Saturday, February 13, 2016

Lithium House Bank - More Thoughts

LUX had 600AH of AGMs as its house bank. We’ve never been very happy with its performance. The charter management company let them run down to 10.5v once, which may have damaged them. We would typically have a max of 200AH available before the voltage dropped to 12.2v with our typical house load of about 6-8A (most refrigeration), so the total capacity was about 450AH. And we found that the charge times were excessively long.

LiFePO4 had several advantages for our use.
Number of cycles: 3000 or more, versus 500 for lead-acid, a 6x factor.
Discharge to 80% (50% for lead-acid), a 1.6 factor.
Weight of 150lb for the entire bank (12 * 12.5 lb), vs 3 * 124 lb for Lifeline AGM.
Charge faster, which translates into shorter engine run times.
Higher operational voltage which makes a variety of marine devices prefer, such as inverters, windlass, and lights.
LiFePO4 don't need to be recharged to 100% every time.

These advantages don’t come without disadvantages.
They cost roughly 3 times more than AGM.
It is a more complex install due to the necessity of battery monitoring and the need to modify charge sources to prevent overcharging the batteries and to not overheat the charging sources (like alternators).

There are some other considerations to include when investigating a switch to Lithium batteries.
First, a battery monitor like the Victron BMV-602 (now BMV-702) will be important so that you know voltage and the number of AH consumed or returned to the battery.

Second is to configure the alternators with external regulators that have alternator temperature sensors. The sensors should tell the regulator to cut back on charge current if the alternator heats up too much (more than 200 deg F). As Troy said in another note, there needs to be a way to charge your starter batteries separately from the house bank. The Balmar DuoCharge is the easiest "drop-in" solution. I’ve spent the time to learn how to modify alternators for external regulation. For regulators, I’ve been using an open source design, see here: Arduino Alternator Regulator. Building one of these is like a hobby for me. I don't recommend it for anyone who is not very familiar with software and hardware. A much simpler installation is based on the Balmar 614 regulators. Read the blog by Compass Marine for a description of his recommended alternator regulator at:

Third is to reprogram your shore power charger. We used a DIP switch setting in the Victron that limits the top voltage to 14.1v in Bulk and Acceptance. Float voltage drops to 13.8v, so the battery isn't charged, and storage is 13.2v. In fact, I noticed yesterday when we were on shore power that our Victron charged until it reached the Float transition point. Then it didn't charge again until the we used 80AH of power and the voltage dropped enough that it decided to switch back into Bulk mode (as I recall we were running the microwave oven, which consumes 80A from the house bank and the voltage sags to about 13v). [Note: We probably need higher ampacity wiring between the battery bank and the BMV-602 shunt.] Instead of using the DIP switches, I should use the computer interface, which allows more detailed programming of the Victron inverter/charger.

From my perspective, LiFePO4 batteries are safe for boats and have a bunch of advantages. I did a lot of reading over several months before deciding on the DIY approach to lithium batteries. If you need to pay someone else to do the design and installation, then expect to pay for their knowledge.

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