Recharging Li-Ion cells

[Lazgoat]

I've purchased some 14.8V Li-ion packs from BatterySpace and their recommended charger. When thhe cells arrived I tested their voltage to see whether I needed to charge them first and they were at a little over 15V.
I used them for about 10 min in total white setting up some lights and put them on charge last night. They were done in 20min or so.

Now, my lights don't have have a battery fuel meter, so how do I know when to stop using them to avoid overdischarging them? For your ref & calculations this is the setup:

3 x Cree Q2 LED's in series
1 x Kaidomain driver (Kennan) @ 700mA
14.8V 2400mAh Li-Ion pack.

Can someone estimate the run time I'd get from this information and give me an idea at what voltage I need to stop using the battery pack & charge it?
Also, are there any "fuel meters" that I can integrate into the light?

Thanks.

 

[imageicon]

arent your packs protected by a pcb with automatic cutoff? else my best guess would be for you to estimate a runtime maybe doing test meassures on your packs with a volt meter to get runtime then just be carefull!

 

[Lazgoat]

Yes they are protected, but I'd like to know what the approximate run time would be rather than be out & hit "the wall" and end up in the dark.

 

[jrbruin]

Ok, I'll take a stab at this. I'm still designing my first bike light, but it's very similar to what you're designing except I may use a bflex to have selectable power output (and be able to go up to 1A)

You've got 3 Cree at a Vf of about 3.5V = 10.5 V driven at 700mA
Total power there is 7.35 W

Your input voltage is 14.8V, so using P=IV:
7.35W = Iin * 14.8V
Iin = about 500mA

Since you have a 2.4mAh pack, you should get a little less than 5 hours on a full charge. The efficiency of that driver is supposedly mid 90%, so I didn't take any extra power into account there, but I think this should give you a good ballpark.

 

[Szemhazai]

Count 80% and you want find yourself in the darkness 20 miles away from home .

 

[TorchBoy]

Since you have a 2.4mAh pack, ...

2.4Ah.

The graph in the datasheet indicates about 86% at that voltage and current for the example circuit. I wouldn't argue with 80% efficiency to give a bit of leeway, but I doubt I could cycle 32km in 6% of the battery capacity - about quarter of an hour. (More likely that the battery capacity would be less than nominal.)