WTB stand-alone pitkin controller

[rjack]

I'm curious if you're using Lithium batteries or not. I suspect that the "drop" has as much to do with thermodynamics and perception as it does with voltage. While it's strictly true that the voltage on a lithium battery drops, it's fairly flat and I'd be suprised if you could tell the difference over time.

My main battery is a 14.8V lithium ion but only 77Wh (in an old diverite slimline canister). I have used a watt's up to measure the draw and output over time in my garage (but I am going from memory here). My tourmaster vest actually starts out at close to 75 watts (at 100% output and full voltage) with a starting voltage around 16.7V. After about 20min the voltage has dropped quite a bit (its a small battery) and its closer to 45W output. I cant really tolerate 75W inside of a weezle extreme+ anyway so I bump it down to 60% right away which was about 35W at 16.7V. However after 30mins at 60% the voltage has dropped to the point where its only about 27W of output. So I bump up the pitkin to 80% and still get ~35W. Somewhere around 55mins of use and 45wh withdrawn from the battery I usually bump it up to 100% and get ~40W of output. The battery is functionally dead at about 70-75mins of use like that because the draw sags the voltage enough for the included BMS to turn it off (11V cutoff). All of my dives using this were in lakes with a thermocline so water temp has been increasing as I ascend, but I have also been chilling more and more and doing less and less activity. Bottom temps were in the 38-39F range, but I've never used the heat on the bottom. Thermoclines up to 60F very near the surface but most of the deco (80 up to 40ftish) was in the 52-55F range. So yes there are a lot of variables all mixed up here, battery, water temps, me, my activity level.

[jlillest]

Very cool to see real data on this! I thought most of this was just arm-waving, kudos.


An important thing to note on this is that the heating tech we use is heat-limited. They work like a polyfuse, in that once a certain temperature is reached then the material itself "clamps" down on the current. It's material science stuff that I can't fully explain, but think of like an automatic valve on the current. The effect is that a small temperature range is maintained when you're in the saturation zone and this is likely what's happening when you see the current lowering. This polyfuse effect is what's limiting the current in this system, not the controller.



The steep drop in voltage (14.8V -> 11V) is because your battery is depleted, you've used the whole safe range of the pack.


To get better control of the heat pack you'd need some sort of temperature feedback, but I'm betting the end result is about the same. Probably not worth the trouble of the extra development work.


It would be interesting to see your watt output plotted against both internal and external temperature to get a general feel for the thermo modeling. You're in cold enough water that I would have expected less of a drop in current that quickly, but in full disclosure I'm mostly arm-waving and don't have any data of my own. Though I have built my own heater controller.

[rjack]

It would be interesting to see your watt output plotted against both internal and external temperature to get a general feel for the thermo modeling. You're in cold enough water that I would have expected less of a drop in current that quickly, but in full disclosure I'm mostly arm-waving and don't have any data of my own. Though I have built my own heater controller.

I can try this in my garage with the watt's up and a thermometer. I don't have a slim enough temperature logger (I sometimes have access to hobos at work) to comfortably put something in my drysuit. But ambient temp in my garage at the moment is only about 48-50F. For modest duration wreck dives in cold water (last march it as 38F all the way to the surface) up to 90-100mins this little 77Wh batteryspace unit works well and doesn't really alter any of my gear rigging or get in the way.

I am actually planning to build a bigger 20Ah 14.4V nominal NiMH battery for longer cave type deco next summer. Typical (year round) cave temps in BC are in the mid to upper 40s and deco might be up to 2.5 hrs so I need something larger. I would use li-ion but the pack price ends up being pretty steep. I am using an old 15Ah SLA canister for this bigger pack anyway so NiMH is easier, cheaper and less of a garage bomb.

[apitkin]

I am actually planning to build a bigger 20Ah 14.4V nominal NiMH battery for longer cave type deco next summer. Typical (year round) cave temps in BC are in the mid to upper 40s and deco might be up to 2.5 hrs so I need something larger. I would use li-ion but the pack price ends up being pretty steep. I am using an old 15Ah SLA canister for this bigger pack anyway so NiMH is easier, cheaper and less of a garage bomb.

If you are building your own NiMh pack, you might find it useful to know that the LM controller has a cutout at 9.6V which was implemented specifically for people using 12V nominal NiMh packs. If you build a 10S rather than a 12S NiMh that feature will prevent you from accidentally killing your cells.

Andy

[rjack]

If you are building your own NiMh pack, you might find it useful to know that the LM controller has a cutout at 9.6V which was implemented specifically for people using 12V nominal NiMh packs. If you build a 10S rather than a 12S NiMh that feature will prevent you from accidentally killing your cells.

Andy

Thanks Andy. I had a typo above... I said 14.4V "nominal" but that would actually be 12V nominal, 14.4 charged. My plan was 2x 10S D-cells to make 20Ah.

[jlillest]

Why use NiMh? Lion is cheap, easy and higher energy density. Unless you had the cells already on hand, I'd just look to make a pack out of something like 18650 cells.

[rjack]

Why use NiMh? Lion is cheap, easy and higher energy density. Unless you had the cells already on hand, I'd just look to make a pack out of something like 18650 cells.

paranoia? lol

The cells are a gift to me, not sure their quality yet but I can zap them together fairly quick and see. I might switch to li-ion just to address the voltage sag issues

[victorzamora]

One note: Everyone thinks Lithium cells are "dangerous" and Nickel cells are "safe." However, I've had ~7 Nickel pack explode pretty aggressively including one going off in my hand. Despite having more experience with LiPo packs, I've only ever had one of them explode on me and I've only seen one other.

[midway]

I'm curious if you're using Lithium batteries or not. I suspect that the "drop" has as much to do with thermodynamics and perception as it does with voltage. While it's strictly true that the voltage on a lithium battery drops, it's fairly flat and I'd be suprised if you could tell the difference over time.

It?s primarily voltage. A Li-ion cell charged to around 4.2V will rapidly drop to 4.1xV then steadily decline (near constant negative slope) to 3.5V below which it is advisable not to go. I repurposed a motorcycle vest and built a Li-Ion battery pack with a voltage controller. It?s a life saver when doing prolonged deco. The only tricky part is water-proving the system if you go the DIY route.

[Jmbarfield]

I have found a controller but if anyone has a set of PST LP45s I?m looking for those too!


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