Li-Ion cell manufacturers do provide some information in relation to their cell performance at different but constant charge/discharge rates and at different and constant temperatures, but hardly any of these can be extended to field conditions, where charge/discharge rates and temperature are continuously varying. This paper attempts to take the primary cell testing data gathered in a lab environment and create a first-order model for battery behavior in real-life conditions. At the same time, real-life data for battery packs in 3-wheeler vehicles are obtained on a continuous basis, when driven or when the battery is being charged. The battery packs used are of 1.25kWh without any forced cooling, and the vehicles are used in the city of Chennai in India, where in 24 hours, the average temperature varies from 25°C to 48°C in summers and from 18°C to 32°C during winters. The State of Health estimate obtained from the model (and the lab data for cells) is then compared with actual field data; further variances between the two and possible reasons for such variance are discussed. The results show that with some care, the battery behavior in real-life can be reasonably predicted.