Environment: it will be in a weathertight enclosure outside: summer sun in Florida & winters in Maine.
Size: 12v, with a device constantly drawing 70ma.
Physical size & weight: not a concern.
Must last about 2 years before replacing.
Looking for lowest cost, type, and mah.
Thanks!

 

70mA per day at 730 days, that's a 1226Ah battery.... Really!!?

 

Smaller battery and solar panel, wind turbine, or water wheel to recharge it?

 

70mA per day at 730 days, that's a 1226Ah battery.... Really!!?

Thanks Doggydave, but I assume that is in an ideal environment. What's the multiplier to compensate for the harsh weather?
Thoughts on what type to withstand the environment at the lowest cost?

 

Your lowest cost is going to be a rechargeable battery, using a solar panel.

 

A large auto battery only goes up to about 100Ah so obviously the idea of using a battery that will provide over 1200 Ah for a two year period is not practical.
So I also suggest a rechargeable battery with a solar panel.

Allowing for say, 5 days of operation without sun, you would need 8.4Ah of battery capacity.
A 12V 10-20Ah deep-cycle type SLA or AGM (lead-acid) battery, such as used in wheel-chairs, etc., should do the job.
(They are much cheaper and easier to charge than Li ion batteries.)

For 4 hours a day average sun time, you would need a panel that could deliver at least 0.5A peak current to the battery in the winter.
A 12V 10-12W solar panel should likely be adequate and those are fairly small and reasonably priced.

 

Smaller battery and solar panel, wind turbine, or water wheel to recharge it?

I've had a motorcycle battery just barely start the motorcycle after a year in storage, quality is a critical factor though. Cheap ones may not stay up more than a month from new.

It has the simplest charging requirements if you supplement it with solar etc.

Unfortunately - if it gets dead flat, it will sulphate and that's game over.

 

I've had a motorcycle battery just barely start the motorcycle after a year in storage.............

Motorcycle or other vehicle batteries do not last long in deep discharge applications such as this.
Deep-cycle batteries, such as use in wheel chairs, scooters, etc. are much better.

But, as with all lead-acid batteries, they have a significant self-discharge rate and do not tolerate long periods of staying discharged.
Any lead-acid battery should be placed on trickle-charge if being stored for more than a month or so without charge.

 

Because of damage to my foot I can't drive at the moment so I've been going out to start and run the car for a while to keep the battery happy. I forgot for a long time The next time I checked the terminal voltage was 5.1V and I thought it had gone to the great charging station in the sky. I put it on a low charge for two days and it is back to life again and able to start the car. No doubt there is some loss of capacity but still it lives

 

Motorcycle or other vehicle batteries do not last long in deep discharge applications such as this.
Deep-cycle batteries, such as use in wheel chairs, scooters, etc. are much better.

But, as with all lead-acid batteries, they have a significant self-discharge rate and do not tolerate long periods of staying discharged.
Any lead-acid battery should be placed on trickle-charge if being stored for more than a month or so without charge.

"deep cycle"? - it was fully charged when I stored it, and (just about) started the motorcycle when I put it back to use a year later.

A good quality 2Ah SLA would probably do what the TS wants, but it'd last a lot longer with something like solar keeping it topped up.

What the TS wants is unlikely with primary cells - lead acid and a renewable charging source is probably the easiest way to make the best of a bad job.

 

"deep cycle"? - it was fully charged when I stored it, and (just about) started the motorcycle when I put it back to use a year later.

A "deep cycle" battery is designed to reliably withstand significant discharge and charge cycles which standard vehicle batteries don't to well.
They are used for marine and other charge-discharge uses such as RVs, power wheelchairs and scooters.
Here are some examples.

Leaving the battery a year without charging likely caused serious sulfation of the plates.

 

A "deep cycle" battery is designed to reliably withstand significant discharge and charge cycles which standard vehicle batteries don't to well.
They are used for marine and other charge-discharge uses such as RVs, power wheelchairs and scooters.
Here are some examples.

Leaving the battery a year without charging likely caused serious sulfation of the plates.

I got lucky with that one, but with turning the starter and normal charging from the generator, the battery was good as new after about a week.

They'll sulphate in less than a week if dead flat - but the TS can avoid that with a small solar panel.

Primary cells probably isn't going to happen. Nickel cells are unlikely to be reliable for long, safely recharging lithium isn't trivial - lead acid is the least bad option.

 

"deep cycle"? - it was fully charged when I stored it, and (just about) started the motorcycle when I put it back to use a year later.

A good quality 2Ah SLA would probably do what the TS wants, but it'd last a lot longer with something like solar keeping it topped up.

What the TS wants is unlikely with primary cells - lead acid and a renewable charging source is probably the easiest way to make the best of a bad job.

First, Thanks for all the feedback.
Note: My devices are not always outside, so I don't want to use a solar panel. I would hope this is possible as a stand alone system with a non-rechargable battery. What about something that can last a year and a half? I've seen some water timers that last for over 5 years that use lithium batteries, but probably don't use the same current draw.

 

A year and a half is only a little better than two years.
For example, one large 100Ah auto battery will last about 2 months at a 70mA draw.

So you either need a power source, such as a solar panel, or you need to significantly reduce your current draw.
Things like water timers likely only draw a few mA or less on the average.
What requires that so much current in your design?

 

A year and a half is only a little better than two years.
For example, one large 100Ah auto battery will last about 2 months at a 70mA draw.

So you either need a power source, such as a solar panel, or you need to significantly reduce your current draw.
Things like water timers likely only draw a few mA or less on the average.
What requires that so much current in your design?

It's a cellular remote switch. I'll try to find a different model.
Do you think SLA batteries leak in extreme summer heat?

 

It's a cellular remote switch. I'll try to find a different model.
Do you think SLA batteries leak in extreme summer heat?

They operate fine in a hot engine compartment on 100°F summer days, so heat should not be a particular problem with any lead-acid battery.

To reduce power can the power be only intermittently provided to the device (for example 1 second on and 10 seconds off).

 

They operate fine in a hot engine compartment on 100°F summer days, so heat should not be a particular problem with any lead-acid battery.

To reduce power can the power be only intermittently provided to the device (for example 1 second on and 10 seconds off).

Great idea, I'll look into that.
Also do SLA batteries have to be upright to prevent leaking?

 

do SLA batteries have to be upright to prevent leaking?

No, they can be mounted in any position.
They are sealed and can't leak in normal operation.
The one in my motorcycle is mounted on its side, for example.

 

First, Thanks for all the feedback.
Note: My devices are not always outside, so I don't want to use a solar panel. I would hope this is possible as a stand alone system with a non-rechargable battery. What about something that can last a year and a half? I've seen some water timers that last for over 5 years that use lithium batteries, but probably don't use the same current draw.

Why not provide a charging jack as well as a solar panel?

 

Why not provide a charging jack as well as a solar panel?

I need this to be as simple as possible for the installers. Power outlets and sun is not always available on site.