Battery Question-2


I have a Way Too Cool 18 watt DC SW-LW portable lamp.  Bill Gardner says each tube uses 2.2 amps.  I plan on running both the SW & LW simultaneously with the fan on.  This should draw a conservative 7 amps.  

I'm not a DIY person, so I would like to purchase a 12V battery pack that would last for 2 - 4 hours.  I've ruled Lead Acid out because of the weight.  This leaves Lithium or NiMH batteries.  Does anyone have a recommendation? Also who would be a good vendor to purchase from?


Brian Walko

ed.a's picture

Re: Battery Question-2

Hi Brian - I've tried various battery types.

My current favorite is the Lithium Iron Phosphate, aka LiFePo4 or LiFe battery. Using four 3.2v cells in series, they are nicely compatible with automotive charging systems. I use them in my motorcycle, for lamps, computer/phone powerbanks, and as an emergency jumpstarter. One small LiFePo4 motorcycle battery was borrowed and started a dead V8 RV. Check out the motorcycle battery specs, and you'll find that they can be regularly charged up to 14.4V. Some manufacturers state a max voltage at 15.2V.

Unlike the typical Lithium Cobalt batteries in phones and notebook computers, the LiFePo4 cells don't require a special battery balancing circuit. When seriously overcharged (or shorted), they will get hot and vent, but don't catch fire like LiCo. You can buy the 12.8V packs with protection circuitry and balancing, but without that parasitic drain they have almost zero self-discharge and can hold a full charge for months. LiFePo4 cells are usually rated for 5000 cycles.

When shopping around, beware of the Amp-Hour ratings. Motorcycle batteries use a Lead-Acid Equivalent or PbEq AH rating that's about 3X the actual battery rating. The little power banks have even worse specsmanship, usually 4x. They magically multiply the mAH rating of each cell by the number of cells. Look for a seller that includes a Watt-Hour rating.
You can divide that by 12.8V (the average voltage during discharge) to get true Amp-Hours. A true rating will spec the AH or WH at a specific discharge rate, like 10C. That would be an amperage of 10X the capacity of the battery.

For example, I have an unprotected 12.8V 20AH 256WH LiFePo4 pack that weighs 4.75 lbs including connectors. It only has 14 gauge wiring so it won't be for jumpstarting, but it was a cheap dealer reject on eBay. It would run your roughly 60W lamp for about four hours, with full brightness.

Another example is the biggest motorcycle battery made by Shorai. It's honest rating is 160WH = 12.5 AH and also weighs 4.8 lb, but is sold as a 36AH PbEq. That should also meet your requirements (about 2.5 Hr runtime). It's very solidly packaged and there's a balancing charger available. It could also be charged with a smart automotive charger, or even from a car socket if you limit the charging current (I use a simple cord with a 12V bulb in series for my packs).

Note that NiMh packs are heavier, take more 1.2v cells to get up to 13V, quickly self-discharge, and are more likely to suffer cell reversal damage near the end of discharge. They also need a special charging algorithm, requiring a 110V inverter plus the custom charger in the field.

I recommend having an inline fuse holder near the battery terminals, and good insulation in case of a short circuit. Ham operators (KE6IZN here) have standardized on Anderson Powerpole connectors for wiring 12v rigs. Get a bunch of red and black PP30 connectors, convert your lamp's wiring, and use an inline light bulb between two connectors as the charge limiter. The bulb has very little resistance (like a wire) when it's cold, and also glows during charging.

Battery Geek Out,

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