[RE-wrenches] discharging Rolls batteries

Wed Jan 13 14:47:17 PST 2010

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 
><http://www.rollsbattery.com>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>http://www.scoraigwind.co.uk
>
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-- 
Hugh Piggott

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