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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?

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    \$\begingroup\$ I agree with explanation about the charge/discharge rates. Bur I think the problem is that the battery is too small to suply that pump. Remember that start current may be 3 times higher than the nominal. \$\endgroup\$
    – Bruno Y
    Commented Oct 21, 2015 at 17:38
  • \$\begingroup\$ I have a 12 volt 9 amp hour battery pack and I use it mostly for charging my phones and a light and a radio but I have used it to run my 2.7 amp water pump from time to time. I noticed it doesn't go down but maybe halfway. After a 15 min shower the battery bank go down maybe from 13.6v to 12.8v I have been living on batteries for the past 5 years. In most times you want to get something that's at least double or triple the amount of amps you going to be pulling from it. As per personal experience I always try for at least double the amps needed to the amp hours. Say I need it 2 amps I would lo \$\endgroup\$
    – user102136
    Commented Mar 2, 2016 at 0:41

3 Answers 3

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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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    \$\begingroup\$ Thanks! Not that I doubt your knowledge (I've accepted your answer), but how do you know this? Is this a well known rule-of-thumb which you can pick up from any "teaching book", or is it something you've learned through experience? \$\endgroup\$
    – sbrattla
    Commented Sep 28, 2014 at 18:44
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    \$\begingroup\$ This is just a rule-of-thumb so please take it with a grain of salt! However it proved very useful to me through the years. It is based on my experience and knowledge of the market, though I have observed some batteries and professional chargers manufacturers using this "C" terminology and setting similar limits when defining a slow/standard/fast rate. \$\endgroup\$
    – jose.angel.jimenez
    Commented Sep 29, 2014 at 7:14
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    \$\begingroup\$ Most deep cycle lead-acid batteries charge at 0.2 to 0.3 C . \$\endgroup\$
    – Nick Alexeev
    Commented Mar 2, 2016 at 5:53
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    \$\begingroup\$ This rule of thumb is problematic as a 12V lead-acid battery is actually 6x2V cells in series. If a 2V cell of a particular size was able to be charged at, say 0.5A, six of them in series (six times the capacity) should also be charged at 0.5A. Voltage and power will need to be higher but the current should be identical. Rules of thumb can be handy but it pays to understand why they apply and when they don't. \$\endgroup\$
    – Richard Thomas
    Commented Jun 26, 2016 at 16:02
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    \$\begingroup\$ @richard-thomas You are right. As any rule of thumb, you are entirely responsible for knowing the underlying physics involved. However, the much less than 1C rule for charging 12V lead-acid batteries is perfectly adequate and according to the recommendation of most manufacturers. Should to want to stay on the safe side, you can limit the charge rate to 0.1C or 0.2C. \$\endgroup\$
    – jose.angel.jimenez
    Commented Jun 27, 2016 at 16:50
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Ideally the manufacturer supplies the discharge rates on the battery datasheet.

A quick point: You mention you have a 12 V 2.4 A SLA (sealed lead acid) battery, but batteries are rated in amp-hours not amperes. Therefore I suspect you have a 12 V 2.4 Ah battery.

Now that we have that out of the way, a 12 V 2.5 Ah SLA battery from Power Sonic, as an example (a company that has datasheets for their batteries) shows several discharge rates that may be of interest:

  • Nominal Capacities:
    • 125 mA discharge rate = 20 hours (2.5 Ah)
    • 220 mA discharge rate = 10 hours (2.2 Ah)
    • 400 mA discharge rate = 5 hours (2 Ah)
    • 1.5 A discharge rate = 1 hour (1.5 Ah)
    • 4.5 A discharge rate = 15 minutes (1.13 Ah)
  • Max Discharge Current (7 Min.) = 7.5 A
  • Max Short-Duration Discharge Current (10 Sec.) = 25.0 A

This means you should expect, at a discharge rate of 2.2 A, that the battery would have a nominal capacity (down to 9 V) between 1.13 Ah and 1.5 Ah, giving you between 15 minutes and 1 hour runtime.

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    \$\begingroup\$ Your'e absolutely right about the Ah vs. A. My mistake - thanks for correcting! And also thank's for the examples above! \$\endgroup\$
    – sbrattla
    Commented Sep 24, 2014 at 5:35
  • \$\begingroup\$ He said that the pump he is interested in draws 2.2A, not the battery! \$\endgroup\$
    – Leon Heller
    Commented Sep 24, 2014 at 8:02
  • \$\begingroup\$ @Leon Oops you're right. I misread. I'll amend that. \$\endgroup\$
    – JYelton
    Commented Sep 24, 2014 at 8:07
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Jose's answer states that the discharge rate isn't related to chemistry. However, this is not correct. It can vary up to a factor of 1000 depending on chemistry.

Different batteries chemistries have different properties.

Beyond the chemistry, this is also related to the battery design itself. Size of the electrodes, the thickness of electrode coatings, electrolyte so it can also vastly vary upon this.

Some are designed for a lower self discharge rate, some for higher energy density or higher instant power output.

Larger electrode with thinner coating will have a higher discharge rate, while the opposite will lead to higher energy density.

The best is to check at the manufacturer datasheet if it is available.

Here is also a table with common values. enter image description here

Concerning specifically on lead-acid, there are also several types, but two are most common, the car starter battery and the stationary battery.

Because of its construction, a starter battery is only suitable for short loads with high current, which most commonly take place when starting an engine of a car, truck … The main characteristic of a starter battery is that they have big, thin, flat plates. Starter batteries are not suitable for cyclic use (continuous charging & discharging) A starter battery is relatively cheap. Source

With your pump, make sure to use a stationary battery, since you are below 1C do that is totally fine.

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