Last year I installed Lithium batteries from Sterling Power onto Lady Cindy. This is how I did it and a summary of how the system is working out in real usage.
The problem with lead
Lead acid batteries traditionally used in boats suffer from a major problem over time… sulphation. This reduces their life span and their capacity. They can also degrade if they are discharged below 50% capacity, they drop voltage as they discharge which means you should only use about half the power stored in them, and they take a long time to charge from 80% to full capacity.
Advantages of Lithium
Lithium batteries can be discharged to near zero and maintain a steady voltage throughout the discharge cycle. Most come with built-in battery management systems that optimise the charging and monitor cell performance. Often, they provide remote monitoring via Bluetooth. Lithium is far lighter than lead, so although they are similarly sized, they are easier to manipulate and can be installed on their side, which gives more install location options.
Lithium is expensive
There are issues with lithium, too; they are very expensive and can not be used in the same bank as lead batteries as they have different charge and discharge characteristics. The established view is that you cannot mix lithium and lead and must throw away perfectly good lead leisure batteries to install Lithium… BUT THIS IS WRONG. I have done it with minimal rewiring, but a special charger is needed.
How to mix lead and lithium batteries
The secret is to have separate battery banks, and the lithium is added as a new bank isolated from the existing lead bank by a battery to battery charger. I chose UK based Sterling Power for both my batteries and my charger after speaking to their technical team to confirm their product could do what I needed. That was to charge the lithium side when the lead side was over 12v (charging or charged) and isolated the lithium when the lead side was less than 12v (discharged below 50%).
All house loads such as lights, fridge, instruments, etc. were moved from the lead leisure battery to the lithium bank. Charging sources (wind, solar, alternator, mains) remain on the lead side, so they are unchanged. The separate lead starter battery and its charging sources are also unchanged.
How the charger works
When there is an active charging source on the lead side, not only are the lead batteries charged, the Sterling charger takes some of this power, regulates the voltage and controls the current to charge the lithium bank.
When the charging sources disappear, the lead battery provides power to charge the lithium until the lead side drops below 12v (50% discharged). When this happens, the Sterling charger isolates the lithium bank, which continues to provide power to the house loads.
Has it worked?
Yes, and better than I hoped. I used to have 2 x 120ah lead leisure batteries that I could use half of (120ah usable power). Adding 2 x 110ah Lithium batteries could have given a total of 340ah of power. In practice, I decided to reduce weight at the aft end (where the lead leisure batteries are located) by removing one of these batteries. So, currently, I have 280ah usable power, and that power is always at a stable voltage. The only negative is that there is a cooling fan on the charger that is noticeable but not loud when it kicks in, so think more carefully than I did as to where to locate this.
I am very happy with the power I have now. I live aboard Lady Cindy and have never run out of power. I charge when I am in a marina, when there is daylight or wind, or when I run the engine. I have a 240v inverter aboard to run various electronic device chargers, and an xbox and have run that for 4 hours before getting to 25% charge and turned it off, so I had power for lights in the evening. The next day was sunny, and I was back to nearly 100 % charge on everything at the end of the day.
How I installed on Lady Cindy
Lady Cindy’s starter battery was installed under the starboard side saloon berth/seat. The two leisure batteries midship under the floor in the galley area. When I rewired the boat, chargers, loads, inverter, and isolation switches were all moved to a locker under the port side map table. This was my start position for the installation.
The lithium batteries and their charger were installed under the port side saloon seat/berth. And charger wired through to the leisure batteries. As all loads were on a single terminal under the map table, it was a simple job to switch the terminal input from the lead leisure battery bank to the new lithium bank. You may have a more untidy setup with lots of loads directly onto your leisure batteries. In this case, put a new terminal near these batteries, move all loads to the new terminal, and then a heavy gauge wire from there to the new lithium battery bank.
Negative terminals of all loads should go to the negative post of the lithium bank. The negative side of all banks and all charging sources should be connected together by a heavy gauge wire.
Although the original design idea was to be able to install Lithium alongside lead and the battery to battery charger allows that, there is another plus from that charger in that it protects the lithium from any excessive voltages that a charge source such as an alternator may generate. This is a useful function even when the lead battery reaches the end of its life and is removed. Any new batteries in the future will be lithium and added to the existing lithium bank.