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Author Topic: Solar / UPS battery monitor  (Read 1340 times)

JeffVolp

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Solar / UPS battery monitor
« on: May 15, 2023, 02:44:17 PM »

When we built this house 20 years ago, we had the builder install a special "electronics" circuit fed by a large UPS to power computers and other electronic devices that we wanted to have reliable power.  That UPS uses a series connected set of 4 deep discharge 12V batteries.  Being frugal, I replace batteries when they fail rather than replacing the entire set.

Several times a battery failed with a shorted cell.  Since that reduced the voltage of the entire stack, the UPS wound up overcharging the batteries.  On two occasions that resulted in a catastrophic failure of one of the batteries.

That battery bank is now part of our backup solar system, and is cycled daily.  To hopefully prevent another catastrophic failure and to give an early warning that something is amiss, I designed a battery match monitor.  It sounds a periodic alarm whenever there is a mismatch of more than .5V between the batteries.  An even more urgent alarm is sounded if any of the batteries go into a deep discharge or overcharge condition.

Some of you may also have solar systems, and I wonder if there would be any interest in this device.  I could supply it as a kit or assembled PCB that would fit into a small Polycase.  It works with 24V, 36V, and 48V systems.  Auto-calibration is done at first power-up with a 13.00V power source on all channels.

Jeff

 
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Tuicemen

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Re: Solar / UPS battery monitor
« Reply #1 on: May 16, 2023, 09:13:36 AM »

Many solar setups use 2 or 6 volt deep cycle batteries in their setups would/could this be made to work in those situations? I have 16, 6volt batteries for a 24 volt system could I monitor each battery, or just each 24 volt bank of 4 batteries? It seems like this is for just monitoring 12volt batteries.
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JeffVolp

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Re: Solar / UPS battery monitor
« Reply #2 on: May 16, 2023, 09:57:01 AM »

Many solar setups use 2 or 6 volt deep cycle batteries in their setups would/could this be made to work in those situations? I have 16, 6volt batteries for a 24 volt system could I monitor each battery, or just each 24 volt bank of 4 batteries? It seems like this is for just monitoring 12volt batteries.

It was intended to monitor up to four 12V batteries in series.  It will work with a 24V, 36V, or 48V system.  Other series connected arrangements could be used, but the monitor points would be the same.  With four 6V batteries in a 24V system, it would still catch a shorted cell due to the mismatch between the 12V and 24V monitor points.

In your case if each set of 6V cells are tied in parallel, a shorted cell should cause that set of cells to drop.  If more than .5V, that 12V monitor point would be recognized as having a shorted cell.

Since it internally halves the voltage for better resolution (7.5V to 15V for a 12V battery), it could monitor four series connected 6V batteries in a 24V system with a firmware change.  That could be automatic, keying off the voltage of the first battery in the series chain.

Jeff
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Tuicemen

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Re: Solar / UPS battery monitor
« Reply #3 on: May 16, 2023, 02:19:41 PM »

So with a firmware change I'd still need 4 units to fully monitor all my batteries?
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JeffVolp

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Re: Solar / UPS battery monitor
« Reply #4 on: May 16, 2023, 03:06:42 PM »

So with a firmware change I'd still need 4 units to fully monitor all my batteries?

It depends on how your batteries are connected.  If each set of four 6V batteries are wired in parallel, just one would monitor all 16 batteries.  But if you have 4 independent series connected banks with only the ground and 24V connections common, then you would normally need one for each independent bank.

It looks feasible to use just one with a resistor summation network at each intermediate point.  But sensitivity would have to be increased because a dead shorted cell would only change the summation voltage by half a volt.

BTW, I already took a first pass at the firmware mod.  But I'll have to assemble another one to test it.  The prototype is presently monitoring my own system.

Jeff
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Re: Solar / UPS battery monitor
« Reply #5 on: May 17, 2023, 07:16:56 AM »

I've had issues with batteries going bad in the past and since it is not recommended to replace just one battery due to your bank is only as good as your worse battery I removed one string of batteries. Testing each so I had the best ones to use. Will this show exactly which battery has the bad cell? If so it will sure make finding the exact bad battery much simpler.
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JeffVolp

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Re: Solar / UPS battery monitor
« Reply #6 on: May 17, 2023, 01:51:58 PM »

I've had issues with batteries going bad in the past and since it is not recommended to replace just one battery due to your bank is only as good as your worse battery I removed one string of batteries. Testing each so I had the best ones to use. Will this show exactly which battery has the bad cell? If so it will sure make finding the exact bad battery much simpler.

No, the intent was to alert me that something was amiss.  I don't want to damage the others due to overcharge.  It just sounds the alarm when there is a mismatch between any of the 4 readings.  Then it is time to get out the DVM.

I know the recommendation is to replace all batteries when one is bad, but some batteries last much longer than others.  So being frugal, I only replace the one that failed.  The last time that happened, overcharging likely took out a second one.

Jeff
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