Ryobi lithium battery with LED flash light

[kinarfi]

I bought the Ryobi tool kit a few years back and modified the flash light to use 4 3 watt LEDs instead of the incandescent bulb and it worked very well with the NiCad batttery. The lithium battery works even better, too much so, I had to add a resistor to cut the current back because the voltage is higher. The real problem is that the lithium battery goes to sleep and drops down to ~ 8 volts if left at rest for a short time and needs to sense that it is being used so it will turn back on. Unfortunately, ~8 volts is to low for the LEDs to draw any current and wake it back up, so I need some method to wake the battery up so it can power the LEDs. I put a 10,000 uF 25v cap across the terminals and that woke it up, but the cap is too big to put in the handle of the flash light, so I'm thinking of ordering some 5.5 v super caps to put in series and parallel to the LEDs, also going to look into a FET design that pulls power at start up and then turns it's self off. Also the Ryobi batteries are rated as 18 volt,but are 20 actually.
Any body got any other ideas?
Ryobi say the cut back is designed into the battery circuit.
Thanks,
Kinarfi

 

[audioguru]

A lithium rechargeable battery cell averages 3.7V. Five in series makes 18.5V but fully charged they total 21.0V. A battery cell is damaged if it is allowed to drop to less than 3.2V which is 16V for your five cells.

We do not know what circuit is there to cause the battery to "go to sleep" and drop its voltage as low as 8V.
We do not know what circuit is there to detect current and "wake up" the battery.

 

[kinarfi]

That problem was a lot easier to fix than I thought it would be, all it needed was a resistor to "wake up" the sleep circuit in the Ryobi Lithium + 18 volt 4 amp hour battery. I found that 30K ohm was just a bit to high to "wake" it up, but 25 K worked fine.