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A vacuum that ran on 10 AAA NiMH batteries in series (15.6v at 1.2A) is nearly dead.

I attempted to replace with 6, EBL 9v 1.2A batteries Lithium... bought off a the internet. I connected them such that 3 groups of 2 9v batteries (series) in parallel which should be 18v at 3.6A.

Unfortunately... the vacuum runs for about 3 seconds and then slowly shuts down with my configuration. If I wait a minute, I can run for another 3 seconds. If I leave the vacuum on after 3 seconds, it doesn't run.

As an experiment I got 2, 9v 1.2A Energizer Alkaline batteries from CVS and connected them in series (18v at 1.2A). It runs strong for about 10 seconds before it starts losing some power... but it still continues going. I imagine if I added some more in parallel it would have more power.

Any idea what is going on? I am at a complete lose.

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  • \$\begingroup\$ the battery array will have a net ESR which must be <5% of the motor DCR to good startup surge load regulation. If the battery voltage drops more than 10% there is a risk that it is an unbalanced drop from the weakest cell. \$\endgroup\$ – Sunnyskyguy EE75 Nov 9 '17 at 6:16
  • \$\begingroup\$ First source of confusion : the motor likely takes far more than 1.2A, probably about 6A. The batteries were 1.2Ah - that is, at 6A they last for 12 minutes, not an hour. 10 second is about as much as you can expect from those 9V batteries. \$\endgroup\$ – Brian Drummond Nov 9 '17 at 7:20
  • \$\begingroup\$ I think Passerby might be onto something in the comments below. With the energizer batteries, I can run it for 10 seconds before it hums. Then I wait a minute... I then am able to run it again for 10 second... rinse and repeat... Possibly a PTC resetable fuse is cooling down during this time. \$\endgroup\$ – slowmo Nov 9 '17 at 16:55
  • \$\begingroup\$ Let me take tonight to see if he is correct and dismantle the hand held vacuum to see if I can find the PTC resetable fuse. \$\endgroup\$ – slowmo Nov 9 '17 at 16:56
  • \$\begingroup\$ @BrianDrummond , Im going to try your suggestion now. Basically those 6, 9v batteries are not able to source enough current for the vacuum needs in comparison to 12 AAA batteries. At this point i just feel like experimenting and adding more 9v batteries in parallel to see what happens :) \$\endgroup\$ – slowmo Nov 10 '17 at 14:53
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The vacuum has a "don't catch on fire" device. Likely a PTC resetable fuse. The higher voltage means a higher current which means a tripped fuse. Or a heat sensor for the same reason.

In either case, lower the voltage or replace with nimh batteries. They are cheap.

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    \$\begingroup\$ Unfortunately I do not see a specific PTC resetable fuse on the circuit board. There is only a voltage regulator and a micro-controller. The rest looks like transistors, resistors, capacitors. Anyway, I'll try lowering the voltage but I wonder if my 6, 9v batteries just aren't able to source enough current. Anyway,,, I'll continue playing around a bit more. \$\endgroup\$ – slowmo Nov 10 '17 at 14:50
  • \$\begingroup\$ The PTC may be in the motor itself. The PTC for my Toyota window motor was inside of it. It's more of a don't crush arms fuse than fire but same thing. \$\endgroup\$ – Passerby Nov 10 '17 at 19:36

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