real low battery voltage

simagic

I teach a 2 hour course on "Basic", "introduction" to solar powered backup systems . Nothing too deep. I have a 12v 9ah small battery .... (link) http://www.amazon.com/UB1290-Sealed-Battery-Terminal-Electronics/dp/B001XVFGFE ... that I use for demonstration purposes with a small panel and small CC. I put it away with the battery fully charged about 3-4 months ago. I just pulled it out and the voltage is reading 1.4 volts. Sooooooo, can it actually go that low. I would think that that is REALLY low I'm under the impression that around 10.5 or so is 0%. Annnnd how did it get so low just sitting (and of course self discharging....but that fast and that far down??????) There were no loads, although the CC ( sungard 4.5 amp) was connected to the battery. I just put it out in the sun to ????charge it up????. and with the CC connected, I'm still getting 1.3 volts. Wouldn't the reading go up on the voltage now that I put it in the sun ( to charge??) indicating the CC was sending (oh lets say) 14.4 v

BB.

Re: real low battery voltage

Was there solar panels attached (and in sun) with that solar charger? If there were no panels, most charge controllers do have some amount of (usually small) leakage current.

It does not take much leakage current to kill a small battery:

9 AH * 1/24 hours * 1/30 days per month * 1/3 months storage = 0.004 amps = 4 milliamps

So, a 4 milliamp load for 3 months will kill that battery (that is a 3,000 ohm load--pretty much nothing).

Note that battery voltage is (partially) a function of specific gravity of the electrolyte. When a battery is near zero volts, that is pretty much water in the cells.

Of course, it is possible that there was a failure inside the battery and you have an open cell/open bus connection.

-Bill

simagic

Re: real low battery voltage

The panel was disconnected and the little case was away in my closet. When you say "kill a battery" ??all the way to almost nothing 1.3 volts?? ( OH, I SEE YOU SAY "PRETTY MUCH NOTHING". sO THAT WOULD BE MY 1.3 VOLTS). I just put it out in the sun to ????charge it up????. (Not thinking it will) and with the CC connected, I'm still getting 1.3 volts. Wouldn't the reading go up on the voltage now that I put it in the sun ( to charge??) indicating the CC was sending it's charge (oh lets say on it's way up to) 14.4 v . I guess I'll disconnect the CC after this next class I teach. Glad it was my little (cheap) battery and not my Concord AGM

Last edited by simagic; June 21st, 2013 at 12:41 PDT.

Cariboocoot

Re: real low battery voltage

simagic wrote: »

The panel was disconnected and the little case was away in my closet. When you say "kill a battery" ??all the way to almost nothing 1.3 volts?? ( OH, I SEE YOU SAY "PRETTY MUCH NOTHING". sO THAT WOULD BE MY 1.3 VOLTS) I just put it out in the sun to ????charge it up????. (Not thinking it will) and with the CC connected, I'm still getting 1.3 volts. Wouldn't the reading go up on the voltage now that I put it in the sun ( to charge??) indicating the CC was sending it's charge (oh lets say) 14.4 v

Last edited by simagic; June 21st, 2013 at 12:41 PDT.

Yep. Controller only connected presents a load to the battery; slowly draining off power over time. All the way down to the unrecoverable level it is now at. If you leave a drain on a battery long enough, not only will it reach 0 Volts but it can actually reverse polarity.

At any rate your battery is toast. Get another one, and next time when you put it away disconnect it from everything. And take it out and charge it once a month as it will lose about 1% per month connected to nothing. Such is the nature of batteries.

BB.

Re: real low battery voltage

Yep, as Marc says, a load is a load and will discharge a battery faster. And a battery taken that low (really below ~12.0 to 11.5 volts resting) is being damaged over time (days/weeks/months of low state of charge will accelerate sulfation).

Batteries will discharge between 1% to 25%+ per month, depending on battery type, age, temperature, etc.).

Charging the battery once a month. Getting a Battery Maintainer type charger, or a cheap "cube" charger with a lamp timer (I set ~1 hour per day) will usually keep a battery charged (many chargers will actually over charge the battery and "boil" them dry on extended float with power 24x7).

It is a pain.

-Bill

PNjunction

Re: real low battery voltage

Treat that UPGI Universal Power Group UB1290 battery like it IS a concord - especially for a class. Here are the docs for it - look for the UB1290F2 as it appears a link for the standard one is missing:

http://upgi.com/products/batterysearch.aspx

You can hit that thing up with 0.3C max inrush current, so about the max size would be a 50w panel and a pwm controller, and when you reach about 90ma (0.1C) of current at 14.7 absorb, you are done (or your controller will float it possibly beforehand).

I'd toss that abused one and start fresh. For an agm, try to get at least 12.6 volts OCV or higher upon receipt. Even small setups like this just need to be scaled mentally and treated like it was a 90ah Concord and the class will go far.

simagic

Re: real low battery voltage

For that "introduction " class that I'll be teaching I have a 25w panel .....VOC 22.41v / VPM 17.9v / ISC 1.54a / IPMM(?) 1.4a........
and I have a 10w panel....VOC 22v / VPM 17.5v / ISC .61a / IMPP .57........ So, If I run these panels separately (each panel having there own set of wires running and joining the + and - on the CC),,, (which I believe would be considered parallel) to the controller (so as to be able to "plug in" the 25w or to "plug in" the 10w or to "plug in them both for a tolal of 35watts), is there any problem that our knowledgeable moderators might find. Often, I ass-u-me, that there isn't any issues but upon inquiring here, I find that there is.........Upon reading another recent post (just today) called "Help me with my expansion" , it says (((("Are you going to be mixing panel types in the string? It would be best not to. If you do the Imp of the string will be limited to that of whichever panel has the lowest current.)))). Well my IMP's are certainly different but if I'm putting them in the way i mention (parallel) does this no-no apply??

Cariboocoot

Re: real low battery voltage

Ah, terminology!

A panel string is panels in series. If you put two different panels in series their Voltage adds up, but their current is limited to the lowest. So 17.9 Vmp @ 1.4 Imp plus 17.5 @ 0.57 Imp would end up being 35.4 Volts but only 0.54 Amps: 19 Watts instead of 35!

But when you put them in parallel as you intend to do the currents add together and the Voltage "remains the same" (very close) so you'd get about 17.9 Volts and 1.9 Amps or 34 Watts. There is always some little loss and how it actually behaves in charging is another matter due to wiring losses and charge Volts.

This is not a whole lot of panel to be working with. It would just about do for a 20 Amp hour 12 Volt battery. What charge controller are you using? Some of them would consume more power than that panel arrangement will produce. Morningstar has a very small one at 4.5 Amps: http://www.solar-electric.com/sg-4.html

simagic

Re: real low battery voltage

It's a 12ah battery and the CC is indeed the SunGuard 4.5 amp 12 Volt Solar Charge Controller. Is the 34 Watts ( If I put them together / the 25 and the 10 )too much for the 12AH battery?

inetdog

Re: real low battery voltage

simagic wrote: »

It's a 12ah battery and the CC is indeed the SunGuard 4.5 amp 12 Volt Solar Charge Controller. Is the 34 Watts ( If I put them together / the 25 and the 10 )too much for the 12AH battery?

There is no such thing as too much panel for the battery if you are using the proper CC (with a low enough Bulk ampere limit AND able to deal with excess panel current capability. Many PWM CCs cannot handle this!)
But there is such a thing as more panel than you need, economically.
In your case, since you have the panels already and your CC can handle it, your only concern will be that you will sometimes be feeding 2 amps to the battery. That is on the high side for FLA, but tolerable. Fine for AGM (which can take 3.6 based on Coot's research.) And possibly destructive to a GEL battery.

simagic

Re: real low battery voltage

thanks "once again" for your help....

PNjunction

Re: real low battery voltage

Well, ups-style agm batteries like this from UPG or PowerSonic etc, can only handle .3C inrush current. It is usually stated on the battery. I have pushed them harder, and what do you know - powersonic was right - hot spots developed, so I cut it back to no more than 0.25C personally.

So for a 12ah agm ups-style battery, that would be 3.6ah maximum inrush.
For a 9ah like the UB1290, that would be 2.7ah maximum inrush

With both of your panels paralelled together, you'd only likely see about 1.9a under best conditions, so you are covered either way.

Note that the Morningstar 4.5a pwm controller has an absorb voltage of 14.2. While this is ok for backyard, camping, and non critical uses, the preferred absorb voltage for the UPG or Powersonic is actually 14.7v when cycling it. This can be achieved by upgrading to the Morningstar 6a SunSaver which has a selectable jumper. (remove the jumper for 14.7v). The smallest Morningstar Sunguard's limited voltage of 14.1 makes it usable, but not the most efficient, across flooded, gel, and agm lines if the user doesn't know the chemistry and recommended absorb voltage for their battery.

Ok, so maybe you don't need to be so critical on these little 9 and 12ah ups agm's. But again, if you want to teach the class the importance of the proper absorb voltages as if these batteries were made of gold, then a small upgrade to the charge controller that allows for a 14.7v choice would be good.

simagic

Re: real low battery voltage

Yehhhhhhh, hmmmmmmm. I got the whole amps / watts / volts thing pretty much down. Now I'm seeing ""0.3C"" mentioned. And this would be ???? what??. My battery does indeed say "initial current 0.3CA". I've been reading these forums for quite awhile and this is the first time I'm "noticing" this reference........ You said ""So for a 12ah agm ups-style battery, that would be 3.6ah maximum inrush.
For a 9ah like the UB1290, that would be 2.7ah maximum inrush. With both of your panels paralelled together, you'd only likely see about 1.9a under best conditions, so you are covered either way"".. What's the math to figure it out that 1.9 that you mention. ?? What does this mean??

Cariboocoot

Re: real low battery voltage

In this context the "C" is "Capacity". In reference to batteries you usually see it used as "C/10" - meaning battery capacity divided by 10 for a charge current. The "0.3C" refers to inrush current: 12 Amp hours * 0.3 = 3.6 Amps.

The "1.9 Amps" is the approximate maximum current from the two panels combined. Since it is below 3.6 Amps it would not be a problem. If you were to hit that same battery with the 7+ Amps available from a 135 Watt panel it would damage the battery: too much inrush current.

simagic

Re: real low battery voltage

Great....got it.. NOW, yesterday I was putting together and testing this small system (with my "new" battery / 12ah, for this class),, and but another question popped up (as I would be discussing this). With my own larger system I have a shunt and a meter telling me what I need to know about what's going on. BUTTTTT, I don't have this info with my shunt-less small set up. I do indeed know that you don't measure battery voltage during or soon after a charge and that it needs to "rest". I also know that you don't measure the battery voltage while it is under load. I know I don't want to bring the DOC down below 50%. So here-in lies the question. If I'm watching my voltage while under load as being my indicator of SOC, and I know I want to 'disconnect" upon reaching 50% (max) SOC, then I'm actually seeing the SOC dropping "while" under load. Upon "disconnecting", my voltage rises bringing the voltage "up"....... (Example). I may disconnect my load upon seeing the voltage on my digital meter reading 12.4, (50% or so) but upon disconnecting, the meter jumps up to 12.6 (75% or so)(OR MORE). So this leaves me thinking that I can bring the SOC down to 25% (during load) because after disconnecting my load it will jump up to 50% (OR MORE) and that the disconnected 50% voltage is the more accurate. Thoughts??

waynefromnscanada

Re: real low battery voltage

"simagic", the battery voltage is only usable for checking battery state of charge if the battery has been sitting, resting, neither being charged not discharged for at least 3 hours.
Voltage readings while under load, or while being charged, tell you basically nothing about the batteries state of charge.
SG measurements however are of value.

simagic

Re: real low battery voltage

Thank you. Yes I mention that I'm well aware of that. What I'm "kind of asking", is it feasible to bring the SOC down to 25% "while under load" knowing that it will bounce back to 50% or more after the load is removed

vtmaps

Re: real low battery voltage

simagic wrote: »

Thank you. Yes I mention that I'm well aware of that. What I'm "kind of asking", is it feasible to bring the SOC down to 25% "while under load" knowing that it will bounce back to 50% or more after the load is removed

If you bring the SOC down to 25% (or any other percent) it will not bounce back. The voltage is what bounces back. The voltage under load does not indicate the SOC.

--vtMaps

simagic

Re: real low battery voltage

Ok. Then in my example " . If i have a load on the battery and see the voltage on my digital meter reading 12.4, (which i would understand as being 50% discharged, entirely based on the voltage of the battery) but then upon disconnecting, the meter jumps up to 12.6 (75% or so)(OR EVEN MORE). So this leaves me thinking that I can bring the SOC down to 25% (during the load) because after disconnecting my load from the battery it will jump up to more voltage. SO, is it "harmful bring the battery past 50% UNDER LOAD orrrrr reading the voltage "after removing the load" and basing my DOD after on that reading. So if the latter is true, then I would believe I can let my voltage continue to drop (via my meter) to the 25% DOD level knowing it will "pop up" soon after the load is removed

Cariboocoot

Re: real low battery voltage

What you're talking about is "Voltage rebound": the increase in Voltage measured after the load is removed. If your load is consistent, you can still use Voltage as a reasonable measure of SOC. All you have to do is test it: draw the battery down with the load until you reach a 50% SOC Voltage reading with the load on, remove the load and let the battery "rebound" for a couple of hours, measure the new Voltage. The difference between the two is approximately the amount you can take the battery down below indicated 50% SOC Voltage under load before you reach "real" 50% SOC.

It is not perfect.

I commonly recommend the opposite procedure to be sure the battery stays above 50% SOC: setting the LVD on inverter to whatever the 50% SOC resting Voltage would be. Then when the load is removed the Voltage will rebound, and the battery is not actually down to 50%. Better safe than sorry.

simagic

Re: real low battery voltage

Hope i'm not being too thick. So, if "under load" my meter reads 12.3v (which according to my chart for this small AGM indicates 50% SOC) and my "resting voltage is 12.9, the difference between those two are .6volts. So are you saying that i can take the 12.3 and subtract .6 which would equal 11.7 ( which is the amount I can take it "below" the under load 50% SOC for an "approximate" real 50% SOC.

BB.

Re: real low battery voltage

Maybe these charts will help:

New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

-Bill

-Bill

simagic

Re: real low battery voltage

No This doesn't help. I'm so very well aware that the resting state of charge is different than the charging or discharging state of charge. What I've been trying to get to, is what can I bring the SOC down to "WHILE DISCHARGING". When do I need to 'disconnect" without harming my battery without going below the 'real" 50% DOC . I think Cariboocoot gave me some insight and I'm waiting to see if he agrees with my figures

Cariboocoot

Re: real low battery voltage

simagic wrote: »

Hope i'm not being too thick. So, if "under load" my meter reads 12.3v (which according to my chart for this small AGM indicates 50% SOC) and my "resting voltage is 12.9, the difference between those two are .6volts. So are you saying that i can take the 12.3 and subtract .6 which would equal 11.7 ( which is the amount I can take it "below" the under load 50% SOC for an "approximate" real 50% SOC.

That's about right.
If you try it you may see that 11.7 Volts under load only rebounds to 12.2, so you will need to experiment a bit. I would start with less Voltage differential and work up, rather than the maximum and work down (i.e. deduct 0.3 Volts to begin with so you go down to 12 under load and see what it rebounds to from there). The lower the Voltage is, the less rebound you will experience.

simagic

Re: real low battery voltage

great. thanks. (again)