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Gday Fencejumper,
I used to use the Coiltek voltage regulated systems on my SD 2200 (7.3vlts) and my GP Extreme (6.8vlts) with a standard 12vlt-7.2 a/h AGM battery.
It is fairly heavy,heavier than the standard 6vlt-12 a/h dome top AGM batteries,but it fits directly into the same pouch. All i did was use a pair of cable ties to secure it to the top of the battery and soldered on a set of wires with a connector.
This set-up never failed me and I could detect for around 16hrs at full power.
If you want to go lighter,i would suggest a 12vlt-5 a/h AGM as the next best bet. It is probably half the weight of the 7.2 amp.
Good luck and remember when you jump them fences,pick the ones without a top strand of barbed wire.

I have used the reeds on my 2200 and getting 14-16 hours out of it is really good.

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G'day Fencejumper.

I too use a Reeds Lucky Lark on my 2200d and can thoroughly recommend them.The fact that it is a 12 volt(14 amp hour +) L/ion battery system,weighs as much as the minelab 4500 Li battery,has a volume controlled inbuilt amp booster,with the added advantage of an internally regulated,voltage controlled regulator(via a brass grub screw) so that you can set your voltage to what ever voltage you desire.In the case of your SD2100 no less than 6.8v for monos 7.4v for DDs.This all comes down to a lightweight,regulated,12 volt system,with plenty of constant power to drive that 2100 all day without worrying about voltage drop.

Cheers kon61.

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G'day Fencejumper.

The reason I say 6.8 volts for monos is because I found it to be the best all round voltage setting on my 2200d when working with mono coils.The mono coils run quieter and smoother over similar ground conditions,than if you were to crank the voltage all the way up to 7.4 or even up to 7.9 volts,in which you'll find works superb with DDs,but on monos,it will do nothing more than raise internal machine noise through your headphones,without actually adding anything to sensitivity or depth through the coil.

Cheers kon61.

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I've gone two days with the reeds before charging but that was with 11" mono.

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G'day Fencejumper.

The answer to your adapter Question is no.Unless the 2100 can handle 8.4v without blowing up.For the SD 2200 shuts down when approaching 8 volts.The 2000 was comfortable and operable at 8.5v but not sure of the 2100.Send a PM to Ismael Jones(a member of this forum) for hes the expert in this field and explain to him if it can be done.

Cheers kon61.

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I agree with you on that one Fencejumper. For you can order the Lucky lark With either a 5 pin (8.3v) or a 4 pin set on either 6.7 or 7.3v and yes if your prepared to chop and change voltage (so as to suit either machine,then I don't see any reason as to why an adapter can't be made to suit.

Cheers kon61.