ML-COVER-3LI, Lithium battery power bank for off-grid data loggers

Lithium battery power bank for off-grid data loggers The ML-COVER-3LI is a power provision cover fitting our ML-x17 off-grid 2G/3G/4G data loggers to power a data logger from up to 3 D-Size Lithium SAFT LSH20 batteries safely, with a combined lossless capacity of 39000mAh.

Although our off-grid data loggers are designed to be as low power as possible, running it from just one single Lithium battery, due to the power demand of mobile communication, has its limits. If you have an application requiring 1 or 2 remote data updates each day a single battery powered data logger can run for years. When you need multiple remote data updates each day, then our data logger with tiny integrated 1Wp solar panel has proven to be a very suitable solution, assuming the data logger can be mounted outside facing the sun. If for obvious reasons a solar powered solution is not possible, then you could consider the use of a lithium battery pack. This is however something to consider with caution!

Lithium batteries are having a high energy density and when used without caution, they can turn into dangerous devices. If you put multiple Lithium batteries with all having the same state of charge in parallel, then it could work, but using it like that is strictly discouraged by the battery manufacturer.

The common save way to put batteries in parallel is by using diodes, but they come with a price. Although battery capacity is specified in mAh’s, it stores energy (mWh’s) and the diodes will dissipated a serious portion of this precious energy. The nominal voltage of a SAFT LSH20 is 3.6V@2mA, but during data logger operation load the voltage drops to 3.5V. A Shottky diode has a forward voltage of 0.3V, so a single diode will dissipate 8.5% of the stored energy. And because the voltage towards the logger drops, the electronics need to draw at least 8.5% more current to compensate the voltage drop. So using diodes will virtual reduce the capacity of a battery by almost 20%. When placing two Lithium’s (with two diodes) in parallel, the capacity increase will be about 65% instead of the expected 100%.

The solar energy industry is experiencing the same kind of problems as blocking and by pass diodes are causing significant system losses. The semi-conductor industry reacted by developing a so called “smart diode”. A “smart diode” is a chip containing comparators, a FET and a regular diode. When the input voltage is lower than the output voltage the FET will not conduct, the battery will not contribute to the energy demand and the regular diode will block the possible reflux from other batteries. When the input voltage is higher than the output voltage, the FET will start to conduct and the battery will contribute to the energy demand. The forward voltage over the FET is just 26mV instead of 300mV.

The ML-COVER-3LI is making use of such “smart diodes” and having three D-size battery holders. You can choose the insert one, two or three Lithium batteries to reach a combined capacity of 39000mAh. When even more capacity is required, multiple boards can be arranged in parallel as well.

Please use our online Data logger power consumption calculator to estimate the battery life of your application.
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