Ideal Energy/Battery Source for a Battery Powered Lamppost

Question

​Hello everyone, Relatively new to electrical engineering (my background is more in coding), so, thank you ahead of time for all your help!

Project:

I was tasked with creating an outdoor lamppost for an event at a campground (IE: no immediate electricity source). At first, it seemed quite simple, but, looking at the necessary requirements, it seems to be more complex than I thought. ​

Requirements:

I need to find a way to power a high-powered LED light bulb (ideally 12W equivalent) for 10 hours a day (for 3-4 nights - for a total of 30-40 hours) without immediate access to an electrical outlet. So, if I were to use my currently provided 12V, 12W bulb, it would be around 120 watt hours a day.

This battery source needs to be able to be outdoors, and face rain/weather. There may be a tarp, tent to cover it.

I may have access to a generator to recharge my battery source at another location if need be.

​I currently have a bright DC 12V light bulb that would be ideal for this project, but, I could also use a AC light bulb if need be.

Potential Solutions:

Connected 6V Lantern Batteries:

Connecting multiple lantern batteries together, and swapping them out on a daily basis. ​Walmart has a set of Rayovac Heavy-Duty Lantern Batteries for around $2.50 each. Each one of these batteries has around 5 ah (not the highest quality, but, they are cheap!)

Pros:

• Connecting these batteries in parallel and series would give me the 20 ah, and 12v specs needed.
• It would come out to around $15 a day.
• Would be easy to put in a large water-proof container and connect to the lamppost

Cons:

• Bit wasteful - I'd burn through around 20 of these batteries, and would have to recycle/dispose of them
• Sealed? - Unsure if I could put them in a waterproof container. Do they need venting?
• Mid-range cost: Would come out to around $50-60 due to the number of batteries needed

12V Deep-Cycle 20Ah+ Battery:

A simple 20ah+ deep-cycle battery would provide me the DC power needed on a daily basis. I would then detach it during the day, and bring it to the generator to charge.

Pro:

• Cost - Relatively cheap, ​at around $50, this battery option would be affordable
• Set it and Forget It - No need to connect multiple batteries together. This would just be an easy wiring job.

Con:

• Venting - Unsure if I could place this in a simple waterproof container outside. Does it need to vent? Or, would a AGM deep-cycle battery be ok in a vent-free container?
• Recharging - Unlike the lantern batteries, I would need to bring this to a generator on a daily basis to recharge. Not the end of the world though.

Ideas​:

I am vacillating between both options. Ideally I would like to go with the deep-cycle battery, but I am quite concerned with the venting problem. I don't believe the 6V batteries need to vent, so they could be placed in a simple container.

Question(s):

• Does one of these solutions stand out more to you as the better fit?
• Are there other battery solutions that you think would work better?
• Do I need to use a deep-cycle battery, or would something like this battery work?

Again, thank you ahead of time for any guidance you could provide! ​

Answer 1

Cons of lead-acid are that they self discharge, and once they go flat are totally destroyed. In practice, the batteries will be dead next year, so factor that in. Or use a medium sized lead-acid battery, and at the end of the job keep it for your car! (BTW there is now a lead-crystal battery that claims to have long cycle life, and survive discharge. Don't know if it is true)

I am assuming that it is summer when you want to do this. Put a solar panel on the battery. It doesn't need to meet 100% - A 10W solar panel will greatly extend the time between charges, even a small 5W panel will help a lot.

Alkalines have 3 cons:

• only give maximum capacity at fairly low currents - you would need to use lots all at once.
• end of life voltage is 0.9V, so you probably need a switching regulator.
• good performance warm, but very poor in the cold.

Pros are:

• readily available and cheap
• long shelf life > 5 years (-3%/year)

Note that non-alkalines ("Heavy Duty") require even lower currents to give rated capacity. They are not useful or economic for this job.

I will consider D-size alkalines, as I know the data for them. They are 15Ah, but achieve full capacity at 100mA discharge capacity, and significantly less at 250mA, and 1/2 at 500mA. They have total energy of 75kJ/cell@100mA.

If you wanted a 12V 1A lamp, you will use 432 kJ/10hrs.

You need

• 6 cells/10hrs to meet the energy capacity
• 80 cells for 100mA for best life
• 40 cells at 200mA for reduced life

So 40 alkaline D cells would do the whole job, probably giving 5 days without recharging, cheapies appear to be about $1 in bulk packs. Make it 36 since they probably come in dozens.

You can slide them into a 2.2m x 1.5" aluminium tube that holds the light up, making 54V as 36x1 or 27V as 18x2, and use a switchmode to step the voltage down to run the leds.

Alkalines are quite rechargeable as long as you only do a shallow discharge (which you would have), so you can still put a small solar panel (5-10W) on to recharge them. With the long shelf life, it will still be working OK next year.

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You could probably hugely reduce the required capacity with a PIR sensor, and run the leds at 1/4 power, until movement is detected, then turn them up to 100%.

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Other rechargables, to run for only 1 day:

• Li 18650's are approx 38kJ (3.6v,3Ah). So you would need 12cells ~ $42
• D size NiMH at 11Ah,1.2V = 47kJ, or 10cells ~$65 (if the specs are true)

So alkaline D's at 5 days for$36 look pretty good.