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

Tue Jun 4 11:46:03 PDT 2013

Hi Allan,

If I can jump in here: using a 60-cell module on a 24V battery is similar to using two (in series) of the old 30-cell "self-regulating" modules that were available way back in the 80's. While it was found that those modules were not nearly as effective at charging 12V batteries as 36-cell modules (or 33-cell or 35-cell which were also available at that time), they did in fact charge batteries up to full voltage. In fact it was found that they could actually overcharge batteries if there were no loads on the system drawing energy from the battery bank, so they were not really "self-regulating" after all, and that is the main reason why they were discontinued. They still needed to have a charge controller in most systems, so it wasn't cost effective to use 30 cells.

What happens is that as the voltage rises above Vmp, the module will operate past the "knee" in the voltage curve. As the voltage moves higher, the output amperage of the module will (rather quickly) decline, until it approaches Voc where there is zero current. This was the concept behind the "self-regulation" idea. However, as you point out, the voltage curve of a module will vary depending on temperature. A cold 30-cell module might overcharge a 12V battery in the winter, but take a lot longer to fully charge a 12V battery in hot weather. Even in hot weather though, a 60-cell module should still have a Voc above 30V, so some charging will still happen, albeit at a greatly reduced current.

This does NOT mean that systems should be designed and installed with single 60-cell module charging a 24V battery, unless the load is so small relative to the module wattage that greatly reduced PV output is acceptable. Two 135W 36-cell modules (or a single 72-cell module) on a PWM controller would be much more charge-effective, at only slightly more cost.

Brian Teitelbaum
AEE Solar

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan Sindelar
Sent: Tuesday, June 04, 2013 10:35 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] 24V charging with 60 cell modules

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<mailto: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<http://www.positiveenergysolar.com/>

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

Mac Lewis <maclewis1 at gmail.com><mailto:maclewis1 at gmail.com>

To:

RE-wrenches <re-wrenches at lists.re-wrenches.org><mailto: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<mailto: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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