Battery Advice Please regarding voltage drop

[Dan-P]

Hello everyone, I have been liveaboard now for nearly 10 years. Recently I've been introduced to something I've not come across before....

 

I have 4no. 12v Numax CXV31MF105 105Ah wet lead acid leisure marine batteries fitted to my narrowboat bought from Tayna October 2024. I have had an issue over the last few weeks where the battery bank is draining what seems very quickly....

Readings from a multimeter as follows with all batteries disconnected from each other and no load:

18th Dec 2025 12:45pm 12.56v

19th Dec 2025 18:45pm 12.40v

21st Dec 2025 12:45pm 12.36v

Average temperatures between 7-11 degrees C.

 

Would a voltage drop between 12.56 and 12.36v seem usual over a 3 day period ?

[cheesegas]

What voltage were they charged at prior to this, and how long were they held there?

[Tony Brooks]

As they were disconnected I agree, but chemical reaction is temperature dependent, but I would have thought that related to amps flowing, not voltage.

 

That is not what is generally called Voltage drop, Voltage drop refers to voltage lost through a cable, but I am sure we all know what you mean.

 

Normally such a drop would be associated with the start of internal cell shorts, but on 14/15-month-old batteries not to be expected unless you have been abusing them - over discharge, failure to fully charge very regularly etc.

 

Try to do an equalisation charge if you can/have the equipment.

 

 

[nicknorman]

We would need to know the recent history in terms of charging and discharging. Bearing in mind the voltage started out at 12.56v this could have been following a short period of charging and the decrease to 12.36v is just the dissipation of surface charge. In order to be sure I think you should fully charge the batteries (14.4v until the current has fallen to 2% of capacity, which will take many hours) and then let them rest for a few days.

Edited December 23, 2025 by nicknorman

[Dan-P]

Thanks guys. I charged the batteries (probably not fully) and left them for 3 hours disconnected which is then when I took the 12.56v measurement. 

They have been looked after properly, charged to float once a fortnight, almost every day through the summer via solar. Only recently they have dropped down below 11.8v hence my post suspecting they are faulty.

In addition to the above, when they were connected, I've had a 10watt cool box plugged in overnight (I disconnected the 12v fridge thinking it faulty and was killing the batteries overnight) and it flattens the batteries from full charge overnight. This is why I also suspected faulty batteries. 

Tayna have been in touch requesting me to send them back at my own expense... but thought I'd post here for advice before sending and finding out theres no problem with them.

When I did the test experiment,  I expected to find one battery with less voltage reading than the others but with them all at similar discharge I am stumped ?!

[Dan-P]

To add to this.... I wouldn't have thought a 420Ah lead acid battery bank would be flattened by a 10 watt / 0.8A cool box overnight. 

[nicknorman]

It would help if you told us how you charged them. Obviously we can’t know from what you have posted so far, but it is a “theme” on here that the cause of problematic batteries is often simply not charging them fully, repeatedly. It can be a mistake to rely on a charger going to float, plenty of chargers are not good at knowing when to go to float and do so prematurely. Really you need to monitor the voltage and current towards the end of charging to be sure it is being charged fully. As you mention, it’s much easier to properly charge in summer when the solar is doing all the work and the consumption is much less.

 

Im also a bit confused by your OP which says that the batteries were all disconnected from each other, but then only give one voltage reading per day. It seems unlikely (but not impossible) that the isolated batteries each had the identical voltage.

 

Way forward is to make sure you properly and fully charge the batteries, then disconnect them all from each other. After a day or so I would expect to see around 12.7 or 12.8v and it should not change over the next 3 days. If there is a problem, it’s highly unlikely that the same problem affects all the batteries in the same way, so you might find one battery has a decreasing voltage due to a partially shorted cell etc, and when they are connected together in normal use this pulls the other batteries down

[Tony Brooks]

1 minute ago, nicknorman said:

Really you need to monitor the voltage and current towards the end of charging to be sure it is being charged fully. As you mention, it’s much easier to properly charge in summer when the solar is doing all the work and the consumption is much less.

 

I am not disagreeing with this, but as solar controllers tend to go into float too early as well as mains chargers, I would rely on neither to know when the batteries are as fully charged as you are likely to get them. Use the alternator, which in the vast majority of cases will not go into float, and keep charging at 1000 rpm+ until (as Nick says) the current flow is between 1% and 2% of the battery capacity, or the current has failed to drop over half to one hour. Say between 4 and 8 amps in your case

 

50 minutes ago, Dan-P said:

hanks guys. I charged the batteries (probably not fully) and left them for 3 hours disconnected which is then when I took the 12.56v measurement. 

 

OK, what you measured was NOT the rested voltage, it was artificially high because they had just been on charge. This is known as surface charge. probably caused by strong acid adjacent to the plates and the plate surface much more fully charged than the material further in. Over the next hours or until a moderate load has been applied the voltage will drop, and the charge evens itself out and the acid circulates a bit. This is normal.

 

If you want to infer the state of charge from the voltage, it MUST be the rested voltage, so either let the batteries stand for at least an hour or (say) run the water pump for a few minutes. The data below gives you the expected readings with no charging taking place - so a wee small hour job in the summer if you have solar.

 

12.7 to 12.8 volts = fully charged

12.5 ish volts = half charges as far as a boater is concerned (actually 3/4 charged but to maximise cyclic life take it a half charged).

12.2 to 12.3 volts = stop all major discharges and recharge ASAP

 

From this you can see that you started with batteries that were nowhere near fully charged and I suspect they were barely half charged, so get the alternator running and keep it running until you hit that 4 to 8 amps, no longer dropping. However, I fear that you may well have sulphated them so they no longer have the capacity you think they have, so they will discharge faster than when the batteries were new. An equalisation charge will do something to restore the capacity, but probably not as much as you would like.

[Dan-P]

Thanks. I charged them briefly with the engine, disconnected them from each other and then let them rest for 3 hours before measuring the voltage. I would normally charge them every week with a generator connected to my Mastervolt charge master 12/50-3 until the charger displayed float.

I will charge them fully this Friday, disconnect each individual battery and measure the voltages. Measure again after 3 hours and then again after 3 days.

[Russ T]

19 hours ago, Tony Brooks said:

 

Try to do an equalisation charge if you can/have the equipment.

Could you clarify this point for me pls Tony. If the batteries in question are sealed lead acid what is the recommended equalisation voltage/duration. I often find it incredibly difficult to find such information from manufacturer data sheets and/or suppliers.

 

Also, to the OP.This is worth a read.....

 

 

[ditchcrawler]

28 minutes ago, Dan-P said:

 

I will charge them fully this Friday, disconnect each individual battery and measure the voltages. Measure again after 3 hours and then again after 3 days.

With the Victron on its standard setting its unlikely they will be fully charged when it goes to float as the chargers are designed for 24/7 power so the batteries would be held a float charge until the next cycle of charging, yours get switched off.

[Russ T]

1 minute ago, ditchcrawler said:

With the Victron on its standard setting its unlikely they will be fully charged when it goes to float as the chargers are designed for 24/7 power so the batteries would be held a float charge until the next cycle of charging, yours get switched off.

He has a mastervolt.

 

Probably much the same though, unless its been programmed with custom settings.

[Tony Brooks]

1 hour ago, Russ T said:

Could you clarify this point for me pls Tony. If the batteries in question are sealed lead acid what is the recommended equalisation voltage/duration. I often find it incredibly difficult to find such information from manufacturer data sheets and/or suppliers.

 

Both are problems, I would never have sealed batteries if I could avoid it because then you can't diagnose with a hydrometer, and then there is the question of a safe equalisation voltage and for how long.

 

For the voltage and time, I am afraid all I can safely say is ask the supplier - as you found, good luck with that.

 

FWIW, Gibbo, our battery guru, the designer of Smartgauge, seemed to have the opinion that boat batteries did not need equalising, but that sort of ignored the facts that equalisation seems to be used to convert, at normal charging voltages, more lead sulphate back into lead oxide. On my boat I never did any equalisation charges and I still got over 6 years life from a set of wet Exide batteries, then we sold the boat. That tends to confirm Gibbo's view. However, I put a great deal of faith in charging until the current dropped to 1 to 2% of capacity and took a great deal of notice of the rested voltage and acted accordingly.

 

I now suspect the OP has sulphated his batteries by consistent undercharging so he has two choices, accept it and put up with the reduced capacity, and perhaps resolve to fully charge them as often as possible, at least once a week, or try on equalisation charge to try to convert a bit more lead sulphate, which may wreck the batteries.

 

In don't know if this helps, but here is a table showing typical gassing voltages for Lead Antinomy batteries like some Trojans. Open cell ones can be topped up to replace the water driven off as gas so at UK type temperatures you could probably go to perhaps 15.5 volts

 

Battery Temperature	Charge Voltage per cell	Charge Voltage for a 12 Volt battery	Gassing Voltage per cell	Gassing Voltage for a 12V battery
-20 °C *	2.60	16.02 to 16.56	2.97	17.82
-10 °C *	25.1	15.66 to 16.2	2.65	15.9
0 ° C *	2.45	15.3 to 15.9	2.54	15.24
10 °C	2.41	14.94 to 15.54	2.47	14.82
20 °C	2.37	14.58 to 15.18	2.415	14.49
25 °C	2.35	14.40 to 15.00	2.39	14.34
30 °C	2.33	14.22 to 14.82	2.365	14.19
40 °C	2.30	13.86 to 14.46	2.33	13.98
50 °C	2.26	13.5 to 14.10	2.30	13.8

 

Typical leisure type batteries are now likely to be lead calcium that have a slightly higher gassing voltage, but again, it varies with temperature, but can be around 14.8 volts so open cell batteries of this type could be equalised at a slightly higher voltage than lead antinomies, but if they are sealed there is no chance to replace any water driven off (unless you drill the cell tops and put corks in) so one would need to be very careful with the voltage.

 

I know this is not much help, but the takeaway is to do all you can not to allow the batteries to sulphate - which hey will do when left standing and not fully charged.

 

 

 

 

[Russ T]

4 minutes ago, Tony Brooks said:

For the voltage and time, I am afraid all I can safely say is ask the supplier - as you found, good luck with that.

 

That sound like good advice prior to trying:

 

5 minutes ago, Tony Brooks said:

I now suspect the OP has sulphated his batteries by consistent undercharging so he has two choices, accept it and put up with the reduced capacity, and perhaps resolve to fully charge them as often as possible, at least once a week, or try on equalisation charge to try to convert a bit more lead sulphate, which may wreck the batteries.

You can bet your bottom dollar that if you do it tother way around, they will find it easier to say that these batteries should not be equalised, and hence we will not replace them FOC.

 

Even if they do suggest its acceptable, I wonder what the chances of getting meaningful values from them are.

 

I wish manufacturers data sheets were in more abundance for these batteries.

[Tony Brooks]

1 minute ago, Russ T said:

Even if they do suggest its acceptable, I wonder what the chances of getting meaningful values from them are.

 

My guess is that they were never designed to be equalised, so no equalisation and values.

[Russ T]

1 minute ago, Tony Brooks said:

 

My guess is that they were never designed to be equalised, so no equalisation and values.

I would guess that too, which may well invalidate any warranty. Not that these warranties are worth much if it is proved you have not charged them to specification.......which obvs is available on the manufacturers data sheets.🙃