Stark Power Lithium Battery Experience

[Cpt Pat]

I've been using the Stark Power 125 AH battery in my sailboat for about a year as a house battery. I have an outboard engine so I don't need a starting battery. After about a year in service and 50 cycles, I manually tested the cell balance of the battery (note: testing cell balance is practically impossible on other brands that use many cylindrical cells). The cells were balanced to with +/- 10 millivolts, which is about as close to perfect as can be achieved. Capacity actually increased slightly after a year.

I have an external low-voltage and high-voltage cutoff circuit that disconnects the battery at 12.7 volts during discharge and 13.8 volts during charge, so I am intentionally not using the bottom and top 20% of the battery capacity. My usable capacity is 72 AH -- my very conservative design focuses on a trade-off of battery longevity over maximum capacity. I am charging at a maximum of 20 amps when on shore power, and less than 10 amps on solar power. Between sails, I store the battery by isolating it from the bank with a circuit breaker and leaving it in storage at a terminal voltage of 13.1 to 13.2 volts open-circuit voltage. The battery is never exposed to ambient temperatures below 10 C or above 35 C, so my operating conditions are very close to optimal.

I still have 330 AH (165 AH usable) of lead-acid AGMs that switch in after the lithium battery reaches 12.7 volts, but the AGMs now mostly sit unused at a full charge during my sails. I'll be gradually replacing the AGMs with lithium batteries.

[Ostinato]

How do you switch in the AGMs - manually?

[Cpt Pat]

Originally Posted by Ostinato

How do you switch in the AGMs - manually?

It's the other way around in my case: the AGMs are always connected to the load bus and chargers. The charger (Mastervolt 40 amp) floats the AGMs at the dock. The Lithium battery is connected to the bus when the AGM terminal voltage drops below 13.1 volts. I use a relay with 50 amp contacts. The relay connects the lithium to the bus, as in the above, at 13.1 volts. It disconnects the lithium when the bus voltage drops below 12.7 volts, reconnects if the bus rises above 12.7 volts, and (during charge) disconnects the lithium from the bus at a bus voltage of 13.8 volts. I use a combination of a Victron BMV-702 Battery Monitor and an APO3 (APO3 technical information - APRS World, LLC) to control the relay. All this is automatic. Note that these switching circuits monitor the bus voltage - not the lithium terminal voltage. The lithium battery has a "last ditch" disconnect built in that operates at 10.5 volts on the lithium battery terminals (this failsafe switch should never operate).

I chose this configuration because:

1) The correct charging regime for lead-acids is completely incompatible with lithium batteries. Yes, you can use that regime, but count on your lithium battery only lasting 1/3rd the expected lifetime due to the overcharging and floating abuse it will receive.
2) This was the least complex configuration I could devise that permits the use of a single charger to charge BOTH the lead acids, the lithium, AND to allow charging by my solar charge controller configured for lead acid batteries. The alternative is two battery chargers and two solar charge controllers.

Naturally, once I replace all my lead acids with lithiums, this complexity can go away. But I still have years left on my 330 AH of Lifeline batteries.

Lithium batteries should spend as little time as possible at 100% SOC. After a sail, I disconnect my lithium from the bus and, provided it has at least 40% SOC, leave it that way until the next sail. I only charge the battery (and then to only 80% SOC) just prior to beginning a sail. The hardest change in mindset pertaining to lithium batteries is that they do not need (and prefer not to be) stored at 100% SOC, nor do they ever need to be recharged to 100% SOC. They have no sulfation issues. You can set whatever "fully charged" threshold you want. For longest life, and reasonable utility, I chose 80% SOC.

When disconnecting shore power, without charge current from the solar array, the lead acids voltage quickly drops to 13.1 volts, and the relay is activated placing the lithium battery on the bus. The lead acids just coast along, neither charging or discharging at 13.1 volts. When the bus is discharged to 12.7 volts, the lead acids are right at their fully charged resting voltage, and the lithium is switched off, placing all the load on the lead acids.

The solar array will do its thing, trying to raise the bus voltage to 14.4 volts, and the lithium will come back on the bus at 12.7 volts to participate in the recharge. The lead acids will recharge beginning at 12.8 volts, while the lithium battery doesn't receive any real recharge current until the bus is above 13.4 volts, with recharge really happening in earnest at 13.6 volts. Once the bus voltage rises to 13.8 volts, the lithium battery is at its desired 80% SOC, and the relay takes it off the bus. The bus voltage then rises to 14.4 volts (as configured on the solar controller) so the lead acids can bask in their extended requirement for an absorption charge. End result: the lead acids get fully charged, while the lithium battery terminal voltage stays within the 20% to 80% "long life" zone.