Battery questions

Hi All. I am wondering (as a do it yourselfer) what is the latest advice on Lithium-Ion batteries for UV field lights. I have recently purchased Ni-MH AA cells on ebay. They were advertised as 3800 mAh capacity, but proved to be less than1400 mAh. There are a bunch of Li-Ion batteries advertised on ebay such as 18650 cells (3.7V) rated at 6800 mAh, and a 12V pack with charger for $20.00 also rated at 6800mAh. Small  light weight, and inexpensive. It seems like they could be used to build a substitute for 7 pound lead-acid batteries. I am wondering if anyone has used them and found them to be as rated. Happy to hear any suggestions.Thanks,

Tom

 

ed.a's picture

Re: Battery questions

Hi Tom,

I've done a bit of tinkering with batteries. I've had more China Syndrome events than I should admit, but I've definitely learned what NOT to do. In a nutshell, NiMH are generally very tolerant of being abused. Lithium on the other hand, need to be treated with great respect and even then they can be rather dangerous.

Lithium cells will happily dump their huge power density into a shorted connection, heating up until they catch fire. Once they catch fire, they don't need oxygen, so good luck snuffing it out. That's why loose lithium cells are banned from air travel and baggage. At the least, you wouldn't want to get bumped off a flight for packing them in your UV lamp.

Lithium cells don't even need to be discharging high current to get hot. If they get overcharged, they can go critical. If they get discharged too far, they can overheat on even normal charging and go critical. Every cell phone and laptop has special circuitry to protect the cells, and even still they can be ... problematic.

The quickest way to get in trouble is to try using lithium cells in series. Laptops do this to get higher voltage, but it takes special electronics to ensure that each cell is kept within a range of 3.05 to 4.15 volts during charge and discharge. Check out the battery gear for model aircraft to see what's involved. Even with that minimal protection, the batteries have to be protected from overdischarging.

The scenario I experienced is that when powering a device with batteries in series, the weakest cell will be exhausted first, and then gets a reverse charge from the other batteries through the load. That's the death of a lithium battery, but more like a time bomb. If the string of cells is later charged without protection on each cell, the weak reversed cell remains effectively shorted, causing any good cells in series to now get overcharged until they outgas/bloat/explode/immolate.

If I were to experiment with LiIon again in a portable lamp, I'd probably try using a spare notebook battery that has all the protection circuitry on board, and can be safely recharged by swapping it into a computer. I could even get some nice battery capacity stats and graphs that way (without the hodgepodge to do it now).

The one application that's perfect for those huge 18650 cells is LED flashlights. I have a 5 watt UV (well, 395nm) flashlight that burns brightly for hours. The cell won't discharge below 2.9v. I carry a spare cell to use if I detect any dimming, but I haven't ever needed it in the field. When I get home, I just swap in the spare and pop the used cell in the charger.

If you're still undeterred and a real DIY, it would probably be relatively safe to uses these fat lithium cells ganged in parallel, hooked up to an efficient DC-DC converter to get 12-15V. (You could boost your lamp output a bit, too). They could then be charged, individually or in parallel, with the regular charger.

Reverse charging is hard on NiMH cells, but the good ones seem to tolerate it fairly well. All the Sanyo Eneloop batteries I have are still in excellent shape. My favorite charger of many is the LaCrosse BC-900. It can temporarily restore some capacity of the other brands of cells, but now I only buy the Eneloop (apparently soon to be a Panasonic brand).

There's an interesting NiZn battery technology that runs at 1.6v rather than the 1.2v of NiCad and NimH. It's great for getting a boost out of small LED flashlights and even handheld UV lamps. The problem appears to be that the NiZn cells suffer badly when reverse charged by running them in series.

Here's a trick I haven't seen around. I have a tiny 4 watt WTC UV lamp that runs on 2 AA cells. NiZn do help over alkaline, but I've found that using a single 3.6V lithium AA cell (14500) plus a dummy cell (actually a dead NiMH) gives it just a bit more boost.  Freshly charged it's just over 4.1 volts, vs 3.4v for NiZn, 3.0v for alkaline, and only 2.5v for NiMH/Nicad.

Another tidbit -  LiIon batteries can hold more energy for a field expedition by charging them to 4.2v, but it reduces their life. Camera and cellphone manufacturers usually charge to 4.1 or 4.15v. (They're unstable about 4.3v). You can extend the life of your LiIon cells by charging to a lower voltage, or possibly pulling them off the charger as soon as you get a green light.

BTW, caveat emptor if you're buying batteries off eBay or from the orient. Almost all of the cheap consumer batteries for cameras and cellphones are repackaged used batteries. They take the recycled "dead" LiIon batteries and bring them above the protection circuitry cutoff so they'll take a charge again. I've peeled the label off of a some wimpy eBay camera batteries to find the original label underneath with a lower (new) mAH rating.

Happy Prospecting,
Ed

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