[RE-wrenches] Needed: Outback setpoints that won't ruin AGM's

[Kent Osterberg]

Mick,

The SunExtender technical manual says to charge at 2.385 ± 0.015 volts 
per cell at 25°C until the charge current drops to 0.5% of the battery 
capacity. That's 28.6 ±0.2 volts for a 24 volt system and 28.8 volts is 
the top end of that range. If you are seeing signs that the batteries 
have vented, the voltage probably has been too high. Since venting 
causes permanent water loss, with these batteries it's probably better 
to have the absorption voltage a little on the low side rather than on 
the high side of the recommended range. It's really important to have a 
temperature sensor and a temperature compensated charge controller too.

It takes a lot more than one hour of absorption time for the current to 
drop down to 0.5% of capacity; three or maybe four hours might do it.

If the end amps is set to 45, then the battery capacity should be 9000 
amphours, otherwise it'll be terminating the charge too soon. 
Realistically, you may as well disable the end amps feature.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com


On 7/7/2012 2:53 PM, Mick Abraham wrote:
> Greetings, All~
>
> My client's string of SunXtender two volt AGM's won't hold a 
> charge...after only two years! The prior AGM batteries also turned to 
> toast prematurely. There must be a better way, so could we review the 
> Outback charge setpoints?
>
> Side note: Delta-v from the highest 2v cell to the lowest shows a gap 
> of only .148v during a charge cycle so cell imbalance is not the 
> likely problem. I think they're all working together equally bad.
>
> A small amount of dry powder--light green color resembling verdegris 
> finish on copper--can be seen around 30% of the +/- posts, in an 
> amount smaller than 1/8 teaspoon. From this sign, I suspect that the 
> cells have vented and dried out. The property is vacant 90% of the 
> time so I am mostly focused on the MX60 solar charge controller as the 
> culprit.
>
> This is an early vintage Outback controller, firmware version 5.11. 
> Temperature compensation is in place; automatic EQ is defeated. Bulk & 
> absorption setpoints exactly match those requested by 
> Concorde/SunXtender: 28.8 volts bulk/absorption & 26.4 volts Float. 
> The controller does regulate at these voltage points so it's 
> technically "working"--just grinding up batteries on a regular basis.
>
> The Concorde people do not furnish suggestions regarding absorption 
> time or "rebulking" so that may be the source of trouble. Controller 
> setpoints have the Absorption Time Limit set for "0 minutes minimum & 
> 1 hour maximum". The controller is also set to "rebulk" the battery at 
> a 23 volt trigger and there's an inscrutable setpoint called "End" 
> that's set for 45 amps. I have a theory about the meaning of that but 
> I'll keep the noise level down...
>
> If I recall correctly, the Outback end of charge routine in early days 
> was to start a clock & see how long it takes to move the battery to 
> the bulk/absorption voltage then apply an equal amount of time for an 
> absorption charge before shifting to float. Steve Higgins may confirm 
> if that is indeed the protocol for a version 5.11 controller but: has 
> that method now been changed in the newer version Flexmax controllers? 
> It seems those now have an absorption duration value that can be 
> changed so maybe that's what we need--a replacement controller along 
> with good information on the absorption duration and/or float 
> setpoints, etc.
>
> I see that some controllers can even be set to not Float at all. Maybe 
> that's better: run the charge then stop it all for the rest of the 
> day. OR: thinking outside the box here...what about a controller that 
> won't initiate charge cycle at all unless the battery voltage crests 
> down to a trigger point. All I know for sure is that two sets of 
> valuable batteries have now been ruined so "normal" charge setpoints 
> are not working well in this situation.
>
> If some better electronics (such as blending in the Outback capacity 
> monitor gizmo) might help, I'm all ears. Next time maybe I'll suggest 
> flooded cells to the client instead of AGM's, but the cells are in a 
> crawlspace that gets freezing cold at times.
>
> Thanks in advance for advice & suggestions. A second off grid client 
> has a similar setup so I need to hotfoot it out there and tweak that 
> (more recent) Outback controller to match the suggested setpoint 
> revisions...before bad things happen again.
>
> Jolliness,
>
> Mick Abraham, Proprietor
> www.abrahamsolar.com <http://www.abrahamsolar.com>
>
> Voice: 970-731-4675 <tel:970-731-4675>
>


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