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I've looked at DC-DC converters for 12v to 18v and they usually have low amperage ratings. One for $50 can use 15A with 25A spikes. But cordless tools can pull much more than that, especially the saws (I've read up to 70A). I want to run a cordless reciprocating saw off my car battery (clearing some thick trunk bushes) far from AC outlet, the portable batteries run out of juice fairly quickly (3-5 minutes operating and they are new NiCd).

Yes, can just use the 12v on cables from battery but not the same zip to saw and blades don't seem to get as much bite (not just slower). I'm unfamiliar with how many amps or current I can put through a resistor over time to split voltage to add additional 6 volts though the ones I see rated at 500w are for brakes and such are 10 inches long and cost $22 each. And I don't know if they are appropriate.

Looking to run the saw for ~2 minutes at a time, brief pause, 30 minutes altogether.

Adding a 6v battery (like golf cart or others) and mount them in my vehicle to tie into regular 12v when needed for tool. Guessing I'd need some rectifiers there but will the car over charge the 6v battery? And how do I go about all that?

Update: Ran it directly from car battery and wasn't good till I replaced wires with heavier gauge. Then acceptable. Power continued to diminish and I questioned whether doing this in 105°F weather affected it. Then I noticed some warm spots within the tool. I opened it and there are some fairly thin wires in there. Not much room to stuff wires (especially around the variable speed switch) but thinking up a plan to beef up the internal wiring and see if I can get much more solid performance at 12v.

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closed as too broad by Brian Carlton, PeterJ, laptop2d, Dmitry Grigoryev, Chetan Bhargava Aug 17 '17 at 20:08

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Would a chainsaw (the electric ones seem to be well-received) be a better solution to the thick trunk bushes? \$\endgroup\$ – Solar Mike Aug 10 '17 at 21:18
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    \$\begingroup\$ Unfortunately, I don't think what you're asking for is practical. 70 Amp 12V to 18V boost converter is just not very practical because of the very high currents involved. Sure you can custom design one; it can be done. But the cost is not worth it and it is definitely advanced design work. You're way better off buying a gas powered chain saw. Or buy a newer cordless saw that uses lithium ion or lithium iron batteries. Those will last much longer at least. \$\endgroup\$ – Vince Patron Aug 10 '17 at 21:23
  • \$\begingroup\$ I would recommend modify your tools to run Lithium RC car batteries. Another guess is, your tools would run just fine on 12V car battery. At that high current you are not getting full 18V out of the NiCd battery anyway. \$\endgroup\$ – user3528438 Aug 10 '17 at 21:49
  • \$\begingroup\$ Vince, avoiding gas powered chainsaw as it's very dry here in southern Oregon and there are even midday bans on using them. \$\endgroup\$ – Hebekiah Aug 12 '17 at 2:33
  • \$\begingroup\$ user3528438, tried running off the truck battery straight and you're right so far, I intend to do some current testing to find out for sure. \$\endgroup\$ – Hebekiah Aug 12 '17 at 2:35
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I think you would be better off with a DC-to-AC inverter to charge the battery packs for cordless tools, or use corded tools with the inverter directly. That is a lot of power so you would probably need a number of 12V batteries depending on how much work you are going to do. Also note that the power levels you are working with can be dangerous and all the components need to be sized properly.

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resistors aren't going to get you to 18V, you need a boost converter eg:

https://www.singtech.com.sg/shop/dc-dc-step-up-power-booster-100amax-80acont-12v-18v/

probably there are similar devices priced cheaper if you look.

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  • \$\begingroup\$ Wow, $500 for the boost converter. I looked around but still minimum $220 for anything that seemed possible, though they looked sketchy and needed heat sinks and better packaging. \$\endgroup\$ – Hebekiah Aug 12 '17 at 2:37
  • \$\begingroup\$ I paid much less for a 1500W 240V AC inverter, I guess the market for 18V DC equipment is much smaller. \$\endgroup\$ – Jasen Aug 13 '17 at 23:07

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