High Battery Reading?

[Robtel]

Some time ago I had a problem with my leisure battery and among the suggestions received was one to purchase a multimeter so that I could gain an accurate reading from the battery. The original problem was resolved but I decided to go ahead and purchase a multimeter anyway. I bought a Gunson make digital one from Halfords yesterday and tried it out last night with some (to me anyway) surprising results.

The caravan is stored on my driveway and has been hooked up to the mains for the last week or so. The battery had been charged indoors on a proper leisure battery charger going up to the recommended 14.5v about 5/6 weeks ago but we have been away with the van since then. We have a mover fitted so the battery would have had some usage and would not have been fully charged when stored.

With the mains hook up unplugged both the onboard dial type volt meter and the hand held digital were showing similar readings i.e approx 13.5 and 13.46. I thought perhaps there might have been some residual power left in the system so left it a good few minutes and tried again with same results.

I went back out this morning and tried again. No hook up, switched on a couple of interior lights to see if that would drain any residual power and got onboard reading of approx 13.25 and digital reading of 13.26.

Then I totally disconnected the battery from the caravan and with the handheld meter got the same reading of 13.26.

I have never totally understood all the techie stuff about the battery but from what I have read on here and else where:

A fully charged battery should read at approx 12.7v

A battery charged from the onboard charger will not be fully charged.

So based on that there are at least two reasons why I should not be getting a reading of 13.26. Any one got any ideas on this?

 

[Chris_374413356]

Hi Rob

Batteries are sometimes a law unto themselves and perhaps if you let the battery rest for an hour or two you may find that a fully charged battery should be around 12.8V.

The problem with a digital multimeter is, yes, it measuring the terminal voltage, but it is not really telling the state of the battery or telling you if you have any faulty cells..... strange as that sounds!

The other checks are taking the SG of the electrolyte, not possible on a sealed battery or using a load tester which will pull load on the battery and tell you its condition.

Hope this makes some sense.

 

[RayS]

Another possibility is the very common falacy of "believing" the digital meter, just because it shows two decimal places. A reading of 12.26 (say) imples an accuracy of between 12.25 and 12.27 - in either case 1 in 1,000.

The dsplay takes it reading from the measuring circuit which - for a low cost commercial meter - is somewhat unlikely to be good to 1 part in 1,000. This sort of accuracy is not easy to achieve even with good quality kit, as a life-time career in measurement and control has taught me.

The same problem arises with calculators, when a given sum is likely to produce an answer running into 5 or 6 decimal places, when the data on which the sum is based is only good to about 0.1% or worse (i.e. less). See also fuel pumps which quote 3 decimal places for a pooound (cash) - have you ever seen a 1/10th of a penny coin ?

Another problem is that of contact. putting a probe onto a battery terminal is not a precise science as changing the pressure applied or scraping the probe such that it penetrates the oxide film on the lead terminal will show ( remember, we are looking for very small changes in voltage for the purpose of this discussion).

I wish the late and very lamented rob_jax was able to add his knowledge to this thread - I'm sure he would have been able to make these points in a much more cogent manner.

 

[Mikey1]

Voltage would tell a faulty cell, as you have 6 cells of around 2volts connected in series.

The failure of 1 cell would show up easily as a voltage drop.

 

[Robtel]

Thanks for the replies.

It is a sealed battery so a Specific Gravity check is out. I think the suggestion to leave it off the charger for a longer period of time than I had done may be the answer. After being off hook up for an hour it was reading 12.63 which is more like what I had expected from an onboard charger. I now have the battery indoors and on a leisure battery charger to see if that will top it up to 100% capacity.

Again thanks for taking the time to reply.

Yes Ray S, I also share your sentiments re Rob_Jax.

 

[xtrailman]

rob-t

i think you are putting far too much thought into the battery state.

My van is on the drive hooked up to the mains, but with the mains OFF.

Although i have a

 

[Daveg1]

I recently had a battery charging problem and found this link useful- http://www.magnacharge.com/mchg_all.htm#Temperature

With reference to the article most modern on board caravan battery chargers have true three stage controllers (bulk, absortion and float)

Regards,

Dave

 

[xtrailman]

You can enter your own figures into this link, to get an idear about battery internal resistance.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dcex6.html

 

[David_374242465]

The longer it takes for a battery, after charging, to drop from the full charging voltage (around 14.5 volts), to settle around 12.7 volts, the better the condition of the battery, as a general statement.

Assuming no load, a battery on its last legs may drop to 12.7 volts within an hour or so. A battery in top notch, excellent condition, may actually take 12 hours or more to settle at this voltage.

As a battery ages, the internal resistance across the plates tends to decrease,(due mainly to sulphation build-up)causing the discharge leakage internally.

A "good" battery will settle at 12.7 volts, and if kept in a dry environment at around 15 - 18 degC, with no load, maintain this voltage for several weeks.

 

[David_374242465]

The longer it takes for a battery, after charging, to drop from the full charging voltage (around 14.5 volts), to settle around 12.7 volts, the better the condition of the battery, as a general statement.

Assuming no load, a battery on its last legs may drop to 12.7 volts within an hour or so. A battery in top notch, excellent condition, may actually take 12 hours or more to settle at this voltage.

As a battery ages, the internal resistance across the plates tends to decrease,(due mainly to sulphation build-up)causing the discharge leakage internally.

A "good" battery will settle at 12.7 volts, and if kept in a dry environment at around 15 - 18 degC, with no load, maintain this voltage for several weeks.

Think about it. If you buy a brand new battery from a garage or Halfords etc, the voltage across all batteries will/should be around 12.7 volts, indicating near fully charged.

These batteries have probably been on the shelf for weeks, even months. The suppliers do not go round these batteries with a charger every few weeks!! In fact they are delivered from the manufacturer and go straight on to the shelf!

 

[David_374242465]

Think about it. If you buy a brand new battery from a garage or Halfords etc, the voltage across all batteries will/should be around 12.7 volts, indicating near fully charged.

These batteries have probably been on the shelf for weeks, even months. The suppliers do not go round these batteries with a charger every few weeks!! In fact they are delivered from the manufacturer and go straight on to the shelf!

Rob-T

If your battery no load voltage reading is still above 13 volts after 48 hours, then you have either got a very, very ,very good battery (I wish I had the patent!) or your Digital Voltmeter is inaccurate.