[Power] Testing A Battery

[Nicholas]

On Sat, May 30, 2015 at 06:17:52PM -0400, Sora Edwards-Thro wrote:
> Hello all,
> 
> My name's Sora Edwards-Thro, and I've been involved with Adam in Haiti
> for a while but this is the first time I've had a reason to post
> directly to this list myself. Thank you in advance for your input.
> 
> I am working with a school in Lascahobas, Haiti where a large solar
> charging system was installed for the XOs back in 2011
> (https://ict4dviewsfromthefield.wordpress.com/2011/09/17/new-video-worlds-largest-school-solar-laptop-charging-olpc-deployment-lascahobas-haiti/)
> 
> However, the system has not been used for a few years now, partly
> because it was implemented specifically to charge the laptops and when
> the support for the laptops ended people failed to see another use for
> it. (The system is DC, so it didn't come in hand for charging cell
> phones...)
> 
> Ideally, we would use the system to power a server (approx 750 watts
> per day) and 45 XO-1 laptops, although if 45 is not feasible we can
> also manage 15.
> 
> The original batteries from the system are still there. There are 6
> 6-volt batteries, wired into a 12V system.They hold 450 AH @ 20Hr, and
> they are "Rolls Battery Engineering" by Surrette, a Canadian company.
> The batteries have been there since 2011, so we suspect that they
> might not be working anymore. However, given that they have not
> actually been charged and discharged too many times, we're hopeful
> that we might not have to replace them.
> 
> The electrician that the school employs claims that they will probably
> work if they are given some "service" (add water and clean them up).
> He's going to be doing that over the weekend and letting me know the
> results. I guess I just want to be able to confirm what he says with
> my own testing - it's always good to get a second opinion.
> 
> Any tips on how to go about checking? I have a voltmeter handy, and
> since the chargers hooked up to them are all working (when the sun is
> shining) we should be able to do some longer-term tests where we
> actually charge the 45 XOs every day for two weeks or so to make sure
> everything is working. I'm just double-checking to see what else can
> be measured / inspected that I'm not experienced enough to think of.
> 
> Thank you,
> 
> Sora
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> Power at lists.laptop.org
> http://lists.laptop.org/listinfo/power

Sora,

Hello, the condensed version. Be careful.

Much about the charge state of a lead acid (Pb-A) battery can be found from voltage. It's good enough for a quick diagnosis anyway. 

>From memory:

A full 12v Pb-A system will rest at room temp (not your room temp but I guess close to 20C) at 12.8v. Half that for 6v cells when checking individually.
A 12v Pb-A will undergo a full charge at 14.4v
A 12v Pb-A battery can sit at 13.8v for a long time (while undergoing a charge)
There are State of Charge (SoC) tables for Pb-A batteries around the net.

However, please try and find some kind of SoC conversion chart for the temperatures you encounter. The voltages I've given are for mild climates and will not be the same as for the tropics.

Things to keep in mind. Even the best Pb-A batteries don't like full discharges. They start to sulphate at lower than full capacity. They will sit at a full charge also called a float charge (13.8v @ 20C) happily for years (temps taken into account).

Watching the voltage rise and fall is often a giveaway as to the health of the cells. You will likely find some better "6volters" than others. If the voltage rises too quickly compared to others of a similar SoC, it will almost certainly have a reduced capacity. Matching batteries (the 6v batteries in the 12v batteries) of similar capacity will be beneficial. Match good 6v batteries to other 6v batteries of approximate capacity in parallel with the others. Some batteries may be duds. Take notes, number the batteries etc..

A useful forum on the net for useful battery knowledge is/was fieldlines.com

Hope this helps,
Nicholas