gel batteries at different volts

49 Posts
Jul 8, 2011 12:12 pm
gel batteries at different volts

 I have 16 gel 12v batteries I bought new 3 yrs ago. I am using them in a 48v off-grid system with 2 inverters and a xantrex charge controller. Now when the system is fully charged at 54 to 57 volts on the charge controller. Some batteries read 13.6v, good. but some are 11.7v and one is 9.8v
 are there bad cells in a gel battery? could it hurt the other batteries? why would some go bad? Ive never let the system get below 45 volts, usually at 46 volts the Generator kicks in and at 44 the system shuts down. Can I or should I replace the batteries with new ones to my old ones or would that be worse.
 
462 Posts
Jul 8, 2011 05:13 pm
Re: gel batteries at different volts

James, it could be that the batteries never get down to the proper depth of discharge voltage. If you never let them discharge low enough, they won't charge properly. Just like a cell phone battery, if you don't let the battery die and then charge, it will never come to a full charge that will last a long time. Instead you will get less and less run time out of it.
 So try letting the batteries come down to their minimum, around 10 - 11 volts, or whatever is recommended for your particular batteries, before recharging.
« Last Edit: Jul 8, 2011 05:15 pm by Tom Mayrand »
 
49 Posts
Jul 8, 2011 11:32 pm
Re: gel batteries at different volts

thanks, Ill try that.
 
46 Posts
Jul 10, 2011 12:26 am
Re: gel batteries at different volts

dear tom,
i beg to differ. i think perhaps an occasional (once a year?) deep discharge may not be terribly bad. but to drain them basically all the way down will do much more harm than good. these are lead acid, not nicad/nimh like the batteries on a cordless phone, or lithium as per a cell phone.
my guess is that they are not getting enough charge. this is why we have an equalizing charge, to bring all cells up to a full (over)charge.
on the plus side, sealed batteries (sla) are maintenance free. the down side they are maintenance free (unmaintainable). they also dont take an equalization charge well (irreversible offgassing, explosions, etc.). without looking, i think james said that he has had these for about 3-4 years. while thats not an exceptionally long life span, i would not expect a whole lot longer for sealed batteries under regular, moderately strenuous charge/discharge cycles (daily use, 30-40% dod, occasional undercharging).
you can apply an equalization charge, but it must be very carefully applied, at the absolute minimum voltage that will create an overcharge condition, and not for very long. i would hope that you do a little more homework to figure out for exactly how long and how much voltage to apply before doing so, if you should choose to take any of my advice (god help you!). -m
 
462 Posts
Jul 10, 2011 09:57 am
Re: gel batteries at different volts

Well, Michael, this is why I put the statement to check with the manufacturers suggestions for charging and discharging for his particular batteries. Note that they are gel type batteries which require a slower charging rate than lead acid.
 Also if you read James' post, he has a back up generator for charging so there should be no reason why he is not getting enough charging power. This can also be a problem since a generator does not comply with a slow charging rate as recommended for these type of batteries. So perhaps it is you who needs to do his homework...Also, as you suggest, the 30-40% of discharge is not being achieved due to the fact that the generator is kicking in at the setpoints he has programmed....
« Last Edit: Jul 10, 2011 11:33 am by Tom Mayrand »
 
46 Posts
Jul 11, 2011 05:54 pm
Re: gel batteries at different volts

whoa tom, easy tiger! not trying to step on any sensitive toes there. my reference to doing more homework was not directed towards you but James. he is the one who is going to harm his batteries if not doing things correctly. the only reason i suggest more homework be done is because my answers (suggestions) are not complete. my assumption is that it is not a problem with discharging, but charging. i realize that these are jells. his solar and generator are probably both sent through a charge controller that has its setpoints for jel batteries (something like 14v max as opposed to 15v or there abouts for an eqalization charge). this would (could) cause chronic undercharging, or at least not bring all cells up to even (ever). as the cells become unbalanced, the problem will be magnified through repeated charge/discharge cycles, growing a bit more pronounced each time. i dont believe the problem to be power (read as amps) but voltage. best of luck either way. -m
 
49 Posts
Jul 11, 2011 09:06 pm
Re: gel batteries at different volts

 Thank you both. Im leaning towards them needing to be equalized. The battery mfg said they dont need equalization so I had turned that option off on the xantrex power panels.
I have 2 sw5548.  the batteries are the mk gel 245 amp.
I will turn on the equalization, It seams a waste that the Gen would run 3 to 4 hrs to equalize but if it saves the batteries or is it to late. The inverters do control the max charge along with an outback controller for the solar.  Thank you both again.  Jim
 
49 Posts
Jul 11, 2011 09:07 pm
Re: gel batteries at different volts

  Wow i just looked at the reciept  455 bucks apeace for a total of 7280
 
462 Posts
Jul 12, 2011 08:34 pm
Re: gel batteries at different volts

No biggie Michael. Just don't want Jim to get to confused.
 Jim another thing you can do is rotate the batteries positions within the string, especially if you are only charging from one end....
 
46 Posts
Jul 18, 2011 05:57 pm
Re: gel batteries at different volts

hey, great idea tom. why did'nt i think of that?
 

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