Fluctuating battery voltage

[Daydream believer]

I have a pair of 12 v 90A AGM batteries linked together as my domestic bank. I have a Dolphin 15 A triple stage charger that charges both starter & domestic batteries. It runs off shore power. If it is giving out max charge the cooling fan inside it goes on. If the fan does not go on I know that the batteries are not particularly low.
I have a NASA BM1 battery volt meter that works on the domestic bank only.
2 days ago I decided to check all was charged up for the laying up period, so I plugged into shore power. All electrics were turned off. The BM1 recorded 6 amp charge for a while at 13.7 V ( fan did not go on so I assume that batteries were fairly well charged)
I left it overnight & on return charge rate was 1 amp. Voltage still recorded 13.7 V
I disconnected the shore power (still showed 13.7V) & to empty the fresh water I turned on the tap & hence the fresh water pump. This then showed 1.3 amp discharge & voltage fell to 12.7 V within the 4-5 minutes I ran it. (I did not re connect shore power)
After about 15 minutes voltage went back up to 12.9 volts
The following day voltage was recorded at 13.1 volts
What I would ask is why such a sudden big drop- Should one expect this? Then why did it slowly go back up?
What is happening? No other electrics connected.
Thanks

 

[macd]

I've not had custody of AGM lead-acids, and those levels are a little different from what you'd expect from flooded batteries. But basically what you're observing is that transient voltages tell you little about a battery's actual state of charge. "Rested" voltage is a far better indicator but, as the expression implies, it takes batteries a while to settle into a rested condition after loads are removed. Many authorities suggest 12 hours, but one at a pinch, let alone 15 minutes.

1.3A for 5 minutes is around 0.1Ah, so a negligible draw. Surely after your pump run your BM1 told you the batteries were still more or less full? Happy batteries, happy skipper? (1.3A is actually improbably low for a domestic water pump. Are you sure solar panels or something wasn't distorting that reading, and others?)

If you want to see a serious voltage drop (and a similarly serious, albeit slower recovery), run your windlass for a minute or two without the engine running.

 

[Daydream believer]

No solar cells & no anchor winch ( I never anchor) no other electrics turned on.

 

[[70521]]

Do you have a multimeter or voltmeter that you can check the batteries with?

 

[halcyon]

I have a pair of 12 v 90A AGM batteries linked together as my domestic bank. I have a Dolphin 15 A triple stage charger that charges both starter & domestic batteries. It runs off shore power. If it is giving out max charge the cooling fan inside it goes on. If the fan does not go on I know that the batteries are not particularly low.
I have a NASA BM1 battery volt meter that works on the domestic bank only.
2 days ago I decided to check all was charged up for the laying up period, so I plugged into shore power. All electrics were turned off. The BM1 recorded 6 amp charge for a while at 13.7 V ( fan did not go on so I assume that batteries were fairly well charged)
I left it overnight & on return charge rate was 1 amp. Voltage still recorded 13.7 V
I disconnected the shore power (still showed 13.7V) & to empty the fresh water I turned on the tap & hence the fresh water pump. This then showed 1.3 amp discharge & voltage fell to 12.7 V within the 4-5 minutes I ran it. (I did not re connect shore power)
After about 15 minutes voltage went back up to 12.9 volts
The following day voltage was recorded at 13.1 volts
What I would ask is why such a sudden big drop- Should one expect this? Then why did it slowly go back up?
What is happening? No other electrics connected.
Thanks

A battery has three basic voltage ranges, the voltage governed by battery state and amperage.

Rested no load, no charge or discharge for 12/24 hours 11.7 - 12.8, new batter voltage can rise to 13.1 volt fully charged.

Load applied, higher the amperage, the bigger the volt drop, but your 12.7 volt would be reasonable.

On charge, the voltage rises with amperage / recharge level, varies but FLA 13.1 - 15.5 volt

Voltage also varies with air / battery temp, age, how you look after the battery and others.

Brian

 

[sailaboutvic]

Wasn't it great in the days when all we had was a needle that shot up as showed it was up to the 12v level and as long as the light came on we knew our batteries had power, if they didn't we just changed them .
Now we seen to spend so much time looking at battery monitors , racking our brain if that extra .5 is making any different to our batteries .
I sure I spend a third of my life day light hours this year looking at the monitor , our come €900 of new batteries, when really I could running then well below the safety margin and they may had lasted another six months .
@ Daydreamer by other posting you sound to me your a weekend sailor and with 180AH batteries bank your no way a long distance cruiser , so here a tip that going to save you some money and stress.
Keep useing your batteries as you do, keep them charge up the best you can and the day you get on board and you flick that switch and nothing happen , buy your self a new set of batteries.
Now I need to get back to my batteries monitor

 

[simonfraser]

sounds perfectly normal + #5

the higher voltages are charging / maintenance, 12.7 or more at rest is fine.

 

[Daydream believer]

deleted

 

[BabaYaga]

The BM1 recorded 6 amp charge for a while at 13.7 V ( fan did not go on so I assume that batteries were fairly well charged)
I left it overnight & on return charge rate was 1 amp. Voltage still recorded 13.7 V

This seems odd to me. 13.7 is a very low voltage, you should surely be able to see a higher figure during some stage of the charging cycle?
That voltage on start up could indicate bulk charging going on, but if so the current should have been much closer to 15A, in my view.
That same voltage the next day is likely a float voltage, so could be normal.
The charger is not locked in night mode by any chance?

 

[Daydream believer]

This seems odd to me. 13.7 is a very low voltage, you should surely be able to see a higher figure during some stage of the charging cycle?
That voltage on start up could indicate bulk charging going on, but if so the current should have been much closer to 15A, in my view.
That same voltage the next day is likely a float voltage, so could be normal.
The charger is not locked in night mode by any chance?

I cannot remember exactly, but if the fan goes on that means it is shovelling loads of amps in & the voltage reads over 14 V.( I think 14.4V from memory) It then goes to a second stage & reads a lower voltage and lower amperage, which gradually reduces. (At this point the fan goes off so I know that point has been reached) It then goes to a float stage. So I assume that the battery was fairly well charged to start with.
There is no " night mode" as such just 3 automatic stages. I have no reason to suspect any fault with the charger.

 

[PaulRainbow]

I cannot remember exactly, but if the fan goes on that means it is shovelling loads of amps in & the voltage reads over 14 V.( I think 14.4V from memory) It then goes to a second stage & reads a lower voltage and lower amperage, which gradually reduces. (At this point the fan goes off so I know that point has been reached) It then goes to a float stage. So I assume that the battery was fairly well charged to start with.
There is no " night mode" as such just 3 automatic stages. I have no reason to suspect any fault with the charger.

I can't see where there is a problem with your figures in post #1.

 

[macd]

I cannot remember exactly, but if the fan goes on that means it is shovelling loads of amps in & the voltage reads over 14 V.( I think 14.4V from memory) It then goes to a second stage & reads a lower voltage and lower amperage, which gradually reduces. (At this point the fan goes off so I know that point has been reached) It then goes to a float stage. So I assume that the battery was fairly well charged to start with.

14.4V is a fairly typical voltage for bulk charge. Most modern alternator regulators are set to this figure, although older ones typically were lower. 14.4 is about the point where flooded batteries just begin to gas.
The gradual reduction you notice is also to do with the rising internal resistance of the batteries as they reach the final 10% or so of charge. They are to a large degree self-regulating in what they'll accept. The downside is that pushing that final few % into the batteries can take as long as the preceding 40%.

 

[BabaYaga]

I have no reason to suspect any fault with the charger.

Did not mean to imply there is one, just found it difficult to understand exactly what the charger was doing having just been turned on, given those figures.
Trying to monitor what is going on by way of the fan might be unreliable, as ambient temperature may play a role also.