The necessity of Charge Control

[SparWeb]

Hello,

I am now using a scavenged set of GNB Absolyte UPS batteries in my system.  They are huge: having a total 1600 AH capacity at 12 Volts.  And that's the 8 hour rating; the 20 hour rating is way more.  The windmill usually only putts out about 8 amps and gets to about 10 when it furls.  This is deliberate because it's very gusty here.

Since the windmill is small compared to the battery bank, I haven't been seeing the recommended Float charge voltage that the manufacturer recommends.  This isn't a big problem because the open-circuit voltage is telling me I can get up to about 90% charged anyway.

So do I realy need a charge controller?  I just bought a Xantrex C40 and now I wonder "Is it going to do anything?".  Several times this summer and fall I overcharged my Trojan T105's.  So I bought the C40 on the premise that you must not overcharge a set of VRLA batteries like my GNB's.  Is that a realistic concern when the charging current is something like C/200 ?

Will I even be able to maintain a nearly-full charge?!

Thanks!

[ghurd]

It depends on how the batteries are used.

Gonna use them every day?  Check the volts every day? Go on vacation for 2 weeks?

I have seen C/50 X 4H/D PVs cook wet batteries in short order.

A C40 won't hurt!

Not sure what you mean by "the open-circuit voltage is telling me I can get up to about 90% charged".

G-

[s4w2099]

If it was me I would play it safe,those batteries are a little treasure. If you install a charge controller you will not have to be monitoring them so often.

[DanG]

Remember the VOC needs to have settled for 24 hours after charging or discharge before you can calculate SOC with less than +/- 10% error, above 72 hours the calculated SOC can be assumed to be less than +/1 2.5%.

You face damage from self discharge below 50% capacity if you lose your float voltage and have no wind; I'd sure leap at setting up a solar maintainer now and then build your system around your VRLA's.

What must be adhered to is the temperature to voltage ratio, there is a temperature where any voltage input will cause gassing and voids on the plates that won't heal. Temperature compensated charging is highly recommended. Exide states Float voltage - 2.23 to 2.27 VPC at 77°F but the correction factor wasn't found on my quickee search. Lock the C40 into that float range with a temperature sensor and don't look back!

[SparWeb]

I have the operation & intallation manual for the batteries, now, thanks to the internet and a lot of patience (11 MB!)  Yes there's a correction factor in the appendix of the manual, but how do I turn that into a useful instruction to the C40?  It came with a temp sensor, but how do I know that the right correction will be applied?  I can't configure this factor.  Maybe that's a question better asked of Trace/Xantrex.

What's really got me squeamish, and I should have explained in my original posting, is that the C40 has "bulk" charge and a "float" charge settings.  As far as I can see in the battery manual, there is no "bulk" charge voltage specification.  It doesn't mention bulk charging at all.

Should I set the "bulk" and "float" knobs to the same voltage, so as not to overcharge these cells?  I've already discovered that equalization charging is clearly not recommended for sealed batteries.

+/- 10% on the SOC, eh?  Then I'm somewhere between 80% and 100% hehe.

The loads currently are pretty light, but I'm on the verge of trying electric water heat in the horses' water troughs (approx 1200Watts).  This is just within the inverter's limits and there may be some significant voltage drops in the wiring I have available.  However, I want to go ahead when I have the rest of the system set up, and that will be soon.  Maybe just from the "management" point of view, it's better to get the C40 in ASAP and then push on to heavy electrical loads.

Thanks, I needed to talk that out.  I feel better now.

<shrink's couch=OFF>

[scoraigwind]

We used to run a lot of systems without charge control in the old days with huge batteries from telephone exchanges and small turbines.  Even so most people would ask a neighbour to come around and turn on the lights if there was a windy period when they were away on holiday.  Nowadays some sort of charge control is the norm.

I don't understand why you guys are using C40s though.  Morningstar Tristar controllers are much better and cost the same (at least here in the UK) and you don't have to deal with Xantrex.  The tech guys there did not even know about the time lag in the C40.  If you get a gust of wind, the voltage will surge up and trip the inverter off.  The Tristar has no time lag.  It is easier to set up using dip switches.  You can plug your computer into it and do custom settings if you wish.  You can also access a lot of data including how many kWh of energy it has sent to dump.

The C40 was a great controller in the days of Trace but now it has been superceded.

[DanG]

Yeah, +/- 10% after 24 hours, the glass mat slowing electrolyte flow needs that extra time - with FLA three hours rest to calculate SOC is permissible.

Most VRLA batteries are a hybrid with semi-gelled electrolyte and glass mats.

With VRLA's the charge rate can be C4 or more using 'voltage limited' temperature compensated charging - the batteries will accept huge inrush of 30% capacity amperage but rapidly their chemistry tapers that off and then accept 5% input while the electrolyte gets redistributed and any minor gassing gets recombined within the AGM mats.

With float maintained VRLA there is sometimes a high-rate charge around 2.4V per cell thats only allowed after a deep discharge when its more important to get SOC back before an expected unavoidable discharge cycle damages batteries worse than a short hot charge.

The C40 charger wants to act as a current limited stage chargers of FLA batteries but the C40 manual says BULK 12-volt system: 13.0 to 15.0 volts in increments of 0.2 volts so set bulk knob to 13.6; and FLOAT 12.5 to 14.5 volts in increments of 0.2 volts,so set float knob to 13.5 with equalization Disabled (manual jumper) and you're doing just fine. I would confirm voltages with a quality digital meter - see the 'Using a Digital Voltmeter to Adjust Voltage Settings' section of manual.

On temperature compensation its an academic point mostly since the charge rate is so small in terms of overheating but shines when batteries have cooled down in winter season to get the most out of your charging hours. I'd just connect the sensor after you have voltages calibrated without it and see where the trend goes...

You have quite the treasure in that string - hope they last you 10's of years!

[Old F]

I just did a quick search  found a Tri-Star 60 for $179 us

For all it can do it's a bargain

Old F

[SparWeb]

Shoot - I don't know much about the industry and competitors.  All I know is that Xantrex comes up highest on google (at least, the way I phrased it).  I've also seen Xantrex in other applications.

Pretty sure a gust of wind will not be capable of causing an over-voltage in my system (given the low ratio between windmill output and battery size), but that's important to bear in mind.

Hugh, if you know of one, is there a spec-to-spec comparison of some of these controllers that you can recommend?

[SparWeb]

"You have quite the treasure in that string - hope they last you 10's of years!"

... No kidding!  I just about had a heart attack when I stumbled across them.  More exactly, after I had looked up the name on the internet and discovered an approximate price!

It feels a bit funny putting them to use before I understand their limitations, but they have already been sitting in my yard for 4 months and there's no type of battery that can be stored forever!

Thanks a lot for your help.  Putting the voltage settings close together like that and then checking sounds like a great place to start and the numbers you suggested are actually about where the maximum float charge voltage-per-cell should be.

Stay tuned - I'll probably be posting about my progress sometime soon.  :^)

[SteveCH]

We run a Trace here at the cabin, and have since the early nineties. It has been running 24/7 all that time with zero problems. However, in the past yr. or so, I've purchased some stuff from Midnite. It is high quality and priced lower than competitors. I haven't dealt with Xantrex since they weren't Trace any more, so have no opinion. But so far, both co's have been very good, no complaints from me and I'd buy more stuff from either.

By the way, I agree with the othes that charge control is the way to go. Back in the earlier days, many people didn't bother, and there weren't many good options anyway unless you were handy enough to build something yourself. [I had a buddy who was a physicist and rose to the challenge.] Then, a lot of systems were very modest, running a few lights and a small radio, maybe a Sunfrost. I would advise anyone to use a controller now. Batteries are not only extremely expensive, they are heavy and a real pain to change out. Besides, you already HAVE a controller...use it.

[SparWeb]

Done!

Built yesterday, installed today.

Lots of wind tonight, putting it all through its paces...