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Simple DIY 16V Li-Ion pack for TDI SL
Posted by: XLOOX
Date: September 13, 2018 04:34AM
Having read all of Reg Sniff, Jim-in-Idaho & Stevenscool posts I figured I would give a ~16V battery pack a shot in my 2017 TDI SL.

I converted the Whites #802-7150 AA battery tray pack from an 8s (8 cells in series) configuration to 4s2p ( 2 lots of 4 cells in series) using a very simple crossover as in the attached photos.

The plastic strip is needed as the insulator for the back of the centre 2 terminals. The outer 2 terminals are conductive both sides. Dont use thicker materials than necessary to make the crossover else it will be too tight a fit but make sure it is robust. Ensure sufficient insulator is left around all sides of the centre 2 terminals to ensure no chance of a short between them or to the pack terminals behind, especially at the bottom & top of the strip. A 2mm surround will do fine.

To assemble the pack with the crossover, put the crossover in first, next to the top pack terminals, then install the top 4 batteries ( 14500 size 3.7V Li-Ion cells), holding all 4 of them against the crossover and making sure the wires are above the terminals, then finally push in the bottom 4 batteries in place 1 x 1. The batteries are installed in their normal direction as printed on the tray. The crossover is now held firmly in place by the batteries and cant move. Check voltage is ~16.8V & slide the cover back down. How easy is that? Total cost ? 10 cents for the crossover & $20-$60 for the batteries.

No damage whatsoever is done to the Whites pack so you can take the 4s2p crossover out & use AA NiMH in 8s config again any time you want to. The crossover is only useful if it is used with 3.2-3.7V 14500 size cells.

I have run the TDI SL half a dozen times at the higher voltage for a total of 20 operating hours and so far so good. I will provide more detail on the battery testing in a month or so.

Re: Simple DIY 16V Li-Ion pack for TDI SL
Posted by: Jim in Idaho
Date: September 13, 2018 09:02AM
That's a nice piece of work, but your runtime is going to be reduced using the 14mm li-ions. Be sure and post how many hours you get. I was thinking of using the 14mm's in a pack for my new 24K, but finally opted for NiMH, instead.

Re: Simple DIY 16V Li-Ion pack for TDI SL
Posted by: Tony
Date: September 13, 2018 08:10PM
Don't the 14500 cells max out at about 900mah?

Nice job though !


Re: Simple DIY 16V Li-Ion pack for TDI SL
Posted by: XLOOX
Date: September 14, 2018 01:38AM
Hi Jim n Tony, pleased to e-meet you.

Yes, 14500 Lion capacities are no higher than 900mAhr (most are 700mAhr) but with 2 strings in parallel we have 1400mAhr. At the nominal TDI SL current draw of 500mA that gives theoretically 2.8 hrs runtime which is what I get in practice.

Without doubt the runtime of the pack is roughly half that of Enerloop Pros but I am happy to change packs half way through the day given the improvement I get in bench test results. I say “”bench“ test because my way of doing an air test is to invert the detector and stack battens of pine on top of the coil so that I can manually run the test piece on top of the battens at a constant distance from the coil. Probably not kosher but very repeatable and should be fine for directional tests. Also my results for the NIMH pack are not far off Karelians.

When comparing NiMH packs with Li-ion packs, the results I get with a Whites 12/7.5 DF coil, Threshold set to barest whisper, Delay10us, Ground Balance 8, Gain 8, Conductivity ALL, Frequency centre, Output-speaker. Ambient temperature 25C are:

1) Depth of detection of 0.38gm spherical nugget increases from 15mm to 40mm
2) Depth of detection of 0.5gm AUS gold coin increases from 160mm to 185mm
3) Depth of detection of AUS penny increases from 300mm to 350mm

As far as run time goes, this is what I have determined so far:

The TDI SL was sat with its coil on the ground and same settings as above. I turned it off every 15 min, removed the battery, checked the battery temp by feel, checked the voltage with a digital voltmeter then reinstalled the battery & turned TDI back on and put it back on the ground. Each stop was less than 30 sec.

All packs tested dropped 0.4V-0.8V within the 1st 15-30 min then had a much slower, steady linear drop until about 85% discharged then a much more rapid drop in voltage. The runtime was determined by when the voltage started going over the cliff. I did this as there is no point in continuing discharge once you touch the cliff edge as you get maybe 15-30min extra runtime at the expense of signficant reduction in cell life.

Results average of 3 runs each were:

Whites NiMH 12V 2200mAhr – Start 11.4V ,4.5hrs runtime, End 9.5V, Average 10V

Panasonic Eneloop Pro AA NiMH 1.2V 2450mAhr 8s- Start 11.4V , 6hrs runtime, End 9.6V, Average 10V

Soshine 14500 Li-Ion 3.7V 900mAhr 4s2p - Start 16.8V, 2.75 hrs runtime, End 14.0V, Average 15V

I think the main risk is that there are so many junk 14500 cells out there on the market. I would like to test the “gold standard” Panasonic UR14500P Li-Ion cells but at $90AUD for 8 I think I will just keep using the Soshine Li-ion ones, which seem to be ok so far and are less than half the price.

I will eventually post a new photo for the crossover with revised dimensions to make it a bit more fool proof.

Re: Simple DIY 16V Li-Ion pack for TDI SL
Posted by: XLOOX
Date: September 15, 2018 03:10AM
Minor upddate to make crossover wider to fill the gap & centralise on the battery terminals exactly and to install so that the wires run at the bottom so they don't get in the way.

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