Battery Voltage

[Constellation1]

I have three battery banks - thrusters 2x8D, house 2x 8D, starter 1x 8D.

Fully charged voltage for the house and starter banks is ~13.4v
Fully charged for the thrusters is ~12.7v and drops to ~11.9v pretty quickly when unplugged, and I had one experience with weakened thruster performance already.

Batteries are all 4 years old. Is it safe to assume that this bank has served it's useful life just replace them?

 

[Iggy]

I would check and clean all of the connection on the thruster side of things first. Use a meter and check battery and charging voltage first. Before thinking you need new ones.

 

[Bacchus]

I have three battery banks - thrusters 2x8D, house 2x 8D, starter 1x 8D.



Fully charged voltage for the house and starter banks is ~13.4v

Fully charged for the thrusters is ~12.7v and drops to ~11.9v pretty quickly when unplugged, and I had one experience with weakened thruster performance already.



Batteries are all 4 years old. Is it safe to assume that this bank has served it's useful life just replace them?

I would doubt it but also wouldn't assume anything w/o confirming.
I will assume FLA as you didn't mention AGM?
If FLA I would charge and check all cells w/ hydrometer to verify.
Start at 13.4 V you are reading charger on float or surface charge if charger recently disconnected.
Thrusters at 12.7 V charged sounds about right unless also charger just removed. Is the 11.9V under load or just after charger removed?
If FLA I would check cells and then consider running an equalization charge and recheck to see if anything improved.

I had a similar set up w/3 8Ds for house/ start & thrusters. Except I recently moved my start from combo w/ house to the thruster bank so my house was only deep cycle house loads. Thrusters only used when engine alt operating & supplementing battys.
FLA 8Ds not great for deep cycle so you would be better off considering other options for house... AGM or FLA GCs
I went with AGM as they are better at both start/thruster and deep cycle. My AGMs are 8 seasons old and still in good shape when I check V in spring after winter storage disconnected but w /o any ability to charge.
I'm done with 8Ds as they are too much bother / effort to move and decided to replace each 8D with a pair of AGM GP31... very close to equal or better CCA and AHs and easier to handle. Trojan and East Penn confirmed to me that with AGM GP31 are equivalent to GCs for both CCA and AHs.
I would also consider combining start & thruster banks so you only have 2 banks and you could enlarge whichever you feel is the weakest vs use.
I have a gennie with its own start batty that is charged from its alternator and is my emergency back- up. Guessing you also have one although not mentioned

 

[SteveK]

How are they kept charged? Curious why they do not start the same voltage 13.4 as the others. An isolator could explain that as it loses .7V
But yes if the cables are removed and it drops quickly to 11.9 then it is shot. You could try an equalize charge on it.

 

[DavidM]

As others have noted if you have a single shorepower charger with switches or combiners connecting your three battery banks, then they should all be at the same voltage.

If you have different chargers for each bank then there could be something wrong with the charger that charges the thrusters. Or as Steve notes above, with a single charger and a diode based isolator that will result in a lesser voltage. If so why not bypass the isolator and fully charge the thruster bank and check its voltage again.

FWIW diode based isolators are always problematic due to their voltage drop. A combiner or ACR is a much better way to isolate them.

David

 

[fgarriso]

How far from the thruster are the batteries, and how are they being charged? Most thruster installations have insufficient power cables for the motor and chargers.

 

[Constellation1]

Great feedback so far - thank you. More details - which I should have provided earlier.

All batteries are AGM
They are kept charged with a singles Xantrex Charger / Inverter
Batteries go through VSRs
I am getting the voltage readings at the battery terminals for each bank
I turned off my charger and the voltage dropped from 12.65 to 11.96 in three minutes. It stabilized there.

Does this give more guidance, or should I get new batteries ASAP. They are not proving easy to find in Key West right now.

Thanks everyone

 

[fgarriso]

Do you have single or twin engines? If twins you don't need thrusters anyway.

 

[Bacchus]

Den
"I turned off my charger and the voltage dropped from 12.65 to 11.96 in three minutes. It stabilized there."

The 12.65V on charge indicates a problem IMO. That is very low for AGM full charge and quick drop to 11.96 w/o load further confirms a problem.
I would think it is time for a new thruster bank but would also investigate possible causes of what seems like an early demise.
What is the charging arrangement while underway from engine Alt?
Do you have any batty monitoring systems aboard?

If AGM 8Ds are scarce (and that's what you are looking for) you might consider some alternatives.
A pair of 6V AGM GCs are roughly equivalent to an 8D AHs & CCA
Likewise a pair of 12 V AGM GP31 are roughly equivalent to an 8D AHs & CCA

Either of the above may be easier to come by than 8Ds...

I would be checking the charging carefully & frequently to make sure batty are not left at low SOC for extended times and are brought up to full charge at least once / week.

 

[SteveK]

I turned off my charger and the voltage dropped from 12.65 to 11.96 in three minutes. It stabilized there.

12.65 is about where a fully charged battery is at rest. The thruster battery dropping to 11.9 and holding suggests a battery issue. Can you charge up to 12.65 and then disconnect battery + terminal of thruster battery, then check voltage.
If you still get 11.9 consider an equalize charge on that battery, or take it to a battery shop, or throw it away and hook up thruster to starter batteries as suggested. JMO

 

[DavidM]

Don't forget that a fully charged FLA battery connected to a three step charger will always be more than 12.65. The charger's float voltage should be 13+, just like two out of the three battery banks reported in the first post.

David

 

[SteveK]

Don't forget that a fully charged FLA battery connected to a three step charger will always be more than 12.65. The charger's float voltage should be 13+, just like two out of the three battery banks reported in the first post.

David

David, was that to me or the OP.
The OP said he turns off charger and the batteries are then at rest voltage 12.65, only one drops to 11.9
Float voltage is 13.5 but varies with temp sensor

 

[Scottwb96]

another question to add into the mix. You mentioned they are all AGM's. But are they the correct type of AGM for the job? Meaning deep cycle vs. start/thruster?
This could also be why some of the batteries went out faster than others. As others mentioned, charge them all up and disconnect all the batteries. Wait a few hours at least then check each battery individually (not bank but the actual separated battery). That will let you know at least where/what has the issue. Then start working backwards from there.
1. connections
2. proper wire size
3. charging capabilities (alternator/charger)
4. proper battery type for the various jobs

etc. etc...

I would also strongly consider ditching the 8D's for Group-31's instead. They are easy to come by and A LOT easier to get in/out of the boat...

 

[Bacchus]

another question to add into the mix. You mentioned they are all AGM's. But are they the correct type of AGM for the job? Meaning deep cycle vs. start/thruster?




4. proper battery type for the various jobs



etc. etc...

When I queried Trojan and East Penn as to which was better for deep cycle and start AGM GCs or AGM GP31... both responded that their performance was equal for BOTH applications.
The guidelines for FLA (different plate thicknesses & designs) apparently doesn't hold for (at least their) AGMs. You might notice most AGMs these days spec both AHs and CCAs reflecting the above.
I have the same East Penn AGM 8Ds in both house and start / thruster use and both have completed 8 seasons with winter storage w/o ability to charge them.

 

[DavidM]

That is my understanding. Unlike FLAs where thicker plates, more room for sulfate below the plates, more room for electrolyte above the plates are all important for true deep cycle performance which is rarely met by anything other than GC batteries, none of these characteristics apply to AGM. An AGM is an AGM whether it is in starting or deep cycle service.

David

 

[Scottwb96]

FWIW, I was told there ARE dual purpose AGM's. But that they also have AGM's that are specific to start (high surge amps) as well as AGM's better suited for long deep cycling uses.
So if you have one that's NOT up to high CCA's then shorter life can/should be expected...

Lol, but who knows, seems like info/tech changes every 90-120 days anyway...

 

[ranger58sb]

I have three battery banks - thrusters 2x8D, house 2x 8D, starter 1x 8D.

Fully charged voltage for the house and starter banks is ~13.4v
Fully charged for the thrusters is ~12.7v and drops to ~11.9v pretty quickly when unplugged, and I had one experience with weakened thruster performance already.

Batteries are all 4 years old. Is it safe to assume that this bank has served it's useful life just replace them?

I turned off my charger and the voltage dropped from 12.65 to 11.96 in three minutes. It stabilized there.

Does this give more guidance, or should I get new batteries ASAP. They are not proving easy to find in Key West right now.

13.4V is likely the charger float voltage. Not necessarily actual battery voltage.

12.65 with the charger off -- and after resting for a decent period -- is more like a fully charged 12V battery.

Voltage drop to 11.96 in three minutes suggests batteries are at end of life.

If that large voltage drop is only specific to one bank, you may not need to replace the other two banks. But better to confirm your tests on all three banks with charger off. Maybe even load test batteries in a bank that drop off but not quite so quickly; might be some individual batteries are temporarily salvageable.

-Chris

 

[Constellation1]

Thanks everyone - I'm confident now that the bank in question is simply at end of life. I'm searching for 2 8D AGMs in Key West / Marathon now. FLA are easy to find - AGMs are proving much harder.

I hear and agree with the Group 31 position - but given that I am away from my home port I will just drop in the same configuration for my thruster bank. I will move to Group 31s with my house and starter banks when I'm back home.

Cheers!

 

[High Wire]

Try this.
Disconnect the thruster bank so that the two batteries are completely separate. Leave them alone for an hour then read the individual terminal voltage with a digital voltmeter. One of them could be bad dragging the other down. Report here the individual voltages.
If it’s a 12 volt thruster you can run on only one good battery for shorter time periods. If it’s a 24v thruster then you should bite the bullet and get a new pair.
My thoughts are one of the two batteries are still good and has value to someone. You are looking at $800 each or so easy for a new 8D agm.