Saturday, February 13, 2016

Experience with a Lithium House Bank

We recently wrote about our new LiFePO4 house bank in Lithium Batteries and Lithium House Bank - More Thoughts.

How well is the system working? Here is a picture of a week of use. The right axis shows voltage while the left axis is used for the remaining measurements (State of Charge, Consumed Energy, in AH, and Current. The maximum use is just over 200AH. A good example of a complete cycle is on Wed, where we had used about 188AH and recharged it to full during an engine run from about 0930 until 1230. The Consumed Energy just reached 0 (full) a few minutes before we shutdown our engines. You can see the charge current (green) is normally 60-80A. (Top Blue line is State of Charge - 0-100%), Green line is Current, bottom Purple line is Consumed Energy in AH, and the Red line is Voltage, measured on the right axis.)
Weekly Battery Usage

The next graph shows six hours of Tuesday, during which we ran the microwave oven (the dip in current to -80A at 0845) The red Voltage graph shows that we only drop below 13.0v when we run the microwave oven.

Tuesday's Charge Cycle
We started charging at about 1205, where we had consumed just over 200AH and stopped charging at -60AH at 1427 (the right edge of the graph). We were idling the engines from 1227 - 1245, so the alternators weren’t charging then. We were actively charging for 207 minutes and most of it was at about 80A. This is with the stock alternators. We’ve not installed the new Arduino alternator regulators yet (so far the charge rates haven’t overheated the alternators).

Things are looking good so far. We've had several nights of use and haven't really stressed the system by using more than about 200AH. Theoretically, we can use 550AH * 80% = 440AH. Using a max of 400AH is probably a more reasonable figure (72% depth of discharge). These cells probably have more capacity than 550AH, so this figure is quite conservative.

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