State-of-charge automatic "full" synchronisation

[ayates]

Over the previous winter I installed a new house battery bank on this ship (St. Lawrence II - Brigantine Inc.). The work was triggered by the existing batteries being toast and a switch to DC refrigeration. The bank comprises 4 x Trojan L16H-AC with a total rated capacity of 12V @ 740 AH (20 hour rate). I also installed a Victron BMV-702 battery/temperature monitor and a Victron CCGX for data logging. The battery bank is typically charged on a daily basis by an on-board generator or shore power; both feed into a Xantrex TrueCharge 40.

The attached graphs show a couple of examples of the actual charging profile, specifically the relationship between voltage, current, and the calculated state-of-charge. I find it very interesting to view what actually happened during the charge cycles.

One can clearly see where the BMV makes the decision to perform its auto-synchronisation with the battery bank, and assume that the bank is at 100% capacity. Given the subsequent voltage rise, and continued high current, I question whether the bank is really even close to 100% at this point. The BMV default configuration is:

the BMV resets to ‘fully charged’ when the following ‘charged parameters’ are met: the voltage exceeds 13.2V and simultaneously the (tail-) charge current is less than 4.0% of the total battery capacity

My friends at Victron came back with this suggestion:

Just adjust the “Tail Current” setting in the BMV set-up menu to a lower value.It is meant to be adjustable for situations like this.

I would tend to agree that makes sense, as the "100% charge" is somewhere on the downward current slope while the voltage is at 14.4V.

Does this make sense to other folk, or am I missing something? What would be a good tail current setting to use? I can adjust the tail current in a range of 0.5 – 10% of the battery capacity in 0.1% steps.


Thanks,

Allan.

[rom]

I have set mine to 1%. Most of the time I can tell the batteries have already reached 100% SOC while the SOC reading on the BMV is say close to 99%.

Problem is if you don't get back to 100% regularly the reading slowly goes away from what's real. On rare occasions it happens that my SOC reading get as low 95% while the batteries have actually reached 100%.

I can not see how this equipment can stay totally in line with the truth, so we have to accept the slight difference.

[Stu Jackson]

Been down this road years ago:

Reset the fault values.

*********************************************
For everyone installing a battery monitor: The "Gotcha Algorithm" thread, a "MUST READ"

Link-series Charging Algorithms -- The "Gotcha" Factor!

DEFAULTS are factory settings that are made to be modified to suit your setup.

Also read this one:

Programming a Battery Monitor (by Maine Sail)

Keeping Your Battery Monitor More Accurate Photo Gallery by Compass Marine How To at pbase.com

[smac999]

Your first issue is charging a 740ah bank with a 40a charger. You should be 100A+

It looks like only 20a is going to the batteries. So you probably have a 20a load on the boat. Effectively leaving you with a 20a charger to charge a 740ah bank.

In 3rd pic I wonder when the current drops more (10a at 13.5v at 80% soc) Is it actually the bank taking less. Or more loads put on, and the charger is still maxed?

The meter thinks your batteries are taking small amps at a float voltage. When in reality the batteries want more. But your charger is maxed out and can't even hit its bulk voltage. In the 3rd graph. The one I looked at. The meter reset before the charger even hit absorption. While the meter probaby thinks you're in float.

Get a bigger charger.

[Thumbs Up]

Keep an eye on the performance of these and if one starts to show low specific gravity and the bank won't stay charged you could have a bad battery. I say this because I think that some of the L16 ac Trojans are defective and if you can catch it in time you might get Trojan to honor the warranty. I had one go bad after two years with no abuse and there is a recent thread here where someone else had the same problem.

[mitiempo]

Originally Posted by smac999

Your first issue is charging a 740ah bank with a 40a charger. You should be 100A+

It looks like only 20a is going to the batteries. So you probably have a 20a load on the boat. Effectively leaving you with a 20a charger to charge a 740ah bank.

In 3rd pic I wonder when the current drops more (10a at 13.5v at 80% soc) Is it actually the bank taking less. Or more loads put on, and the charger is still maxed?

The meter thinks your batteries are taking small amps at a float voltage. When in reality the batteries want more. But your charger is maxed out and can't even hit its bulk voltage. In the 3rd graph. The one I looked at. The meter reset before the charger even hit absorption. While the meter probaby thinks you're in float.

Get a bigger charger.

Agree 100%.

Get a larger charger. Your 40 amp charger is only capable of just over 5% of the bank size - and this is without any loads on the boat. That is basically a float voltage for a battery bank of that size.

[DeepFrz]

Isn't your bank really 870 Ah? Maybe you have set the Ah rating to low.

[mitiempo]

Originally Posted by DeepFrz

Isn't your bank really 870 Ah? Maybe you have set the Ah rating to low.

Good catch. Trojan's spec sheet does state 435 AH @ the 20 hour rate.

http://www.trojanbattery.com/pdf/dat...ata_Sheets.pdf

[ayates]

Originally Posted by Stu Jackson

For everyone installing a battery monitor: The "Gotcha Algorithm" thread, a "MUST READ"

Link-series Charging Algorithms -- The "Gotcha" Factor!

DEFAULTS are factory settings that are made to be modified to suit your setup.

Also read this one:

Programming a Battery Monitor (by Maine Sail)

Keeping Your Battery Monitor More Accurate Photo Gallery by Compass Marine How To at pbase.com

Brilliant information! I have devoured many article's on Maine Sail's site, I am surprised I missed that one.


Thanks,

Allan.

[ayates]

Originally Posted by mitiempo

Good catch.

Very good point. I had stuck in my mind the capacity of the L16RE-B which was my original plan. I'll double check the BMV configuration.


Thanks,

Allan.

[ayates]

Originally Posted by mitiempo

Agree 100%.

Get a larger charger. Your 40 amp charger is only capable of just over 5% of the bank size - and this is without any loads on the boat. That is basically a float voltage for a battery bank of that size.

Totally agree. Unfortunately I couldn't fit in that upgrade last year. My thoughts are to upgrade to 2 x Victron Skylla 12/60 running in parallel. That will give me 120A of charging capacity, along with redundancy in the event one unit fails.

That would cover generator and shore power driven charging. Ideally a high output alternator would also be used, so as to get similar capacity for engine driven charging.


Thanks,

Allan.

[ayates]

Originally Posted by Stu Jackson

DEFAULTS are factory settings that are made to be modified to suit your setup.

Greating reading.

My Trojan rep came back with (for the L16-HC):

• Peukert factor is 1.79
• Charge efficiency is 85%

Thanks,

Allan.

[ayates]

FYI... Found an nice instance of a complete charge cycle in the logged data. See attached graph. The Xantrex charger is putting out its full output of 40A (29A+12A; the dips lower than 12A are from the fridge which runs on 120V when available). Not actually sure what triggered the switch from absorption to float, as neither the time nor the current line up with the published Xantrex algorithm.

I will definitely be updating the battery monitor configuration parameters.


Allan

[billknny]

I am going back to repeat what has already been said: The battery charger is woefully inadequate for both your battery bank and your rate of power use.

Let's do just a little arithmetic:

***You frequently pull power of of the batteries at a pretty constant 20 amp rate. If you do that for 12 hours, then you have withdrawn 240 amp-hrs.

***If you then start to charge with your 40 amp charger, you only have 20 amps available to feed to the batteries, the rest gets used by your loads.

***If your charging was 100% efficient (and it is nowhere close) you would need 12 hours of charging to replace the 240 amp-hrs you pulled out in the previous 12 hours!

Basically, you need to run this charger at least 18 hours a day just to keep up. Look at the voltage your batteries are dropping to: less than 12V! Ouch... they are nearly dead already.

[smac999]

As above. At under 12v. With a 10-20a load. You are under 50% soc. either the meter is way off. Or you have already toasted the new bank. 12.8v full battery. 12v 50% battery.

Get a bigger charger.