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Author Topic: Thinking about batteries - terminology  (Read 1786 times)
charlieb
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« on: January 23, 2015, 02:25:06 PM »

I'm building a tiny off grid place and am considering a basic battery system (prob for a few watts of lighting overnight, and maybe music.  Nothing else initially).    I've done quite a lot of work on utility-scale energy storage, and the two metrics that matter there are Capacity (in W) and Energy (in Wh).  (Along with discharge cycles, etc, but the basic energy/power is all in MWh and MWs).     On those terms I know exactly what I need, but leisure batteries never seem to be rated in Watts or Watt hours.  Is there a simple way to (approximately) link energy and power needs (or inputs, if I get a small PV)  to AmpereHours?
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Countrypaul
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« Reply #1 on: January 23, 2015, 02:45:08 PM »

The main formula is simple P = V * I - that is power (Watts) = Volts x Amps.

The energy is then simple E = Power (Watts) x Time (Seconds)

If your battery is given as AmpHours then multiply by the battery voltage and 3600 to get Joules (Watt seconds), or divide the Amphours tmes volts by 1000 to get kWh.

Paul
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charlieb
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« Reply #2 on: January 23, 2015, 03:40:26 PM »

Perfect.  THanks Paul.   So, a 12V x 65Ah mobility scooter / golf buggy battery has an energy capacity of 0.78 kWh.  (12x65/1000).  Seems surprisingly little , considering how long they say they can drive for.   Although I suppose it's also a tenth or so of average household daily consumption.  (It would certainly be enough for me I think.)     Or am I missing a zero somewhere?   
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Blodders
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« Reply #3 on: January 23, 2015, 04:11:40 PM »

Ah, but rather than the "headline" capacity, you need to know the capacity based on only discharging to a certain level (and a certain rate) to ensure longevity.

I'll let others with more experience guide you further, my battery knowledge is from maintaining and installing 40' shipping container sized UPS systems
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Billy
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« Reply #4 on: January 23, 2015, 04:40:03 PM »

No that's fine and dandy.   fingers crossed!

I try to only discharge to 15% of my rated capacity if I can help it.  Discharging to 50% is ok but your cycle life is bad.  It might be ok for a cheap battery for experimenting as it won't cost you a lot when it dies but to replace mine will be in excess of 2k so I want to do what I can for them.   Grin
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charlieb
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« Reply #5 on: January 23, 2015, 04:48:59 PM »

Got it. Thanks Billy.  Obvious next question is how do I tell what the discharge level is?  (I can feel myself getting into complicated territory already!  That's largely why I began looking at golfcart, etc, batteries; because they come with standard mains chargers.  For now I'm not even going to think about what I'd need to link it to a PV panel).
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RIT
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« Reply #6 on: January 23, 2015, 05:15:13 PM »

Got it. Thanks Billy.  Obvious next question is how do I tell what the discharge level is?  (I can feel myself getting into complicated territory already!  That's largely why I began looking at golfcart, etc, batteries; because they come with standard mains chargers.  For now I'm not even going to think about what I'd need to link it to a PV panel).

The normal way is to monitor the voltage of the battery, as a battery is discharged its voltage will drop. If you are lucky the maker of your battery will publish some details you can work from.

As for charging the batteries, Navitron has a range of charge controller's, so they are a good place to start. They are not the cheapest devices, but they are fit for purpose as they will handle things like overcharging and battery conditioning.

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