Maximum current draw from 12v sealed lead acid battery?

Question

I am currently building a "battery box", for camping, and kayaking, etc. It has a few different features on it, such as a 150W inverter, a 48W spotlight, 12V accessory outlet, and strobe lights. The inverter and the spotlight seem to draw quite a bit of power, 7 Amps and 3 Amps respectively. (correct me if that doesn't sound right.)

This will all be powered by an ExpertPower 12v 8ah sealed lead acid battery, however I do not know if it can handle the load that I will put on it. I tried looking around, but most people were using this battery for some sort of home security system.

I found a datasheet on the bottom of their website that I think would tell me that (http://www.expertpower.us/exp1280), but I cannot interpret it because of its complexity. If someone could also show me the process of calculating this, that would be great, as I am very eager to learn.

Answer 1

I think you will be disappointed with that battery.

Lead acid batteries are best on low rate discharge. Most these days are rated at 20hrs. That battery is rated 8Ah, so will deliver that capacity when discharged over a 20hr period, at 400mA. At higher currents, the capacity will be less. Here are a few lines taken from the discharge capacity table in the data sheet, for constant current discharge, down to a cell voltage of 1.75v (more of that later!)

current  period  capacity
  0.4A     20Hr    8.0Ah
  4.8A      1Hr    4.8Ah
 16.5A     10min   2.8Ah

so there's quite a capacity penalty to high rates of discharge.

A 150W inverter will take around 15A (assuming 85% efficiency) to deliver full power, 7A is only around half maximum load.

The lifetime of a lead acid battery, before it wears out, is strongly related to its depth of discharge. That battery rates 260 cycles at 100% DOD, ie to 1.75v. You can double that lifetime if you only discharge to 50%, and x5 if you go to 30%, that is, stop discharge at a higher voltage. Depending on how you want to use it, weight and capacity may be more important than cycle lifetime to you.

It's not clear whether you have a vehicle with you, or everything is carried in your kayak. If you have the capacity to carry more weight, then a cheap auto battery is by far the best bang for buck for the sort of currents you are talking about.

Both nickel and lithium chemistries stand up to high rates of discharge better than lead, and lithium in particular gives a much better power and energy per unit weight.

Answer 2

According to the data sheet, that battery can withstand quite high discharge currents. The Terminal Voltage (V) and Discharge Time curves go up to 3C, which for your battery is 24A*.

But you may be very disappointed with how long the battery lasts. Even at 8A, the battery will be flat after half an hour.

And be aware that lead-acid batteries don't like being left flat. Once run down, they should be recharged as soon as possible, or they may be permanently damaged.

*1C is a current numerically equal to the amp-hour rating of a battery. So for an 8Ah battery, 1C is 8A.