# Charging a lithium ion battery pack with a thermoelectic generator

## Main Question or Discussion Point

So I was wondering if I could get some help and advice on a project I am working on. I have next to no experience or knowledge of electrical circuits so I appreciate your patience.

I am trying to power a structural health monitoring system "accelerometer" unit on a bridge using thermoelectric energy. The unit is typically powered by 3 AAA batteries. I was wanting to use the thermo generators to charge a lithium ion battery pack that would then provide the power to the accelerometer day and night. The user guide says that the nominal voltage for the accelerometer is 4.5 volts and has a cutoff limit of 4.7 volts. I have ordered 1 thermo generator from TEG Power so I can test its voltage and power ouput under typical outdoor temperatures. I will probably have to be wiring a whole series of these things together....

So, I guess I have some questions on what I should be doing. First, how do I regulate the amount of voltage that the lithium ion battery pack will supply to the accelerometer? If I somehow string enough generators together to charge the battery pack at say 5 volts.... how do I regulate the output voltage from the batteries to the sensing unit to 4.5 volts? Should I be concerned with the amperage output? Do lithium ion batteries come in specific ratings? For instance, 3.7 volts or 4.3 volts? Any other advice you can provide would be great. I'm sure I will have plenty of questions.

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Pooty,

I assume this is a continuation of this thread:

From the TEG website, the minimum temperature differential sited is:

38C (100F) - 2.1 V @ 70 mA = 0.147 watt / Delta T = 20C (68F)

Price $499.00 So, my first question is: where do you think you are going to find a 68F temperature differential under a bridge that isn't exposed to sunlight? A 68F temperature differential is not typical in the natural environment where one might find a bridge. If you have exposure to sunlight, why wouldn't you simply use a solar array? My final question: why is someone who is NOT familiar with electronics tasked with designing a power supply for what would appear to be a well-funded government project? I don't mean any disrespect by my questions, but it appears you have chosen a highly improbable means of powering a rather sophisticated piece of monitoring equipment. You might be better off to give some details about the environmental conditions of the location and any site restrictions that are pertinent, and simply ask for help devising a means of powering your unit. For instance, a small wind turbine or solar array would seem far more practical for battery charging in most places around the world. If the bridge requires stress monitoring from an accelerometer, then you might be able to use the motion of the bridge itself to power a small generator. Anyway, about your OP: AAA Alkaline batteries have a capacity of 1.2Ah and a nominal voltage of 1.5V. So, the first question is, How long will the unit operate on one set of AAA batteries? Knowing this will tell us what it's power requirements are, and give us some idea of how much power your generators will need to produce. Charge controller technology and design are well-documented, if you are unfamiliar with electronics it is unlikely you will be able to DIY a charge controller, so you will likely need to find an "off-the-shelf" unit, which most likely means starting with a 12V supply. I would strongly suggest you test the operation of your unit @ 3.7V. If the unit will operate @ 3.7V then it will greatly simplify your situation. Lithium Ion Batteries have a nominal voltage of 3.7V. The 18650 form factor is the "cheapest" wrt capacity/$ because it is a very high volume unit used in laptops and the "Tesla" electric vehicles. The capacity of the 18650 form factor ranges from 2.2Ah to 3.0Ah. Two of these in series would produce a nominal 7.4V which you should be able to find an "off-the-shelf" 12V charge controller for.

Back to what I really think:

I just can't help but think you are in over your head. You have ordered a \$500 piece of equipment and are ready to order "a whole string of these things" and I simply am not convinced you have the environmental conditions requisite to make them work. I simply cannot imagine this technology is a good fit for your application. If you really want help engineering a solution, please post a datasheet on your accelerometer and as detailed a description as possible on the environment the bridge is in and any other design specifications you have and let the forum suggest power supply options.

Fish

uart