Battery condition

[Tom Price]

Having found out the hard way that battery failure can happen without notice, how do you check their condition?
FWIW mine are sealed. And there is a voltmeter on the dash - but which of three batteries does it monitor?

 

[NickRobinson]

'How old?' is one question-

Which battery does the voltmeter monitor?

Disconnect a lead from each battery (engine off) in turn- when voltmeter reads zero, you've found it/them.

Nick

 

[savageseadog]

A battery tester measuring conductance such as Midtronics

 

[Tom Price]

A battery tester measuring conductance such as Midtronics

Apparently open circuit voltage is not a very accurate guide to a battery's state of charge and ability to provide cold cranking amps.
Well?

 

[NickRobinson]

I agree about voltage not relating directly to capacity. After all, a new but tiny motorcycle battery would read higher than a slightly discharged huge, healthy, truck battery.

Can you crank the engine with the 'ignition' off? If you can see the starter, a screwdriver can simply do this. (Watch your dangly bits!) If so, see how long you can crank until the revs start to drop. It's reassuring to know you have 30/60/120 secs cranking when your cold engine starts in (eg) 5 seconds- (Note this time, you can compare next season)

Don't overheat the starter and have the water feed off to avoid water entering the engine via the exhaust valve.

If you don't have a 1/2/both switch, a single car jumper lead can feed power from house to engine battery in an emergency

Nick

 

[Tom Price]

[QUOTE=NickRobinson:

"If you don't have a 1/2/both switch . . "

But I do; one for motor, another for domestic (that's two newish batteries in parallel) and a third switch for both.
So I continue until the engine is reluctant to start, bring in back-up, then replace the motor battery.
Panic over!

Apologies, reluctant to start interfering with the guts of a 6-cylinder diesel - after all, what do ex-raggies know about engines?

 

[NickRobinson]

Sounds like a plan-

That'll work, and save money too (impecunious raggies again!)

I did have a 9 ltr Gardner six cyl, used to be able to cut out individual cylinders while running.. No EM system though .

Nick

 

[Plevier]

Open circuit voltage is not a good guide at all.
You can get an idea of the cranking capability from a heavy current discharge tester measuring the current and voltage drop but with a big modern high performance battery it really has to be something heavy, not just the old Nolten tester that you press onto the terminals for 10 seconds.
http://www.thetoolboxshop.com/0-524...tester-for-12v-lead-acid-batteries-3194-p.asp
For domestic load capability there is no substitute for a discharge test at something like the average working load.
This is assuming you don't have a battery monitor to watch how the system is behaving over a period of time. On a mobo I would have thought that was pretty essential.

 

[VicS]

Having found out the hard way that battery failure can happen without notice, how do you check their condition?
FWIW mine are sealed. And there is a voltmeter on the dash - but which of three batteries does it monitor?

The voltmeter will give you a rough indication of the % state of charge of a battery but that is all and only if you make the measurement when the battery has rested with no charging and no load for 12 hours or so.

A good battery in a good state of charge will give a reading of something around 12.7 volts maybe more. If only half charged you will see around 12.2 volts. Anything below about 11.9 is effectively flat!

If by charging you cannot bring a battery up to a rested reading of 12.7 ish then it is probably getting towards the end of its useful life.

It is also helpful to monitor the battery volts over a longer period of non use. A good battery should hold up to 12.6 volts for several weeks.

The diagram below is useful, but you will be able to find many others, tables too, all varying slightly

The open circuit volts tells you nothing about the battery's ability to supply the high current required by the starter except that if you cannot charge it to a good value it is not likely to perform well as a starter battery.

You say you have three batteries. 2 in one bank for domestics and a single one for engine starting.

A volts reading on the two in parallel will give an overall %SOC for the bank as a whole. It may be dragged down by a dodgy battery in the bank therefore disconnect them let then rest and check them individually.

I do not understand your description of your battery switching.
Normally a 1,2,both,off switch will select battery bank No.1 or battery bank No.2 or both banks in parallel for use and charging or switch it all off.
You seem to be suggesting that you have three separate switches ... presumably just on/off switches ???

You need to determine if your panel meter is reading the volts of the domestic bank or of the starter battery.

The best meter to use is a digital one ( eg an inexpensive multimeter ) it will give better precision than an analogue one, but do not be tempted to read too much into the second decimal place. It will probably also give a more accurate reading than a panel meter.

 

[jordanbasset]

The voltmeter will give you a rough indication of the % state of charge of a battery but that is all and only if you make the measurement when the battery has rested with no charging and no load for 12 hours or so.

A good battery in a good state of charge will give a reading of something around 12.7 volts maybe more. If only half charged you will see around 12.2 volts. Anything below about 11.9 is effectively flat!

If by charging you cannot bring a battery up to a rested reading of 12.7 ish then it is probably getting towards the end of its useful life.

It is also helpful to monitor the battery volts over a longer period of non use. A good battery should hold up to 12.6 volts for several weeks.

The diagram below is useful, but you will be able to find many others, tables too, all varying slightly

The open circuit volts tells you nothing about the battery's ability to supply the high current required by the starter except that if you cannot charge it to a good value it is not likely to perform well as a starter battery.

You say you have three batteries. 2 in one bank for domestics and a single one for engine starting.

A volts reading on the two in parallel will give an overall %SOC for the bank as a whole. It may be dragged down by a dodgy battery in the bank therefore disconnect them let then rest and check them individually.

I do not understand your description of your battery switching.
Normally a 1,2,both,off switch will select battery bank No.1 or battery bank No.2 or both banks in parallel for use and charging or switch it all off.
You seem to be suggesting that you have three separate switches ... presumably just on/off switches ???

You need to determine if your panel meter is reading the volts of the domestic bank or of the starter battery.

The best meter to use is a digital one ( eg an inexpensive multimeter ) it will give better precision than an analogue one, but do not be tempted to read too much into the second decimal place. It will probably also give a more accurate reading than a panel meter.

Good useful post, especially liked the graph

 

[VicS]

Good useful post, especially liked the graph

I ought to acknowledge the source of the graph . G B ATTFIELD & Co who at one time made an LED based battery and charging level indicator called "VoltWatch"

Stupid idea I thought when they were available. Now they are not I'd like to buy one!

 

[Plevier]

That graph describes reasonably the typical behaviour of a conventional flooded lead acid battery in good condition and helps you understand the design basis of battery and charger systems.

However you cannot use it to diagnose the health of a battery. A severely deteriorated battery can follow much the same profile without very obvious voltage changes - but relative to what Ah capacity and what current capability? You just don't know.

A nice new 100Ah wet battery of typical design probably does have around 50Ah in it at that voltage (12.2). But take an aged deteriorated one and it might only have 20Ah in it, and a very limited high current capability.

In other words, you can't say that because it's showing 12.2V it's healthy and half charged and has 50Ah in it. It works the other way round. If you have taken out 50Ah from full, and the voltage is still 12.2, you can say yes it's OK. If it's gone a lot lower you can say it's knackered! But you cannot use voltage as a predictor of available capacity unless you ALREADY know the battery is in good health and capable of giving full capacity.

The straight centre section of the graph is just based on the relationship that open circuit volts per cell = (specific gravity of the acid + 0.85). Automotive type batteries are typically 1.28s.g. fully charged so you get about 12.8V from a 6 cell 12V battery. Deep discharged might be about 1.1s.g. giving you 11.7V. The relationship of sg to state of charge is reasonably linear while everything is healthy. Note those are rested open circuit voltages not under load voltages.

However the voltage is not directly related to the state of charge of the plates. Take a nearly fully discharged pair of plates and put the acid sg back up to 1.28 and as long as there is some active material in the plates you will see the full voltage. It doesn't mean there is full capacity there.

If the battery is in poor condition with lots of corrosion, paste loss and sulphation it upsets these relationships, but you'll have to look very carefully and use trends and gradients not just instantaneous values to derive anything meaningful from voltage measurements. (I'm fairly sure that is how the SmartGauge works; it is emphatically not just a voltmeter.) As you are looking at a total change of less than 1 volt on top of a base value of 12v, i.e. less than 8% range, a slight shift down of the voltage line through lowered s.g. due to acid consumption in sulphation is not very obvious.

That graph will also look somewhat different for valve regulated sealed lead acid batteries that have much less electrolyte in them and may start at up to 1.3sg and go virtually to water if deeply discharged.

It is a useful graph but don't misuse it!

 

[Bosun Higgs]

And there is a voltmeter on the dash - but which of three batteries does it monitor?

If you dont know, how do we?

 

[NickRobinson]

I ought to acknowledge the source of the graph . G B ATTFIELD & Co who at one time made an LED based battery and charging level indicator called "VoltWatch"

Stupid idea I thought when they were available. Now they are not I'd like to buy one!

Currently on sale in Aldi- less than 4 pounds.. I've built in two, one for each bank.

Nick

 

[pappaecho]

One of the best load discharge systems is the 12 volt fridge. If you rig one battery on its own to the fridge, and monitor the voltage as it goes down, you will quickly find the culprit. the advantage of this method is that you have cold beer and ice for the gin and tonic as you conduct the tests

 

[VicS]

Currently on sale in Aldi- less than 4 pounds.. I've built in two, one for each bank.

Nick

Yes I got one of the Aldi ones.

I should have got two so that I could wreck one to find out what's inside and perhaps make an "improved" version.

the VoltWatch ones were properly calibrated .. I bet the Aldi ones are not.

 

[Tom Price]

Welcome to my world!


Does this help clarify the 3 switches? LH = motor, RH = domestic, centre = both. This is before I added a second domestic battery under the floor, now with the orange cables running for'ard to a bow thruster.

Thanks for your input, sans pareil in your specialist areas.

 

[VicS]

Id assume from the picture that you have three simple on/off isolating switches.

One for the dedicated starter battery, one for the dedicated domestic battery and a third ( both) that parallels them for emergencies.

presumably you have some form of split charging system ( diodes or VSR or something more fancy) so that they are both charged simultaneously without closing the "both" switch

 

[ostell]

Did I see doubled up cables into one crimp, the thick orange pair going between batteries?

 

[powerman]

Testing domestic batteries

A simple way to test the domestic battery (if you don't have an ah meter like a Nasa , Victron etc) is to charge the battery as normal and then apply a load which will drain it over 20 hours. i.e. for a 100ah battery apply a load which will use 5 amps - something like a 60 watt head lamp bulb would do the trick. You will loose power in cabling, connection etc but it still gives you an idea of what's happening. Quite often you'll find a domestic battery will pass a CCA test but somehow still not provide many amp hours under a slow discharge. On a 100ah battery you might see that it works fine, voltage dropping slowly for the first few hours and then suddenly the voltage drops like a stone.