Battery charging

[PaulRainbow]

State of charge monitors tell lies unless they have been reset recently with FULLY charged batteries.
You have not been fully charging them for a while and I would expect them to be shot - sulphated.
The only way to ensure that they are fully charged is to measure the charge current going in. Keep charging until the current is less than 2% of the total battery bank capacity, preferably 1% even better. This will take many hours, a couple of hours is nothing like enough.
Taken off charge and no load for several hours the voltage of a fully charged lead acid 12v battery should be at least 12.7v.
For best life you should never discharge to below 11v. And even then, recharge immediately.

11v is flatter than flat, that's a ridiculous lower limit.

12.1 is 50%, a much more sensible lower limit.

 

[geem]

Hi first time on this forum and I’m not technical.

I’ve spent two hours with the engine running to charge my boat domestic batteries today.

The battery monitor said they were down to 45% and we causing fire up the heater.

After 2 hrs charging they were still only at 56% according to the monitor.

Went ashore for two hours and came back having left only the fridge on and everything else off.

The monitor now shows 100% charged!

Any ideas what’s going on? Is the battery monitor faulty?

A heavily discharged battery will take a minimum of about 7 hours on charge to fully charge it, regardless of how big the alternator is. It is very inefficient to charge with an alternator unless you happen to need to engine to go somewhere at the same time.
It would be worth investing in some solar to ensure you get your batteries fully charged on a regular basis

 

[Wild Goose400]

I have an engineer coming later and will report back.

Turns out the batteries are fine but the diode splitter wasn’t letting enough charge to the domestic bank. Replaced with a relay and all working well now. Thankyou all

If Tranona is correct, and he is seldom wrong, then you will need new batteries. But if you have an engineer coming he will probably want to sell you batteries. The question you have to ask him, or yourself, how did the batteries 'destroy' themselves. You don't want to buy new batteries until you are sure you can keep them sufficiently charged to offer a long lifespan.

You said originally that the batteries were at 45%, then after 2 hrs charging 56% and then 2 hours later at 100% - but the picture of the mon seems to suggest 56% or did it drop from fully charged to 56% - in which case the batteries have given up the ghost.

If the engineer is coming 'later' and this gives you the time - describe your battery practice/usage, size of batteries, and the assembled members will be able to offer you advice on what they might do in your situation. You can weigh up the answers offered here with the suggestion from the engineer. ......That's what the forum is for . But if you have a big bank you may need to rely on shore power or solar and the latter might need some imaginative solutions as to where to instal - so details of the vessel might also be useful.

I'm not prying - but we are groping in the dark.

Jonathan

The engineer load tested the batteries and declared them good.

 

[B27]

11v is flatter than flat, that's a ridiculous lower limit.

12.1 is 50%, a much more sensible lower limit.

A lot of people come out with numbers like 12.1, but voltage under load is more useful.
If you've got a fairly small bank and a fairly big load like a heater igniting, 12.1V becomes a less useful limit.
More so when it's measured via wiring shared with said heater or whatever.

 

[PaulRainbow]

A lot of people come out with numbers like 12.1, but voltage under load is more useful.
If you've got a fairly small bank and a fairly big load like a heater igniting, 12.1V becomes a less useful limit.
More so when it's measured via wiring shared with said heater or whatever.

The post i replied to said "Taken off charge and no load for several hours" So 12.1v is the relevant voltage.

Voltage under load doesn't tell you much, unless you know the SOC and the load applied. Hit the bow thruster control and the voltage under load is likely to be 10.5-11v, that doesn't mean the batteries are flat or knackered.

 

[Neeves]

A lot of people come out with numbers like 12.1, but voltage under load is more useful.
If you've got a fairly small bank and a fairly big load like a heater igniting, 12.1V becomes a less useful limit.
More so when it's measured via wiring shared with said heater or whatever.

We don't know how big the battery bank is.

Hopefully the engineer knows his stuff and the OP will not have problems into the future.

I did wonder at the OP's comment on use of the heater as being mid/late June I would not have expected it being used. I now understand you have had a very cool June - so it becomes a bit clearer.

Jonathan

 

[B27]

We don't know how big the battery bank is.

Hopefully the engineer knows his stuff and the OP will not have problems into the future.

I did wonder at the OP's comment on use of the heater as being mid/late June I would not have expected it being used. I now understand you have had a very cool June - so it becomes a bit clearer.

Jonathan

We don't know how big the batteries were when new, or how much capacity they've lost to age.
It's been wet here, so some heater to dry things out has been useful.

 

[Neeves]

We don't know how big the batteries were when new, or how much capacity they've lost to age.
It's been wet here, so some heater to dry things out has been useful.

Having seen your weather (and the pictures) for the RTI I've stopped complaining about our cold June. Air temps in the morning on the coast here are around 10 degC - quite bearable (if you dress correctly) and with sea temps of around 20 degC - I'm grateful for what we've got. For us in a few more days and the daylight hours will start to slowly climb.

I'd worry about what capacity the batteries have lost through being at 45% of capacity. If the diode splitter is/was at fault I wonder how long the undercharging has been going on. If the batteries have been load tested and the engineer did not suggest he could supply new batteries - then it might be all good. No explanation for the batteries moving from 65% charge to 100% in 2 hours of charging (using a faulty diode splitter).

Jonathan

 

[PaulRainbow]

We don't know how big the battery bank is.

Hopefully the engineer knows his stuff and the OP will not have problems into the future.

I did wonder at the OP's comment on use of the heater as being mid/late June I would not have expected it being used. I now understand you have had a very cool June - so it becomes a bit clearer.

Jonathan

Is it possible that the "engineer", on site, knows more than you ?

 

[Neeves]

Is it possible that the "engineer", on site, knows more than you

I'm sure he does (and I certainly hope so). I'm not an electrical engineer, nor advertise as such.

Jonathan

 

[Tranona]

We don't know how big the battery bank is.



Jonathan

Post#10 does say 3 battery domestic bank so most likely 300Ah+ common in cruising boats