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# Battery Life - 12V Inverter

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#### Berserk87

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If I have a 12V lead-acid battery, hooked up to a DC inverter, how do I calculate how long the battery will last?

In the scenario im thinking of, it would be a 12V 24Ah lead-acid battery, and a 400W inverter.

I know on the output it would be 120V @ 3.6Amps to get the 400W.

Are inverters of this power only intended to be used in a car while its running?(so the alternator is doing the work)

If you are very fortunate, the inverter will be 75-80% efficient (including losses in your cable if you minimize them). If the engine is not running, the battery voltage will only be 12V.

For 400W out, then you'll need 500-533W in. At 12V, that's almost 45 amperes. If your 24AH battery could be 100% efficient, that would be only 30 minutes.

However at 2C (twice the Ampere-Hour number) it probably will only give half of that time (e.g. 15 minutes) before either going flat or exploding.

It will last longer if you don't actually use the full 400W from the inverter. If the motor is running, the good news is the input voltage is about 13.8 volts, the current is only 39A, and if the charging system can keep up, everything is happy.

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ok, so i did have it properly(in my head) then.

if the adapter was 100%(impossible) then it would be 400W draw from the battery at 12V, which makes it 400/12 = 33Amp draw.

also, being that it wouldnt be a deep cycle battery, the battery would be toast pretty soon.

just to add something here, any idea what the maximum draw on a non deep cycle, lead acid, battery would be? if it was kept at ideal temps. and without shortening its life.

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No idea. Time to do some research. Maybe some of the manufacturers publish specs.

Your 24Ah battery will aos probably not be 24Ah when 40A is drawn.

I reckon the battery will last for less than 20miniutes.

just to add something here, any idea what the maximum draw on a non deep cycle, lead acid, battery would be? if it was kept at ideal temps. and without shortening its life.

About 15 seconds at 400A, (about 2Ah) which about what it takes to start a car. 99% of Car batteries are not "storage" batteries; they are "starting" batteries. They have many thin plates designed to produce Cold Cranking Amps. If repeatedly discharged beyond about 10% of their rated Ah capacity and then recharged (7Ah for a 70Ah battery), their plates do not reform properly, and they will have a very short life.

To power portable electronics and lighting, I use "deep cycle" type batteries. The only time I would use a "car" battery for this is if the car battery is so far gone that it is not cranking properly. Then, what have you got to lose?

Most inverters have a 'drop out' voltage, some are set at 11.5 volts. What that means is that the inverter will shut off when battery voltage reaches (drops) 11.5 volts.

Depends on the quality of the car battery. I have several old Sears Die hard batteries that are 8 - 10 years old and were severely abused during their life time and they still have good cranking power and cranking time. My dad buys the cheap low end Wal-Mart batteries and most of them dont even make it a year or two in good conditions.

As far as draining a battery there seems to be a unrealistic concern about the amps draw blowing them up around here. I have yet to ever have a Lead acid battery explode from either high amperage drains or fast recharging and I have done that to countless numbers of batteries.
Yes there are idealistic what if scenarios but then what if everything else happened too!

In hobby electronics 40+ amps is a big number. In the industrial electronics I typically work with 40 amps is not even worth worrying about. Use the proper size wire, fuses, connectors, dont over drain your battery and you will be fine.
And over all if you buy cheap junk you get what you payed for in the end!

The inverter was just an idea that i was curious about.

In reality im not going to be drawing any more than 2-3Amps.

...
In reality im not going to be drawing any more than 2-3Amps.
Its not how many Amps you draw; its how many Amp-Hours (Ah) you draw!

Its not how many Amps you draw; its how many Amp-Hours (Ah) you draw!

i thought if i had a 24Ah battery, i would be able to draw 1Amp for 24 hours

=S

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Yes, that is correct. However, if you draw 24Ah out of a 24Ah battery, you have used its entire capacity. That is ok if it is a battery rated for repeated deep-cycle discharge.

If it is not specifically rated for deep cycle use, it is a starting battery. Starting batteries are ruined by just one deep-cycle discharge. They will never regain their rated capacity after one or two deep discharges...

If it is not specifically rated for deep cycle use, it is a starting battery. Starting batteries are ruined by just one deep-cycle discharge. They will never regain their rated capacity after one or two deep discharges...

thats what i figured, but there was a couple people telling me that i could use normal 12 lead acid batterys, that arnt rated for deep cycle use as long as there kept at normal temps, and you dont draw to much.

the issue here being that deep cycle batterys cost a ton :\

...

the issue here being that deep cycle batterys cost a ton :\

That is because they are not made in the quantities that starting batteries are. They have more lead in them. Their plates are thicker.

It is ok to repeatedly draw out/put back about the top 15% of the capacity of a starting battery. After all, that is the use model in a car. A normal car battery is typically ~50 to 80Ah, so you could cycle ~9Ah in/out.

Most Sealed Lead Acid (SLA, VRLA, AGM) batteries are rated for deep discharge, but not typical flooded-cell car starting batteries.

blah.

whats the cheapest deep cycle battery then, and where can i get it?

If I have a 12V lead-acid battery, hooked up to a DC inverter, how do I calculate how long the battery will last?
Clip an ammeter on the battery lead.

just to add something here, any idea what the maximum draw on a non deep cycle, lead acid, battery would be? if it was kept at ideal temps. and without shortening its life.
Lead acid batteries actually are "fixed energy" devices for life span. The deeper you discharge them (in terms of A-hr), the sooner they go away. "Deep discharge" batteries are built to allow deeper cycling over their lives, but use still equals life span for these guys.

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whats the cheapest deep cycle battery then, and where can i get it?

For my travel trailer, the house battery on my boat, backup power for my ham station, I use 100Ah Sealed Lead Acid (AGM) batteries designed for UPS service. I happen to have a source of these that are removed from commercial service after they get two years old. They typically have a capacity of about 85Ah when I get them. I use them for about three years and discard them when their capacity goes below ~50Ah. To get three years, I constantly float them per the manufacturer's instructions (13.5V to 13.8V, depending on ambient temperature), other wise, just sitting around, they go dead in six months to a year, and cannot be recovered.

Check with local government or businesses that have standby power systems. For critical applications, they routinely remove from service batteries that still have a lot of useful life for a hobbyist.

BATT. BACKUP TIME = AH[capicity of batt.]*12[batt voltage]*N[no. of batt.]*0.8[power fuctor]*0.9[eficiancy of inverter]\ VA[ o\p load ]

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Your 24Ah battery is rated to provide only 2.4A for 10 hours or 1.2A for 20 hours.
With a load of 24A then it might last only a few minutes.

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