One for the battery buffs!

[Thepipdoc]

I recently (before Christmas) fitted a Smart Gauge battery monitor and it transpires that my 2x 105AH batteries lose 2% of their capacity per day when the batteries are isolated and nothing is running? (The bilge pumps would run if they needed to but the bilges are bone dry)
Is this normal or are the batteries on the way out?
Thanks

 

[SolentPhill]

Sounds about right, no more than if you leave a car alone for month or so it won't start as battery flat. You will find the first few days/weeks it will go down quicker so it will go from 100% to 80% fairly quick then it will slow down, then it will stay at around 50/60% that's when you can tell a good battery from a bad one

 

[sarabande]

yes, though that is a bit much. It's called "self-discharge", and depends on many variables such as type of battery, designed life cycle, material and method of construction.

Very generally speaking, the more sophisticated the design and materials, them ore a battery costs and the lower the rate of s-d.

 

[Thepipdoc]

Both answers make sense.
Thanks.

 

[mjf]

Both answers make sense.
Thanks.

Oddly enough I have just purchased a new battery similar to one I got in the summer 'cos I was surprised how good it was.

The 1st battery was fitted to a little used car in July and its not been used very much since
Then over Christmas I was charging another vehicle and thought I would charge up the other one afterwards; as it had sat for over 2/3 months - smartcharger on and within a couple of seconds up lights the fully charged light. Brilliant! Turned the engine over just to prove it. Thus no loss on new battery at all

 

[SolentPhill]

It does depend on where its stored as well, a battery stored outside in the cold and damp will go down quicker than one sitting in a warm workshop. Also one stored on a metal surface/floor will go down quicker.

 

[Maree]

Would a battery monitor show the self-discharge though?

Surely there has to be current through the shunt for them to register.

My boat was laid up end of June with the monitor showing 91%, 6 months later it showed 84% (450Ah bank). The only thing it's running is the monitor itself.

I don't now what the actual state of the batteries is, due to self-discharge, but I had readings of 12.4v using a multi-meter so I'm not worried yet.

I would be very concerned about 2% per day.

 

[SolentPhill]

Hi Maree A battery monitor would show in the fact it would go down daily its atmospherics (is that spelt right) that uses the power it is very low even the monitor uses a small amount, also damp air will hit the terminals and use power again very low. if your battery dropped from 91 to 84 in 6 months thats good but you have a big bank,

remember the 2% per day is the first stage, your batteries probably went from 91% to say 85% fairly quickly and then it leveled out, if your bank is all good quality batteries which it sounds like it is then that should'nt drop much more than 84%, but will over time, thats why even torch batteries have a shelf life.

Voltage isnt always a good reading a old battery that shows 50-70% could still show 12volts but when you turn the key to start engine it goes to zero. Thats why a good battery tester you see those things with a wooden handle it has a big coil and when you put it on the battery it puts the battery under load. those yellow or black things are not really good for battery volatage

using a meter is ok when the battery is standing, put a meter on it, make note of reading then turn on charger and put a meter on it again, the difference is what your battery is charging at, then turn off the charger and start engine and put meter on again, this will tell you how much your alternator is putting in thats your charging side, as long as it goes up on both and neither makes your battery go past 14.4 volts then your ok.

Im sure someone is better at the amps side on here as you can do that as well

Most battery monitors need to be set up Im not sure about all of them but the important item to set up is your tail current and if this is not set up Im not sure how they can monitor the acurate % of a battery. I know Im opening a can of worms here but there are many types of battery, 110AH 90 etc and you have that guy Peukert had some theory which will totally confuse you. Those monitors that you plug in and work, how do these work how does it know what battery you have and what AH how does it know when your battery is full. so it uses an average, it has a default so it will set its own tail current which is ok if your battery and charger meet that.

ok fuse burning I will stand back

 

[Si Dude]

Ok it sound to me like all boat batteries its a combination of multi factors

1. if your batteries are isolated yet your smart charger is displaying battery percentage, first of all you need to ask your self is the mains connected (shore Power), if not the obvious the a small power draw is being used to power the displays.

2. don't get confused between voltage and ampage, a battery can show 12v's but have no amp's or power so hence flat battery. we're now getting into the complicated stuff so I won't bore everyone.

3. you batteries could be showing signs of what we call in the industry "sulphation" every battery will suffer from this and our batteries on boats will certainly have it. When a battery is not being used i.e. not being put under load or heavy discharge we get cristals forming on the plates, this is call sulphation. what this does is it slowly decreases the over all amp per hour of the battery. you only really notice this for instants starting an engine that you've metered 12volts but nothing happens. the sulphation is blocking/covering the plates in the cell, not allowing the power to be used.

4. this can be slowed down by breaking down the sulphation by deeply discharging the batteries, full charging, deeply discharging and so on.

5. unfortuatly this does not cure the problem completely, once your batteries start to sulphate after being stood for extreme lengths of time they will continue to develop the cristals.

6. temperture does have an effect. most batteries loose 5% overall Ah every 5 degrees below freezing you go.

7. gel batteries are completely different altogether.

so to conclude, with out testing the gravity of the acid in the cells, a combination of lack of use, extreame tempetures, small power draw and possible sulphation of the plates is the problem

just give them a good discharge and re charge

 

[dt4134]

If your battery monitor works out discharge by summing up the current flow through a shunt (as I expect it does) it isn't going to show self discharge which is entirely within the battery.

However two percent is a large drop for a day. Have a look at what current it shows coming out of the batteries? Perhaps something is drawing current still when you think the battery is isolated.

You also have to sanity test what the monitor says. If I leave my batteries isolated for a week the monitor might say something like 0.3% discharge (but if you look at the amps it flicks between 0.0 and 0.01 and over a week that would sum up to maybe an Amp Hour). However the voltage reading indicates that the batteries are fine.

It takes about 1Ah of load for my batteries to drop from 13.8V to 13.2V, which is the normal voltage for a good fully charged battery (left alone & isolated they would slowly self-discharge down to this, and if left alone for a long time down below it).

Si Dude also has a point about the effect of the shorepower charger. I've a smart charger so it'll put a bit of power into the batteries then switch to trickle charge and repeat the boost occasionally. So if it is boosting it might be putting out a voltage say 14.5V, but when it switches to trickle the batteries won't hold that voltage and some current will flow out (that does require a connected load for the measurement to be made by the battery monitor).

Mine settle down at trickle charge at about 13.8V and when they first get there the monitor gives the impression that they are at less than 100% even though they're fully charged. Over time the trickle charge leads the monitor to think they're back up to 100%.