Start Battery issues

[SeaAir 3]

Hello everybody...

I'm looking for advice about a vexing issue with my start battery.

Onboard I have 2x AGM house batts (in series.. 320ah) and a wet cell crank (770cca) charged by a standard 80ah alternator and a 7 stage charger when connected to shore power when in the marina.

The house batts are twice the age of the crank and appear to be pretty healthy as we can be on the pick (eg.. overnight)  using energy and they're maintaining a good state.  However if the crank battery has been idle for a similar period,  it's usually 'sluggish' and can often struggle to turn over the motor when she's cold.  

I have a suspicion that my charge setup isn't fit for purpose due to the mix of AGM and wet cell technologies and that the start batt is getting 'short changed'. 

I'd appreciate any ideas or comments from you gurus who are used to playing with all this electronic black magic stuff..  many thanks. 

 

[CarpeDiem 510]

Is there another alternator?

If yes, is your house supply 24v or 12v?

If no, how are you charging the start battery currently? 

[SeaAir 3]

No.. the only charge source when not connected to shore power is the single 80amp alternator via a VCR.

It's a 12v system. 

[Leftred 45]

VCR? Ah that's your problem then, Beta or VHS?

Hilarious. Seriously though, that's basically my setup, and I've had the same issue in the past with sluggish start when out sailing for a few hours. I've assumed it was a dodgy earth return from the engine, and that when I was sitting at 13+ volts fresh off the charger/alternator it was OK, but as the start battery settled at 12.8v or whatever it became more of a problem.

That's a only guess though, and I haven't been out since replacing my connections and cables. Would love to hear from more knowledgeable people.

[LBD 169]

Have you checked all the cables and connections between start Battery and Motor...Cold motor takes more cranking and poor connections or old 1/2 broken cable ends exacerbate the problem which will be more noticeable when the engine is cold. 

[CarpeDiem 510]

Does your vsr go both ways? Eg if your 7 stage charger is plugged in, does the vsr let it charge your start battery? 

It's curious that you are only having this issue when you are out. Eg you have not indicated that it happens after the boat has been sitting at the marina for a couple of weeks.

[SeaAir 3]

Oops.. yep you got it Leftred...  VSR!

I've checked all the cables LBD.. especially the earth and they all seem A1.  

CarpeD ... yep the VSR does prioritise the charge between the house and crank batts.  I probably didn't make it clear, but the issue arises whenever the batts are off the shore power (charger)  ... that's whether the boat's in the marina .. or out'n'about. 

[aardvarkash10 1,065]

There is nothing like getting back to basics.  Check the battery properly - if its open cell, check them all for level and balance.  Check the open circuit voltage (after a decent rest period).  Check the cranking current loaded voltage.

Use a voltmeter and check the circuit voltage drop from the alternator to the VSR and then the VSR to the battery - live side and earth side, under load (20 A or better).  Visual is fast, but voltage drop testing is accurate and relevant.

Check the charging voltage and current to the engine battery.

Check for a low-current draw off the battery when everything is supposedly switched off.

Nothing like this is ever complicated - its just a matter of gathering the right data and then being logical in understanding it.

[CarpeDiem 510]

1 minute ago, SeaAir said:

Oops.. yep you got it Leftred...  VSR!

I've checked all the cables LBD.. especially the earth and they all seem A1.  

CarpeD ... yep the VSR does prioritise the charge between the house and crank batts.  I probably didn't make it clear, but the issue arises whenever the batts are off the shore power (charger)  ... that's whether the boat's in the marina .. or out'n'about. 

Firstly check for parasitic loads.

Anything attached to the start battery that shouldn't be.   The VSR is obvisouly a parasitic load, but that should be the only one.

Then check the battery... 

(1) what's the voltage on the start battery immediately before cranking when it has this problem? Less than 12.4v then the battery wasn't fully charged, or if it was it isn't holding charge.

To confirm this, turn off the shore charger and let the battery rest for 48hrs.  Now check the voltage... Should be 12.6.

(2) What voltage does it drop to during cranking? (you will need an analogue volt meter) less then 9.6v then the battery has lost sufficient capacity

(3) get the start battery load tested professionally to confirm above. 

Your charging setup isn't optimal.  You unfortunately cannot charge two different batteries optimally from a single charging source. There will always be a compromise.

To charge your 320Ah of AGMs optimally your charger should be bringing the voltage up to 14.7v this is really too high for a substantially smaller fla. You'll be over charging the fla frequently which will shorten its life.  Guessing some, but as it is a 7 stage charger it probably does periodic equalization charges as well, potentially at 15.4v which is way to much for fla. 

There's nearly limitless ways of better setting this up... It really depends on budget and appetite for change.

A splitter and two chargers would probably make the most sense.  You could also disconnect the house>start side of the vsr. 

Out of interest does your alternator have a smart regulator on it? If not you'll be slowly walking down the capacity of your AGMs...