I've got a 12V 2.4Ah lead acid battery which I plan to connect a water pump to. I've looked at various pumps, but the one I'm most interested in draws 2.2A.
I'm not so interested in how long the pump can run, as it only will need to run for about 5 - 10 minutes/day. So, I'm assuming the battery is plenty for that. The battery will be charged via solar cell panels.
However, I'm more concerned about the discharge rate. I've read that lead acid battery not should be discharged too quickly, as this might result in overheating the battery (and cause damage to it).
How do I figure out what a safe maximum discharge rate is for a 12V lead acid battery?
Answer
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.
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