My New Battery Bank Seems to Sag in Voltage

[Matrix]

So the thread I started asking about LBCO for my inverter at times seemed to deviate, and so I wanted to start a different thread specifically about my new battery bank. I am needing help understanding various characteristics and what is considered normal battery behavior and what might be a problem with the batteries or the system.

Here is the set up:
- 4 New Trojan L16H-AC 435Ah batteries in series at 24v.

• Fully charged at 25.7v
• Connected to each other with 4/0 cable that is 9" long each connection x3 cables ... and to the inverter from the battery bank with 4/0 cable that is 9 feet long.
• All the individual batteries at open circuit are balanced with in a few 1/100th's of a volt.

- Schneider Conext SW 4024 Inverter / Charger

• AC out on #8 with a 30 amp breaker
• AC in on #8 with a 30 amp breaker (for back up support AC - which does not seem to be working correctly so I am working with Schneider about that - and charger operation)

- 6 Selectable Circuits coming in from the inverter with various loads.

1. Fridge 20amp breaker (runs between 100-300 watts using 2350 Wh's / day)
2. Well pump 20 amp breaker (Start up of 800w / running at 735w using 550 Wh's / day. )

(All other loads below less than 600 Wh's / day)

3. LED lights 15 amp breaker
4. LED lights 15 amp breaker
5. Various receptacles 15 amp breaker (most not being used)
6. LED light and receptacles 15 amp breaker (not in use or turned on on the battery)

The most watts I can load up on the entire system with everything I can find to run on every circuit is less than 2000 watts (excluding fridge and pump start up surges that last 30 secs or less).

Here are the Conditions in Question: When I am running this 24v system WITHOUT the AC in Support at between 500 and 600 watts on the system running on the batteries only, my 4 fully charge Trojans voltage will begin to drop rather quickly. At 600 watts, that 25.7 voltage starts to drop right before my eyes down to 25.2v and if the pump kicks on taking the system to 1300 watts for 30 seconds the batteries will Sag to 24.6v at the that 1300 watt load. When the startup surge relaxes and things setting down to about 600-800 watts with the pump running (usually less than a 30 seconds) the battery voltage be about 25v or just below.

But during that surge of start up, the SW 4024 inverter gets a fault that says "DC Under Voltage Fault" (which means according to the manual the DC voltage at the inverter terminals dropped below LBCO for 10 seconds). I have the LBCO set to 24v and I have a delay on LBCO for 30 seconds ... so the Inverter does not shut off. But the inverter records a momentary volt drop below 24v. I do not see it on the display because it happens so fast. But in the surge for 15-25 seconds I am seeing a voltage of 24.5 +/-

When the pump kicks off, and the load goes to 200-300 watts, the battery voltage goes up to about 25.2v and as soon as I turn off the inverter and return the system to an AC Line in for supply, (that is the batteries are just sitting ideal) the battery voltage returns to 25.4 to 25.6v +/- with in a minute or two .

Here are the questions:

• Given internal resistance, would this be normal behavior for New Trojan L16H-AC 435Ah batteries x 4 in series for 24v?
• Am I correct in assuming that the 435Ah 24v battery bank should be good for 10.4 kw / 2 (50%) = 5.2 kw of storage?
• My total daily load that I calculated is 3400 watts? Should I be able to go for 24 hours with my existing battery bank? (3400/24v = 142 Ah)
• But how much of the time you can run on battery is effected by heavy loads, like start up loads and extended heavy run loads? Say 1300-1800 watts for 10 to 15 minutes 2 times a day
• How long could I expect the system to operate under battery power alone if I am drawing an average of 200 - 300 watts per hour? (at 300 watts I am figuring I could run for 17 hours with out any charge input before I hit the 5.2 Kw of storage in the battery - is that correct?)
• Does all this sound typical ... Or Do I need a larger battery bank to handle this load for 1 day of autonomy and especially the the larger start up loads and brief run times even if not very long because the voltage will sag? ( say 8 L16s at 870Ah)

Help me make sense of this. Is this Typical and I am undersized ... or do I possibly have a problem with the system or batteries? Any comments or suggestions about this would be appreciated. Thanks for your time.

[Matrix]

OK ... so far I have found this link about Amp Hours vs Kilowatt hours ... and this link about Peukerts Law to be pretty helpful and in simple English

[Matrix]

I have found these links to be helpful

http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-fundamentals/amp-hours-vs-kilowatt-hours/

http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-fundamentals/peukerts-law-and-exponent-explained/

[Raul]

I had in the past similar behaviour with a new bank FLA Rolls S480 and a lot of voltage sag with anything higher than C/10 discharge . When new, the plates are forming , so it takes few cycles to form. Mine is half your inverter but double the battery, almost. Now I can chuck at it a C/5 draw and is solid as a rock. I did checked the SG and upped the absorb by quite a bit . I follow the new updated charging parameters that are higher than the old ones.
To keep the SG at 1,27-1,28 it takes me 2,45-2,55vpc.

I think you are undersized for peak loads you don't want to discharge under C/10 for to long. The efficiency will suffer the higher you discharge . I oversized the bank 6x daily loads and that gives me a real 3 day reserve to 50% DOD and draw anything from C/100 to C/10 Max ; mostly C/25. That gives me good eficiency and low pewkert taxation.
Also I think your 9' cable to inverter is to thin for its length.
I'm sure the pro's will enlighten you better ;
i just share my expirience.

[Matrix]

I think also I have done what Moderator Mike once said ... "you have re-discovered Peukert's law".

I have been doing a lot of reading since this post and what you are saying coupled with the link I posted here looks like the root of my "concern". Am I on track?

So help me understand this. I see in the Trojan specs that a 24v battery bank is at 100% charge at 25.48v (I do not yet have a way to test SC ... that arrives tomorrow and is first on my list to test). But I have seen some threads here saying fully charge off the charger is more than that. Like upwards of 27v+ (I can't now find specific threads) What is the fully charged state of my battery bank?

I question this, because I read last night after this post that Midnite solar actually ships the Classic 150 with very low charge specs by default to protect sealed batteries. I have been charging all week since Monday with the defaults. I think I may be set too low ... so my battery, while coming off charge at 25.5v might be being under charged.

The Classic will not let me change Bulk charge settings. Only Absorb and Float. Absorb was set to under 28.5v / Float to 27.2

Trojan says I should set my Bulk to 29.67v and my Float to 27 But there is NO Absorb spec on the L16H-AC spec sheet. And the classic does not have a Bulk adjustment only absorb and float.

Without a Bulk setting adjustment, should I set my Absorb to 29.6v? Or more Or less? I think on top of anything else, I may be starting off with a battery that is not fully charged.

Right now I have my Absorb set to 29.6v and 2hrs 15min and my Float set to 27v.

[Matrix]

To keep the SG at 1,27-1,28 it takes me 2,45-2,55vpc.

Please elaborate more. I do not fully understand this statement.

[littleharbor]

Trojan says I should set my Bulk to 29.67v and my Float to 27 But there is NO Absorb spec on the L16H-AC spec sheet. And the classic does not have a Bulk adjustment only absorb and float.

Without a Bulk setting adjustment, should I set my Absorb to 29.6v? Or more Or less? I think on top of anything else, I may be starting off with a battery that is not fully charged.

Right now I have my Absorb set to 29.6v and 2hrs 15min and my Float set to 27v.

Bulk and Absorb are interrelated. When in bulk the voltage rises from whatever it begins charging at up to the set absorb voltage point.There is no "Bulk setting because bulk is allowing everything your array can give. The cutoff voltage transition from bulk to absorb IS the absorb setting for example 28.8 vabs. At 28.8 the controller will hold the voltage steady while the battery continues absorbing current. At a timed or metered current level your controller will transition to float when it determines your battery is full and wont absorb any more meaningful current.

[Matrix]

So would it be OK to set my absorb to 29.6v as the specs say from Trojan say Bulk is 29.6 but I have not Bulk adjustment. Or back off Absorb to 28.8?

[littleharbor]

Yes, go with Trojan's numbers. As I said, there isn't a bulk setting. Bulk will charge the batteries up to the absorb voltage. The battery voltage at the beginning of the charging cycle is whatever it is. In bulk mode it will rise up to that set absorb voltage point. Put it this way That setting could also be considered the BULK CUTOFF voltage point.

[Raul]

What I meant is 2,45-2,55 volt per cell. If 24v bank that would be 29,4v to 30,6v depending on temperature. I have temp sensor in middle of the bank. Setting absorb at 29,6v is a good start , specially that is specified by Trojan. The 28,8v could be OK for summer where you have more time with solar charging and elevated temperatures.
Your hydrometer will tell you what is your voltage. With the L16's , being a tall box you need to give it some spirited charge otherwise the electrolite can stratify.

[Matrix]

Littleharbor, I see u have a Trace SW4024. Is that similar to the Conext SW 4024? I have a question about that inverter and AC in. Are u using AC in and AC support mode?

[littleharbor]

Trace SW4024.jpg

Littleharbor, I see u have a Trace SW4024. Is that similar to the Conext SW 4024? I have a question about that inverter and AC in. Are u using AC in and AC support mode?

No they are different inverters. Here's a picture of a Trace SW 4024. I don't have any AC in or Gen support. Fortunately I have never needed it.

[Matrix]

OK thanks ... I tried to PM you but I cannot see where this forum allows for Private Messages.

[Matrix]

OK getting the charge voltage up to 29.6v absorb charge as referenced in the spec sheet from Trojan seemed to help. Now can someone answer me about the Absorb time. I have it set for 2hrs 15min at the moment. I think the absorb time goes with time or charge voltage whichever comes first. But Looking at my solar charging graph today, I noticed it stopped absorb charging right at 2hr and 15 minutes. Does that mean that the batteries may or may not have been fully charged, but that charging stopped due to the time setting?

What is a good number to set for Absorb time?

And what would be a good number to set for Equalization time?

[Mike90250]

i would:
a) increase the delay to 2 min (from 30 sec)
b) re-confirm you ARE getting a full charge, gradually increase absorb time (15 min chunks) to see if it helps your state of charge.