OK this is a difficult article to write, but I have been having to explain these concepts to so many people recently .... so here goes.
Many times a year I see somebody say - "the battery is full" - it's 13.2v. This may be true, but usually it isn't.
Lead acid batteries are strange beasts, and you have to understand their quirks. You also have to understand the relationship between volts and state of charge.
Volts “on charge”
A battery "on charge" will usually always show over 13v. That is because charging voltage is always higher. That's why you always see the number 13.8v so often - this is the traditional charging voltage for a lead acid battery. Some batteries like AGM like a higher voltage, but that's another story.
So nowadays most owners have solar, and if the sun is shining, then most likely your battery will be showing over 13v all day long, but this does not mean it is full. So you can never look at any battery that is on charge, see 13v and assume it is full.
So how do you tell the state of charge of a battery? “Resting voltage”
You need to know about "resting voltage". Resting voltage is when a battery is doing nothing - not being charged and not being used. When resting, a battery will settle to a voltage between 12.0v and 13v. 12.0v is technically empty, and 12.8 to 13.0v is technically full. So every point one of a volt represents about 10% - so at 12v, decimal points of a volt are important!
So how do you get at and measure resting voltage? If you have solar, or are plugged in, you will never see it, because the voltage is always over 13v when "on charge" - so to get at resting voltage you need to remove all charge sources - and the easiest way to do this is to wait until the sun goes down, and be not on EHU - and anyway if you are on EHU, then your battery is fully charged anyway, so no need to bother. Resting voltage is only really relevant in off grid situations.
And of course you need to be able to see, so having a light on is fine - 1 or 2 amps of discharge doesn't really affect the measurement.
The next problem is meters - classic motorhomes designed in the 70s 80s and 90s have old fashioned analogue meters - and they are not as accurate as modern digital meters. So you have to calibrate them, or better still, install a permanent digital meter. One of the best ways of doing this is to simply have a modern MPPT solar controller with a digital display. This will display battery voltage to 2 decimal places. It is my general suggestion these days that every van should have some sort of reasonably accurate digital volt display.
So if at sunset you see a resting voltage of 12.8v, then you know your battery is full. If you then watch 5 hours of TV and have the lights on, and you check again just before bed, you might see 12.4v - that means you have used 50% of your battery that evening.
Resting voltage is the ONLY way to gauge state of charge, empty or full, of a lead acid battery - in a standard van. There are other ways, but they all involve installation of extras which I discuss later.
Understand Volts and Amps
Volts are not amps! Volts only represent potential - ie availability. But only amps represent the actual transfer of power. This gives rise to some confusing situations.
Example - a battery CAN be full, and show over 13v, but because it is full it will take no amps. This can be confusing to owners who have installed modern dogital MPPT solar - the display says 13v but no amps, so they assume a fault. No ... the battery is full, it does not need and cannot take any more amps. There is no fault.
Example - you know that a battery is full, but when you use it the volts drop and you think there is a problem. This is because a battery under load always drops the volts, and the heavier the load, the bigger the drop. A "heavy load" is about 10 to 20% of the battery's capacity - so thats 10a to 20a on a 100ah battery. Light loads - 2 to 5a, do not affect voltage as much. This why "resting voltage" is important - a battery on charge shows a high voltage, a battery under load shows a lower voltage - but when the battery "rests" the voltage is an accurate indication of it's remaining capacity.
100ah is never 100ah …
A 100ah battery is not 100ah. Most people know this, but it bears repeating. The "technical" rule book says that a battery should not be discharged below 12v, and 12v is often reached at around 50% or so. But this varies greatly from battery to battery and is entirely dependent on quality and history. A lead acid battery changes with use and time, and how they respond to use and time is down to quality. That's you will often read that cheap batteries are not worth it - except for very light use. But cheap batteries are usually fine for those who are almost always plugged in to EHU.
Not many people know that a lead acid battery is only usually capable of so many "cycles" - a cycle is being fully charged then discharged. This number is not often published, but for a lead acid battery it is around 200 - that's not a lot. So if you fully charged and then discharged (down to 12.0v) a battery every day, it would be end of life in under a year!
The other problem is that there are hardly any legal standards for battery specifications, so the manufacturers can be economical with the truth. This is again because of the "quirky" nature of lead acid batteries. For example - it's not simple maths - you would expect a 100ah battery to deliver 1a for 100 hours, or 100a for 1 hour, or 20a for 5 hours - but it doesn't - in fact it is different almost every time, because the harder you work the battery, the more time it needs to recover, and the more you force it to recover the less it's lifespan will be. So the same battery that is asked to discharge 20a for 5 hours every day, will have a shorter life than one that is asked to discharge 5a for 20 hours. And to make it even more complicated, temperature is also a factor!
Reputable manufacturers - ie Yuasa, Bosch etc, usually quote either a 10a or a 20a figure. But most manufacturers, especially the cheap Chinese ones, don't. So they can say it's 100ah, and if anybody complains they just hide behind what I have just explained, and say it's only 100ah when doing nothing!!
The only thing that guarantees you a half decent battery is lots of lead - the metal - in plenty of quality chemical - the acid - and designed and manufactured to the highest quality. That's why if you weigh a cheap noname 100ah leisure battery and an expensive professional 100ah fork lift truck traction battery side by side, the pro one will be heavier by quite a factor.
Over the years this has been a bit of a scandal in the leisure industry, and in the last few years the caravan clubs and trade associations have finally got their act together and introduced some better standards.
So now you know all that ….
So how can you make use of all this knowledge? The answer is that knowledge is power, scuse the pun. The only time that most owners know that their battery is running out of juice is when the lights start to dim - and usually by this time it is too late - the voltage is way below 11v ... and it is a sad fact that nothing shortens the life of a lead acid battery worse than multiple complete discharges below 11v. But by monitoring your resting voltage, and getting into the habit of checking it each night before bed, you will soon get a good notion of whether your battery system is adequate for your needs. If you only use the van in summer, and solar is pitting it back the next day as fast as you are taking it out, then happy days, your battery should last its full lifetime.
But if not, and you often see resting voltages late at night of 12.1 or below, then you can make use of that info, and hopefully save some future problems.
Battery monitors
Finally, let me touch on a couple of modern inventions that can make the whole thing easier. The first is called a battery monitor - this is an accessory that attaches to your battery, and comes with a display, and inside the display is a little computer that is programmed with all those quirky variables I just mentioned. It counts every amp in, and every amp out, in real time, taking into account the temperature and the depth of discharge - ie how hard the battery has worked, and it calculates all this and displays a state of charge - either as a percentage or a graphic - also in real time. This means you never have to bother with "resting voltage" - the gadget does it for you. These cost around £50 for a Chinese one, and around £120 for the king of them all - the Victron Smart Shunt, which beams the info to your phone in a pretty app. But I must stress that you can have all this for free by using the "resting voltage" method - but if like me, you like a gadget, then a digital battery monitor is a nice thing to have in the van - not only does it show state of charge, but how much time you have remaining, how much solar you are getting, and as you switch things on and off the numbers change so that you soon understand how much power everything in the van uses.
Lithium
The second thing to mention is that no article on leisure batteries is complete these days without a mention of lithium. The new breed of lithium batteries are quite different to lead acid batteries because the chemistry is quite different. Lithium batteries deliver their power without the falling voltage that characterises lead acid. A 100ah lithium battery will deliver 12v for over 90% of it's capacity, but then it will just stop. And they have another problem feature - they don't take abuse like their acid cousins - if you over or under discharge them they can get damaged. So the clever manufacturers of lithium batteries came up with something called a BMS - Battery Management System. This is a board of electronics and transistor switches that sits between the battery and the outside world. It monitors the battery in real time, counts all the amps in and out, and if at any time it's not happy, it shuts down the battery before any damage can happen. All lithium batteries have a BMS - even those packages to look like old fashioned batteries - there is a BMS board inside. The best of these have bluetooth and an app (just like the Smart Shunt I mentioned) and this makes them into a "smart battery" - so with a smart battery all you have to do is look at your phone or tablet and you know exactly what is going on.
This in my opinion is the "killer" feature of smart lithium - the information. You get to know exactly where the power is going and coming from, in real time. Practical examples are that you soon get to know how long you can stay off grid, or how much solar you can get on a cloudy day, and how the seasons affect the available sun light. In the case of smart lithium, knowledge really is power!
Add to this that an average lithium battery is capable of at least 2000 cycles, and that they are now no more than double the cost of decent lead acid batteries, and the financial argument is compelling. But while lithium is a great thing to have if you need the power, there is nothing wrong with good old fashioned lead acid, especially if you take on board the characteristics of lead acid that I have tried to explain in this article.
