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 <[email protected]>
Date: Wed, January 13, 2010 5:47 pm
To: RE-wrenches <[email protected]>
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 wishesHughHi 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 <[email protected]> 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
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
This email and its attachments have been scanned by iConnection E-Mail Firewall for viruses, spam, and malicious content.
The information transmitted in this email is intended only for the entity or person to which it is addressed and may contain confidential/privileged material. If you received this in error, please contact the sender and delete the material from any computer. Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is strictly prohibited. %^^%
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________ List sponsored by Home Power magazine List Address: [email protected] Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
