Using 12v SLA battery (w/ LM317 regulator) in place of 7.4v Li-Ion battery?

I have an existing internal Li-Ion camera battery which is 7.4v 2200 mAh, and although I can make an external Li-Ion battery pack of 7.4v 9600mAh without a problem, I would be interested in also making this external battery using a SLA (sealed lead acid) battery as an alternative. When the camera is being powered via an AC adapter that came with the camera, the rating of the AC adapter is 8.4v, 3 A.

If I connect the LM317 voltage regulator (https://www.electro-tech-online.com/custompdfs/2010/08/LM317M-DPDF.pdf) to the 12 (7 AH or other size) SLA, it seems that the LM317 can not handle this amount of current, without generating excessive heat.

Suggestions are most appreciated to drop the 12 volts to the 8.4 needed, with minimizing the losses, but without having to use a complicated or expensive circuit.

I have also seen 4 volt SLA batteries which could be used in series to supply a straight 8 volts without having to use a regulator, although they are not as common as the 12 volt SLA bateries.

Michael

Look at an LM338 or an LT1085. Put an external power transistor around an LM317 or other adjustable regulator.

The camera is likely making use of the AC adaptors 3 amp current limit. This pushes the charging power dissipation to the external AC pack relieving the camera of large heat dissipation of an internal charge regulator. 8.4 vdc is exactly two series connected LiIon batteries' full charge cut off voltage. This is also critically importance.

The camera's internal charging path is probably nothing more then a MOSFET switch to the external connector with safety circuit monitoring battery temp and low end discharge voltage. This would make the AC adaptor an 8.4 vdc current limited voltage source. You don't have to supply 3 amps but can not be over 3 amps. It would be desireable to be able to supply worse case camera run current so you don't rely on battery.

Keep in mind power dissipation on your external regulator. Going from 12.5 vdc to 8.4 vdc with 1.5 amp output is over 6 watts of heat dissipation. That requires a moderately large heat sink.

I would suggest a switching regulator as going from 12 to 7.4 is wasting nearly half of your battery life.
look at national semiconductors simple switcher line: **broken link removed**

if you want to do it the easy way this 25$ drop in lm78xx replacement that you adjust with a screw pot on the IC looks pretty good. up to 25 watts output!
3A Adjustable step down switching voltage regulator

Thanks for the suggestions, I have seen the smaller 10 watt version of the switching regulator, nice to see a 25 watt version. I will do some testing to see what happens.

There are third party smart phone cigarette lighter chargers available pretty cheap capable of 1.5 amps. You may be able to change value of one resistor in their output voltage feedback sense divider to raise the switcher's output voltage to 8.4 vdc.