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Ενδιαφέρουσες πληροφορίες για το τι συμβαίνει στις μπαταρίες lipo&ni-mh σε θερμοκρασίες πέρα των 25C και προτεινόμενες λύσεις.

Tim’s Tips Tim Wilkerson

LiPos and Cold Temperature Operation

Have you ever been flying on a cooler day and on takeoff the electric motor seems to drop out then reengage? Typically this is caused by the battery voltage sagging to below the LVC (Low Voltage Cutoff) that is programmed into your ESC (Electronic Speed Control). As a point of reference, I set my LVC to 3.1v per cell.

The forums are full of examples of battery failure during low temp operation. We enjoy rather mild winter temperatures here compared to other places in the country. Even in our climate it’s good to be aware of the temperature effects on our LiPos.

While it’s usually not such a big deal to have a momentary motor sag unless you’re doing some low altitude 3D stuff at the critical time. The real risk is that the voltage sag could affect the receiver, that is, like turn the rx off. Consider if the battery voltage sags to below where the BEC can operate reliably and the receiver supply minimum supply voltage is not met; then the receiver stops receiving. On many receivers it can take seconds to reconnect to the transmitter. Seconds are a long time when flying RC airplanes and can seem even longer if the plane is fast.

This potential long “reconnect time” is one reason why I fly dual battery setups my larger planes, but that is the topic of another article.

The Problem

Take a look at the picture below which is a graph of LiPo voltage over temperature. The graph is scaled in degrees Celcius which is a little inconvenient. Recall that 5 deg C is ~40 deg F and 25 deg C is ~70 deg F.

LiPo Discharge Performance over Temperature

The graph shows LiPo discharge performance at a 10C rate at varying temperatures. The 10C for a 2200maH battery is 22 Amps. Note that the battery discharge voltage is pretty flat at room temp and higher (the blue and purple graphs), but at lower temp the voltage sags initially (yellow graph). As the discharge continues the terminal voltage increases due to self heating temp rise, then all graphs fall off as the battery charge is depleted. If the discharge current is higher than 10C, the voltage sag will be greater. As a point of reference on the discharge current, my electrostik pulls about 42 amps from 4S2500 which is ~16C (42amps/2500maH), so I would expect a higher sag than the graph depicts.

Clearly, if using higher capacity batteries this effect will be less. For example, if I were 5000maH batts, operation would be at 8C and I would expect less voltage sag. In this case, I would also be carrying extra weight resulting in a higher landing speed.

Electrical discussion

This effect can be explained using simple modeling. Electrically, a practical battery (behind the battery terminals) can be modeled as an ideal battery in series with a resistance. As the current delivered from the battery increases, less of the ideal battery voltage is apparent at the terminals due to the series IR drop across the resistance. Using Ohms Law ( E=IR) where I means current, R means resistance and E means voltage so IR means current * resistance.

Clearly there is not conventional resistor inside the battery, but we can think of the chemical and physical elements as a resistor for easier conceptualization. For convenience, let’s call this resistance by a common name, Equivalent Series Resistance or ESR which is commonly used when describing the quality of a capacitor.

Below is a graph of a NiMH vs LiPo batteries as noted over temp. Consider the change in ESR between 10 and 30 deg C as a convenient point. For LiPo we see (60-30= 30 mΩ) which is a 100% change in resistance for 20 degC change in temperature. Compare that to NiMH change over the same range which is (47-35=12 mΩ) 34% change over the same range. LiPo ESR temperature change is certainly significant as compared to NiMH.

LiPo ESR over temperature

As shown above the ESR doubles between room temp and 5 deg C. This temperature dependant variation of ESR is the reason the battery terminal voltage sags as the temperature declines.

Interestingly enough, the ultimate battery failure mechanism at colder temps is the result of this additional self heating and the battery fails due to internal high temp even though it is a cold temp environment.

A Solution

Clearly the central idea is to keep your batts warm during cold operation to compensate for the inherent battery cold temperature performance. Here is a passive solution that I’ve been using. Some people are even talking of active heaters which could be done with a thermostat and a heating element but I went the inexpensive way.

I have been taking several precautions to keep my batteries warm this season, a) use battery heaters, b) preserve battery heat in a cooler in transit to the field and c) insulate batts in the plane to minimize thermal effect of airflow.

For the heaters, I talked my wife into making some bags of similar size to the batts and filled them with rice to use as heaters. Before flying I pop them in the microwave to heat them up then put them in my LiPo sacks with batts. I took some temp measurements and the heat lasts about an hour in an open air environment. Below are some pictures with 4S2650 on the left and 3S2150s on the right. I have the 4x 3S batts in one LiPo sack with two heaters and 4x 4S batts in another LiPo sack with two heaters.

3s 4s with rice bags.JPG

4S and 3S LiPos with Rice Bag Heaters

Going to and from the field I put my batts in a Styrofoam cooler shown below. I had this laying around so I use it.

LiPo cooler.JPG

LiPos in cooler on their way to the field

Even though the heater bag temp only lasts for an hour in an open environment, inside the cooler my batts are warm when I get home which can be well over 4 hours.

Regarding “in plane” precautions, I use a bubbled mailing bag cut to batt size as shown below.

photo1.JPGbattery bottom side with insulation.JPG

3S LiPo and in flight insulation from padded mailing bag

When I fly I take the batts from the cooler immediately prior to the flight, wrap them in insulation and fly. I usually get two flights back to back and remove two batts from the cooler, put one in my pants pocket where it stays warm and one in the plane inside some insulation. Here is a picture on my 41edge with Insulated batts just before a flight.

insulated batts in edge41.JPG

Insulated 3S in Tim’s 41 Edge

I heard some people online talking about blocking airflow in the plane but I don’t like that as it also blocks airflow from the ESC.

Here is a reference for those interested. http://www.rcgroups.com/forums/showthread.php?t=1343928

While we do enjoy relatively mild temperatures during the winter season here, there is still an effect on our LiPo batteries worthy of consideration. Hopefully the information in this article is worth the read.

Happy flying