how to double battery bank?

[birdhouse]

hello all-
my current battery bank is (4) trojan t-125's wired in series for 24v nominal.  they are about one year old, and fed by 390w of solar. 

i want to add (4) more t-125's to double my capacity.  my new bank would consist of two strings of four batteries each, then paralleled to get 24v. 

what's the best way to do this?  i was thinking it might be wise to pull two of the old batteries from their string, and put in two new batteries into that string.  idea being that each of the two strings would have two new, and two old.  maybe do it every other.  does this sound like a good idea? 

was planning on using 3/0 copper for the paralleled connections.  existing #4 is being used for interconnects, and i've got more professionally crimped #4 for the new interconnects.

am i on a good track?

adam

[mab]

As the 'old' batteries are only one year old I would suppose they are still in near new condition, so it probably doesn't make a lot of difference.

If you can measure the fully charged rest voltage (after a couple of days disconnected) of your old ones and compare to the new it would help - if the difference is less than 0.1 I'd not worry.

If we suppose there is a small difference between new & old then:-  I, personally, would put the new 4 in 1 string and the old 4 in another. That way when the voltage reg cuts in they are all seeing the right voltage, though the amps in the two strings may be slightly different. If you connect them 2 old 2 new then any small difference between them will result in 2 seeing too-high a voltage and the other 2 too-low.

Ironically I myself have a 24v battery made from two very different sets of 12v batteries in series, but that is because they are very old and have very different voltage requirements (13.9 & 13.3 v) so they cannot be paralleled with each other. To avoid overcharging the ones that 'fill up' first I use two separate 12v shunt regulators.

If your rest voltages are substantially different, then you might want to do it the other way to prevent cross-charging, but then you may need to do what I did and use separate regulators for the two halves.

Or you may prefer to keep the strings separate and switch your loads between them.


mab

[dnix71]

Why the heavy wire connections?  I use 4 ought only to connect an inverter I use occasionally. The rest of the wires are 10 gauge. My string of 8 12v deep cycle batteries in parallel is center tapped for the inverter, and the adjacent bettery is connected by 4 ought to it. Under a heavy load there will be a .2v difference between the ends but that evens out when the load is off.

I use the 10 gauge wires as a fusible links. I put my fridge off to one end and a small inverter for lights, fan and computer off to the other end.

All it takes in a string is one weak battery to bring them all down to it's level. I just swapped out 2 for that reason. One acted like it would take a charge, but if it was allowed to rest alone it would drop off below 12v after a while. The other was just old and would rest at 12.8 I took that one to work and put it in an old pallet lift and went and bought a new replacement.

The two weak ones were a different brand. All the rest will rest at 13.0 if properly charged.

You might consider keeping the new batteries together in a string and the originals together in a string. Charge both from the same source, but put a heavy duty isolation diode on the older string and use each string separately for different purposes.

[Madscientist267]

I wouldn't run old and new in series.

Leave the original bank together, parallel the new bank to it.

Use identical wiring on each string, and pull your positive and negative from opposite ends of the strings to eliminate as much balancing disturbance as possible.

If your original bank is healthy, there's no reason to isolate the two strings from each other. If not, isolation would be a must.

You want to avoid anything that will create differences in the strings, as this will cause balancing issues, leading to premature failure of one bank or the other, whichever is worked harder.

dnix - That .2v difference may not seem like much, but it indicates that the center batteries are working harder than the outer ones are. You'd be better off running equal length jumpers from each battery individually to a common point for both your positive and negative. This will even out the load distribution, and reduce the stress on the center batteries.

The alternative is to 'rotate' them physically so that they all eventually 'see' the same charge/discharge patterns. That's a PITA though considering the alternative isn't really all that expensive to implement.

My 0.02c, FWIW

Steve

[Rover]

Dnix, I've never seen my battery bank rest at 13 V, typically at 12.4 to 12.8 depending on temperature. Mine are outside, 6 g27 FLA'a  (ok walwart) .

Then again maybe I don't  see em rest, too many scheduled loads. .. need to hunt through the logger database. 

What kind of batteries you have?

[birdhouse]

wow-  this thread took off all a sudden. 

thanks for all the replies.  i believe the current bank is in great shape.  i never have the chance to fully charge them, then let them rest for a few days to measure resting voltage because they are up at my ranch, and i'm always using them during the time i'm up there. 

last time up, i got up there late.  probably midnight.  so they had a chance to rest some once the sun went down (5 hours), but their reading was 26.8v.  they also have probably only seen 25-30 charge/discharge cycles, and NEVER brought below 24.5v. 

i use the professionally crimped #4's for interconnects because they were free from a salvaged battery bank.  i'm planning on using the 3/0 for the parallel connections because it was also free, and that's where my 1800w  PSW inverter connects in. 

i was def. planning on pulling juice from opposite ends of the bank.  ie- + from end of one string, and - from the end of the other string, right where the parallel connections are. 

i really don't want to use diodes, isolation switches ect. because i'm not the only person to use my place, and if it becomes too tricky for others to use, they may accidentally pull too much from one bank or the other.  eventually causing damage to one bank or the other.   

i do need to clean the terminals, so jockeying the inside batteries to the outside spots wouldn't be hard.  plus is probably about time to do that! 

sounds like the consensus is (so long as the 1 year old bank is in good shape which i think it is) to leave the old in a string, and the new in a separate string. 

thanks for all advice!

adam   

[dnix71]

Rover the batteries I have now are Autocraft marine/deep cycle flooded lead acid hybrids. The largest that Advance Auto sells. Group 27 DC-2's. They are a few dollars more than the other size, having a 90AH/175 minute rating. $90 each with exchange.

I have two solar mppt's. The smaller is set to cut out around 14.6, the other won't go that high no matter where I set the trim pot adjustment. The set hits float almost every day, but having two controllers with different float points makes the system a 3-stage charger, which is ideal. The batteries are on the floor inside a room that doesn't get that hot. South Florida isn't Texas or Arizona. We have the Atlantic to moderate things a bit. The humidity can be nasty, but heat, not humidity is what kills batteries.

As I type this I have 12.9. My laptop is on, and the fridge is always plugged in. By tomorrow morning the bank will be 12.7 or 12.8 depending on how hard I run the fan while I sleep.
When the fridge kicks on the voltage drops to 12.8, but then recovers when it turns off.

The other set were bought from Pep Boys and they never seemed to hold a charge. They would rest at 12.6. So I have 720AH of batteries and 710 rated watts of panel.

[Madscientist267]

birdhouse -

Sounds like you're primed for parallel... Keep everything 'equal', and you should be in good shape.

You're right about the diodes/switches - If I haven't learned anything else about batteries, I've learned that KISS is the only way to go... What works on paper doesn't do so hot in reality sometimes.

The trick with multiple batteries is to view each connection to the 'outside' as the batteries would see them. Ideally, they all need to electrically be the same 'length'.

From the way you describe your intentions, I think you understand how they need to be wired.

Just in case, some crappy ASCII art for your entertainment:

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Steve