[RE-wrenches] discharging Rolls batteries

Thu Jan 14 11:15:25 PST 2010

Hi Dan, 

Good point. 

A quick chart on freezing temperatures vs SG; 

Specific Gravity 

(cor. to 80Â° F/26Â° C)Freezing Temp 
1.280 -92Â° F (-69Â° C) 
1.265-72.3Â° F (-57Â° C) 
1.250-62Â° F (-52Â° C) 
1.200-16Â° F (-27Â° C) 
1.150+5Â° F (-15Â° C) 
1.100+19Â° F (-7.2Â° C) 

Regards, 

Jamie

>>> <dan at foxfire-energy.com> 1/14/2010 10:56 AM >>>

More important than loss of capacity at low temps, (at least in my
world), is the fact that battery freeze points rise considerably -- with
a reduced State of charge... So you get a sort of double whammy... not
only do you get less capacity, but they freeze easier.

Dan Brown
President
Foxfire Energy Corp.
Renewable Energy Systems
(802)-483-2564
www.Foxfire-Energy.com
NABCEP #092907-44

-------- Original Message --------
Subject: Re: [RE-wrenches] discharging Rolls batteries
From: Hugh <hugh at scoraigwind.co.uk>
Date: Wed, January 13, 2010 5:47 pm
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>

#wmMessage blockquote, #wmMessage dl, #wmMessage ul, #wmMessage ol,
#wmMessage li { padding-top: 0 ; padding-bottom: 0 } 
Hi Jamie, 

Thanks for the very quick response! 

I note that the temp compensation slope for charging is more like a
curve.  It would be great to have a few more data points on this curve
to match it against the linear compensation programmed into a Tristar
controller. 

But I guess there is some leeway anyway?  My personal 48-V household
battery is rather large (2 x S530 in parallel) and the max charge rate
is low (around 20-30 amps).  Would that be a reason to set a higher
charging voltage maybe, or does it not make a difference? 

As far as the disconnect point, I now have one data point for 50%
discharge: 1.93V at C/20 and 25 degrees C.  Again it would be great to
have a whole family of curves to work from in this area.  Most of the
systems I work on have batteries that are sized for several days usage
rather than 20 hours.  I don't really like to see the LBCO setpoint as
low as 46.3 volts. 

Capacity is reduced at low temperatures.  Now does this mean that there
really is less energy in the battery, so that if I take out 50% of this
reduced capacity I will only end up with a 50% state of charge once it
has warmed, or is this just a performance hit that reduces the voltage
and makes the battery capacity appear to be less?  Will the battery
actually have less amphours to deliver, or will it just be delivering at
a lower voltage?  My point is to ask whether it is legitimate to push
the battery to a lower voltage in cold weather on the assumption that it
is not really running that low, but just sluggish with cold? 

I hope you don't mind all the questions.  I hope the others find the
answers interesting too! 

In an ideal world we could control our end-of-discharge based on
amphours measured, but I have no faith in the calibration of such
devices after a few partial charge swings up and down over period of
days.  So the reality is that we rely on inverter LBCO settings or rules
of thumb such as "never let the voltage drop below 47" to ensure that
the batteries do not get abused. 

Best wishes 

Hugh 

Hi Hugh & Wrenches,

Regarding the charge voltages you sited, you've got the gist.

As battery temperature drops, you need to increase the charge voltage
to properly charge and, conversely, as temperatures rise, voltage needs
to be decreased to reduce the risk of overcharging.

I believe the range we site to accommodate for temperature is (in C);

0-16C: 2.5VPC

17-27C: 2.4VPC

28-40C: 2.36VPC

These are your max bulk / absorb voltage settings.

Regarding a low voltage disconnect, there is a significant difference
between on load and at rest voltages at 50% SOC.

Below is a table of a cell being discharged at a known constant load
(C/20) over time at 25C;

SOC        VPC

100%       2.1

75%         2.01

50%         1.93

25%        1.84

0%          1.75

Please note, this is for a cell(s) under constant load and at 25C.  If
the load increased, you would see lower voltages more quickly, 
Moreover, once the load is removed, resting voltages will increase
dramatically, i.e. 1.75VPC on load will equal ~1.95VPC at rest.  If you
were using 2.0VPC (on load) as your cut off, you would only have been
able to discharge ~25% - which does not include capacity reduction due
to temps <25C.......and I know they are lower than that here!

Hope this is of some assistance.

Regards,

Jamie

James Surrette

Surrette Battery Co. Ltd
1 Station Rd.
Springhill, NS, CAN
B0M 1X0

Direct: 902.597.4027
Fax: 902.597.8447

>>> Hugh <hugh at scoraigwind.co.uk> 1/13/2010 2:26 PM >>>

Hi wrenches,

I have been selling and using a few sets of Rolls 4000 series S530

batteries lately.

I have downloaded the Solar Battery manual from www.rollsbattery.com

It's very interesting.  At 17 degrees C you need to charge at 14.4

volts.  At 16 degrees it's 15 volts.  Hmm, Ok I get the idea.  I am

certainly learning to compensate my temperatures this winter.  

I did a search in the document for the word 'discharge'.  I am

cutting off at just below nominal voltage and starting my generator.

I do not get much capacity.  A customer has suggested that a lot of

the discharge capacity is to be had at 11.5 volts (he has a 12v

system obviously).  Hmm.  Sounds a bit harsh.  But what voltage

should I be considering to be 50% discharged, and time to start the

genny?  And in a hard frost, the same voltage?

No mention of discharge in the solar battery manual.  Any ideas
anyone?

Please don't tell me to buy an amphour meter (or worse still to sell

them).  Those things are way too confusing to calibrate.   I can tell

you that I am not getting many amphours out of my batteries in these

low temperatures before the voltage per cell drops below 2.

What do I tell my customers about the end-of-discharge voltage for

sustainable cycling of their Rolls batteries?  And is it temp

compensated?

thanks

--

Hugh Piggott

Scoraig Wind Electric

Scotland

http://www.scoraigwind.co.uk

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Hugh Piggott

Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk 

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