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Topic: Converting watts to amps (Read 389 times) previous topic - next topic

tjones9163

Hello, I am trying to go over the math to see how long I can run appliances with my 2000w pure-sine wave inverter and a deep-cycle 12v battery.

Is this correct the way I am going about it?


Let us say I want to run my TV and Xbox which combined use around 250w for 5 hours from an inverter.
Watt-hours= Watts x Hours= 250w x 5hours =1250wh
Account for inverter effiencey lets say 85% ------>1250/0.85 =1470wh

Since watts = Amps x Volts, divide the watt-hours by the voltage of the battery to get amp-hours of battery.
Amp-hours (at 12v) = watt-hours/12v = 1470/12= 122.5amp-hours

and let us say i got a deep-cycle battery that is 130amp-hours, I would be able to theoretically run the appliances for 5 hours?(I later plan on implementing arduino)

DVDdoug

Your calculations look correct but there are a couple more unknowns... 

When you get to the end of the amp-hour life, what's the voltage?   (I think the standard is 70 or 80%.)

How low can the "12V" go and still operate the inverter at its rated voltage?

MarkT

#2
Sep 07, 2019, 07:16 pm Last Edit: Sep 07, 2019, 07:18 pm by MarkT
Firstly derate any battery capacity by a factor of two or so - no battery retains its as-new capacity for very long, expect it to degrade over time.

The capacity of most batteries is measured at the 10 hour or 20 hour rate of discharge.  At a 5 hour rate the
capacity will be somewhat less (batteries are very imperfect devices, that's how the chemistry works).

So the answer is no, you'll be dissappointed.  Choose 200Ah perhaps.

And another bit of bad news, lead acid batteries don't make very good deep-cycle batteries, but they are
the cheapest option.  Their faults include that they must be regularly charged when not in use - you have
to remember to do this or they'll be ruined (they drop in capacity and in power handling a _lot_).  They need to be immediately recharged after discharge, not left for days discharged.  They also last much better if never
discharged very much - another reason to over-size them for your calculated load.

Vehicle batteries are an ideal use for lead-acid - always recharged after discharge, frequently recharged to full...

batteryuniversity.com is a great resource.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

tjones9163

Firstly derate any battery capacity by a factor of two or so - no battery retains its as-new capacity for very long, expect it to degrade over time.

The capacity of most batteries is measured at the 10 hour or 20 hour rate of discharge.  At a 5 hour rate the
capacity will be somewhat less (batteries are very imperfect devices, that's how the chemistry works).

So the answer is no, you'll be dissappointed.  Choose 200Ah perhaps.

And another bit of bad news, lead acid batteries don't make very good deep-cycle batteries, but they are
the cheapest option.  Their faults include that they must be regularly charged when not in use - you have
to remember to do this or they'll be ruined (they drop in capacity and in power handling a _lot_).  They need to be immediately recharged after discharge, not left for days discharged.  They also last much better if never
discharged very much - another reason to over-size them for your calculated load.

Vehicle batteries are an ideal use for lead-acid - always recharged after discharge, frequently recharged to full...

batteryuniversity.com is a great resource.
Thank you for your reply and information. I could be wrong but i thought that i read somewhere that deep-cycle batteries are better for a constant load and car batteries are better for higher amperage for a short amount of time, like starting a car?

MarkT

Yes that's true, deep cycle are not rated for heavy currents, car batteries are.  That doesn't change the fact that the lead-acid system is not the ideal chemistry for deep-cycle, but it is the cheapest.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

jackrae

i thought that i read somewhere that deep-cycle batteries are better for a constant load and car batteries are better for higher amperage for a short amount of time, like starting a car?
If that's what you read then the information source whilst factually correct didn't really give the correct message.

What it should have said was something along the lines of  "car batteries, which are designed to provide short duration high currents (engine cranking), are not really suitable for providing long duration constant loads"

They will provide a constant load, just don't expect too much out of them with respect to supply duration, especially if you are discharging at greater than C/20.   Their duration performance declines quite rapidly as the load current increases.   See https://en.wikipedia.org/wiki/Peukert%27s_law for a fuller explanation

slipstick

Deep cycle lead-acid batteries, sometimes called Leisure batteries, do exist. They have different construction from car starter batteries and are often used as caravan and motorhome habitation batteries and for things like golf carts where no high cranking currents are required. They are much better at handling sustained relatively low currents.

But they do still need to be treated fairly carefully and preferably not discharged by more than about 75% capacity before recharging. They often specify a number of charge/discharge cycles and these are typically rated at 50% discharge. Greater discharge levels will reduce the number of cycles available.

Steve

MarkT

Or put another way lead-acid is very poor at deep-discharge.  I use LiFePO4, but they are much more expensive (although since they are expected to survive thousands of cycles, perhaps they are much cheaper in the long run).
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

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