Safe discharge cut off point for 12v SLI calcium lead acid battery

An easy rule-of-thumb for determining the slow/intermediate/fast rates for charging/discharging a rechargeable chemical battery, mostly independent of the actual manufacturing technology: lead acid, NiCd, NiMH, Li...

• We will call C (unitless) to the numerical value of the capacity of our battery, measured in Ah (Ampere-hour).
  • In your question, the capacity of the battery is 2.4 Ah, hence, C=2.4 (unitless).
• The vast majority of the batteries in the market will safely charge/discharge at a rate of less than 1C Amperes.
  • In an ideal world (without losses), this would translate into a 1 hour charge/discharge process. In practice, the charging/discharging operation may require up to twice/half the time.
  • Without further information (datasheet), I would not charge/discharge any battery at a rate higher than 1C, for safety and endurance reasons.
  • In your question, less than 2.4 A would be a nice charge/discharge rate, as the manufacturer datasheet confirms.
• By applying a charge/discharge rate much less than 1C, you usually extend considerably the life of a chemical battery.
  • Rates << 1C are commonly known as "SLOW" rates: 0.5C, 0.2C, 0.1C...
• Charge/discharge rates higher than 1C are best avoided unless working with a properly known battery.
  • Rates >> 1C rates are commonly known as "FAST" rates: 2C, 3C...
  • In the past, batteries designed for rates >1C were usually marketed as "high current" batteries, because not all batteries were capable of sustaining such rates safely or without compromising its endurance.
  • Nowadays, most batteries can safely be used at rates >1C, up to the rating specified by the manufacturer. However, a reduction in the battery life is to be expected.
  • Forcing a battery to rates >5-10C involves serious risks.

Disclaimer: this is a rule-of-thumb, useful as an starting point when the datasheet is not available or when dealing with a no-brand/unknown battery.

You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&A meters, solar charge controller, & a power regulator).

Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate. Most often people use a generic table that shows V & SOC %. The problem with that data set is that it applies to a new/good battery and as a battery ages & deteriorates the internal battery resistance increases, charge capacity decreases, & charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off & power is being drawn from the battery.

If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off: 1. measure the battery V 2. measure the A draw of the system 3. To maintain the battery's current SOC, your power system must produce as much power as the system is consuming. Your program would calculate the optimal charge A & V to provide sufficient charge current to offset the power consumption rate. 4. To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).

Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery & maintain other electrical loads. Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).