[Power] [support-gang] solar power design for 5-to-10 XO-1 laptops in Haiti (or elsewhere!)

[Richard A. Smith]

On 07/23/2013 09:22 PM, George Hunt wrote:

> I'm concerned about the battery you're planning to use.  The small UPS
> batteries are usually 9AHr -- or said differently, can nominally store
> 108 watt hours.  Each XO1 battery requires about 30 Watthrs.

+1 and the 10 XO peak draw at 12V will be 14 Amps with a rough average 
of 8 to 9 Amps.  For a 9 or 10 Ah battery thats a C/1 discharge rate.

That will pretty much destroy that battery.

> I am imagining that a "charger squid/bank" might include the MPPT
> controller, and the small battery to limit the voltage applied directly
> to the XO's (with the help of the MPPT controller, and it's DC-DC
> converter). The power from the solar panels will pass through the
> controller, go through a dc-dc down conversion, for the most part bypass
> the lead acid battery, and charge the XO's.

If you think of the battery as disposable and don't rely on it to 
provide backup power then a direct setup will probably get you by.  You 
should think of the system as not really having a battery.

You have to have some sort of battery though because the controller will 
require the battery voltage to operate.  I've not seen one yet that will 
work from just the solar power input.  The UPS battery will work fine 
for that but you need to only charge the XO's when you have good sun and 
you will want some sort of voltage monitor on the battery so you can 
tell when you are in charge vs discharge.  If you are discharging the 
battery then you need to disconnect some of the XO's.

If you don't have the funds to buy a real deep cycle solar battery then 
try to find a battery out of a heavy truck.  Truck batteries have to 
provide power for various power setups when the engine is off thus they 
are designed with deeper discharge that what you would get in say a car 
battery.  Some place that services heavy trucks may have some used ones 
they can can sell cheap.

Since you may not have any of the capacity information for a used 
battery I came up with a metric using a large list of truck batteries 
and got a conservative measurement of at least 3Ah/kg with batteries 
15kg or heavier coming in a 3.5-4 Ah/kg.  I created it for the Ethiopia 
project but we ended up finding a source for solar rated batteries in 
country and didn't need it.

You need to try and find a battery in the 15kg range.

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child