Electronic – Is it ok to use a car battery to power an inverter

batteriesinverter

I need a battery for low power inverter application but it seems that it's not easy for the following reason:

If i need 100 W output from 12v battery then i need to pull about 10A out of it(considering the losses). So if i need it to work for about 2 hours then that's 20Ah and considering the discharge rate of batteries then the final battery rate will be higher than 20Ah.

The problem is it's hard (for me) to get a battery with that amp rating with a good price and the only solution(as it seems) is to use a car battery since it has a high amperage capacity with a reasonable price.

Is a car battery is a good idea for this application?

Best Answer

Only you can decide if a car battery is a suitable solution for the application. Is it a workable solution from a technical standpoint? Yes it will work but there are factors that you do need to consider to determine suitability.

Car batteries are heavy and large size.
Lead acid car batteries are not usually sealed so there is danger of acid spill or escape.
Car batteries can source lots of current and if there is accidental shorting without use of adequate protection devices the massive current can do a lot of damage.
Charging lead acid car batteries leads to hydrogen gas production which can be dangerous if done indoors and/or in a confined space.
Connection of low current load wires to a car battery can be somewhat of a pain as regular battery terminals do not normally accommodate small wires.
You will have to evaluate the discharge rate versus available charging rate to determine if the down time duty cycle of the battery is too large for your application.
If usage of the battery capacity is done over a long time period there may be self discharge loss of overall capacity to consider.

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Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

Your hunch that batteries have a current limitation is correct. In general, it's hard to tell the current rating [A] from capacity [A·h]. You have to look it up in the datasheet. A lot depends on the design of the battery.

For example: coin cells with 500mAh capacity may have only 3mA max current.
Another (opposite) example: automotive starter battery with 40Ah capacity may have 500A max current.

Lead-acid batteries are interesting in this respect, because there are two distinct types.

Starter lead-acid batteries are designed specifically to deliver high peak current for a short period of time. Deep discharge, however, dramatically shortens the life of a starter battery. So, it's not suited for routine operation at high depths of discharge. Your typical starter battery in the automobile works at very shallow depth of discharge.
Deep cycle lead-acid batteries are designed (as name suggests) to discharge further. But they can not provide as much instantaneous current.
Here's an example datasheet for a deep cycle battery. Have a look at the nominal capacity on p.1. Notice that capacity depends on discharge current (i.e. the rate of discharge).

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

(Source.)

p.s. If you want to read-up, here's an excellent web site on batteries - Battery University.

Electronic – Extending the backup runtime of a UPS with an external car battery

Good quality (APC) UPS can be used like that. I have two units running at the moment. I use them for different purpose, they trickle charge my motorcycle battery and the battery I have left from the previous car. APC UPSes are very good storage chargers for Lead acid batteries (after all, that's one of their primary purposes) and I have a shop nearby giving them away for free.

The unit will run non-stop with any battery, and you can even swap batteries while running on them. The car battery might not be too happy though, they don't appreciate deep discharge, but if you're planning to drain it to the floor only once a year it won't make much difference.

Charging will be quite slow since the unit is set to the battery which comes with it. The battery inside 600 VA APC Back UPS is 7 Ah so the car battery will be charged with 700 mA current.

Best Answer

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Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

Electronic – Extending the backup runtime of a UPS with an external car battery

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