Battery Question for the Experts

[Mad Hatter]

OK, so I'm working on my first saber, and trying to figure out how to pack all the features I want into an MHS hilt without making it 6 feet long :grin:, and I'm hoping someone can answer some technical questions about battery packs...

I see that it is possible to make a 7.2v pack from 6 AAA batteries. And it looks like 1000 mAh is the upper end for capacity. So, since the cells are all in series, we get 7.2v and 1000 mAh. BUT.... it doesn't seem like any AAA batteries are anywhere near rated for 1.5 amp continuous discharge, so I'm guessing that the 1000 mAh capacity is going to go extra fast because of severely over-taxing the batteries. So here's my question:

Is it possible to wire 6 AAA-sized li-ions in a 2S3P configuration and increase the run time?

We still get 7.2v from the "2S". And we get around 1050 mAh from the "3P" (350 mAh X 3). But since the 2S's are wired in parallel, each should only contribute about 500 mA of the overall current - which is under the 700 mA max discharge rating.

Assuming that the 2S3P would last longer than the 6 normal AAA's wired in series, the next question is:

Is it still okay to hook the 2S3P li-ion pack to a smart charger?

[xwingband]

By 1.5A I'm guessing you are trying a K2 or Red III? If you use a driver it won't pull 1.5A under that voltage... it can take the extra and run on less amperage.

If you're not going to drive it you should rethink the 7.2V because it will be wasted as excess heat. That way you save space there!

I think a bit more info would help greatly... the LED and your space. AAA size li-ions aren't very good batteries where something else might work out. Like if you can fit 6AAA of li-ions... that could also be 1000mAh of NiMH that do a lot better with current draw. Then usually where you can fit 6AAA, 8AAA might be fine too if you say want to run a V. See how I'm thinking?

[Mad Hatter]

I plan on using various leds for the saber. A red III, and will do a green and royal blue - probably V's, but I haven't decided yet. They will be driven by a crystal focus 3 board. I was using 1.5 amp as a max value for lux + accessories.

The reason I was looking at the AAA is because I was told the CF won't fit on top of two AA's in an MHS. If I can fit the board on top of 6 AAA's (like in the image below from my other thread), then I will only need about 3.5 inches hilt length for both the power pack, and the CF. That would give me enough room for a motor and crystal chamber without making the hilt real long.

As for current requirements, I'm surprised by the idea that the nimh's would outperform the li-ions when they would appear to have less initial voltage, less capacity (1000mAh vs. 1050mAh), and, presumably, lower max discharge current... ?

I thought that the li-ions would outlive the nimh's, since wiring them 2S3P would only draw 2-500mA (of 700mA max) from each cell - as opposed to drawing the full .7-1.4mA from each cell for nimh. (On the assumption that the max rating is less than that, and the battery would be over-taxed.)

Unless there are AAA's out there that are made to deliver that much current?

[Novastar]

I might be wrong in interpreting what you're saying... but... try not to confuse "mah" with "ma", as they are different things.

Batteries rated at (for example) "2400 mah" have NOTHING to do with the maximum amount of current that can be DRAWN at any given moment from the cells. Most cells can handle at least a draw of 1A, and it's further likely that 2A or 3A can be drawn as well... no matter WHAT the "mah" rating.

Anyhow... another thing... if you're trying to drive THREE high-powered LEDs all at the same time with CF... that's probably not going to work very well. ESPECIALLY if you are going to try:

CF v3 trying to drive: Lux III + Lux V + Lux V

That's just an INSANE amount of voltage to be trying to feed... never minding the current for the moment!

But... again... I may not be comprehending what you're saying very well!

As to cells "out-performing" another... well, when it comes to POWER, there isn't any major difference between 2v of Li-Ion vs. 2v of Ni-Mh or Alk or whatever. Voltage is voltage, current is current.

The fact of the matter is that Li-Ions have more "energy density" ounce for ounce, size vs. size than most any other cell. This is not to say that they are "the best" per every application anywhere in the world hands down. In fact, like so many things, they have advantages as WELL as disadvantages.

But it is true that a single cell of Li-Ion is 3.6 or 3.7v, while a single cell of Ni-Mh is 1.2v. There is no getting around THAT. :)

[Mad Hatter]

Yikes! Ok. I'm not explaining myself well... (And thank you both btw for jumping in :D)

First, I plan on swapping led's, not trying to run all three at once. So, always just one led. I'm just planning ahead to be able to switch between led's with different power requirements.

On the capacities, I'm using the terms correctly: Voltage is obvious. Capacity in mAh is how long the battery can maintain a load (1000 mAh = 1 A for 1 hour). Max current draw given in Amps (or C's, which is a multiplier applied to capacity) is how much load a battery can handle.

So for 6 nimh wired in series:
Voltage: 7.2v (1.2 x 6)
Capacity: 1000 mAh (1000 per cell, but wired in series, so 1000 total as well)
Max Current Draw: 1 A max (1C rating, and C is 1000 mAh for this cell)

So if we use this pack to drive a 1.4 amp load, we are exceeding the battery's max current draw by 400mA, and we are not going to get the full 1000mAh capacity because we are overloading the battery.


For 2S3P li-ion (2S=2 cells in series, 3P=3 2-cell sets wired in parallel):
Voltage: 7.2v (3.6v x 2 for the 2S)
Capacity: 1050 mAh (350mAh x 3 for the 3P)
Max Current Draw: 2.1 A max (2C cell, so .7 x 3)

Assuming I've got that correct, if we use this pack to drive the same 1.4 amp load, we are well under the max current draw, even though a single li-ion can only handle 700mA draw (2C for this cell, and C=350). Since each parallel 2S "cell" provides 1/3 the current (467mA), we are under the max rating and should get the full 1050mAh capacity.

So, even though the 2 packs have similar voltage and capacity, they have very different max loads, and it seems the li-ions should outperform because they are within their rated limit. Unless someone knows where to get nimh's with better "C" rating? 1C 1000mAh is the best I've found...

[erv]

so, in a nutshell

capacity (C) isn't discharge rate amperage.
Basically, you can bet that a battery will "last" 10 hours if it's 1Ah but it's used at 100 mA only. Using it at a discharge rate of C/10 make this law working quite well.
As soon as you start using a current close to the capacity, it's starts being really wrong, especially for nimh.
Then, batteries will be giving less current and less voltage over time : duration of how long a battery last DEPENDS on the minimum the board can handle and the MINIMUM voltage the luxeon needs to operate at a certain current.
For the same battery, a RED luxeon saber last LONGER than a BLUE one for that reason.

Using the battery at a rate close to the capacity (1A for a 1Ah battery) will lead to incredible ripple on the output voltage, which is BAD for the board.
Safety general precaution : use the "TWICE PRINCIPLE". You need 1A, then take a 2Ah battery.

Having more batteries (AAA) in serie to increase the autonomy won't change that voltage ripple cause the capacity isn't increased.

Personnally I use a pack of 4AA nimh 2600 or 2700 mAh, and it's fine, more than 2 hours for a blue luxeon at 1.5A, I got close to 3h40 for a red one (constant brightness, however, the sound gets lower and lower).

Now, it's great also to have more voltage to get loud sound. My preference is now 2 cell li-ion pack, 2400 mAh. Starts at 8.4V and last almost forever.

Lithium-ion : use packs or cells WITH protection PCBs. They make sure the voltage is totally cut when you reach a certain low voltage, and this makes sure that you won't undersdischarge the cell (which then losts about 60% of its capacity). 2400 mAh cells are bigger than AA, and won't fit hilt in most case : I use the stick battery made by battery space.

Using 14500 cells / batt packs (li-ion) : they are only 900 mA but... lithium is better for providing current without ripple, and you might use them at 1A, for a lux V (overdriven) for instance. I haven't tried them at 1.5A but it might still work.

now PLEASE stop mixing Capacity (C, in Ah or mAh) and current (A) and stop pretending 1Ah is like 1A for an hour, it's NOT the case.

Erv'

Erv'

[Mad Hatter]

Thanks Erv.

So I get that 1000mAh can probably push 100mA for 10 hours, but not 1A for 1 hour, because the higher load will lead to fluctuations in output and (in effect) drain it faster.

But I'm still confused on the max current rating - only because the battery specs seemed so straight-forward. Here's from the nimh specs:

Nominal Voltage 1.2V
Typical Capacity 1000mAh
Max.Discharging current 1.0 A max. (1C rate)

The li-ion had:
Nominal Voltage Average 3.6 V
NominalCapacity 350 mAh
Max.Discharging current 0.7 A (2c) max.

I took this to mean that when max discharge current is given as something like 1.5C or 2C, that you multiply the mAh capacity to get the max current. Not because mA and mAh are the same thing - just because the max discharge current (A) was given in multiples of "C".

Is that not correct? :confused:

If it is, it seems like you are confirming my point... The 6S nimh can't handle the load, but the 2S3P li-ion can (though it doesn't quite make the "twice principle").

[erv]

the max discharge rate gives a clue on the performance of the battery when requesting a lot of current for a short time. Depending on the battery technology, ripple on the voltage will more or less important. It's most of the time a feature about a temporary burst of power.
Nicads are great for that and this is why they are still used for RC models (like 60 A for starting a little motor), or portable power tools. But in those cases, it's only about current, the voltage ripple isn't a problem. For a sound board + luxeon driver, it embeds some electronic parts and microcontroller, plus sensors, and the voltage shouldn't play yoyo to much.

so... if you want to stay away from problems with with sound board + luxeon, take li-ion with a capacity > twice the used current.
Also, don't take a 350 mAh li-ion cell to wire several of those in parallel. Take directly a bigger one and wire several cells in serie, I'm not trusting home made parrallel packs for li-ion, especially during the charging process.

If your hilt is too small in diameter... change it :mrgreen: (ok, I'm kind of kidding... I mean, don't take the problem in the reverse way. Proper operation requiere certain batteries, hilt should be adapted to that, not the opposite)

Erv'

[Mad Hatter]

I'm not trusting home made parrallel packs for li-ion, especially during the charging process.

Even with PCB? That's the main thing I was worried about. :(

I've seen some forums where people said to run 3 cells in parallel to keep the cells at equal voltage and then put two of those in series. But I haven't seen a lot about charging a 2S3P pack...

[erv]

individual cells with individual PCBs put in parallel isn't a good idea. While it's not really a problem with nimh or nicd, and since batteries are never totally equivalent, you always have the possibility to get one cell repissing into the neighbour, leading to a pseudo charge, but not in the state of the art. Be carefull, charging a li-ion is a complex process having a protection PCB does not solve everything.
It's true that some packs are made of // cells, but with a specific PCB inside I think.
My point of view : use bigger capacity cells instead of putting several weak ones in //.

Erv'

[Mad Hatter]

I was just looking over at BatterySpace, and it seems that they have 2S2P and 2S3P packs, and that they are using multiple PCB's and polyswitches in them.

If I decide to do anything with parallel cells, I'll either have them make the pack, or post on their forum to make sure the wiring/charging will be good.

I was also checking out the 14650's (7/5 AA). Two of those in series would have 920mAh capacity, which is better than the AA's. It seems that those are the only option left that doesn't involve parallel cells, or cells with a much larger diameter...

Thanks for all your help Erv.

[erv]

check also the packs sold by TCSS, there is a small one at 900 mAh that works fine for a lux V (just in case). The bigger one might not fit in your hilt.
At battery space, check the green STICK battery.
All-battery also have a great choice of batteries.
And finally, individual cells can be bought at deal extreme !