I have an appliance that needs to run for around 10 hours, even if there's a power cut. I don't need a UPS, as I'll just connect the appliance to a battery if needed.
The appliance uses around 50w of electricity, and it happily runs off
12v. So, a 40Ah car battery seems at first glance like it will do, except it won't survive being discharged down to nothing too well.
The standard solution is a leisure battery, although these are very expensive. Could I use two standard car batteries (in parallel) instead? Are they okay with being discharged 50%?
An 80Ahr leisure battery should do it - you should discharge even a leisure battery below 50% if you want long life. A decent one will cost about £70 or so from Tanya. I like Hankook batteries with a 4 year warranty. Ideally a smart charger but a simple float charger at worst. Don’t cook it by leaving it on charge at too high a voltage too long.
Leisure batteries aren't all *that* expensive if you shop around. Just check that they really are leisure batteries, there's a scheme called the NCC scheme to rate them according to how many discharge and charge cycles they will manage. So NCC A is best, then NCC B and lowest grade is NCC C. Even a C battery will probably be better than a 'normal' starter battery.
How often do you expect to need this? Is it just during power cuts? If so I doubt you'll need anything much better than a C rated leisure battery, but go for some extra capacity, like about twice the capacity you think you want.
That is what a float charged lead acid battery will be at when you first switch over. Unless you look after your lead acid batteries properly they will be dead or near dead when you actually need them to work.
Leisure batteries are a lot more robust in this respect.
Two in series and a switched mode regulator to output a regulated 12v supply at the required current might be a better bet.
I recently bought a 'UPS' battery to use in a hybrid car, where it doesn't need to crank the engine but there is a constant parasitic drain:
formatting link
Supposedly much more robust than the official AGM battery, and roughly half the price. The official battery usually dies in 2-3 years, while somebody says they've been running the UPS battery for over a decade.
Most 12V equipment will happily work off a higher voltage, but I do agree the OP should confirm.
Most are actually semi-traction and are primarily batteries capable of starting engines. True NCC A rated batteries are made differently and are more rugged.
50W is a nominal 4A at 12V. There really is no need for the extra cost of 24V to 12V DC converter. At most you might need a LDO 12V regulator if there is a sensitivity to a voltage higher than 14V.
A standard size of Leisure battery is 100Ahr, anything above 80Ahr would be fine for the OP, as long as he can detect a voltage of 12.2V (depending on chemistry) and disconnect the load. If the voltage is allowed to go much lower, the battery will be irreparably damaged.
A figure I've heard in the past, is to shoot for 25% discharge. The allocation here seems more generous.
formatting link
Depth of Discharge Starter Battery Deep-Cycle Battery
100% 12–15 cycles 150–200 cycles
50% 100–120 cycles 400–500 cycles
30% 130–150 cycles 1,000 and more cycles
Table 3: Cycle performance of starter and deep-cycle batteries.
*******
Personally, I would not touch automotive batteries with a barge pole. They're s**te.
Get a leisure battery. Get one of a decent capacity for the job. You can see in the table, there is still a dd dependency. The leisure battery just has more plate to waste.
I've had starter batteries die in as little as two deep discharges. That is why my preference is not to use them as power sources. (Lithium might be better, but Lithium doesn't like the cold like lead acid does.)
Make sure you have a wheeled device of some sort, to carry your new purchase. Something to lift and move it. The transport is all part of the purchase plan.
You will also need to look up what a "deep discharge" actually is. Is it 12.8V settled, down to 12.0V under load ? Or some other figure. You will see various figures, like someone running a 12.8V battery down to 11V. Is that good for them ? More research required.
Battery voltage is temperature sensitive. You can mis-judge charge state and remaining capacity, by 50%, if you don't temperature compensate. Battery temperature is either "ambient temperature" or it is the electrode temperature on top of the battery casing. You decide. On vented cells, an acid density measurement device may make for a better measure of remaining capacity. Even the acid density device has a temperature compensation table on the back of the package.
A battery must sit for 24 hours, after a charge, to reach "true terminal voltage". Don't take measurements too soon after disconnecting the charger.
BatteryUniversity would probably recommend coulomb-counting as a preferred capacity method. But that does not include any measure of how battery capacity drops, from one usage cycle to the next. Laptop batteries use a formula for the chemistry, to automatically compensate for whichever charge cycle this is. Doing this sort of work by hand, would require careful bookkeeping.
*******
Batteries are a kind of religion.
You have to worship every battery pack you own. They need to be checked for charge level, like every three months.
Lead acid, some of them can be left on float. Ask the person you are buying it from, for "best practice" in this regard.
My next door neighbour did not worship the battery on his rechargeable mower, which is why he threw it out after only three years (pack was a built-in, but I'm pretty sure you could buy a replacement). He probably did not look at the State Of Charge, over the winter, and that's why it died.
If you buy a battery from an automotive store, they haven't a clue what constitutes good charging techniques or anything else. They do know that "boiling a battery", removes stratification on a flooded battery. If there is a brutal technique, a garage knows it.
That's why, for your leisure battery, you want a "full service" vendor who can provide advice as well as a very heavy purchase.
Some very large stationary batteries, they come with a datasheet on what the charge should be, what the float should be, and so on. Good charging devices, sometimes have settings you can adjust for best results. Battery construction (coil wound versus flat plate), makes a difference to the voltage settings to be used.
I would be really surprised, if you got anywhere near the table values for lifespan.
Rated capacity is at a 20hr long discharge. You get a bit less capacity at 10hrs. I wouldn't discharge a leisure type below 50% and expect it to last well.
You say nothing about the load: the first thing is always to look at every possible way to reduce energy consumption. Maybe you've done that already.
Lithium isn't short lived like lead acid, if you truly need this for many years you're better off paying the big extra for li.
The datasheet for a battery will have a "Duration of discharge vs. discharge current" curve.
Also a capacity for a 20 hour rate, one for a 10 hour rate etc. The rating of a battery, i.e. 80Ah is usually for the 20 hour rate, or even longer. 10 hour rate
formatting link
is a datasheet for a 28 Ah Panasonic SAL battery (chosen pretty much at random). Dig through that, and: it needs to be at the very least double, better triple the capacity. (Yeah: "wot e said"... ~ 90 Ah.)
Different datasheets available for different battery types: cycle use, trickle use, etc. Read, and then see how much the appliance not failing is worth paying for.
How many time do you expect your battery to do that before it's toast? Do you have the make and number at hand? Once you get to Trojan quality you might as well go lithium with an in built BMS on the basis of cost vs life-time.
Holding a battery at 14.4V for extended periods is not good. I'm sure your warranty details will have something to say.
The last NCC A Lead Acid battery I purchased had a warranty of 2 years.
No they don't. Lead Acid of a specific type has a well-defined charge / discharge characteristic. They will keep step.
It is very common to parallel batteries up, the issue is to ensure that they all have the same leas resistances. This article explains:
formatting link
The only issue is that some battery types should not be mixed AGM-Gel batteries tend to have a slightly higher open circuit voltage than flooded, and perhaps this is what you're thinking of?
Float is 13.8v. 14.4v is the voltage needed to reasonably quickly recharge a battery.
I would not recommend leaving a battery even on the 13.8v permanently, with little or no need. Better is a regime of bringing it back to full charge once every month or two, if unused.
'Float' is maybe a little less than 13.8v, my 'smart' chargers seem to settle down at 13.4 to 13.5 volts.
I have two 100Ah starter batteries and three 100Ah leisure batteries in my boat. They are permanently connected to a 'smart' charger which also manages the solar panels which provide the charging electricity. The starter battery just sits at the float voltage all day when not in use, obviously it goes off charge at night when there's no sun. The batteries have stayed in good condition for five years or more now. I recently had to really work the starter batteries very hard because of a problem with the engine and they performed admirably.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.