[RE-wrenches] 24V charging with 60 cell modules

Fri Jun 7 06:35:27 PDT 2013

Eric, 
You may not be aware but some (not sure, perhaps all) MPPT controllers will switch to simple PWM mode if there is insufficient differential voltage to operate the MPPT circuit.

To answer Mac's question, no, there is no "good" method to use a 60 cell module to charge a 24 volt bank, especially if he is referring to charging flooded batteries used in deep cycle service. Temperature coefficient losses play a role here since you are depending on power output up to Voc. If a battery is kept in float service, it may work. Or, the example David Katz outlined. If Mac has options, go for a higher voltage module.

Larry Crutcher

On Jun 4, 2013, at 1:31 PM, Eric.Bentsen at schneider-electric.com wrote:

Hi Allan, 

The C-series behaves in the same way as if you were to connect the module directly to the battery, 
then watch a DC volt meter and disconnect/reconnect when the bulk voltage is reached to "hold" the battery 
at that level. It will hold at the bulk level for 1 hour, then transition to float. 
If left connected without disconnecting, the voltage would eventually climb to Voc (approx 36VDC). So, while it is not 
going to be as efficient as the MPPT controller in the bulk/absorption stages, once the PWM controller transitions to float, 
it will operate right about the sweet spot (Vmp). The question was making a single 24V module work with a 24V battery. 
 PWM is the only way to do it, as the MPPT requires a higher PV voltage due to the use of a buck transformer. 
 I will certainly pass on any recommendations for improvement to our engineering team, and 
I appreciate the feedback. 

Eric Bentsen 

_____________________________________________________________________________________ 

Eric Bentsen  |   Schneider Electric   |  Solar Business  |   UNITED STATES  |   Technical Support Representative 
Phone: +(650) 351-8237 ext. 001#  |   
Email: eric.bentsen at schneider-electric.com  |   Site: www.schneider-electric.com/solar  |   Address: 250 South Vasco Rd., Livermore, CA 94551 

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From:	Allan Sindelar <allan at positiveenergysolar.com>
To:	RE-wrenches <re-wrenches at lists.re-wrenches.org>
Date:	06/04/2013 10:35 AM
Subject:	Re: [RE-wrenches] 24V charging with 60 cell modules
Sent by:	re-wrenches-bounces at lists.re-wrenches.org

Eric,
Please explain further, as what you wrote makes no sense to me. Mac didn't indicate whether he wanted to charge a flooded battery (~29.2V) or sealed (~28.4V). The C-series of PWM controllers has a fixed two-hour absorption, which it only counts down once the bulk voltage is reached. If the bulk voltage setpoint is above the MPP of the module, as it is in this case, how can it ever reach float, as you suggest?

And further, this is spec-sheet calculation. In all but the coldest winter weather, the Vmp of the module will be lower still as the module temperature rises above 25ºC, and is likely to be even below the float voltage in hot weather. And in the winter, if the battery is also cold, it will need a higher voltage still to get fully charged.

So I'm with Bob-O on this one. You can't charge a 24V battery with a 20V module. Please tell us how I'm wrong.
Thanks, Allan

P.S. - And while I'm on a virtual roll, please tell your engineering folks to build a charge controller with a higher voltage input window. I don't mean 600 volts at $1,500 or whatever it is; just higher than 150V. On a 48V system, we're stuck with series strings of three modules if we use the lowest-cost-per-watt 60-cell modules. That means that systems have to be designed with arrays in ~750-watt increments. Give us another 50-100V or so of headroom, so we can design with strings in multiples of 3, 4, or 5. Outback, are you listening too? The Midnite Classic does this for us... Thanks.

Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com

On 6/4/2013 10:43 AM, Eric.Bentsen at schneider-electric.com wrote: 

Hi Mac, 

A PWM charge controller, such as the C35, will work in this application, because it will 
act like a switch that connects the panel to the battery. While the bulk voltage will likely be slightly 
higher than Vmp, once the controller transitions to float, which is approx 27V for most battery types, 
it should allow the module to operate very close to Vmp. 
Typically, MPPT controllers require a higher Vmp than the target charge voltage. 

Eric Bentsen
_____________________________________________________________________________________ 

Eric Bentsen  |   Schneider Electric   |  Solar Business  |   UNITED STATES  |   Technical Support Representative 
Phone: +(650) 351-8237 ext. 001#  |   
Email: eric.bentsen at schneider-electric.com  |   Site: www.schneider-electric.com/solar  |   Address: 250 South Vasco Rd., Livermore, CA 94551 

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From:	Mac Lewis <maclewis1 at gmail.com>
To:	RE-wrenches <re-wrenches at lists.re-wrenches.org>
Date:	06/04/2013 08:05 AM
Subject:	[RE-wrenches] 24V charging with 60 cell modules
Sent by:	re-wrenches-bounces at lists.re-wrenches.org

Hello wrenches, 

Is there a good method to charge 24V battery bank with single 60 cell modules?  Ideally, I'd like to charge a 24 V bank with a Kyocera 245GX-LFB.  However, at NOTC the Vmp is 26.8V.  Any good way to do this? 

Thanks in advance.
-- 

Mac Lewis
"Yo solo sé que no sé nada." -Sócrates

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