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Old 19-10-2018, 17:53   #1
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Battery voltage with Inverter on.

I have a battery bank with approximately 1300 amp hours. Charger keeps them at 100% SOC. When I go out on the boat and have the inverter on, the voltage drops in a few hours to 12.4 or so. I am drawing about 40 amps 12 volt.
Is this OK, normal?
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Old 19-10-2018, 17:59   #2
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Re: Battery voltage with Inverter on.

If the battery voltage reads 12.4 WHILE you are drawing 40 Amps, and after you have been for a "few" hours, yes that is normal.

After 4 hours drawing 40 amps, you have pulled 160 Amp-hours out of the battery, so they are down about 15%.

If that is the voltage after a significant amount of time "resting" without putting out any power, that would be lower than I'd expect.
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Old 19-10-2018, 18:02   #3
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Re: Battery voltage with Inverter on.

I understand your point. This is not a resting voltage, this is a reading when inverter is on supporting a load of 30-40 amps. It has gone as low as 12.0 volts when under a relatively high load.
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Old 19-10-2018, 19:12   #4
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Re: Battery voltage with Inverter on.

It is normal, but can be a problem. Gets worse as batts age, lose capacity.

Bigger bank will help, also some AGM batts are better than most.

LFP much better at this than lead.
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Old 19-10-2018, 22:23   #5
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Re: Battery voltage with Inverter on.

Seems a bit low for such a big bank.

I would do a 20 hour discharge test on it. Google it
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Old 20-10-2018, 10:40   #6
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Re: Battery voltage with Inverter on.

What's drawing 40 Amps?....that's a pretty big draw....what size boat do you have?
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Old 20-10-2018, 10:44   #7
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Re: Battery voltage with Inverter on.

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What's drawing 40 Amps?....that's a pretty big draw....
@ 1300Ah it's a pretty big bank too ;-)

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Old 20-10-2018, 10:51   #8
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Re: Battery voltage with Inverter on.

You don't say what size charger you have. I would expect a bank that size to have at least a 40a charger. If the voltage is dropping while the charger is on it should still maintain full charge so voltage should not drop. If the charger cant keep the batteries at full charge you have a charger fault. If not all seams OK.

But as others say, what draws 40a continuously? That is in the realm of always having to be hooked up to shore power or a generator, its 1000a/h day. Only system that will handle that is a large lithium bank with genset and large solar array costing sever $K
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Old 20-10-2018, 11:32   #9
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Re: Battery voltage with Inverter on.

Seems high for a 1300Ahr bank as the 20 hr rate is 65A so you are not pushing too hard with a 40A load. The age and subsequent state of health (SOH) of your bank is coming into play here. How old are your batteries? Do you always charge to 100% after a discharge cycle? Do you discharge to <50% often? Do you leave them in a partial state of charge (PSOC) condition?
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Old 20-10-2018, 16:46   #10
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Re: Battery voltage with Inverter on.

Quote:
Originally Posted by spudsdailey View Post
I have a battery bank with approximately 1300 amp hours. Charger keeps them at 100% SOC. When I go out on the boat and have the inverter on, the voltage drops in a few hours to 12.4 or so. I am drawing about 40 amps 12 volt.
Is this OK, normal?

===


It sounds normal to me. We have about 1,000 amp-hours and draw 30 to 40 amps more or less continuously. Voltage will be down to 12.4 or so after a few hours, and down to 12.2 over night. It takes an hour or more of bulk charging at about 110 amps, and another hour of absorb charging starting at 110 amps, tapering down to 40 amps to get them back to nearly a full charge.
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Old 20-10-2018, 17:14   #11
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Re: Battery voltage with Inverter on.

Those numbers don't add up.

What model batteries?
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Old 20-10-2018, 22:19   #12
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Re: Battery voltage with Inverter on.

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Originally Posted by john61ct View Post
Those numbers don't add up.

What model batteries?

===


Eight Trojan T-145s in series/parallel (lead acid, 12 volt bank).
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Old 20-10-2018, 23:45   #13
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Re: Battery voltage with Inverter on.

Quote:
Originally Posted by spudsdailey View Post
I have a battery bank with approximately 1300 amp hours. Charger keeps them at 100% SOC. When I go out on the boat and have the inverter on, the voltage drops in a few hours to 12.4 or so. I am drawing about 40 amps 12 volt.
Is this OK, normal?
What is "a few hours"? If you can find out more precisely how many hours it takes to drop to 12.4 volts at 40A, then you will get better answers about the health of your battery bank.
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Old 21-10-2018, 06:43   #14
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Re: Battery voltage with Inverter on.

[QUOTE=spudsdailey;2744967]I have a battery bank with approximately 1300 amp hours. .
QUOTE]

Do you mean 1300 Cold Cranking Amps (CCA) or have you missed a decimal point? A 4WD battery would typically have 700 CCA

I have two quite large AGM batteries in my 43 ft yacht and each are 135 Amp Hours (AH)


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Old 21-10-2018, 06:53   #15
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Re: Battery voltage with Inverter on.

Quote:
Originally Posted by wayne.b View Post
===


Eight Trojan T-145s in series/parallel (lead acid, 12 volt bank).

According to my research each T-145 weighs 28kg so you have app 250kg of batteries?

WOW!

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