[RE-wrenches] Absolyte Battery Question

[Brian Teitelbaum]

Allen,

First a minor correction: Those 100-33 cells are rated at 1920Ah at the 20hr rate.

It’s interesting that their Sr App Engineer would say 9A per 100Ah (C/11 rate), since the operation manual and spec sheets state that the max charging current is “5% of nominal C/100 Amp-hour rating” at 25⁰C, or C/20. The 100-33 cells have a 100-hr rating of 2100Ah to a cell voltage of 1.75V, so the max charge rate should be 105A. However, that 5% rate is based on using a constant-voltage charging source at the 2.35V per cell voltage setting. I’m sure that you can charge at a higher rate if the battery voltage is lower (still partially discharged), and that is probably what the Engineer was thinking of.

You don’t have to push AGM batteries as hard as flooded batteries since you don’t need to stir up the electrolyte.

I don’t think that this is particularly relevant though, since any charge rate that replaces the used amp-hours, plus the self-discharge rate, will work fine. So really the more important question is “what is the rate of self-discharge for these batteries”.  The spec sheet states that the self-discharge rate is between 0.5 and 1.0% per week, but that’s with a new battery. What the rate is on your older batteries would not be easy to find out. The only way that I can think of would be to charge them up completely, and let them sit, with no charge or discharge, for a few weeks to a month, and measure the static voltage drop over that time. From that you can get an idea of what the lost percent of charge is, but I doubt that this is a practical experiment to do.

Ultimately, the issue is whether the charging sources can keep up with the discharge over time. Only observation and time will tell that. A good amp-hour meter will sure help.  If the batteries can’t be brought up to a full state of charge within a few days of a major discharge, you need to add more PV, reduce loads, or run a genny.

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: Wednesday, February 10, 2010 9:19 PM
To: RE-wrenches
Subject: [RE-wrenches] Absolyte Battery Question

Wrenches,
I am working extensive cleanup on a five year old 24V Outback off-grid system for a neighbor. The original installation work was pretty amateur. The BOS is standard PS2, with 2100 watts of tracked PV and a single VFX3524 and MX60 (which will need to be replaced with an FM80 to handle this much array). The batteries are GNB Absolyte II, the 100/33s with "1600 amp-hours, 8 hour rate" on each.

I believe the 8 hour rate multiplies by 1.23 to determine our more familiar 20 hour rate, so each set is rated 1968 amp-hours. There are two sets; apparently a crane was used and the stairs removed (and replaced) in order to get these bad boys settled in the basement. The batteries are used telecomm batteries, and the one string currently in operation appears to be in quite good condition, as it is holding up well under extended discharge due to cloudy weather and weak input during rewiring.

My questions: I can safely assume that when the rework is done PV input will be about 70A PV input in good sun. That's a C/28 charge rate on one string; C/56 if both are hooked up.
1. What is the minimum charge rate needed to properly charge these batteries? Flooded batteries like at least a C/20 or higher to overcome internal resistance and get the voltage well up, but I don't have much experience with these.
2. Put the question another way: given that the batteries are already in the basement, will the customer get better performance and longer life with one string or both connected in this particular situation?


Back story: I tried to get answers first from GNB. I asked for the minimum necessary charge rate and got this back: "I did discuss this with engineering, and they recommend 2.35 volts per cell as the charge rate."

So I replied: " I'm sorry to be a pain, but let's try again. A cell voltage is not a charge rate. That's a charge voltage. We already discussed this last week. I am asking for a minimum current - in amps, not volts - necessary for good battery care. Could you ask them again, please?"

And got back: "I spoke with Sr. App Engineer, and said 9 amps per 100 amp-hours."

OK, so if I use the C/20 rate, I need to charge at 177 amps. Double that if both strings are used. Right.

I need better advice, so it's time to request it here.

Thank you.
--

Allan Sindelar
Allan at positiveenergysolar.com<mailto:Allan at positiveenergysolar.com>
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com<http://www.positiveenergysolar.com/>
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