Time to charge battery

System · January 2008

Hello,

I'm new to the forum and I was hoping someone could answer my questions. I'm an industrial design student looking to build a personal power supply device for long term power outages. I'm looking at using a few ways to charge the device, solar panels being one of them.

First, how long would it take to charge a smaller sized deep cycle battery. I'm looking for someone to maybe give some physics equations that involve ways to calculate things such as this using Amps, Watts, volts, etc.

How long would it take to charge the battery of a laptop. My laptop battery is rated at 11.1v and 80WH. What does this mean and could a portable PV panel charge this?

This is more about batteries. Does anyone know what could be run off of a 44AH 12v deep cycle Optima battery? I'm trying to get an idea of what sort of home appliances might be able to run off of a battery like this. Thanks

Ben

mike95490 · January 2008

Re: Time to charge battery

IDstudent wrote: »

Does anyone know what could be run off of a 44AH 12v deep cycle Optima battery? I'm trying to get an idea of what sort of home appliances might be able to run off of a battery like this. Thanks
Ben

A small fan or a small light, for just a couple hours.

Here's the lowdown (my version, other folks - correct any glareing errors I make)

You only want to deplete your battery to 50% discharge. More than that usually starts to damage the battery, and shortens it's lifetime. So now you 44AH battery is a 22AH battery.

Take a 25w 12V Bulb (CFL, Halagon, Incandescent, LED) 25W is 25 watts. Light output (brightness/lumens) is what varys. (count on a modern laptop pulling 100W)

It draws 2.083 amps from your battery. After 1 hour, that's 2.1 (rounding up) AH.
So you could light that 25W light for about 10 hours. (21ah drain from your allowed 22ah)

Then your battery needs recharging, to prevent it from sulphating.
An average day gives 5 good solar hours of charging light. You need a panel that can produce slightly more than a full charge, in that 5 hour window.
battery charging losses are about 20%, [4.2ah] so you need to replenish with about 25.2 Amp hours.

You would think at first, a 5A panel would do this just fine, but actually, the battery adsorbs the last 20% of charge slowly, so you need to get 80% of your charge, into the battery within 4 hours, leaving that last hour to slow charge it. [6.25A panel]

And, your x Watt panel, unless it is aimed PERFECTLY for all 5 hours, will only give you about 75% of it's nameplate (STC) wattage [8.5A panel]

A panel's voltage is about 19V, and your battery recharges about 15V The charge controller will regulate the voltage, so you don't cook your battery, but this mis-match in voltages, will also reduce your panels effective wattage output. (another 7% or so of loss) [9A]

So my educated guess would be, that you need a [9A, 19V] 170W panel, to recharge the battery, after using a 25 light for 11 hours.

I've done some rounding, and it's always done worse case, because that's the way the world usually works.

Time to charge battery

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