Lead Acid battery in mobile equipment?

[mc]

I've been working on some equipment, and for one sensor I need it remotely/internally powered to avoid cables, so I've been considering various battery options.

Looking at the various options, sealed lead acid appears to be the best option.
Power requirements are 9-24V at 20mA for 12hours max, so I only need around 250mAh, but aiming for at least 500mAh to allow a good safety margin, as it could be used in subzero conditions so need to allow for reduced capacity.

Cost wise I can get a sealed lead acid 12V 1.2Ah battery for under £10.
NiMh the cheapest I costed works out at least twice that to get enough volts. Lithium gets even more expensive, and that's before you cost in charging. Lead acid just seems to be the cheapest/easiest option, unless I've missed something?

So, is it acceptable to use a lead acid battery in what will be a reasonably well sealed container (it's not going to be perfectly sealed, but will have minimal ventilation as it needs to withstand rain), along with a PCB and sensor assembly?
I can design the enclosure so things are seperated, but there is always going to be some passages between areas.

[saturation]

Except for low power density, size and weight, SLA is well established as a reliable power source, and easy to charge, maintains output in freezing temperatures and is robust.  If you are designing a sensor station to be left in the field for days at a time, the extra bulk also helps stabilizes the whole package.  A lot depends on how these units should be deployed, are they going to be delivered by vehicle or backpacked by humans?  For humans, anything to decrease weight is a plus.  Li with removable separately charged packs is a better option to design very small deployment packages.

[codeboy2k]

On my previous house, the driveway gate controller used (2) small 12V SLA batteries in an outdoor NEMA4X box mounted by the gate.
The gate controller and gate motors were 24V DC. The batteries were 7500mAh I think, or 6500mAh I can't remember now

The batteries were trickle charged by a solar panel.  They lasted 10 years or more, and the originals were still working when I moved out.

SLA batteries are definitely reliable workhorses, as long as they are not allowed to drain, and trickle charged properly. The charge controller is very important.

[SeanB]

SLA is cheap, rugged and simple to use and charge. You can go for the 1.2Ah battery you saw, but better to use a 7Ah unit if possible, as they are both a lot more reliable and are very common when it comes to a charger ( off the shelf are available) and to availability, as well many IP rated cases are available that have a built in shelf for them As they generally do not outgas you can have them in the same box as the electronics, just place it at the bottom, and keep it with the terminals and the cell covers upright for best operation. If possible use sleeved spade connections, and place a plastic sheet over the terminals so that any dropped metal parts do not create a short.

[mc]

Thanks for the replies.

I don't know why I never thought of using a small lead acid battery before now. It's just that I happened to be pricing them for something else when I realised they were available in such small sizes, and are pretty cheap.
Weight isn't that major an issue, at worst the equipment will have to be carried a kilometer or so, but that will be a rare occurence.
The 1.2Ah size works out perfect for this, as it'll be going into a custom machined housing

Charging isn't too big a concern, as that will be done back at base, and I've already got a suitable charger.

[daedalus]

Another option would be PP3 sized 9v batteries (250-300mAh a battery). two should hit your runtime requirements, and you can get a 2 ni-mh pp3s with a mains charger for around £10 on ebay.

[Rick Law]

...
So, is it acceptable to use a lead acid battery in what will be a reasonably well sealed container (it's not going to be perfectly sealed, but will have minimal ventilation as it needs to withstand rain), along with a PCB and sensor assembly?
I can design the enclosure so things are seperated, but there is always going to be some passages between areas.

Almost all the wheelchairs in the USA uses SLA.  Some are 6V and some 12V.  Almost all cars in the USA has an SLA.

Some wheelchairs have electronics they carry to control steering, most cars have electronics from radio to air conditioning.

So, it is just about as tried-and-trusted a method as you can find.

[mikeselectricstuff]

Many years ago I had a cassette walkman (aiwa I think) that used a very small SLA battery - it was a fairly thin rectangular profile, from memory something like 10x5mm - maybe this was more efficient than NiCad at the time. 

[SeanB]

More due to being able to make a flat low profile battery, as this was before the development of prismatic nicad cells, and well before pouched lithium cells. That the cell was flat was more important than the low energy density or the mass.

[mc]

daedalus, the PP3 are too low a voltage. When I say 9V, it really means 9V, and NiMh nominal voltage for a PP3 is 8.4V.  Once the sensor voltage drops below 9V, it's outwith it's guaranteed operational zone, and combined with cold conditions it's likely to fail giving me lots of hassle.
I could pair them up in a 2S2P configuration, but then it's more expensive than a comparable size SLA.

Rick, I'm well aware how much SLA batteries are used in transport of many sizes, but there they're normally well vented and not enclosed with electronics, which was my main concern. I've never knowingly came across any smaller enclosed equipment that has contained a SLA, so had wrongly assumed it wasn't good practice.

[NiHaoMike]

Almost all the wheelchairs in the USA uses SLA.  Some are 6V and some 12V.  Almost all cars in the USA has an SLA.

Almost all of the car batteries I have seen are just regular lead acid, not the sealed kind.

[mc]

Alot of sealed/maintenance free car batteries are just regular lead acid batteries with a big sticker over the top so you can't easily remove the caps. Some have the caps glued shut (I deal with them on a regular basis, with AGM batteries being my current headache)

With modern charging systems, any battery that needs topped up is most likely starting to fail anyway. Plus with modern capacitive discharge testers, you never need into them to test the acid, so being sealed isn't an issue.

[Rick Law]

Rick, I'm well aware how much SLA batteries are used in transport of many sizes, but there they're normally well vented and not enclosed with electronics, which was my main concern....

Ah, I see...  I did not discern the enclosed with electronics part.

On that note, I have been driving a car with the car battery enclosed in the trunk with other electronic like the CD changer all in one corner of the trunk.  An old car, but I have driven it for over a decade with no leak, and no other problem.

[Paul Moir]

Damage to lead acid batteries accelerates under 50% charge, so as long as you don't mind taking the hit on having some extra capacity, it can be the way to go.

Examples of extremely common SLA applications with electronics nearby is emergency exit signs (rather complex charger these days to make the batteries last a lot longer) and security systems.  It's just a non-issue.

[peter.mitchell]

I'm with SeanB on this, 7Ah SLA. It's pretty much the goto. if thats too big, there are 3.5~Ah ones, but they are likely the same price just due to being less common. I wouldn't go too low, if you start discharging them moderately they won't last.

[SeanB]

Common and I will be buying 144 of them soon, UPS is due for a battery change, and it uses 144 7Ah units in 2 banks of seriously high voltage and current capability, not to mention bloody heavy as well. Good thing it is on wheels, along with the server rack next to it. Just have to move everything else out of the server room though to get the floor to change the racks out in, not going to carry further than the front of the UPS. About $15 each in bulk, and i get $1 from the scrapyard for them.

[codeboy2k]

I'm with SeanB on this, 7Ah SLA. It's pretty much the goto. if thats too big, there are 3.5~Ah ones, but they are likely the same price just due to being less common. I wouldn't go too low, if you start discharging them moderately they won't last.

that's probably why my cheapo UPS at home keeps killing it's batteries.. they are 1.5Ah and I guess they just get drained down during a power fail that they are damaged... I have had to replace them twice, but my gate uses 7AH batteries and they seemed to last forever.

[SeanB]

I got tired of buying the 14Ah ones in my UPS, so replaced with a slightly used 40Ah car battery. Not the best to discharge it to cut off if power fails, but is is the same price as a 14Ah unit, and has a lot more capacity, plus it can be refilled with water if needed. added a capacitor in parallel to help with the vpower demands of the UPS, around 19000uF of capacitors on a veroboard ( only used for the holes, not that the traces can handle the current, I added a few flyleads that end in terminals) to help as the battery charge goes down. Lasts 45 minutes now when power goes, enough time to shut down gracefully.

[mc]

I've ordered up one of the 1.2Ah to try. Under normal use, it should only ever be drained about 25%, and should fit nicely in the planned enclosure. It's only about £8, which does mean it's nearly 4 times more expensive per Ah than the 7Ah batteries I've ordered to power other things, but it's just not practical to use a 7Ah for this application.


Rick, BMW by any chance?

SeanB, try getting a semi-traction battery. They're designed for cyclical use, but can still be charged using normal methods (i.e. vehicle alternator/standard charger). They are a bit more expensive than a standard battery, but will outlast them in cyclical applications. I deal with some vehicle systems that use them, and they last 2-3 times longer than a standard battery in the same application. They usually get totally flattened every few months when drivers forget to shutdown vehicles properly, yet 20min running after a jump start and they've got enough life in them to work again, and by the end of shift are back to normal charge levels.

[SeanB]

Mc, the battery was free, I had a scrap casing to trade in for it so kept it, as the fault was in the wiring that caused the battery to go flat, and a new battery was the quickest way to get running temporarily until i could do the wiring properly ( always in the middle of a shift or just at end of day it seems) and thus I charged it, left it around the shop for a week, then put it in the UPS. Will look at the scrapyard for some UPS batteries, as they often come there, nice 200Ah units that may work, just have to buy a half doxen, test then take the best ones. Still looking for a fork lift battery though, which will do about 10 hours of runtime on the UPS.

[Rick Law]

Rick, BMW by any chance?

Yeah, very old by now.  Still runs well but surely not like when I took it out of the lot with single digit on the odometer.  That was at a time when I had a different life...

Rick