[RE-wrenches] C60 setpoint question

[Brian Teitelbaum]

Allan,

Do you know what the daily Ah consumption is, or alternatively what the daily DOD is on the batteries?

830W of PV on a PWM controller should have a peak amperage of about 48A (830W ÷ 17Vmp = 48.82A). This is only about a C/18 charge rate, which is fairly low but should be workable as long as solar production keeps up with load demand. Of course, with old modules you may not see that many amps, especially in hot weather.

If they don't have an amp-hour meter installed, that should be an upgrade priority. Every off-grid system should have an amp-hour meter, especially a rental house.

Without some DOD battery info it's hard to tell exactly what's going on, but I think that as long as some energy is consumed each night I don't see any reason why you couldn't set the float voltage the same as the bulk/absorption voltage, at least in this case. The charging day is limited to hours of sunlight, and as long as you stay under the gassing voltage it should be fine. But as Eric Benson noted, the battery manufacturer should have the final word.

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: Thursday, August 01, 2013 1:20 PM
To: RE-wrenches
Subject: [RE-wrenches] C60 setpoint question

Wrenches,
Eden, our main off grid field tech, came across a situation about which I want to ask the group for advice, please.

The system: existing old/funky off grid 12V residential system, done by others over many years; this was our first visit. Owner out of state, currently rented, owner's complaint is that the old Heart 2800 EMS inverter is shutting down at times. Six four year old Concorde SunXtender PVX-3050Ts, so 900 rated amp-hours in three strings at 12V. Approximately 830 watts of a hodgepodge of older modules, but well below rated output at present, though no obvious failures. Trace C60 charge controller. Our assessment after a site visit is that the batteries have been chronically undercharged and are showing signs of premature sulfation.

With AGM batteries such as these, we have been encouraged (by Midnite tech support and others) to set really long absorption times - we have some systems with six-hour absorptions. The logic is that theese batteries can tolerate - indeed thrive on - staying at absorption voltage of 14.2 - 14.4 V (for 12 V nominal) indefinitely, and given the fickle nature of off grid PV charging, this maximizes the likelihood of their getting full most days. This logic makes total sense to me.

Most modern MPPT charge controllers have a programmable absorption time, and some have a done amps (a.k.a. float transition current), to allow them to transition to float if the charge current necessary to maintain absorption voltage drops below a set threshold (typically 1-3% of battery capacity). The C60 has neither, but rather has a nonadjustable two-hour timed absorption. This brings the question: is there any good reason that the float voltage on the C60 should not be set to the same voltage as the bulk/absorption voltage, such that they never drop into float? When sufficient input is available to get the batteries full, they are just allowed to stay in absorption mode indefinitely, slowly getting the energy necessary to reach 100%.

We have installed many Concorde AGM batteries, but never with an old C-series controller. This idea seems wacky, but the more I think about it, the more I can't see any problem with it. What do the rest of you advise?
Thanks, Allan
--

Allan Sindelar
Allan at positiveenergysolar.com<mailto:Allan at positiveenergysolar.com>
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc., a Certified B CorporationTM
3209 Richards Lane
Santa Fe, New Mexico 87507
505 424-1112 office 780-2738 cell
www.positiveenergysolar.com<http://www.positiveenergysolar.com/>
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