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​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! ​

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  • \$\begingroup\$ There are relatively cheap plastic containers for deep-cycle batteries that provide the necessary shelter/ventilation. They are typically used to power trolling motors on fishing boats, among other things. Check your local sporting goods store. \$\endgroup\$ – Dave Tweed Jun 24 '18 at 2:14
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    \$\begingroup\$ Deep cycle is designed to survive hundred of cycles of deep discharge. Any gel cell, can survive a dozen deep cycles. What you should do is to /reduce the light to 25% when the battery voltage hits 11.5V, then totally turn it off at 10V. Any lead acid battery is destroyed by being left for just 1 day fully discharged. \$\endgroup\$ – Henry Crun Jun 24 '18 at 8:51
  • \$\begingroup\$ Why not LiFePO4 packs like those used in electric bikes? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 24 '18 at 9:00
  • \$\begingroup\$ When I bought an ebike battery I got the bullet type with the screw connector, as it is a great small portable power pack: 36V, 17Ah. Unfortunately expensive, but good if you have an ebike already. \$\endgroup\$ – Henry Crun Jun 24 '18 at 9:17
  • \$\begingroup\$ Dave - Thanks, I'll look into those battery boxes. Henry/Ignacio - Good call. I'll look into some alternatives to deep-cycle batteries. Would something like this work? amazon.com/dp/B00KC39BE6/?tag=stackoverflow17-20 Thanks again everyone! \$\endgroup\$ – JakeZ Jun 25 '18 at 3:15
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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.


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%.


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.

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  • \$\begingroup\$ Henry - This is incredibly helpful. The Rayovac 9v's initially looked like a good idea, but, something seemed a bit off about their cost and ah specs. I actually started looking at D batteries - and your mapping out their benefits makes a great deal of sense. They are surprisingly cheap too! If I go that route, would you recommend the use of a boost converter? (Something akin to this: a.co/cOaIRzf) \$\endgroup\$ – JakeZ Jun 25 '18 at 3:24
  • \$\begingroup\$ no, you series the batteries to get a high voltage, and use a buck (step down) convertor. So if you had 36 batteries in series, for 54V in, using a convertor like this: aliexpress.com/item/…. This is nice, you put them all inside an 1 1/2" tube 2.5m long, that holds the light. Too easy. \$\endgroup\$ – Henry Crun Jun 25 '18 at 8:33
  • \$\begingroup\$ You could provide some solar charging with either a step-up from a 5V panel, or just put 10-15 of these little guys in series. They will only put 60mA, but thats ok to extend the alkaline life, alkalines work recharging with low current. aliexpress.com/item/… . The charging only gets economic at >10 full sun days usage however \$\endgroup\$ – Henry Crun Jun 25 '18 at 17:21
  • \$\begingroup\$ important that batteries are replaced as a full set of new , identical cells. \$\endgroup\$ – Henry Crun Jun 25 '18 at 17:23

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