On the CF power requirements and battery life

[Rayataz]

Hey guys,

I've been tinkering around with an idea for a new saber that I want to work on. And in doing so, I've been trying to figure out the power requirements for a CF 7.5 and what the best power solution it would need to give me optimal battery life balanced with an acceptable number of features. I've only built one saber thus far - one that uses a NBv2 with a 3400mAh 3.7 li-ion battery - and to say that I didn't know what I was doing when I started working on it is probably kind of an understatement. So this time around, I'm going to try to better understand the internals starting with how to calculate the current load and thus the battery life of the saber. I've done several searches on this forum but I can't seem to find the answers I'm looking for.

First question: How do I calculate the current load of the entire saber? I understand I'd probably have to add the current from each LED but does the manual specify somewhere any additional current that needs to be factored in?

Second question: If a 3400mAh battery pack lasts longer than a 1400mAh battery pack, will converting a CF to a single cell battery mean I maximize my potential battery life (assuming I use a 3400mAh battery pack)?

Third question: What are the disadvantages to converting a CF to a single cell battery? The manual specifies 30% reduced audio output but nothing else.

Thanks in advance for any answers!

[NanoRex]

Okay first of all you should know that the capacity ratings are not exact. The Panasonic NCR18650B cells (the 3400mAh ones) actually have a capacity of around 3100-3300mAh (or something like that, can't remember exactly what the numbers were).

1. Battery life is usually not a massive deal and the run time can't usually be predicted accurately. When calculating the current load, I usually just add up the current used by the main LEDs and add a little bit. It seems to be a decent enough approximation.
2. Technically yes. Although you could make a two-cell 7.4V 3400mAh pack if you wanted (I am doing this for my saber).
3. I believe it also affects the LED current regulation somehow. As well you can't wire your LEDs in series, for obvious reasons.

As far as choosing whether to do the hack or not, that's ultimately up to you. Personally, I would never do the 3.7V hack unless I absolutely had no way to fit two cells in the hilt. It's not something I would do just to get longer battery life, because to me it's like purposefully crippling an amazing sound board.

[Rayataz]

Thank you! This was very informative!

[atomsk]

the listed mah "battery life" rating for li-ion batteries is definitely a pretty shifty number. i dont doubt that many manufacturers exaggerate these numbers. but its equally common for ppl just to misundertand them. if you look at the spec sheet for any of these batteries you will find some very important values for nominal/max charge and discharge rates. These are reported as a value C, which you can use as a multiplier for estimation purposes. Also listed typically but not always will be the discharge rate used for the testing they did to arrive at that mah rating. Almost always you will find this is quite low compared to what you would operate at in a saber, usually around .2C or .5C. most li-ion battery chemistry will lose effeciency as the current draw approaches its max. They will also often list the values of this curve in the spec sheet, so if you feel like doing the algebra you might be able to get a pretty close estimate to run-time for your application this way. Most of us here either hate algebra (raises hand) or are simply satisfied to plug it in and find out how long it lasts.

personally im not too worried about run-times. most of the battery, electronics combos on offer at tcss provide varying degrees of adequate run-time. the options exist mainly for electronics compatibility and to adapt to different hilt sizes. The max charge/discharge values i think are definitely worth looking up though.. and the relevant values for the protection pcbs as well. they are very useful if trying to source a compatible charger or simply want to know how much current you can draw b4 the pcb shuts everything off.