldn't work well. I'm guessing I'll have to make due with something that puts out more volts per cell, but with less mAh. I guess that works out in the end though, but can't explain it. I'd like to hear more about it though.

You can always re-think the whole battery/driver thing and go with 2 C / 2 D mag with protected LiIon batts. More expensive then reg NiMH batts, but will push hella current and more volts, then you'll have to get a driver that will handle upwards of 8V - then again, when the batteries are drained you're looking at a bottom of 5.6V or so. If you go with a 3 C / 3 D LiIon setup you're looking at a top end of ~12V and bottom of 8.4V. Lots of mAh from those LiIon Cs and Ds. 4 C / 4D would be ~16V...and 11.2V...etc.

You have a lot of options here - looking forward to hearing others chime in with possibilities since I plan to do the same thing soon myself. I'm not certain about all of this, so others chiming in will help certainly - but with the availability of protected LiIon Cs and Ds the possibilities have expanded a bit.

[JamisonM]

The driver will take Power in watts. Watts = volts x amps.
If you raise the volts (add another battery cell) you will lower the amps.
The driver will probably have a limit in how many amps it can take in. To lower the amps, you raise the volts. I just looked at the Sandwich shoppe and it says 4 amp input current max for the shark.

The 4.0 volts for the battery is for when they are nearly dead. As the batteries run down the voltage falls, so the amps has to rise to get the same number of watts.

Yes bringing the input voltage closer to the output makes it a bit more efficient but it is not a big difference unless you are running very low voltage (1 cell lights).

14 watts is pushing the limit for these small boards. I think you would be happier and would have a much more reliable light if you lowered the current from 1000 to 750mA. It would only be ~15% less bright. You could not tell the difference unless you compared side by side.

Ah, so adding the extra battery, or batteries, will keep amps down and keep the driver working properly and from possibly damaging itself (that's a total guess)?

[JamisonM]

You can always re-think the whole battery/driver thing and go with 2 C / 2 D mag with protected LiIon batts. More expensive then reg NiMH batts, but will push hella current and more volts, then you'll have to get a driver that will handle upwards of 8V - then again, when the batteries are drained you're looking at a bottom of 5.6V or so. If you go with a 3 C / 3 D LiIon setup you're looking at a top end of ~12V and bottom of 8.4V. Lots of mAh from those LiIon Cs and Ds. 4 C / 4D would be ~16V...and 11.2V...etc.

You have a lot of options here - looking forward to hearing others chime in with possibilities since I plan to do the same thing soon myself. I'm not certain about all of this, so others chiming in will help certainly - but with the availability of protected LiIon Cs and Ds the possibilities have expanded a bit.

I'm already rethinking this. That's why I started this thread; I should at least know a little bit about what I'm messing with. One thing I like about the D cells is that have upward of 20,000mAh, but everything I've seen as far as rechargeables go has been bellow 10,000mAh. I don't mind the weight of my 4D mag either. As I said in an early in this thread (though I didn't say exactly this; (kinda hinted at it), mAh isn't everything. Heh, just while writing this reply, I think I see why a lot of you guys have the AA to D adapters. BTW wintermute, where can I buy said batteries and if you don’t mind I’d like get more information on those batteries.

[yellow]

all my type is with considering 4 Ni-Mhs as batts and strongly uprounded
(to be on the safe side).

with the parts You have, heat is no problem. Just mount the emitters as good as possible (thermal paste at slug + put something "heavy" onto them while letting the epoxy cure)

Changing batts to Li-Ion most likely needs expensive and not this easy change of the Flashlight; might result in needing another driver, charger, ...

[wintermute]

I'm already rethinking this. That's why I started this thread; I should at least know a little bit about what I'm messing with. One thing I like about the D cells is that have upward of 20,000mAh, but everything I've seen as far as rechargeables go has been bellow 10,000mAh. I don't mind the weight of my 4D mag either. As I said in an early in this thread (though I didn't say exactly this; (kinda hinted at it), mAh isn't everything. Heh, just while writing this reply, I think I see why a lot of you guys have the AA to D adapters. BTW wintermute, where can I buy said batteries and if you don’t mind I’d like get more information on those batteries.

C's are here:
http://www.cpfmarketplace.com/mp/sho...d.php?t=157613

D's are here, but not available yet - although not available in this thread, I swear they are somewhere in the CPF marketplace: http://www.cpfmarketplace.com/mp/sho...d.php?t=137182

But yeah, AA -> D adapters make sense now, eh...depending on the adapter and configuration you can do a lot with them. Plus, a 10 x AA NiMH charger does wonders for quick recharges compared to charging D cells. (I don't mean quick, like 30 min, I mean quick like 2-hours)

all my type is with considering 4 Ni-Mhs as batts and strongly uprounded
(to be on the safe side).

with the parts You have, heat is no problem. Just mount the emitters as good as possible (thermal paste at slug + put something "heavy" onto them while letting the epoxy cure)

Changing batts to Li-Ion most likely needs expensive and not this easy change of the Flashlight; might result in needing another driver, charger, ...

I agree with this in some ways - but maybe he just wants to be different and explore his options. He hasn't bought a driver yet - so wouldn't need to change the driver.

Bottom line, the possibilities are endless and just see what direction you want to do - I was thinking about a cutdown mag with a 1 x C LiIon with a single LED and an aspherical lens in one project and a 2 x C LiIon 3 x Cree setup for another. Just exploring options here, and the info from others will help me in my future builds as well.

[wintermute]

Add - Mag D battery adapters are available here:

http://www.cpfmarketplace.com/mp/showthread.php?t=96929

So why not a 3xD with a 12xAA adapter w/ 1 dummy battery for 13.2v or 2 dummy batteries for 12V. Then you wouldn't be pulling so many amps when the batteries go down (i.e. down to 11-10V). Fully charged, the 11xAA would put out like 14.3v and the 10xAA would put out like 13V - so you'd still be ok.

I say 10xAA over 11xAA mainly because they have good 10xAA chargers available, like this one: http://www.amondotech.com/index.asp?...ROD&ProdID=279 - so you can charge all of the batteries for the light at one time.

Shark + Remora looks good.

Will be nice when George80 finishes the MaxFlex: http://www.taskled.com/maxflex.html

Have fun - This thread is good for my own ideas though. I still think I might go C-sized LiIons though for my build. They have C adapters for regular LiIon chargers now: http://www.cpfmarketplace.com/mp/sho...d.php?t=165418

One more note - if you went with 3 x Cree instead of 4 x Cree, some of the overheating problems could be avoided and you could get by with a 2 x C LiIon or 2 x D (w/8xAA adapter) easily and never run such high currents from the batteries (although you could still probably do the 2 x C LiIon if you wanted to and get ok runtime).

[JamisonM]

Thanks for all the help guys. I think I'll drive the LEDs a bit lighter; at 750mA. I"ll be using modamag's PXR-19De heatsink. I'm guess it'll be alright at that level, but I'll put arctic silver 5 in the threads to help things along. I have no evidence, but it looks like the way he designed it; it could use the whole head of the maglite as a heatsink. Again, I can't tell you how well this would work.

As far as batteries are concerned; I rather go for the cheaper solution, but I see the advantages of rechargables. I'm thinking of maybe, a dummy D cell, 8 AA eneloop cells and the sandwich shoppe's 8AA to 2D adapter. I'd like to hear ideas of a charger for them if possible as well as competing batteries for my application.

Two things I still would like to hear more about is efficiency and how to make it better as well as what kind of effect would throwing the max input voltage have on the driver. Basically, how do I find the efficiency and can I affect it in a positive way (as much as I can really).

[Cydonia]

I'm already rethinking this. That's why I started this thread; I should at least know a little bit about what I'm messing with. One thing I like about the D cells is that have upward of 20,000mAh, but everything I've seen as far as rechargeables go has been bellow 10,000mAh. I don't mind the weight of my 4D mag either. As I said in an early in this thread (though I didn't say exactly this; (kinda hinted at it), mAh isn't everything. Heh, just while writing this reply, I think I see why a lot of you guys have the AA to D adapters. BTW wintermute, where can I buy said batteries and if you don’t mind I’d like get more information on those batteries.

Download the Energizer data sheet for D cell alkalines. There are some graphs shown. The first bar graph on page 1 titled "Milliamp-Hours Capacity" is amazing. It shows how the D cell's capacity goes down as the current draw goes up. At a 600ma draw, the D's capacity in effect goes down from 20,000 to what looks like 10,000. You can imagine what happens when you try to draw several Amps continuously out of it!

So a NiMh D cell, (they have ones that claim 12,000mah now) with around 10,000mah, in fact, ends up having way more capacity when drawn upon heavily. It can give you the big currents without breaking a sweat. And that's why all multi LED lights just have to use NiMh. Those 4 D alkalines will end up giving you the energy of an AA cell otherwise, if you try to pull a few amps out of em'.

Very excellent quality battery chargers are made by Maha. Check out their awesome one for C and D cells.

[wintermute]

MAHA chargers are the best - but can be expensive as hell (some $80 or more). Can almost double your build cost from chargers plus D-Cell NiMH batts.

You can get 8 x Eneloops from Batterybob.com using the discount code "Sheep" for about $19

A 10-bay decent AA charger from Amondtech for $23: http://www.amondotech.com/index.asp?...ROD&ProdID=279

Or an 8-bay faster charger for $25 here : http://www.amondotech.com/index.asp?...OD&ProdID=1026
--this 8-bay is nice with a charge rate of 700mA for AA/AAA.

If you want to go the 8 x AA Eneloops - I would recommend that second charger...good specs, good price.

[wintermute]

NiMH
12 AA => 3 D adapter = 14.4v => 12v dead
--- 1 dummy AA = 13.2v => 11v dead
--- 2 dummy AA = 12.0v => 10v dead
9 AA => 3 D adapter = 10.8v => 9V dead
8 AA => 2 D adapter = 9.6v => 8V dead
-----------------------------------------------------

4 x Crees 14.8v

Theoretical possible lumens, not what you'll get.

---------------------------
4 x Q2 @750mA = 628 lumens
4 x Q2 @1000mA = 768 lumens
---------------------------
4 x Q4 @750mA = 720 lumens
4 x Q4 @1000mA = 880 lumens
---------------------------

Do you already have the 4D maglight?? Or can you mod a 2D/3D maglight instead? Because if you used a 12AA => 3D adapter, you could use 10 eneloops to get 13v fully charged and 10v when dead.

Not sure how what the Sandwich Shoppe's 8AA=>2D adapter - but the thread I posted above from modamag has excellent adapters. (edit - they are the modamad adapters) The best they offer is an 8xAA=>2D where you can get a 12xAA=>3D adapter from that thread above and use 2 x AA dummy batts.

-----------------------------------------------------------------

I really don't know what would give you longer runtimes:
10 x 2000mAh Eneloops giving 12-13v when charged boosting to ~14.8v
or 4 x D cell NiMH batteries giving 4.8v when charged boosting to ~14.8v

I could crunch the numbers - but since the D-cell option doesn't appeal to me, I'll let someone else do that.

[JamisonM]

NiMH
12 AA => 3 D adapter = 14.4v => 12v dead
--- 1 dummy AA = 13.2v => 11v dead
--- 2 dummy AA = 12.0v => 10v dead
9 AA => 3 D adapter = 10.8v => 9V dead
8 AA => 2 D adapter = 9.6v => 8V dead
-----------------------------------------------------

4 x Crees 14.8v

Theoretical possible lumens, not what you'll get.

---------------------------
4 x Q2 @750mA = 628 lumens
4 x Q2 @1000mA = 768 lumens
---------------------------
4 x Q4 @750mA = 720 lumens
4 x Q4 @1000mA = 880 lumens
---------------------------

Do you already have the 4D maglight?? Or can you mod a 2D/3D maglight instead? Because if you used a 12AA => 3D adapter, you could use 10 eneloops to get 13v fully charged and 10v when dead.

Not sure how what the Sandwich Shoppe's 8AA=>2D adapter - but the thread I posted above from modamag has excellent adapters. (edit - they are the modamad adapters) The best they offer is an 8xAA=>2D where you can get a 12xAA=>3D adapter from that thread above and use 2 x AA dummy batts.

-----------------------------------------------------------------

I really don't know what would give you longer runtimes:
10 x 2000mAh Eneloops giving 12-13v when charged boosting to ~14.8v
or 4 x D cell NiMH batteries giving 4.8v when charged boosting to ~14.8v

I could crunch the numbers - but since the D-cell option doesn't appeal to me, I'll let someone else do that.

wintermute, I have both a 4D and 3D ready for modding. I’m thinking of using the fatman from TaskLED. If I do, I’ll use the 3D with 8 AA eneloop cells and the 8AA to 2D adapter with a single dummy D cell. If I do, looks like I’ll get some good efficiency at 750mA and more then enough light. I’d still like to hear more about efficiency though. My questions are in my last post before this one.

[wintermute]

OK, I'm bored and you've forced me to do it.

4 x Crees @ 1000mA and 14.8v = 14.8 W / 0.9 (approx driver efficiency) = ~16.5 W from batteries
4 x Crees @ 750mA and 14.8v = 11.1 W / 0.9 (approx driver efficiency) = ~12.5 W from batteries

4 x NiMH = 4.8v charged
~16.5 W / 4.8 = 3.44 A from the batteries
~12.5 W / 4.8 = 2.60 A from the batteries

4 x NiMH = 4.0v near dead
~16.5 W / 4.0 = 4.13 A from the batteries
~12.5 W / 4.0 = 3.13 A from the batteries

10 x NiMH (Eneloop) = 13v charged
~16.5 W / 13 = 1.27 A from the batteries
~12.5 W / 13 = 0.96 A from the batteries

10 x NiMH (Eneloop) = 10v near dead
~16.5 W / 10 = 1.65 A from the batteries
~12.5 W / 10 = 1.25 A from the batteries

So 10 x Eneloop AA in a 12xAA=>3D adapter with the Crees running at 750mA would only pull 0.96 A. Considering the Eneloops are 2000mAh - this would make them last for a decent amount of time. If any of my math is wrong, someone please correct me. The added benefit of the Eneloop batteries is they do not exhibit much of a voltage drop at 1A as you can see from this graph by SilverFox - they should stay above 1.25v for most of their runtime (@ 1A at least):

Thread can be found here: http://www.candlepowerforums.com/vb/...hlight=eneloop

So, what should run better - 10 x Eneloop AA batts or 4 x D cell batts??

[wintermute]

wintermute, I have both a 4D and 3D ready for modding. I’m thinking of using the fatman from TaskLED. If I do, I’ll use the 3D with 8 AA eneloop cells and the 8AA to 2D adapter with a single dummy D cell. If I do, looks like I’ll get some good efficiency at 750mA and more then enough light. I’d still like to hear more about efficiency though. My questions are in my last post before this one.

Fatman is a different animal then the shark - what made you change from the Shark with the Remora (I think it's a great combo) - you get switch selectable brightness!

Why not use 10 eneloop AAs with the 12AA=>3D adapter from modamag (w/2 dummy AA batts), instead of 8 x AAs with a dummy D-cell??.

But for information regarding the 8 x Eneloop AAs w/ 4 Crees at 750mA:
8 x NiMH (Eneloop) = 10.4v charged
~12.5 W / 13 = 1.20 A from the batteries

8 x NiMH (Eneloop) = 8.0v near dead
~12.5 W / 8 = 1.56 A from the batteries

10 x AA chargers are widely available, so running 10 instead of 8 will keep the current draw from the batteries at <1A and limit the higher current draw at the end of the life of the battery.

As far as efficiency goes, when it comes to a boost driver I would think that the closer you are to the output voltage the more efficient it would be. If I am wrong about this, I welcome correction. It just seems that the more the driver has to up the voltage the more you lose in efficiency.