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Lithium Iron Phosphate batteries for Ice Fishing

3/10/15 @ 8:46 PM
MEMBER since 12/19/01
I recently ran across Ted Peck’s mention of using Lithium batteries on this website, and then I ran across some AA-sized LiFePo4 (Lithium Iron Phosphate) batteries for sale in of all places, Walmart. They were $9.99 for a pack of 4, 600mAh capacity.

As it turned out, I was in the market to replace my 12V AGM/Gel batteries for both my Vex and underwater camera, which were pretty much done.

I did a bit of research, then went back and bought 2 packs. Went to Radio Shack and got a couple 4 cell AA battery holders, about $5 for the pair. Taped the 2 battery holders together and wired the leads in parallel to make an 8-cell, 12V, 1.2Ah battery pack. Added shrouded male QD tabs, so that both my Aqua-vu and Vex would plug in, and I had a nice solution to my battery problem.

I had previously measured the current draw of my camera at 450mA, and the sonar at 210mA. I tried running the camera on a single 4-battery pack right out of the package, and it ran for 45 minutes; after recharging the batteries, it ran 50 minutes. In other words, the batteries were at about 90% charge even after sitting on the shelf at Walmart for however long. Pretty impressive, for a rechargeable battery.

The 8-battery pack runs the camera for 110 minutes (measured). It should run the sonar for about 4 hours.

The major advantage is that the battery pack I built weighs 7 ounces including the foam insulation; the battery it replaced weighs 5 lbs. Since I mostly only use the sonar and camera for locational purposes, the run times are plenty long for me.

It is handy that I can use the very same chargers that I used for the 12V 7Ah Gel/AGM batteries. These are “wall-wart” DC supplies that put out 400-600mA, and top out at about 14.5V. They were made for safely being connected to a Gel/Agm battery for indefinite periods, but they also work great for fast charging LiFePo4 battery packs, getting a full charge done in about 3 hours.

These lithium batteries are a different chemistry from the lithium-ion batteries in your phone, laptop, and car. They share a lot of the good characteristics – fast charge and recharge, low self-discharge. They are not quite as energy-dense, but they are also apparently extremely safe – they are not susceptible to the thermal runaway issues that plague lithium ion. Oh, and they are rated for 2000 cycles as well…….

I will add a post to provide more detailed information on these batteries, as it can be challenging to find. Please note you cannot use these to replace the AA batteries in your GPS or other product, as the cell voltage is over 2X, and you will fry your toy.

10/13/15 @ 11:59 AM
MEMBER since 12/19/01
Very nice; I like the charge control chip mounted directly to the cell holder, slick idea, and should make charging idiot proof.

Yes, your Vex charger will probably work fine. The one caveat would be whether it can withstand the charge current that these 18650-sized cells will draw. It is possible that you might break the charger; you will find out when you try it.

I would not shy away from the higher capacity cells for this application. The cells you referenced are 1.5Ah, you can find 2.6Ah in the same size battery. The relatively low discharge currents needed by the Vex are ideal for those larger capacity cells. (But, the higher capacity cells will stress your charger more...)

10/6/15 @ 12:59 PM
User since 12/22/04
Mendota - I hope you are out there. Here is what I am proposing to do:

A little more money into solution for PCB to not have to worry about battery damage because of over use or poor charging habits.

I read alot of the Ebay batteries are over stated in capacity, and that it is better to go through a real place. They also have a wider operating temperature better suited for ice fishing then what you reported.

I believe with this setup I can use the same charging system I have on my Vexilar now.

3/12/15 @ 10:24 AM
User since 12/22/04
thank you sir....I was just complaining yesterday about how much my stupid vex weighs for hole hopping.

3/10/15 @ 11:49 PM
User since 10/31/06
Mendota, Thanks for your info, electrical engineer. Slick idea.

3/10/15 @ 8:49 PM
MEMBER since 12/19/01
Ya, I know those were long posts, but I thought I might as well get it all out there. I know there are a few folks that like to travel light, and are probably just as tired of hauling those batteries as I am.

3/10/15 @ 8:48 PM
MEMBER since 12/19/01
Using these LiFePO4 batteries for your equipment will require you buy a dedicated LiFe PO4 battery charger, which charges each cell individually, or you can use your existing 12V battery charger, but only if you are very careful to monitor the battery voltage and terminate the charge at the proper time. You will need a digital voltmeter!

Here are the rules for LiFePO4 battery use: Nominal voltage, 3.33V fully charged, rested. (x4 = 13.32V on the battery) Do not discharge below 2.0V per cell (8V on the battery) or else permanent damage can occur! Do not overcharge above 4.2V per cell (16.8V on the battery) or else permanent damage can occur!

Charge 32F-104F only. Discharge 15F-140F. Capacity 85% at 32F, 50% at 15F. Perhaps as low as 20% at 0F. Keep these batteries warm!

The wall-wart battery charger you used for the 12V 7AH Gel battery will work fine to charge these lithium battery packs, as long as you monitor the battery voltage and disconnect when the battery reaches the required voltage:

While charging, 3.50V per cell (14V on the battery) is 90% charge; 3.65V per cell (14.6V on the battery) is 100% charge. Disconnect the charger when you see anything above 14.6V. The MOST time the entire charge cycle will take is about 3 hours, at least with 600mAh cells.

The energy storage of the 600mAh LiFePO4 cell is about 2.6W-h; this is about identical to the energy storage of a Duracell Ultra Power alkaline battery, which is non-rechargable.

Scanning Ebay reveals that 600mAh, 700mAh, 1200mAh, and 1500mAh AA cells are available, along with chargers. I saw 1200mAh cells selling for $13.29 for 4 pieces; this is an even better deal than the Walmart price, from a capacity point of view. 8 of these cells will make a 2.4Ah battery; that would power a Vex for 8+ hours easily, and weigh considerably less than 1#.

4 cell battery holders are available via ebay or Radio Shack; don’t be tempted to buy an 8 cell holder unless you want to build 24V batteries. Paralleling 2 4-cell holders is the way to go; the #22 leads can be twisted together, and the parallel #22’s crimp in the tab connector better than a single #22 does anyway.

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