96v/200Ah Battery bank imbalance

11JAGAN

Hi Friends,

i have a problem with 96v-200Ah battery bank of 5Kva backup system

Details of the backup system:

Application:Backup

Inverter/charger : 5Kva / 96V
Brand :Luminous Power technology (A Schneider Electric company)
It has its own AC and DC MCB .

Battery bank :8 x 12v ,200Ah deep cycle Agm battery (Ritar power Corp)

Room temperature :22-28 degree Celsius

Problem :

The battery bank is unbalanced
The backup time of the system was reduced more than half.so i checked the battery bank voltage it was about 104.9Vdc. when i checked the voltage of individual batteries i find some thing odd.
Though all the batteries are connected in series the first four batteries have one set of voltage ~13.2 V(4 batteries in bottom rack) and second four batteries has another set of voltage ~12.52V( 4 batteries in top rack).

Attachment not found.

can you please tell me what could be the reason?

Thanks in advance!

BB.

OK... Just guessing here.

First, AGM Batteries cannot be "high voltage" equalized". Flooded cell batteries, you charge at a high voltage (15.0 to 16.0 volts or so on a 12 volt battery bank) so that some cells are "over charged" (and gas/generate hydrogen+oxygen) so that the "leakage current" can charge the other "low cells" in the string.

With AGM/Sealed batteries, over charging generates too much hydrogen and can overheat the catalyst (which combines Oxygen and Hydrogen back into water) and/or cause the cells to vent from over pressure. So--AGM "equalization" is just leaving the batteries on extended absorb (~14.4 volts) for many hours extra to cause the cells to equalize. For batteries this far apart in state of charge--I am not sure that AGM equalization will do anything useful.

So, you are left with getting a 12 volt AC battery charge and charging each of the low state of charge batteries until each is full... Then try "equalizing" every 2-6 months and/or if you see battery voltage spread again.

Next question, why did it happen.... Again, guessing... The batteries with lower state of charge are in the upper rack. They may be in "hotter air" (warm air rises, heat from the lower bank "warming" the the upper bank, etc.). More or less, for every 10C increase in battery temperature, you probably see a 2x increase in self discharge rate. So--The upper bank, being hotter, is simply self discharging faster vs the cooler/lower bank.

Long term fix--You can move the lower bank to the upper bank every year... More "AGM equalization", using a 12 volt battery charger to bring up discharged cells, re-rack so that all batteries are at the same level (equal temperatures)--Some people still find they need to shuffle batteries even in equal elevation racks.

If you don't have a 12 volt AC battery charger handy... You could try clipping 12 volt @ ~2 amp lamps (brake light bulbs, or similar) to each "full battery"--That will let some charging current bypass the full batteries so that the low batteries can recharge... If your low batteries need ~20 AH of charging, then the bypass bulbs should be (20 AH / 2 amps ~10 hours to bring up the low batteries....

You can do this when you are charging the bank and monitor battery voltages closely (check every hour). Or you can do this at night/not charging, and dishcharge the "full batteries" down to the state of charge of the low batteries (all battery voltage equal to ~0.10 volt or less). And next time you recharge, all should be well.

Cells going out of balance can be an issue--And for AGM, LiFePO4 and other low loss/efficient batteries--Cells going out of balance is an issue and why frequently battery management systems are used for these types of batteries (Lithium). And why I sort of have a problem with very high voltage battery banks... More battery cells in series, more things that can "go wrong" undetected (you need ot check battery voltage balance every 1-4 weeks or so to catch problems early). Flooded cell lead acid batteries go "out of balance faster/easier", but are also easier to "equalize" with simple over voltage charging.

Anyway, some thoughts on your question/problem.

-Bill

11JAGAN

Thanks BB for your response.

when i find this problem.
To fix the imbalnce i rearranged the battery bank like below .

Attachment not found.

i going to absorb the bank for 1 week.

can i find any difference in this short period of time?

11JAGAN

48v-200Ah battery bank


i also have similar problem with another battery bank ,in this case its a 48v/200Ah battery bank.
all the batteries are in the same rack like below .please see the voltage of individual batteries.
Attachment not found.


what could be the reason and how can i overcome this?
can i sort it out by charging that particular battery(12.5v) with a 12v battery charger?

awaiting for reaponse.

Dave Angelini

The more you post about this the better the advice you will get. Give some dates and how old the batteries are, who makes them......... Since both systems have similar problems it is a battery problem. Have you load tested the batteries? I would be careful about running a week long absorb without a means of measuring battery temperature. What does the battery manufacturer suggest? What charge voltages are you using?

11JAGAN

In both cases ,we installed same age of batteries and from same brand.

Dave Angelini wrote: »

Have you load tested the batteries?

i didn't get you.

i have attached the battery data sheet .

The age of battery bank is about 6-8 months.

Regarding charger
Phase1- Boost voltage charging ~14.25V
Phase2- Float charging ~13.25V
Phase3- Trickle charging

BB.

Regarding batteries all on the floor (for example)--A few people here have had problems (like the battery closest to the + bus connection of the string did not charge as well as the rest of the cells in the bank)--And they would pull the cables and reshufle the batteries (move the weak/lower charging batteries to the other end of the string).

Series string batteries should not matter the wire lengths between cells.. But in your picture, the lower shelf batteries have their jumper cables very short... And the upper shelf batteries have longer jumper cables. If you can rotate the upper shelf batteries 180 degrees, then you can use shorter jumpers (and have less voltage drop due to cable resistance--usually a good thing).

Load testing--I think is what Dave may be asking. Just take a large load resistor and place it across one battery at a time--Check the voltage after XX seconds and see if the battery is able to sustain the load... Those with poor load response (low voltage under/after load) either need charging and/or have some other problem.

Dave could also be asking if you discharge your system to be worst case normal use (say two days of normal loads with no AC/Solar recharging)--How do the batteries perform?

In your case, you already have 4 "under performing" batteries (for some reason). I believe you need to get those batteries fully charged... Whether you do that with a 12 volt charge controller connected to each battery, or use brake lamps across the "high voltage" batteries (to shunt current around the high voltage batteries)--Your choice.

And swapping top shelf batteries with the bottom shelf batteries (or shuffle for the single shelf batteries) to see if battery location affects ability to charge for your case.

AGM batteries are very nice for many reasons--However, estimating state of charge/balancing/equalization/diagnosing problems (no specific gravity/per cell measurements possible) makes them difficult to maintain/figure out what is wrong if there are problems. Using a brake lamp to shunt around high voltage batteries is pretty easy and relatively safe (avoid sparks/wear goggles)... But it does need you to monitor the batteries to make sure you do not create other problems.

And remember that elevated charging voltage (or in AGM's case, lengthening time of absorb) is difficult too--Especially with "high voltage" battery strings. 96 volts or 8x 12 volt batteries in series... You want around 14.4 volts maximum across each battery... But with that many cells/batteries in series, it is very easy to get 14.2 volts across 4 "low voltage" batteries, and 15.2 volts across the 4 "good batteries".

And as Dave says/asks--That means that 4 batteries (in the above case) are "over charging" and possibly running hot (too much gassing, catalyst recombining gases back into water--heating up the catalysts/batteries).