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Old 07-08-2020, 18:05   #1
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Battery Question

I have a 650amp hour house battery bank (6 Trojan T105 plusses) and a single 50 amp hour AGM starting battery. When my house batteries are reading anything below 13 volts they are not strong enough to start the engine, in fact they can barely operate the low oil pressure alarm. The start battery is usually at around 13.01 and has no trouble turning the engine over. Why are the house batteries so weak? Is it the nature of the beast, or a poor connection somewhere or is it because the house batteries are old (8 years) and due for replacement?? Thanks.
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Old 07-08-2020, 18:22   #2
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Re: Battery Question

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Originally Posted by osprey877 View Post
Why are the house batteries so weak? Is it the nature of the beast, or a poor connection somewhere or is it because the house batteries are old (8 years) and due for replacement?? Thanks.
Assuming you've checked there are no loose/high resistance connections, it sounds like you are losing the race with Peukert and the house set is worn out.
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Old 07-08-2020, 18:31   #3
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Re: Battery Question

I would agree with Checkswrecks. If it isn't a bad connection somewhere between the house bank and the engine (don't forget the ground which is a common problem) then the bank is going out.

I have the same setup, six 6V batteries for 650 amp hours with a separate starting battery and my house bank will spin the engine like a top.
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Old 07-08-2020, 18:48   #4
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Re: Battery Question

A large deep cycle bank will start much larger engines than any single little starting battery.
Watch the volt drop measured at the banks pos terminal, if it’s a large drop as in less than 10V. Then your banks shot, if it’s a loose / dirty connection there will be very little voltage drop.

But as it’s not even being able to power the low oil pressure alarm, that has to be a bad connection, no bank could run anything in the boat and not be able to run the alarm.
When checking connections don’t forget the grounds, they are most often the culprit, and once you have checked the connections, jumper past the battery switch, I’ve seen them go bad and look like a bad battery before.
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Old 07-08-2020, 19:04   #5
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Re: Battery Question

I have a similar set up (6 ea Sams's Club GC2 best, no starting battery). Each pair can be switched on/off. That is how I determine replacement time. When any one set struggles to start my 3GM time to head back to Sams (typically 5 or 6 years).

While starting, get a meter and check the voltage at the lead terminals. Assuming no bad connections/ charging problems, time to replace.


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Old 07-08-2020, 19:35   #6
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Re: Battery Question

[QUOTE=a64pilot;3203680]A large deep cycle bank will start much larger engines than any single little starting battery.
Watch the volt drop measured at the banks pos terminal, if itís a large drop as in less than 10V. Then your banks shot, if itís a loose / dirty connection there will be very little voltage drop..[/

NOT FOLLOWING YOU HERE. DO YOU
Mean to watch the voltage drop when the starter button is pushed??
I HAVE A BaATTERY MONITOR which displays voltage and amperage for both banks. Not sure where itís measured from though.
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Old 07-08-2020, 19:58   #7
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Re: Battery Question

[QUOTE=a64pilot;3203680]A large deep cycle bank will start much larger engines than any single little starting battery.
Watch the volt drop measured at the banks pos terminal, if itís a large drop as in less than 10V. Then your banks shot, if itís a loose / dirty connection there will be very little voltage drop..[/

Thanks but NOT FOLLOWING YOU HERE. DO YOU
Mean to watch the voltage drop when the starter button is pushed?? Drop to 10 volts or a 1volt drop?
I HAVE A BaATTERY MONITOR which displays voltage and amperage for both banks. Not sure where itís measured from though.
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Old 08-08-2020, 06:02   #8
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Re: Battery Question

Quote:
Originally Posted by osprey877 View Post
I have a 650amp hour house battery bank (6 Trojan T105 plusses) and a single 50 amp hour AGM starting battery. When my house batteries are reading anything below 13 volts they are not strong enough to start the engine, in fact they can barely operate the low oil pressure alarm. The start battery is usually at around 13.01 and has no trouble turning the engine over. Why are the house batteries so weak? Is it the nature of the beast, or a poor connection somewhere or is it because the house batteries are old (8 years) and due for replacement?? Thanks.
AH ratings and voltage don't really mean anything about the capacity of used batteries. And an 8 year old set of T105s is probably past due for replacement.

Not all house banks are designed to be able to fully supply the start circuit and there may be a smaller cable between the 2. I suspect you possibly have a cable/connector issue also.

My 440AH T105 house could start my main engine just fine. In fact my set is 4 years old and 1/2 have been taken out so I only have 220AH and they still start the engine. But the boat wasn't really designed for this and I rerouted the cables on my FET to allow for this.
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Old 08-08-2020, 06:29   #9
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Re: Battery Question

[QUOTE=osprey877;3203735]
Quote:
Originally Posted by a64pilot View Post
A large deep cycle bank will start much larger engines than any single little starting battery.
Watch the volt drop measured at the banks pos terminal, if itís a large drop as in less than 10V. Then your banks shot, if itís a loose / dirty connection there will be very little voltage drop..[/

Thanks but NOT FOLLOWING YOU HERE. DO YOU
Mean to watch the voltage drop when the starter button is pushed?? Drop to 10 volts or a 1volt drop?
I HAVE A BaATTERY MONITOR which displays voltage and amperage for both banks. Not sure where itís measured from though.
Yes you must watch the voltage when starting the engine, or attempting to do so anyway, and to determine battery condition, the voltage must be measured at both terminals, donít connect to anywhere else. If connected to anywhere else, then your not measuring battery voltage, your measuring battery voltage after it goes through a circuit.
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Old 08-08-2020, 06:54   #10
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Re: Battery Question

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Originally Posted by osprey877 View Post
I have a 650amp hour house battery bank (6 Trojan T105 plusses) and a single 50 amp hour AGM starting battery. When my house batteries are reading anything below 13 volts they are not strong enough to start the engine, in fact they can barely operate the low oil pressure alarm.
You have some combination of
  • poor connections
  • undersized conductors
  • shot batteries

As suggested, to understand what's going on, you need to take a bunch of voltage measurements - at the battery, at the starter, maybe at the batt selector switch. In such cases I like to measure the voltage drops between battery and starter.

You should also test the house batteries for peak current capability. Simple loading testers are pretty cheap, and battery analyzers are getting inexpensive as well.
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