Re: Alternator charging a Lithium bank

Dean Gillies

Hi Dominique,
Scott does have a good solution.

Regarding the Cyrix disconnect, I'm not convinced you will see that problem in real use. It is very unusual for a lead acid battery (gel, agm or flooded) to rest for long at 13.2V+ volts. As soon as you apply any load at all to the battery, the voltage will drop to 12.7-12.8V.  I suspect that simply connecting your battery to the Cyrix will cause the voltage to drop quickly to less than 13.2V (per battery) and therefore the Cyrix unit will disconnect. 

Maybe you can test this?

Personally I like the idea of retaining some Lead Acid capacity for a few reasons, but mainly as a fail safe. The system I am working on at the moment has lead acid and Lithium connected directly together. Ie no Cyrix unit in the system. As you correctly noted this does cause the Li battery to top-off the Pb at the end of a charge cycle, and the Pb does almost nothing on a discharge cycle until the Li reaches around 5% SOC.  If you think about these two characteristics for a bit, you will realise that they are very good things. Li batteries don't like to remain at high SOC for too long (they won't), and the Pb batteries provide an extra buffer at low SOC to support the Lithium batteries as they near low Voltage disconnect.

When operating the system in real life, I would try to maintain the Li system above 20% SOC, this means that the Pb batteries are almost always close to full charge.

If/when the Li batteries disconnect (for any of the reasons that can happen) the Pb batteries continue to power the boat systems with no outage.

The alternator disconnect issue is solved of course, because the Pb is always in the system.

When leaving the boat or simply being alongside for long periods, the Li system can be dropped to 50% SOC and simply turned off, leaving the Pb system to operate in the usual float mode. 

Finally, as with your system, weight of the Pb batteries helps with the port side list of the 54. 

Best regards, Dean

SV Stella




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