Author Topic: Hoping for the best..  (Read 1300 times)

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Phssthpok

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Hoping for the best..
« on: March 05, 2009, 06:56:42 PM »
Since I got laid off last week I've been devoting some time to sprucing up the 24'TT that I plan to make my home for a season or two when I move to my dream off-grid site in Montana (hopefully in short order).


One of the first things I did was to check how the batteries fared over the winter. I have (8) 6 volt, 200Ah AGM batteries and I left them in the trailer hooked up to (2) Uni-Solar 64 watt panels ( http://i2.photobucket.com/albums/y32/Phssthpok/trailer006.jpg ) through a PWM CC with a simple 'bar' type LED display ( http://i2.photobucket.com/albums/y32/Phssthpok/trailer012.jpg ). The only load on the bank was the LED display on the CC....no other ghost loads.


I figured that even with the grey skies we usually have in winter I should get enough power flowing in to keep things topped off. As you can see they were initially quite happy, but here's where it gets strange. When placing a multi-meter on the individual batteries they each register 7.3v, but if I check ANY combination of batteries in series (2, 4, 6, or all 8), I get a reading of 11.5 which is about what the CC 'LED bar' meter shows.


At first I had sinking feeling that I had mis-wired something when doing the series/parallel of the batteries, but the wiring checks out. Then I thought maybe I had a dead battery in the string, but given that they each read 'healthy' on an individual basis, I can't fathom why the 'bank' reads so low. I can't even kick-off ONE of the 5w HF Compact Fluorescent lights!


Admittedly the interconnecting wires are only 16ga speaker wire, but since I'm not placing any kind of real load on the system, I can't see how that could be causing any kind of false reading. It's all just in a state of 'maintenance' right now.


So...are my batteries toast? They are about three years old (Dec '05 Mfg date) but have never had any real load put on them. I got them for $35/ea back in April '06 so I'm not out a whole LOT of money, but it would suck to have to replace my bank just as I'm getting ready to go remote.....that's several hundred $$ I could do without spending right now.


« Last Edit: March 05, 2009, 06:56:42 PM by (unknown) »

Flux

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Re: Hoping for the best..
« Reply #1 on: March 05, 2009, 12:39:52 PM »
Nearly impossible to follow this.


"When placing a multi-meter on the individual batteries they each register 7.3v, but if I check ANY combination of batteries in series (2, 4, 6, or all 8), I get a reading of 11.5 which is about what the CC 'LED bar' meter shows."


I assume this is connected for 12v.


Firstly the only way you will see 7.3v across any cell is if it is fully charged and gassing. If that figure is with panels feeding it may make sense. If not charging or with any form of load then suspect your meter.


" but if I check ANY combination of batteries in series (2, 4, 6, or all 8), I get a reading of 11.5 which is about what the CC 'LED bar' meter shows."


Now what does that mean? two cells in series are nominal 12v but no other combination makes 12v.


Disconnect these things and leave them stand for a few hours. Connect two in series and ignore the rest. You now have a string of 2 cells and the total voltage must be double that of a single cell ( or perhaps to put it more accurately the 12v reading should be the sum of the 6 volt readings ( should be very close but may not be identical). If this is the case and they have been standing long enough to loose the surface charge then the 12v reading should be something like 12.8v if fully charged ( 6.4v per cell). If you still have the 11.5v then they are flat and each cell should be about 5.75v.


Try a load at 12v such as a car headlamp bulb and see what happens to the volts. If it was at 12.6v or more to start with it should hold up with a small load unless you have unexplained resistance somewhere ( including in the batteries). If it started below 12v then it will fall very quickly on load.


Until you sort what is going on with a simple two battery string there is no point in adding confusion with all the cells. If that pair behave then try the others in pairs and you should start to make sense.


Multimeters have a very high impedance and will measure nearly correct volts even if you have a high contact resistance somewhere but the lamp load will soon show any bad connection. The CFL will just add another layer of confusion at this stage. A filament lamp will glow with only a few volts and let you really see what is happening.


Flux

« Last Edit: March 05, 2009, 12:39:52 PM by Flux »

Ungrounded Lightning Rod

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Re: Hoping for the best..
« Reply #2 on: March 05, 2009, 06:38:11 PM »
I'd worry that the panels overcharged your AGMs and gassed off the water.
« Last Edit: March 05, 2009, 06:38:11 PM by Ungrounded Lightning Rod »

vtpeaknik

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Re: Hoping for the best..
« Reply #3 on: March 06, 2009, 11:13:46 AM »
Your volt-meter may be confused by the pulsing effects from the controller.  Suggest you disconnect the charge controller and then measure (or measure at night).  If the batteries' voltage does not add up even with the CC disconnected, then perhaps the meter is no good, or is still affected by nearby induction from something else (inverter?).  Try an old analog meter.  Or move the batteries some feet away.
« Last Edit: March 06, 2009, 11:13:46 AM by vtpeaknik »

ghurd

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Re: Hoping for the best..
« Reply #4 on: March 06, 2009, 12:35:37 PM »
Confused meter!

Is this the $3 HF meter with the original battery?

They can do some funny things when the battery is low.  Things like you are seeing.

G-
« Last Edit: March 06, 2009, 12:35:37 PM by ghurd »
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