Understanding battery voltage

[aandy]

According to the Yuasa chart that has been referred to on here a few times, a conventional lead acid battery has 75% capcity remaining at 12.4 volts. Can anyone tell me whether that relates to the battery's original or actual capacity? If a 100ah battery has deteriorated with age such that it has only 80% of it's original capacity, does a reading of 12.4v indicate 75% of 100 amps or of 80.

[aandncaravan]

I would love to know a definitive answer to this one, and it was one of the questions I asked when on my tour of the certified battery test facility.

 

I got two different answers from two experts, both started with there "being so many variables when taking SOC readings". The best explanation I have found of just how many different factors have an impact, from Electrolyte concentration to temperature, is this : http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

 

 

One expert told me the voltage will drop as the Plate material degrades, but I have never seen this in practice, so not sure it is true.

 

The second said that so long as the battery wasn't suffering a 'mechanical failure' (more on that below) the voltage will reflect the capacity of the 'working' material. So in your example 12.4v indicates 75% of 80Ah.

Without a load test or one of the sophisticated impedance spectro meters there is no real way of knowing what the actual capacity is. Although the first expert above said that battery recharge time would drop, as the amount of active material reduced. The second expert said that the recharge time (as in elapsed time) may not change but the amount of overall current drawn would. Again, I am inclined to believe the second expert as that seems more logical. If true a Wattmeter style device might give a guide?

 

 

A 'mechanical failure' would be something in addition to the Exhaustion of the Plate Active Material/Paste like a build up of Sulphate crystals (Sulphation), Antimony poisoning, Paste that has detached and may be causing 'shorting', buckled plates, severe corrosion, etc.

 

The problem is that many poorer quality batteries suffer mechanical degradation almost as soon as the Paste starts to become exhausted through use. Sometimes, more than one mechanical failure will be present.

 

It was generally agreed that Corrosion of the grids was the mechanical failure that impacted more than most other faults, at least initially.

 

[aandy]

Thanks. I suspected it would be the lesser of the two. Trying to decide whether to risk 6 nights off hook up (and without moving the van) with my my 2 year old Banners. I rarely stop for more than two nights at a time, so very much in uncharted territory. In theory they should be more than adequate, so I'll probably just hope for the best.

Andy

[aandncaravan]

Andy, A battery down at 80% capacity is regarded as 'exhausted' by the industry. Although it might initially seem it hasn't lost much, don't forget that most advice says don't drop below 50% DOD, that is leaving 50% in the battery.

In your case that means you have just 30Ah to draw out before you hit 50% DOD.

That isn't a lot for someone who wants to manage 3 days off grid, let alone 6.

 

I would suggest it would be very easy to drop below 50% DOD with the potential for some quite serious life shortening damage to occur.

One of the reasons that batteries below 80% capacity tend to degrade very quickly, is that people take out the 50Ah they have got used to drawing, leaving a paltry 30% left in the battery or 70% + DOD.

 

 

In the case of Banner, the chances of yours currently suffering 'mechanical failure' from Antimony poisoning or severe corrosion, is high, which will also rob some of the 80% you think you have.

 

We don't see many Banner Energy Bull's make it much past two years. The AGM Running Bull's rarely reach even two years.

 

Don't know which Banners you have but 6 days is a long time. I would suggest you consider new batteries?

 

[aandy]

The 80% was purely hypothetical. I wouldn't know where to start working out the actual state of deterioration. Because I rarely stay in the same place for more than one or two nights, the batteries have rarely dropped below 12.4 volts. At the same event last year (albeit 5 nights) they only went down to 12.3). My logic is that they are unlikely to fail altogether, and if they do go so low as to damage them, then they were scrap anyway. If, on the other hand, they don't, because they are usually so lightly used I'll get another year out of them.

 

As you've probably gathered, I know pretty much nothing about batteries, so feel free to point out any flaws in my logic.

 

The batteries are energy bulls. When I do come to change them (probably for varta lfds) would a single 180 be any better or worse than 2 90s?

 

Andy

[aandncaravan]

You say you know little about batteries so may not be aware that although the Energy Bull was often sold as maintenance free, it is probably one of the highest maintenance Leisure batteries going. Therefore, if you don't already check the fluid level regularly, suggest you do so now.

If you find the plates are exposed they are likely to be damaged which may mean that the way the batteries performed in the past, might not be appropriate to their performance now.

 

So with that proviso, I agree with your logic.

 

One disadvantage of one very big battery is the weight and size making them difficult to handle.

 

But the major disadvantage is the very low turnover of any none 'mainstream battery' having been on the shelf a long time and already lost some of it's life by the time you buy it.

LFD90's turnover is very high, when the new Yuasa L36-EFB rolls out in August it will obviously be straight from the factory.

 

 

[aandy]

I've actually got 2x100ah, so if at 80% I'd have 10ah per day which in theory should suffice, with the same again in reserve if not bothered about killing them (we'll be out most of the time, only sleeping and eating in the van). I read in some of your previous posts that the EBs can be thirsty, but had always checked them every few months even before that. One of the reasons I wanted to put off replacing them was that I remembered you saying a while back that Yuasa would be bringing out a new range, which looked promising. If they are available in August I may get them anyway just to be sure. No matter how hard I try to convince myself, if I was that confident in the Banners I wouldn't have asked the question.

 

Thanks for your help

Andy