Battery for modest loads?

[sidpost]

I am looking for a load that runs 18W~27W. How do I convert this to battery amp hours and what are my better options for batteries that aren't super expensive but, hopefully, aren't maintenance intensive.

I'm wondering about the cheap lead-acid batteries I see on Amazon which are sealed but, I wonder if they will last over time and provide enough reserve power for overcast days during winter. I should note I live in East Texas.

Would an AGM car battery work in this application? I would consider Marine batteries but, they rarely last more than one year in my experience. I will most likely use a Midnite Solar Brat but, I can be swayed in a different direction as long as the cost doesn't become excessive.

TIA,
Sid

 

[MPRanger]

• https://learnmetrics.com/watt-hours-to-amp-hours/.
• I never recommend lead acid batteries for anything except low temperature solutions.
• AGM and Lead Acid are long-term money wasted if you have a long term power need. LiFePO4 presently its the most cost effective long term solution available.

 

[MisterSandals]

load that runs 18W~27W. How do I convert this to battery amp hours

Assuming your battery is 12.8V nominal...

18W / 12.8V = 1.4A
How long you running the load? 5 hours?
1.4A x 5h = 7Ah

(27W / 12.8V) x 5h = 10.54Ah

 

[DThames]

Sealed lead acid work but after 3 or 4 years will often show signs of getting week. I would favor something like these LiFePo4 batteries. You should not charge them when they are below freezing, so be aware of that. MisterSandals shows 10.5 amp hours needed for 18w, 12v for 5 hours. A 50 amp hour battery would give you about 5 days with no sun. Double the size if you plan on using lead acid as they don't do as well (less cycle life) if you discharge more than about 50%.

 

[brewmatic]

Assuming your battery is 12.8V nominal...

18W / 12.8V = 1.4A
How long you running the load? 5 hours?
1.4A x 5h = 7Ah

(27W / 12.8V) x 5h = 10.54Ah

And if you are going to use a Lead Acif battery you should multiply calculated Ah by 2 since they shouldn't be discarded below 50% if you want to keep them in a good shape.

 

[sidpost]

LiFePo4 batteries would be nice but, man they are expensive. 20aH x 2 for $350 is pretty spendy though they do appear to have a built-in BMS.

I looked at some on Alibaba but, man shipping is terribly expensive right now.

Would a 27DC or 29DC marine lead-acid battery work very well? I can get a 29DC for ~$90 with 122aH but, I'm not sure how well they would work. With my 24hr load (camera with radio/antenna) that would give me a ~60aH working capacity assuming worst-case power consumption.

 

[Short_Shot]

How long do you need this to last for? A year? Two?

 

[sidpost]

How long do you need this to last for? A year? Two?

I expect to have it in service for ~2 years. Battery technology is evolving pretty fast right now so, I suspect in a year or two, battery options will be much better and more cost-effective. So, paying a large premium for something today that has marginally better performance or lifespan doesn't seem warranted.

 

[Short_Shot]

I expect to have it in service for ~2 years. Battery technology is evolving pretty fast right now so, I suspect in a year or two, battery options will be much better and more cost-effective. So, paying a large premium for something today that has marginally better performance or lifespan doesn't seem warranted.

Lithium isn't marginally better. It's 5 to 10x the lifespan when capacity and charge is managed comparably (which is to say appropriately for the type) and you can pay for what you need for the most part instead of buying twice the battery. There are other concerns with lead battery behavior but you're not operating at a level which makes them an issue.

If you only need two years because you really only need 2 years then get lead acid.

If you're planning to use this device longer than that assuming you'll have a better battery in two years for the same price and will then replace the lead battery then lithium will still be cheaper to have purchased up front.

Always waiting for the next big improvement means you'll never buy anything worth keeping. Lead acid will still be around in two years and it won't have improved, and lithium might come down in price by then but then you're still buying twice and you're still subject to the lead acid battery failing fairly quickly if you're using it to its limits daily.

Just food for thought.

Lead acid isn't inherently 'bad' and when properly sized will give you what you need for less up front cost.

 

[sidpost]

The large lead-acid batteries we have used for years were/are a pretty good value. One of them died when I was gone and solar power was low. Other than that, I think we are at about 8 years with those batteries.

These batteries won't see as gentle a service life so, I'm not expecting 8 years from them. The low initial purchase price is attractive too as I need charge controllers and panels. In 2 years, I will be in a better position to buy Lithium batteries or whatever the new hotness is.

 

[DThames]

4 or 8 of these maybe....and a BMS. 4 will get you 12v, 25ah, 8 would get you 12v 50ah.