[RE-wrenches] Magnum System - Discharged Battery Bank

Wed Dec 17 10:36:13 PST 2014

Wrenches,

I’m guessing that there is a problem with the battery, or the user just over discharged this bank.   I doubt the inverter is at fault here.

Reverse charge batteries are common in two cases from what I’ve seen.   You can take any battery, discharge it to zero volts and reverse charge it.. you’ll lose efficiency but the battery will work… By way I don’t advise doing this just because it might be fun!

1.      Low Battery Cut Out situations where the system in unsupervised, Low battery is hit and the inverter turns off, but doesn’t isolate from the battery bank.
LBCO is a great feature, but too often it’s left at the factory defaults which in most cases is way too low.   Most inverter/charger manufactures set this at 1.75 volts per cell.   1.75 vpc is essentially a 95-100% Dead battery.   Even a little bit of current load, say the 3-5 watts an inverter, charge controller or any other DC loads connected will quickly draw the battery bank down to 0 volts.    Depending on the internal resistances of each cell, this can actually cause the cells when the load is finally removed to reverse charge!

2.      Excessive parallel string battery banks.

This usually happens when you have a dead or bad cell in a parallel string that is not caught for a long period of time.   Over time this string acts like a load to the rest of the stings, and eventually reaches zero volts, at that point there is a possibility that it can reverse charge.

Best way to avoid these situations,  Raise the LVCO voltage to a higher point, (Usually 1.85 to 1.90volts per cell) and instruct the customer to where the status screen is on their device.  Most of these devices out there tell you if it shut off during LBCO or not…   If this is a cabin or a site where the customer is away for an extended period of time, have them disconnect the inverter, and only leave the charging sources connected to the battery bank.

Installing a voltage controlled breaker, this acts as a back up to the LBCO… in most military applications they needed these installed due to they didn’t want to kill a combat vehicles battery system.   Installing this breaker can be pretty expensive though.   An last but not least, avoid excessive parallel strings.

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Corey Shalanski
Sent: Wednesday, December 17, 2014 9:37 AM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Magnum System - Discharged Battery Bank

Clarification and Update:
- I believe I may have inadvertently thrown Magnum under the bus in in my initial post.
[https://mailfoogae.appspot.com/t?sender=aY3NoYWxhbnNraUBqb3VsZS1lbmVyZ3kuY29t&type=zerocontent&guid=dab0a56b-2f33-4542-ab89-2eb7e81192f3]ᐧ
While on site yesterday I did speak with Magnum's customer support about my findings and they advised to try to trickle charge the batteries up to the minimum threshold at which the Magnum inverters would turn back on. From there we will hopefully be able to run some more determinate tests. My question about the low-battery-cutout was more intended to explore whether others have ever experienced any similar issues with this protection feature. I am finding out that the feature is only relevant during inverting (not charging) mode and so would imply that the utility feed had been cut, which does not appear to be the case.
- Our battery distributor has lent us a 12-circuit trickle charger - one set of alligator clips per battery. I am planning to re-visit the jobsite later today to put each battery on a trickle charge, which I am told will take 1-2 weeks for any lasting change to take effect. The distributor advised to reverse the leads on the batteries with negative voltage, hoping that they may be recoverable as "reversed pole" batteries thenceforth.

--
Corey

On Tue, Dec 16, 2014 at 8:27 PM, <cshalanski at joule-energy.com<mailto:cshalanski at joule-energy.com>> wrote:

Wrenches,

I visited a customer today whose grid-tied battery backup system suffered
some sort of catastrophic failure, and now I'm trying to play detective to
figure out what happened and whose equipment/design is to blame.

Here is what I know so far:
- System is ac-coupled with a Sunny Boy 7000 (7.85kW array), Magnum
MS4448PAE (x2), 3 strings of 4 12v batts (705Ah total)
- Site visit was triggered by the Sunny Boy registering zero output via
online monitoring.
- SPST Solid state relay installed on Sunny Boy output, controlled via
Magnum router, serves as secondary overcharge protection to Magnum's
frequency shift feature. Technician who initially responded claims he
measured 240v across the relay's terminals - relay manufacturer claims this
is "100% impossible". We removed the relay from the circuit, and the Sunny
Boy is again operational.
- On arrival I found the Magnum display showing a "Low Battery Charge"
message with the bank measuring ~4Vdc. The individual batteries were in
various states of charge, ranging from high of 6.3v to low of -2.6v. These
measurements were taken at rest, all battery cables disconnected, and yes
three of the batts were registering a negative voltage.
- Customer reports that he was not aware of any recent prolonged power
outages.

The three potential suspects would seem to be:
- SS relay: The customer is convinced that this is the weak link in the
system and somehow triggered this failure. I am not so convinced but would
be interested to hear recommendations for properly testing its
functionality.
- Magnum equipment: Since there is low-battery-cutout protection, why would
this feature not have activated and prevented the batteries from draining
so low?
- Batteries: I am not very familiar with modes of failure, but our
distributor suggested that an internal short could be a possibility - what
might have triggered this?

Clearly I am fishing for potential leads here, so any suggestions are
welcome.

Thanks!
--
Corey Shalanski
Joule Energy
New Orleans, LA

Steve Higgins
Technical Services Manager
P: +1.902.597.4020
M: +1.206.790.5840
F: +1.902.597.8447
Surrette Battery Company
Exclusive manufacturer of
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