where to place new batteries in an older string?

pedro65 · November 2011

I have a 48V system made up of 16 350 amp/h LA batteries. 4 strings of 4. The original system was badly undercharged for a couple of years probably. Through diligent charging and increasing the number of panels, I have gotten them up and running pretty well considering.
I replaced the worst of the older batteries with new ones about a year ago. Essentially putting new batteries into a two year old damaged string. I just replaced the worst of the batteries, spreading the new ones into the remaining older ones.

Any thoughts as to whether I am better off putting together one string of the "new" batteries together, or will it really make much difference? Currently I can get all the old batteries up to about 1.65 sp/gr the new ones are higher in the 1.75 range.

BB. · November 2011

Re: where to place new batteries in an older string?

I am not sure there is a "Better" way...

Placing all the new batteries in series will have a slightly different charging voltage (51.7 vs 51.9 volt or whatever) difference between the old an new string... You may get uneven current sharing between old and new batteries (perhaps the new ones will discharge first/faster and charge a bit slower--i.e., they will cycle first and probably "age" to match your old set).

If you put two old and two new in each string--You should get better current balance but you do run the risk of a deep discharge (to 20% state of charge or less) of the "lower capacity" batteries in the series string going to 0 volts (dead) first (and can even reverse charge the "weak cells" which pretty much kills them forever).

Do you have a DC Current Clamp meter (cheap but serviceable meter here) where you can measure current sharing?

Otherwise--I would take your best guess and just run them and not agonize over the decision.

Keep a close eye on your batteries and if you see one with low voltage or low current (or high or low relative currents between strings during charging/discharging), you want to take that one out of service before it damages the rest of your batteries (again, voltmeter, current clamp meter, hydrometer).

An "open cell" will cause a string to never charge/discharge (and damage the other batteries from never charging).

A "shorted cell" will slowly (or quickly) discharge the rest of your battery bank--and damage the rest of the cells with under charging/deficit charging--and possibly over charge the same series string cells/batteries (too much charging current).

A single failed cell is a bit easier to see in a 12 volt system (12 volt - 1 cell = 10 volts pretty quickly dead system). A 48 volt system - 1 cell is 46 volts--things still work.

-Bill

Vic · November 2011

Re: where to place new batteries in an older string?

Hi Pedro,

Well if you really getting the "new" batteries up to the "1.75" range, an not sure that the new ones are that good.

And, I do not under the four strings of four. S-460s are L-16 -- 6 volt batteries. So if you have strings of four, that is a 24 volt bank.

If you have enough "new" batteries to make one string, I wonder about taking just the new ones, amking one string and trying to charge/EQ those to see what happens.

An SG of 1.750 is a relatively dead battery in my book.

Vic

Cariboocoot · November 2011

Re: where to place new batteries in an older string?

Vic wrote: »

An SG of 1.750 is a relatively dead battery in my book.

Vic

Methinks you're having one of my days, Vic: normal fully charged SG is 1.265.
1.75 Volts per cell is dead.

Mind you, the OP's numbers (including configuration) don't make sense either.

pedro65 · November 2011

Re: where to place new batteries in an older string?

sorry, significant figures are your friends...

1.275.....My dyslexia is acting up.

Seems like the new batteries aren't drawing down much on spec grav. I don't have a clamp meter, but think that needs to be remedied.

I have two strings of eight, they are laid out in groups of four each.

Ill check with a clamp meter to see where the current is going....might help determine whether I need any changes at all.

thanks all.

Vic · November 2011

Re: where to place new batteries in an older string?

Coot,

Yea, but I did read what was written, "1.65 sp/gr the new ones are higher in the 1.75 range" I was thinking that this was SG, as stated.

So, Pedro, think that you may be saying that each string of eight batteries is composed of four old, and four newer cells, for a total of eight batts in each string. With two strings.

Terminal voltage is not a very good indication of State Of Charge. BUT, I would arrange one string of only new batteries (assuming that you have eight total newer batts). Charge these, and then EQ them. It is generally nothing but trouble to mix new and old batteries, in any configuration. You might then string the older batteries in one string and charge those, by themselves, and RQ those and see what happens. Vic

solarvic · November 2011

Re: where to place new batteries in an older string?

I don,t have much battery experience but have a sugestion that someone might confirm if they think it a good idea. Wht not use a battery switch and use all the old batterys in one string and the new ones in another string. solarvic:D

where to place new batteries in an older string?

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