Assessing battery condition

[sebastiannr]

Hi there,

What is the best way to assess the condition of your batteries?

I think that my bank of (3 year old) 2 x 120AH AGM leisure batteries (which are charged by 2 x 80W panels + about 1 hour motoring per day average) are reaching the end of their life. My reason for thinking this is that when a current draws from the battery, the voltage seems to drop more than it used to. E.g. when the 5 amp fridge pump kicks in, the voltage will drop from 12.7 to 12.25, then return to 12.60 when the pump turns off (3-4 minutes). I'm sure the voltage drop was less than that 3 months ago. Also (though it's difficult to gauge exactly), the battery bank seems considerably quicker to deplete / charge than before.

I've only been on this boat for 3 months and the batteries are usually kept topped up by the panels, so it's not that easy to gauge real changes in depletion / charge times. Before I rush out and buy new batteries it would be good to know if there's a concrete way of testing the batteries' condition.

Thanks!

 

[Rigger Mortice]

If your batteries only ever get up to 12.7 volts there's probably something wrong with your charging system or you are just not giving them the chance to get a decent charge. There is a case for saying that it's probably best to not even use the fridge when the batteries have got down to that voltage.

You can do a capacity test (google will throw up several "how to" sites) on the batteries to assess them but you still need to get them fully charged for that test to have any meaning.

 

[sebastiannr]

If your batteries only ever get up to 12.7 volts there's probably something wrong with your charging system or you are just not giving them the chance to get a decent charge.

I thought that 12.7 was fully charged? Assuming there is no charge going into the battery when the voltage is checked of course.

 

[nigelmercier]

I thought that 12.7 was fully charged? Assuming there is no charge going into the battery when the voltage is checked of course.

Yes, but it does depend on temperature. The best way is to use a hydrometer, if you can get at the acid.

More info here: http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

 

[cryan]

You can test the electrolyte condition with a turkey baster type hydrometer. Get one with a traffic light tri-colour scale as thats easier to understand than just numbers. You can also by a load tester from the likes of Halfords or Machine Mart which will tell you the condition of batteries load handling abilities.

 

[cryan]

Actually forget the turkey baster I just realised your using sealed batteries. But you can still load test them.

 

[Rigger Mortice]

I thought that 12.7 was fully charged? Assuming there is no charge going into the battery when the voltage is checked of course.

You're right. My wording was clumsy as I had picked up on your reference to charging with solar panels which I would expect to keep the batteries above 13 volts pretty much throughout the day....especially if you are in the Med.

 

[pheran]

I've always thought of 12.8v as being fully charged, 12.5v as half discharged and 11.7v as flat. Is that correct?

 

[macd]

I've always thought of 12.8v as being fully charged, 12.5v as half discharged and 11.7v as flat. Is that correct?

It's a convenient measure, if not especially accurate and won't necessarily reveal whether the battery's capacity has declined (for that you need a load tester). For a typical wet-cell battery, the range is a little narrower: around 11.8 to 12.7V. Half-charged (does that make me an optimist?) is around 12.3V. Gels will show maybe 0.2 to 0.3V higher at both ends of the scale. Readings are only reliable if the batteries have rested (zero charge or discharge) for at least an hour, preferably longer.

 

[vyv_cox]

During the day when the solar panels are charging my domestic battery bank voltage is invariably close to, or over, 14.0 volts. First thing in the morning before the sun comes up, fridge having run all night, the battery bank voltage is usually about 12.6 and with the fridge running it is 12.1 to 12.4, dependent upon how much power we used on the previous evening.

 

[sebastiannr]

You're right. My wording was clumsy as I had picked up on your reference to charging with solar panels which I would expect to keep the batteries above 13 volts pretty much throughout the day....especially if you are in the Med.

They do during the day when there's nothing drawing. I should have mentioned that my example with the fridge and 12.7 to 12.25 was assuming there was no charge (night time) and no other loads.

Is there any info on, for example, what the voltage drop would be on a healthy battery if a given load was put on it, say 5A, or am I asking the wrong question? Would the drop be the same no matter what the state of the battery?

 

[rob2]

It sounds like you are in the right sort of area with your figures - it is not possible to accurately guage condition from a set of figures from a moment in time. The professionals use a drop tester which needs to know the type of battery (lead/acid,AGM,etc) design capacity, CCA and then it can match the drop in voltage against load against time to a curve based on the type and starting voltage - and even then, a battery with a surface charge gives erroneus results! I was very impressed when the tester said one of my "knackerd" batteries was down on charge but still giving around 80% capacity - the other battery was foobarred and dragging the "good" one down. I don't think there is really any other reasonably accurate way to assess a battery.

Rob.

 

[TQA]

You asked for the best way to assess the condition of your batteries?

Get them fully charged. Give them an equalising charge if possible. Be real carefull doing that on sealed or AGM batteries, follow the manufacturers protocol exactly ideally with a temperature monitor.

Leave them for 3 to 7 days.

Apply a load to drain 40% of their capacity in 3 to 4 hours. I used to use headlamp bulbs.

Measure the voltage on load and off load at the end of the discharge time.

Minimum of 12volts on load and 12.25 off load.

If the battery meets or exceeds those then I would regard it as fit for further service.

The load tester is really designed to measure the kind of performance required during engine start up. If you are looking at house bateries then the above method is better, time consuming but better.

 

[sebastiannr]

It sounds like you are in the right sort of area with your figures - it is not possible to accurately guage condition from a set of figures from a moment in time. The professionals use a drop tester which needs to know the type of battery (lead/acid,AGM,etc) design capacity, CCA and then it can match the drop in voltage against load against time to a curve based on the type and starting voltage - and even then, a battery with a surface charge gives erroneus results! I was very impressed when the tester said one of my "knackerd" batteries was down on charge but still giving around 80% capacity - the other battery was foobarred and dragging the "good" one down. I don't think there is really any other reasonably accurate way to assess a battery.
Rob.

Thanks for that info, it answers my question completely. Once batteries begin to go, is it a rapid decline after that or a gradual loss of capacity? (Just thinking if I should replace soon or wait until the last minute)

 

[Rigger Mortice]

They do during the day when there's nothing drawing. I should have mentioned that my example with the fridge and 12.7 to 12.25 was assuming there was no charge (night time) and no other loads.

Is there any info on, for example, what the voltage drop would be on a healthy battery if a given load was put on it, say 5A, or am I asking the wrong question? Would the drop be the same no matter what the state of the battery?

The problem seems to be that all the guidelines you see are just that, guidelines, rather than absolute figures that you can rely on without question. One battery I had gave a figure of 12.84 volts after 24hrs stabilisation when it was new. Another gave 12.92 volts and my present battery gave 13.02 volts.....and whenever I've checked my meter against others it has always been reasonably accurate. Temperature may account for some of the variations but other than setting up a laboratory test these things it's difficult to know.

All you can do is carry out a capacity test but even that probably does no more than give you an indication of the rough state of the battery.

In answer to your question though. I would expect the voltage drop on an unhealthy battery to be greater than thre drop on a healthy battery but others may disagree.

 

[William_H]

A battery is basically good if it will do the job you need. If they are cactus you will have no doubts.
You are perhaps worrying too much. That is unless you are departing cross Atlantic or similar.
You should periodically ensure that each battery alone will start the engine.
If you want to assess condition then disconnect charging and run your domestic services and see if they will supply your needed power for the needed time.
Yes as said you need to ensure that they are being fully charged. ie check charging voltage from engine and solar regulator. good luck olewill

 

[mocruising]

Never had less than 7 years out of my conventional lead acid batteries. We have 9 x 120 AH on my present boat and half are 4 years old already. (6 x 24 volt 3 in series and 3 in parallel) 1 x 12 volt ME start 1 x 12 volt gen. set start and 1 x 12 volt nav. service.