Battery Condition

[MauriceMynah]

Here I sit at Fleet dyke with no umbilical cord feeding my boat with leccy. I now have a NASA battery monitor which, when I arrived on Friday morning read 13.2 volts. Now, midday Monday,  it reads 12.5, dropping to 12.4 when the fridge is running.

My first question is...

How low should I let the voltage drop to before damage to the batteries might occur?

[Cheesey69]

When I had a sail boat on the Medway, I considered 12.1 as flat. These were two 110 amp deep cycle batteries  They recovered well because of victron smart charger I had installed 

[Meantime]

Unfortunately voltage is no more than a guide, amps are your real test of capacity. Lead acid batteries should not be discharged beyond 50% of their capacity and where possible not left discharged for too long. So if you have 2 x 110ah batteries, then in reality you have a usable 110ah.

A typical lead acid battery cell will give 2.1V and there are 6 cells in a battery meaning a fully charged battery will read 12.6V. However this is a measurement taken when the battery is under no load, as you are witnessing the fridge kicking in is making the voltage drop. The other thing is when a battery is fresh off charge it will often have a surface or float voltage above 12.6V, hence you saying you had 13.2V when you first arrived. Surface voltage quickly disappears to reveal the true voltage after a small load has been applied for a short period.

To add to the complications some batteries give slightly more or slightly less than the figures quoted above for the average lead acid battery. So you need to find out what your true fully charged starting voltage is. When your batteries are fully charged and for argument sake reading 13.2V, turn on a small load for 5 minutes or so and then turn it off. See where the battery voltage settles and this is as good as your fully charged voltage. I would expect this to be around 12.6V - 12.7V. If you consider 12V to be full discharged then somewhere halfway between the two is your 50% discharge point.

[MauriceMynah]

Somehow I just knew there wouldn't be a simple answer.

I have 4 x 120ah batteries and I would like a simple indication of their state at any one time. 

Without anyone going into rocket science,  is there a way?

[BrundallNavy]

Your fridge should cut out before you do any lasting damage to the batteries. 

[grendel]

 

so

So from the above chart you can drop to 12.25V and still remain with 50% charge (note under a heavy load it may well drop lower while the load is connected.)

[Meantime]

7 minutes ago, BrundallNavy said:

Your fridge should cut out before you do any lasting damage to the batteries. 

There is no way your fridge will cut out at 50% depth of discharge. Regularly exceeding this will do lasting damage to your batteries.

From the Isotherm Cruise manual;

To prevent the batteries from becoming completely discharged, a battery voltage sensor switches off the compressor automatically at the following levels. 10.4V cuts out and 11.7 cuts back in again.

[NeilB]

I have one of these on my domestic battery bank - https://www.victronenergy.com/upload/documents/Datasheet-BMV-700-series-EN.pdf 

There are a couple of versions available and I've not seen less than 80% and that was after a cold night in March.  My batteries were brand new then, 4 x 135ah.

[Regulo]

I can't recall now the exact values, but my fridge could be set to cut out at different voltages by the value of a resistor across two terminals at the rear. As far as battery voltage:

12.7 we're in heaven,

12.5 we're still alive,

12.2 there's a lot to do 

11.8 It's too late! 

 

[MargeandParge]

If you're still dropping cubes in the libation then life is sweet. 

Kindest Regards Marge and Parge 

[MauriceMynah]

My old fridge would only cut off under 3 circumstances. 

A. Over voltage

B. Under voltage or

C. I'd put something in it.

[grendel]

the answer to all 3 of those conditions could be summed up with 1 word- Shock.

[annv]

The only really accurate test is to disconnect all the battery's then check each cell with a hydrometer! Only really practicably once a year, But accurate. checking each battery's voltage when disconnected is next best way. John

[annv]

The only really accurate test is to disconnect all the battery's then check each cell with a hydrometer! Only really practicably once a year, But accurate. checking each battery's voltage when disconnected is next best way.  It also depends if your battery's are connected correctly in the right order with clean greased connections .John

[MauriceMynah]

My batteries are wired as the second diagram shows with bat b positive and bat c ground.

[Meantime]

You might find the following useful! 

[LondonRascal]

This is a fascinating topic, full of opinions but if you want some professional insight, this website is the best there is and you can really find out all there is to know about batteries, charging, chemistry and looking after them... https://batteryuniversity.com/articles

[MauriceMynah]

1 hour ago, Meantime said:

You might find the following useful! 

Watt ?

[LondonRascal]

On the subject of power, I have been surprised just how much of it I have saved since upgrading things. All the lighting is LED now, but the fridge and heater were old (1992 vintage). In wintertime needing the run the heater (and I'd have it on low 24hrs if was cold enough to warrant that) I'd get through around 57Ah from my theoretical 330Ah of stored energy overnight before I started the engine. But, in all honesty I see it as 165Ah of usable energy as I never like to draw more than half the capacity out of the batteries.  In summertime, I'd use less without the heater being run, and it would come down to around 33Ah overnight usage. When I last used the boat with the new fridge, but no heating as it is summertime, I used just 16Ah overnight! It truly sips power, runs less often and for less time. I was amazed how much a modern, efficient fridge could change power usage.

I once considered all sorts of fancy things like Lithium batteries, but these are pointless unless you have high amperage drainage that other batteries can't sustain over time (and I do not) and want rapid re-charge times too. If you do have Lithium batteries to really take advantage of them you need a way to re-charge at very high amp rates too as one of the great things about them is the ability to fast charge. Even if you do not believe that 'fast charging' is needed, never connect Lithium batteries without current limiting, using a Battery to Battery Charger to a normal Alternator, as they will suck all the Alternator has to give and overheat the poor thing (video here)

You could get AGM batteries, but then you need to adjust your charging profile for those, and a Gel battery is even more susceptible to being ruined by wrong charging or being left discharged - even a little - for a period of time. So I stick with good old fashion Lead Acid batteries and replace them every 4 years come what may. That might seem unusual to some, but I only have to budget £80.00 a year for battery replacement costs and doing it every 4 years means their cyclic life is not exceeded and I have top performing batteries, so they are to my mind a 'replaceable service item' just as coolant, oil, impellers, filters etc are.

[MauriceMynah]

Hmmm, what do you do with the old batteries? I expect a modern battery to last nearer 10 years rather than 4. Maybe I've just been lucky.  

It has been interesting being moored without running the engine. I'm approaching 5 days now and the drop in voltage has been pretty consistently 0.16 per day. I'm down to 12.3 volts or 12.2 under fridge load. 

I shall be under way tomorrow, the batteries are holding up rather better than the poo tank. Deffo time for a pumpout.  

Thanks for the link to that battery website  I actually understood some of it, though it basically said that without a hydrometer  knowing the remaining amp hourage is the black art of guesswork and optimism.

[LondonRascal]

There is very little 'modern' about Lead Acid batteries, while small improvements have been made the chemistry is much the same, and every time you use some power from one and re-charge it - they degrade just a bit more. An average battery will come with a limited two year warranty and in the small print might only have a cyclic life of 50-70 cycles and many manufactures will state a cycle is taking it down to 50% discharge and re-charging it. So imagine how quickly 70 odd cycles will take place an after this, the life of the battery is very different to a brand new one despite it being perfectly good for the average user for many, many more cycles before they begin to see issues creeping in (faster discharge times under the same loads).

I personally would not have left my batteries discharging for the 5 days you have as I prefer to always re-charge daily - but that is me. Because an Alternator is not a battery charger (think of it more as a battery maintainer) they won't fully recharge a battery unless you've run it for many, many hours. You can see on your NASA Battery Monitor (and this should be set up correctly to the average temperature of battery, battery type, capacity etc) that when you turn the engine on your Alternator will be putting out a fair amount of amperage 40+Amps - but as the voltage increases, say 13.5volts, the amperage will decrease rapidly as the regulator on the Alternator does its thing. This is why the batteries are not really being 'charged' whereas a stand alone, multi-stage battery charger would begin straight off on bulk charge at 14volts and, say it was a 50Amp charger, at this maximum amperage output too - then as the batteries are recharged, the amperage is reduced (to stop boiling of the electrolyte) but the voltage is maintained until 'full' where it will reduce to 'float' anywhere from 12.9volts to 13.2volts.

If you are harpy with what you have, that is the main thing - because boy, can it be a deep pit to throw money into if you take it too far - from solar panels and charge controllers to DC to DC chargers to take the power from the Alternator and then this box of tricks sends it on tot he batteries in a 'multi-stage' way as a shore powered charger would to charge the batteries correctly and then there is a whole world of shore powered charges to dive into - some now with WiFi and Bluetooth connectivity! I confess to being pretty nerdy with batteries and goodness knows what I'd be like if I had an Electric Car lol.

[MauriceMynah]

Robin, it is your nerdiness I'm relying on here, and yes, solar panels are being considered.  I too do not care to test the batteries often. Just the once really, unless something gets changed. 

It is sometimes handy to know what you can do if, for example, the boat had to be left away from a power source for a few days.

One day, when it's convenient to you,  I'd like to have a chat over a pint or two about solar panels as I have a few ideas which may or may not be realistic.

[grendel]

As some may have noticed I recently fitted a solar panel to Water Rail, it came as a kit and is only a 50W flexible monocrystaline panel 

https://lowenergysupermarket.com/product/flexible-50w-white-solar-panel-kit-1/

this was less than £70 for the kit (other sizes are available). and was installed to the engine battery just to top it up and keep it fully charged

I went with this size purely on the basis that the size was perfect for the space on Water Rails front roof with the screen down.

I chose monocrystaline because they are better at charging in bad weather (I have a similar panel at home and regularly see it charging at 5am as soon as its daylight and even before the sun rises, so it even charges on overcast days).

I would want larger capacity panels should I expect them to be able to keep the domestic bank topped up from usages such as the fridge and lights

[grendel]

just checked my home install and at 5.30am on a wet miserable day (10 minutes after nominal sunrise) the panels are up and running, I dont expect them to be pushing more than a couple of watts into the battery this early (remember the 50W will be at full sunshine at midday).

[Smoggy]

I have a 130w panel on the roof of my boat and although it won't keep up with full load usage it really does boost things during the day, photonic universe is a good place to look and go for mppt controller,  I have pwm type as I wanted two outputs with level control so engine battery was also maintained (mainly due to lockdowns) but as a live aboard your engine battery will be fine standing between runs so no need.

Mine would do better with mppt and may change at some point.

With your roof space that would have been one of my first purchases, go as big as you can afford.