2 Chargers, 1 Battery Bank - Different Voltage?

KenBrown

Hi, I've been looking around trying to get this question answered - but have been unsuccessful so far.  My issue is that I have 12x12v 250ah Deka batteries wired for a 48v system, 4s3p.  I am having a hard time keeping the 12 batteries with matching voltage, my parallel strings stay right in line with each other, but not each 12v battery.  I'm currently using 2 flexmax 80 charge controllers to bring me back up to full charge each day.

My idea, is to wire the battery bank 1s12p and connect that to one of my two charge controllers and let that one charge it at 12v for however long it takes to equalize each 12v battery, and leave the other controller charging at 48v.  I've read that you don't want to apply different loads and 'tap' the batteries to make them go out of sync, but that's not what I'm trying to do.

Is there anything I'm missing as to why this wouldn't bring all 12 batteries into alignment with each other?

Thanks in advance for any/all feedback.

KenBrown

Or, even just wiring the batteries themselves 1s12p and leaving everything else alone and just let them 'self adjust' with time?

vtmaps

Your basic problem is that parallel batteries are a very poor design choice for a system that cycles frequently.  Putting 12 batteries in parallel will only make your problems worse.   When you charge parallel batteries it is difficult to get the charging current to divide evenly among the parallel paths.  With your existing configuration, the batteries should be combined with a heavy bus bar.  Each string of batteries should have its own fuse or circuit breaker. 

Your best approach to equalize the batteries is to charge each 12 volt battery one at a time.

--vtMaps

BB.

Do you have a good quality hydrometer? Have you measured the SG when the battery bank is "full"?

Usually, the difference in battery (and cell) voltages are a reflection of different specific gravity levels of each battery/cell. And, the usual answer is to equalize the batteries (controlled over charging the entire battery string so that the current going through the "full cells" is used to charge the less than full cells.

Generally, for a 12 volt flooded cell deep cycle battery, you are looking at 15.0 to even 16.0 volts (higher voltage for "industrial" type batteries). And around 5% down to 2.5% current during equalization (2.5% of a 100 AH cell would be ~2.5 amps). And watch the battery temperature while equalizing (you may need to stop and let the bank cool, then start again).

Also, new battery banks do seem to have an issue with individual battery balance vs one that has been cycled 20-100 times or so.

-Bill

KenBrown

They are sealed gel batteries, so without going a little crazy on them drilling holes, not happening for SG check  Does it matter that I'm only proposing a temporary 12v parallel and then going back to everything 48v?

vtmaps

KenBrown said:

They are sealed gel batteries, so without going a little crazy on them drilling holes, not happening for SG check  Does it matter that I'm only proposing a temporary 12v parallel and then going back to everything 48v?

It won't hurt to charge them with a 12 volt (nominal) charger... in fact, if you do it correctly it's better than charging them 4 to a string at 48 volts.   The key is to charge them one at a time.  If you put 12 batteries in parallel you will make your problems worse.

--vtMaps

BB.

You might need to charge each battery separately.

GEL and AGM batteries do not do well with higher voltage equalization (especially GEL batteries).

-Bill

South Africa

Here by us, a few guys found these and are using them with great success on 48v banks, where they have midpoint differences.
See here: http://www.ebay.com/itm/Battery-equalizer-balancer-for-48v-lead-acid-LiFePO4-balanced-charging-/252370442152?hash=item3ac27357a8

Most of the guys said after a week or so the bank was in near perfect balance.

Victron has this: https://www.victronenergy.com/batteries/battery-balancer

And BMV702's are used with great success to warn you about the midpoint mismatches: https://www.victronenergy.com/battery-monitors/bmv-700

Connections look similar to this: https://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf

Hope this helps.

vtmaps

South Africa said:

Here by us, a few guys found these and are using them with great success on 48v banks, where they have midpoint differences.

Those all work to balance batteries that are in series. 

In Victron's example of six 12 volt batteries in a 24 volt system, they made a bank of three 12 volt batteries in parallel, and then put that in series with another 12 volt bank composed of three parallel 12 volt batteries.  The balancer can try to keep the two banks at the same voltage, but it can do nothing to equalize the current flows through all the parallel pathways.

--vtMaps

South Africa

vtMaps, is this not what you would do for 12 x 12v batteries, 48v system?

vtmaps

South Africa said:

vtMaps, is this not what you would do for 12 x 12v batteries, 48v system?

The point is that I would never have 12 x 12v batteries in a 48v system.  If I need that AH capacity, I can achieve it with a single string of cells. 

Designing a system almost always involves compromises.  Parallel batteries are not an optimal design choice.   I would make many other compromises before I would use parallel batteries.

--vtMaps

South Africa

Yes, I agree, same here for the more batteries, the more problems one has.

But, having said that, the problem is there, and needs a solution, that being my focus.