TDI SL Battery mods

[Digger_O'Dell]

There's been a lot of talk on the net about the stock NiMh TDI SL battery pack. Though I haven't had mine very long I've realized pretty quickly much of what was being said was true. Mostly the complaint is that there just isn't enough "oomph" to properly power the coil which affects the depth to a degree, but more so the sensitivity. So here's how I addressed the problem on a budget and had a very noticeable improvement.

First off, here's the meter reading using NiMh batteries in the AA battery holder. These run at only 1.2V per cell. This is the same as the stock battery.



Next is the same pack filled with Alkaline batteries which start at around 1.5V per cell. It's a marked improvement, plus it will give 5 to 6 hours of use (depending on which battery) as opposed to only about 4 hours on the NiMh pack.



Now here is a pack of 8 18650 Li Ion cells (2 banks of 4 cells in parallel). These run 3.7V nominal but around 4.2V fully charged giving almost 17V out.



I was able to get the cheap holders for just over $5 a pair on ebay. The batteries I bought some time back for my flashlight but never got around to using them. There are a good off brand rated on the label at 3400mAh but in reality are only 2000-2200mAh, but still was a great price for what they are.

The pack itself with these batteries gives me around 4400mAh which should get me 5 to 6 hours run time in theory. If I upgrade to the real Panasonic high drain 3500mAh ($60 set of 8) batteries, that would give me 7000 mah and upwards of 8 hours run time.

So going beyond the math, here's how I have it put together. I found a plug set with pigtails from an old obsolete surveillance camera setup I was able to repurpose. (Free!!) Instead of having to open the control box snd wire into the power wires themselves I used the AA battery pack. I was able to simply drill a small hole, thread the wire and solder the wires to the contacts inside. Plenty of room for everything!





This leaves the plug just extending outside the box.



The battery pack I was able to repurpose an old microphone curly cable to attach to the pack and to the plug. I used shrink tubing for more insulation and to add strength to the splices.

Here's the finished pack and battery tray hooked up. As you can see the voltage is almost double the stock NiMh batteries. No problem with circuits in the TDI. According to Whites engineering it can safely handle 18V. I've heard of someone hooking 24V to it, but I wouldn't recommend it.



When using this setup the battery case just slides in as normal and I run the pigtail out the side of the battery door. The door will close and latch but will only lock on the one side without the wire. I haven't decided yet if I want to clip the battery door for clearance.

Here's the complete setup. All I have to do is slip the case into the slot in the TDI SL and pop the battery pack into the pouch on my belt. Almost too simple! I may look for a better fitting plastic hard case that fits the battery pack if I can find one.



As for performance the differences were as night to day! In the old church yard I tested it at j couldn't get any ground reading to see the ground balance using the stock battery pack. When swinging I would often get a lot of false tones as the ground would change ever so slightly because the GB was way off. Best I could do was guess and tweak as I went along.

Now add the additional voltage, I suddenly was able to get a fair tone change while doing my GB in the very neutral soil and was able to fine tune the machine before going off willy-nilly across the grass. The real signals just popped, while the irregular trashy signals were more pronounced aiding immensely in deciding to dig or not. So in my opinion a very worthwhile project!

[del]

Very nice battery mod Chris

[Digger_O'Dell]

Thanks Dan! Certainly much more economical than the RnB battery. I checked and it appears the Li Ion pack for the SL us to available any more. Only the big one for the TDI or TDI pro. Their replacement is a 12v 3100mAh for about $150. If I go with the high capacity batteries for $60, I'll have more than twice the capacity, full power, and all for about $70 total. Sounds like a deal to me!

[MangoAve]

Nice setup, Chris. Idk if you looked into blanks or not. But they make them. I used a blank and a Li-Ion 14650 and it sure worked well instead of 2AA. This way I do have the rechargeable option. The only downside is that I think there is half the capacity with Li-Ion compared to Alkaline. At least I can say it worked on my machine 4x, three of which were trial for a few min. The fourth time it lasted for 4 or 5 hr (when alkaline are like 15-18hr). Once I got to the second cellar, It almost looked like the battery died. I had spare alkaline so I just switched over and have not checked it. I think the battery curve for them show that they hold a steady charge for a while, then just drop off. Quicker than alkaline, so its not easy to gauge remaining power based upon what the display says. And I have not powered up the machine since, due to the snow, so Idk the status. But I will try again.

That and I am also looking to do something with the line up. Maybe try the MX. Then I can try 4 Li-Ion and 4 blanks. 4 x 3.7 = 14.8v. Maybe I could always modify a battery tray to have 8, but parallel in a 4 x 2 grid.

[del]

Thanks Dan! Certainly much more economical than the RnB battery. I checked and it appears the Li Ion pack for the SL us to available any more. Only the big one for the TDI or TDI pro. Their replacement is a 12v 3100mAh for about $150. If I go with the high capacity batteries for $60, I'll have more than twice the capacity, full power, and all for about $70 total. Sounds like a deal to me!

Your not worried the 17v rating is not to much power for the machine , I think the original TDI and Pro models only run at 14v .

[Digger_O'Dell]

Your not worried the 17v rating is not to much power for the machine , I think the original TDI and Pro models only run at 14v .

Like I mentioned in the original post, according to Whites it's safe to go up to 18V. That was one question of the other forums where that question was answered by Reg after he conferred with Whites engineering.

[del]

Like I mentioned in the original post, according to Whites it's safe to go up to 18V. That was one question of the other forums where that question was answered by Reg after he conferred with Whites engineering.

Ooops ! I thought that voltage number was for the other two machines and not the sl Chris . I'll have to reread that post again .

[MangoAve]

Your not worried the 17v rating is not to much power for the machine , I think the original TDI and Pro models only run at 14v .

Most things nowadays have a PWM for a supply which can take a variable input range. I think for my machine it's a 3.3V (2AA is ~3.3 when new) input supply, but can handle up to 5.5V absolute maximum. There's some leeway, however it is not a huge leeway, and I do believe changing the battery pack to a higher voltage kinda screws with the power indicator.

I did check my machine out last night, kinda getting it ready for the upcoming hunt. I tried all the LI-Ion batteries. Even the ones I was using that gave me an issue last time worked fine and was showing full charge. Maybe Li-Ion is more of an issue with temp than alkaline are? Mine does kinda take a little while to update the power status. It is not instantaneous. Turn it on and it's full, 10 min later it drops to three bars...

[Digger_O'Dell]

Jim, what kind of machine are you running? I think you're comparing apples to oranges here. A vlf machine is a set voltage range to run the circuitry which is digital. The coil really doesn't draw much power. On the other hand a PI machine like the TDI or Minelab GPX/GPZ run higher voltage because it's needed to drive the coil. The TDI also draws over a half amp of current which is why batteries don't last long, typically only about 4 hours for the rechargeable. If you look into the Minelab GPX series you'll find they have a huge external battery pack for that reason.

[MangoAve]

Jim, what kind of machine are you running? I think you're comparing apples to oranges here. A vlf machine is a set voltage range to run the circuitry which is digital. The coil really doesn't draw much power. On the other hand a PI machine like the TDI or Minelab GPX/GPZ run higher voltage because it's needed to drive the coil. The TDI also draws over a half amp of current which is why batteries don't last long, typically only about 4 hours for the rechargeable. If you look into the Minelab GPX series you'll find they have a huge external battery pack for that reason.

Where's the gasps from others 'did he just question Mango?" It's more like comparing oranges to grapefruits.

I'll start off saying no I'm not 'waiving it around', but I do hold a EE which shows credibility on the matter. Yes, I do know this is a detecting forum, not an electronics forum. Lol. Yes, I do have a VLF, but I hate to tell you that all machines, including your PI, have set internal power supplies and both run on digital circuitry. There is a microprocessor that controls the pulse frequency, and handles the display functions, and RAM and Operational programming software, and also the A/D functions. And you can't quite say VLF machines don't utilize higher voltage. My Tpro may only have 2AA, but it most certainly requires a set regulator, whether buck, buck-boost, or charge pump. Pretty certain it takes advantage of the LVL so that it is more power efficient. Pretty sure there is one internal switch-mode regulator at 93-95% efficiency for the 3.3V logic and one that prob doubles that voltage to 6.6V specifically to run the coil sine wave. It has a lower transmit coil amplitude than most machines, but makes up for it with higher sensitivity on the receive coil. Umm, but the MX (a VLF as well) runs on 8AA.. Both the Tpro and MX have specs stating approx. 20 hr run time. Guess what, that means they both use roughly 125mA.

Your PI machine, you say will last 4hr on rechargeable, and maybe 5hr on alkaline. Yeah, that's ~500mA. Most prob for the transmit function, as you said, but you still require a step-down regulator to run the circuitry. They haven't used 10.5v logic since the 80's, which is that is even damaged beyond 12v, so that's def not it. And 5v logic is pretty much a relic as well. So your machine has an internal PWM that converts 11~18V into 3.3V to run the digital logic sectiion, and prob a separate bank of a steady 12v for the coil drive. Battery power is not steady, so there needs to be something to create a steady supply. All machines alike.

I really wasn't making a comparison, kinda just stating what happens with my machine and how you can still utilize the same concept on Li-ion for any machine, however there can be at least an intuitive comparison. No matter what machine you use, a battery in series only increases the voltage, but does not increase capacity (amperage). Eight AA in your machine still only has ~2500mAh to work with, just like the VLF MX, or GTI2500. It's just at 12V instead of 3.2V like the Tpro. A battery in parallel increases the capacity, or amperage, but not the voltage. Li-Ion are said to have roughly 1/2 capacity of an alkaline, but at twice the voltage. One comparison you can make is: if you use 4 Li-Ion 14650 in your box utilizing 4 dummy AA, you will still be drawing 500mA, but your run time prob cut in half as each 14650 has half the capacity. If you parallel two Li-Ion, it's the same as two AA in series. If you use the below configuration, it will give you about the same run time, power, etc. as using 8 AA, but gives you the recharge capability. And no issues like Ni-MH issues with memory. Sure you still have 8pc in there, but you have the benefits of the Li-Ion. I think they are a tad lighter as well. You are modifying a separate box, so you can do this. Each parallel 2pc gives ~2400mAh @ 3.7V. Overall gives you ~2400mAh @ 14.8V.


Oh, I almost forgot. No matter what machine, they say to use Alkaline (vs carbon-zinc heavy duty) as they heavy duty have poor shelf life. They have a different discharge curve. They can leak easy. Heavy duty can't handle the drain rate.

[Digger_O'Dell]

Yes, I understand what you're saying here. The TDI does use regulated voltage for the circuitry of course. As an EE you know different components have differing operating ranges hence the 18v max input on the TDI, and much less in other machines. The TDI was originally designed to run a Li Ion pack at the higher voltage, but the battery pack was changed to a smaller compact size with NiMh batteries in the SL model after the issues with battery travel restrictions and overheating problems with charging wrong, plus issues with Li Ion safety if cells are damaged. PI coils inherently need more power due to their nature of operation.
With the pack I run its 4 in series, 2 in parallel effectively doubling the mah capacity for a longer run time. Upping the voltage brings back the performance edge of the TDI pro. Figuring typical PI circuits, the coil is unregulated and fired at straight battery voltage regardless what that may be. Then a comparator looks at the reference voltage and the signal coming back measuring the charge decay. Very old and simple technology, but effective.

[MangoAve]

I saw those DIY schematics for PI machines, but those can't be pre-production designs. Those are kinda crude, non-software driven designs. Whether PI or VLF, the strength of the transmit signal is proportional to the power input. Think of a transformer where the magnetic flux is proportional to the input current. You'd automatically lose depth within the first 10 min. Battery drop off is not linear, but it occurs and therefore would affect performance significantly if the pulse supply wasn't regulated. Something post production has to include a regulation on the pulse supply, that way you get a constant depth regardless of the charge of the battery.

[Digger_O'Dell]

I know that's what one would expect, but I'm pretty sure that's not the case. One reason being is the differences seen in air testing for one, secondly the difference I've noticed in detecting using the different batteries. One advantage of good Li Ion batteries is they do drop from 17v when topped off to their running voltagr of 14.8v in a short time. Then they hold that voltage until about 90% discharge, then drop like a rock. Alkaline on the other hand are as you said, a much more linear drop in voltage throughout the operating life.

[MangoAve]

Yeah, Chris. That's what I was initially getting at when I said the battery indicator might be off with Li-Ion. The machine may 'think' there is still 50% charge, but it's more like 20% charge and will just drop off and kinda surprise you.

A bookface friend was thinking on the enerloop. Ni-MH has close to the current capacity of the alkaline, but they take too long to charge and have the memory effect. Someone mentioned Li-polymer batteries. Boy, they are pricey, tho. For a 'new' technology that hasn't been fully tested. It says they are light weight and can be charged more time than Li-Ion. I do believe cellphones been using the Li-polymer type. But I'm sure you have seen the news about how many cells been blowing up, and not by the texting from a drunk ex. Just know those are mWh ratings, so divide it by 1.5 and you get an approx. mAh rating.

[Digger_O'Dell]

Lithium is an unstable element and reacts violently when exposed to oxygen. Lipo batteries are more reactive that Li Ion which does allow a higher capacity. But on the flip side, they are more dangerous and fragile. Go below the recommended voltage (depending on cell count) and the Lipo may be junk. Charging us a challenge as well. Different batteries have different discharge ratings (C rating) limited by internal resistance and design. Charge too fast, they overheat and BOOM!
I actually have a couple large LiPo batteries I had thought to use from my drone. I have a 6S (22.2V) 16,000 mah battery that could run for a couple days. But I would have to put it in an enclosure, add a buck voltage reducer and/or voltage regulator to 15V. A voltage alarm, and low voltage shutoff. Just getting set up tk charge LiPo batteries can get crazy expensive quickly.

[Digger_O'Dell]

Photo of my LiPo battery, next to an 18650 cell with quarter for size comparison. Yep, it's a brick!

Attachment 57839

[reidman]

Good job on the mod. I have been making TDI batteries for years. I sell SL and TDI batteries for those that are looking for them. [email protected]
Both used 18650 Panasonic 3400mah cells, the SL is 14.8 volts and adds depth, the TDI I run at 18.5 volts which adds depth. Both detectors have no issues with added voltage as the SL was designed on the TDI circuit. And the TDI was rated at 21 volts. I also use a new protection board with balance function for both keeps cells equal adds to run time and battery life.All cells are welded together no soldering onto Li-Ion New chargers come with both for the new voltage. Thanks for looking Reid

[Digger_O'Dell]

Reidman, looks like a great fit! Very tidy and well thought out. What kind of run time do you get with the TDI SL?

Here's my latest and hopefully final version. I've added the balance plug and the voltage monitor/alarm. The batteries I have are sort of oddball as far as 18650s. They were salvaged from some scrapped laptops which had good battery packs. I kept the best ones of course. They charge to 4.35v instead of the normal 4.2v maximum, and discharge to 2.75v per cell giving them an approximate capacity of 3200mAh. Though I have a top Opus charger it won't charge that high. But I should able to use my big iCharger which is fully programmable and will go that high.





I was able to repurpose the molle leg holster to use my spare pouch instead. It's an easy snap on addition to the battle belt I use. Turns out to be a very clean secure setup, and a perfect fit.



I was also able to find some nice nylon battery pouches on the cheap. Surprisingly good quality too!