Point of Beginning

GIAA Mailbag

August 1, 2005
Tool tips on Li-ion battery charging and PPM corrections.

Q: I have several products that use the new Lithium-ion (Li-ion) batteries. The people who sold me the equipment implied better performance compared to the Ni-Cds with respect to "memory" (when the battery loses capacity with repeated charges, especially when not completely discharged before recharging). However, my Li-ion batteries seem to have a memory problem similar to what I experienced with the Ni-Cds. Are these new batteries defective or am I the victim of hype?

A: Lithium-ion batteries are indeed higher capacity and much less susceptible to "memory" problems. But this highly advanced battery technology requires a more advanced charger as well as some understanding of how this battery's characteristics vary from those of Ni-Cds. Li-ion batteries are more particular about charging conditions. It is this aspect that may be the cause of your problems. You didn't say at what temperatures you charge your batteries. Li-ion battery chargers have intelligent circuitry that prevents damage to the batteries. If the ambient temperatures are above 119° F (45° C) or below 32° F (0° C), the charger may actually shut off or greatly reduce the charging process. So the cause of your problem may be due to the conditions under which you charge your batteries. For instance, if you re-charge your batteries in the field (i.e. your truck) on a hot summer day with the windows closed and the air conditioning off, or on an extremely cold winter day, it may be that there is little to no charging occurring because the intelligent circuitry of the Li-ion battery charger has limited or completely stopped the charging current to prevent damage to the battery. There is also an optimal Li-ion battery operating temperature range, but that is considerably wider than the operating range. Since battery technologies are constantly being improved (even for Ni-Cd), we strongly suggest you contact your manufacturer for more information specific to your batteries. Finally, be aware that even though it is outside the scope of this column's discussion, all battery technologies have optimal charging and operating temperature ranges.

Q: Instead of using a thermometer and barometer to measure atmospheric conditions to set the PPM atmospheric correction value on my EDM, I use a 100-ft tape to determine the correction. I stretch it out and set my EDM at one end and my prism pole at the other end. Then I vary the PPM settings until I read 100.000 ft. I attended a seminar recently where the speaker said this wasn't the right thing to do. Why?

Photo courtesy of Western Latitudes.
A: You didn't say how accurately you were trying to determine the PPM value for your EDM, or how accurately you desired to make measurements. Regardless, it is highly unlikely that your method is providing any benefit, and it may actually be decreasing the accuracy of your surveys. First, the tape itself must be calibrated and its true length determined for the conditions under which it is laid out to compare with your EDM (tension, temperature, etc.). You didn't say whether you apply tension to the tape during your EDM comparison, but it is essential that you do so and correct for tension if you used a different tension when you calibrated your tape. EDMs usually have two types of random uncertainty, which is why their accuracy is usually defined by an expression that looks like this: ±(x mm + y PPM). In a short distance, the x value is the meaningful one. If, for example, the x value is 3 mm, the potential uncertainty in 100 ft results in a precision of 1:10,000. This uncertainty corresponds to 100 PPM. Thus, more typical PPM corrections for temperature and pressure are overshadowed by the potential error in poor calibration techniques. Remember that inaccuracies resulting from poor tape calibration or measurement techniques will only potentially worsen the error overshadowing effect.