Help with battery pack

[herculino]

Hi all,

I'm designing a video light and need a bit of help with the needed battery pack.

The main trouble I find is that all battery packs already made I've find, have a from not suitable for my needs, so I'll build my own.

The idea is to build a 4 cell (18650) 2200-2800 mAh battery pack with a PCB to protect and charge the pack.

The system will have 9 XML leds 3s3p driven by a Taskled H6Flex.

I've make some fast calculations of runtime and current flowing and find out that:

Lumen	Watts	Current A	Time min
3477,6	26,28	3	81,10
3780	29,7	3,3	71,76
4284	32,94	3,6	64,70
4914	41,85	4,5	50,92
5619,6	50,868	5,4	41,90
6174	57,24	6	37,23
6930	64,152	6,6	33,22

So if I go to the max Iout of the driver (6.6A) I'll need a battery pack able to draw 6.6 Amp, isn't it?

Looking for cells with tabs I've find that most of them are rated 2C maximum discharge rate that means 5.4 Amp for the biggest ones.

I'm not sure if I'm understanding things well, I read a lot of articles in Battery University website and other sources of information about Li-ion cells.

If I'm not wrong, the max current I'll be able to draw form this cells is 4.4 to 5.4 Amp. Is there any way to solve that.

I'm not sure if this cells are "protected" ones and I should try to find "unprotected" ones as I'm going to use a PCB for safe balance charge and safe discharge.

Please would you be so kind to "illuminate" me?

Best regards from Zaragoza

 

[gearhead1972]

How many volts are you looking to get out? Did you look at the Li-Ion packs at battery junction? What about an RC car pack, they can pull large amounts of amps..

 

[herculino]

I didn't put the volts as is another row and don't give too much information.

XML as all leds drop different voltage at different current, going form about 2.9v at 1A to 3.3v at 2.2A as there are 3s3p the voltage trough the 3 leds in series is between 8.7v to 9.9v.

The battery pack will be nominally 14.8v the driver is a buck driver and I estimate an efficiency around 92-94%.

RC car packs have different dimensions not suitable for my design, in the other hand are more expensive than 18650 based packs.

Thanks anyway.

 

[alpg88]

something like this?

 

[gearhead1972]

As far as I can find, if you get 4 AW 2900mah protected batteries you will be limited to a max of 5.8 amp discharge. Now an IMR cells you can get 10 amp discharge, but you wont get the mah, unless you go 26650 like the lighthound ones, 4000mah 10 amp discharge. Also AW has a 26500 that is 2300 mah and can do 20 amps.What dimensions do you need for the pack?

 

[herculino]

The pack must fit into a cylinder of 55mm diameter.

I would like to know if it's possible to obtain non protected cells. I'm thinking on this LG ones that are sold in some web's after adding a protection circuit and repacking.

I have the idea that the amp limitation comes form the protection circuit. Anyone can confirm please?

Best regards form Zaragoza.

 

[alpg88]

you need 4 unprotected cells, and pcb,

 

[herculino]

I'm considering this PCM/PCB the only trouble is that is too long.

It would be nice if anyone how has try it could tell me if you can trim it a little or is not possible, looking at the pictures it seems that you could trim some milliliters in each side.

The one you recommend looks interesting also, the max amp are just there and has no equilibrium options that sounds interesting for a pack.

Still looking for unprotected cells...

Best regards from Zaragoza.

 

[alpg88]

I'm considering this PCM/PCB the only trouble is that is too long.

.

it is not your only trouble. your pcb has equilibrium option, that means it discharges, cells to equal state of charge, it turns extra voltage into heat, lots of it, you can not have it sealed, especially next to batteries. in first post you posed max draw of 6+ amps, pcb with 8A should work just fine.
btw that site cells unprotected cells as well, good quality japan\korea cells, get them with tabs, solder the tabs, do not solder directly to cell itself, you will kill it, i used to solder to them, every other cell i soldered died within month or so.

 

[herculino]

Thanks for your answer.

I got surprised with that information, I never thought that this could work that way, that makes the idea of a balancing PCB quite useless as is not going to be efficient at all unless you have batteries with identical rate of discharge.

I'm moving to a 5 cell pack looking for a bigger runtime in the torch and it's even more difficult to find a PCB with the right measures and values.

I was mailing with George (Taskled) about the current draw by the cells to the driver and he told me that is not the same amps as the leds are consuming as the voltage is quite higher.

I made some fast calculations and got that (in the ideal world of 100% efficiency as so on) I need no more than 4.28Amp to have the max current draw of the driver (6.6A) and this when the cells are near death (3V).

I rethink the idea and got 3 different options:

1st build a battery pack 5s with protected cells, install wires and connector for charging whit a hobby charger able to balance them. The main point is that I need to open my lights (and change the design in accordance) for charging the pack.

2nd build a battery pack 5s with UNPROTECTED cells and use this PCB just for charging the pack and connect the batteries directly to the driver for discharge, the driver can cut off at a selected voltage so it will give a kind of discharge protection, also provides open-circuit protection. This system will not balance the batteries but permits to charge through wet contacts and let's me keep the torch sealed easier.

3rd use that PCB from ebay and pray that it works. Same troubles than in 2nd option but at least seems that I could use the PCB also for discharging.

My good what a mess.

 

[alpg88]

i would take option 2.
99% of all electornics that use li ion cells, use that set up, even with those pcb's you still get cells charged, and have potection against shorts, overdischarge, and overcharge, it also has ballance function, not to mistake with equalibrium function, that pcb charges all cells equaly, it is not pcb that shuts charging off, but a charger itself (unless the voltage is too high, than pcb kicks in) but in that case you need a new charger.
don't overthink\overcomplicate things, i used those pcbs many times, all work just fine.

 

[herculino]

Thanks again alpg88.

So I will go for option 2.

Just another newbie questions...It's possible to "reuse" the NTC Thermistor of a old laptop battery pack?

Which battery charger could you recommend me? I've been reading a bit in the forum about it but got confused with so many options. I'll use it for charging the 3 packs I'm planning to build and I need it to be "light" as I'll take it with me in my diving travels and luggage wait is a big concern for flying divers.

What about this one?

Thanks again for your help.

I would like too much to be able to follow you recommendation...but my brain don't let me:duh2:

Best regards from Zaragoza.