(battery charging, and) Cells in a Battery

When I built my shed I knew I'd need power down there and so it immediately occurred to me that this was an excellent opportunity to conduct a practical (and beneficial) experiment in "Off Grid" solar. 

I knew that while I understood a lot of the theory, its often the case that some surprises occur in the actual operation. This blog post is about an expectation that was fulfilled: cell imbalance in flooded lead acid batteries.

This issue is important to anyone choosing lead acid chemistry in an off grid application because it leads to premature death of your (often expensive) investment in batteries. The reason for 24V is that I want to use the energy in the battery to power things which are made for AC, thus I need an inverter. The above mentioned blog post has a few more details.

the invisible (to most people) issue

People never think past words, like "battery".  A battery is not just an indivisible block, but basically a connected series of cells each depending on the other (check the word battery in a dictionary).

Charging a cell is straightforward, but charging batteries is more complex than people usually consider. This is because while each cell needs to be charged equally we apply a voltage across the entire battery and "magic happens"; where the battery magically stores the energy. 

If only it was this simple there wouldn't be any problems.

Above is a picture of my 24V battery system for my solar shed, the battery system was commissioned  (fancy word for the relatively simple process, sorry) in early March but as you see the battery dates are 4th of Feb (which is why dear American readers the rest of the world writes 4/2/22 and your Month Day Year format is frankly 0.o) 

As you can see I have two 12V batteries connected in series to make a 24V battery, and each 12V battery has a small volt meter on it. I did this as I wanted make voltage checking a simple matter (rather than kneel down on the floor with my volt meter). I wanted to check because I wanted this to be an experiement in exploring why batteries fail early. 

I believe batteries fail  early because of unaddressed issue in imbalances. Of course I'd like to improve on this situation, but that's pretty much impossible with the way modern batteries are made and (a big hint to off gridders to select batteries appropriately) how batteries are connected for charging.

Probably the first thing you can see there is that the voltages are different between each 12V lead acid battery. 

• B1 is 13.3V
• B2 is 13.6V

This wasn't always like this, as back in Feb they were both new and both within 0.01V of each other. Somehow that balance has changed. Leading me to my point about balance:

The Balancing Act

So we get to the thing I'd like to talk about here cell balance, and the word cell will need to be clarified here because I'd say more than 90% of readers have really never given this any thought. When the battery is made each cell is made pretty accurately to be identical, but unless this is a battery made for NASA some small variances in all the parts can mean that each cell has a slightly different reaction to charge and discharge. Over time these small variations can add up.

The nature of these differences usually is in the resistance each cell has (in the chain of cells in series) and perhaps in its actual capacity for charge and discharge. If left unchecked and unaccounted for inevitably at least one cell starts to get more stress.

The Cell

As you (should) know a 12V battery (as above) is actually a collection of 6 cells (or electro-chemical cells) each which contains some sulfuric acid and some lead plates. This cross section diagram (borrowed and altered from here)

Its tempting to just see this as a black box (well and its often in a black plastic box) but the reality is that each cell may well need individual attention.

In that above cutaway you can see that the battery is a series of  6 cells linked together inside connecting postive to negative to make a 12V battery. By joining two batteries you can make a bigger battery and double the voltage available. An off grider with much nouse would probably find themselves joining 4 12V batteries into a 48V battery, thus reducing the amount of amps that need to be carried to feed their inverter (which makes 240V AC which powers things that plug into the wall).

But returning to my more simple model (having 2 batteries or 12 cells) lets go back to that 0.3V voltage difference.

Once upon a time we had tools to measure the chemistry of each cell, this dates back to a time when (for various reasons) nobody had volt meters. The most common tool was a hydrometer and you can see how its used at this Wikipedia link. You'll notice that in that picture that each "cell" is by itself and has a hatch to allow you to suck up some of the chemistry (sulphuric acid) and measure how much is there.

You may even recall having seen batteries with 6 little caps along the top to allow you to make this measurement. Indeed more expensive and larger batteries usually still have them (see below)

But because less and less people have a clue what to do with these the makers target these to the applications where people should have a clue (and sadly this results in less and less people having a clue).

Lets get back to that 0.3V and work through this a bit more.

Because those little LED meters are cheapies ($2 each) I know they aren't perfect and I know there is about 0.1V difference on one of them (and perhaps something under 0.5V on the other meaning I can't actually see it easily). This means that when I use my Fluke digital volt meter (pretty accurate and provides 2 decimal places) I can see that the voltages are more like 13.18 and 13.38V so that's a bit better.but back on the 8th of the 3rd they were already drifting apart and were 12.91 and 12.89V (which the astute will observe is the other way around).

I noticed that differences were creeping in and so I wanted to observe these more carefully thus I bought those small LED's and fitted them.

• 14/04/22 I noted 0.1V difference when the batteries were on trickle charge
• 14/05/22 I noted 0.5V difference when the batteries were on trickle charge

I recently cycled them both down and individually (<<note that point) groomed them back up with a smart charger. Yes, this means I physically dis-connected them (so no more power coming from them if you're off grid) and charged and allowed to settle post removal from the charger. 

Both sat nicely at 13.36V when on the smart charger on "trickle". This is one measurement, but another is what's 'rested' voltage and this was 13.18 and 13.38 ... so this difference now seems to be permanent. 

While this may seem like a small small things may grow.

Is this a problem?

For me, in this situation, no not yet, because (importantly) when the batteries are under load (like when I'm running a vacuum cleaner which draws a lot of power) everything holds up ok and within expectation.

Why is this important?

well we know that the voltage of the battery is 13.18 or 13.38 depending on which we're looking at, what we don't know is what each cell is doing because on these batteries we can't measure that. If we imagine that we could only see the pair of batteries (which importantly is what the inverter or charger sees) we would see 26.56V - its only because we can measure each component we know something is amiss.

But where and by how much?

Lets say only one cell is down and that B1 has one cell that is 0.2V different. 6 of the cells are ideal at 2.23 and one cell is 2.03V ... that means that this cell will now become the weak link in the chain and will suffer more stress and eventually result in the early death of that battery because it is the weaker link.

Important note: a battery is a chain of cells ...

Worse, if I replace B1 then the new battery will be stronger than the then worn B2 (that didn't fail) so that will cause that link to fail sooner too. Meaning I'm minimising the returns on my power from what I spent on the battery.

As it happens (and partly because this is a learning exersize) I paid $90 each for these batteries (they were bought new), but they are, as you can see, small. But then so too is the load and so is the charging. Further I'd say that they are the perfect relationship between needs and cost.

However, f you were in an off-grid house and you were in this situation each battery could well be around $500 and you may have 4 or 8 of them (at least). For instance here is a freinds off grid house 48V system with each "battery" component being a 6V battery (of three 2V cells).

however you can see that each has an inspection cap so you can measure the state of charge and fix up any chemistry. Each of those boxes is nearly a $1000 now, so you can see that caring for and understanding this is crucial for your investment (unless you've got more money than sense).

So ... is there a better way to charge?

Yes, each cell needs to be monitored. However lead acid chemistry is more tolerant of over charging in terms of how it fails, Lithium ION on the other hand is not, and has a nature of catching on fire if over charged and dying (irreversibly) if discharged too deeply. Since Li-ION is expensive and fires inconvenient people have developed a management system for the battery (called a Battery Management System or BMS). This not only monitors the charge but monitors every cell being charged. This is the one out of my scooter still attached to the cells. It manages 13 cells.

As you can imagine with 13 cells in series (compared to just 6 in a regular car battery) the chances of anything getting out of balance is not just high, its practically inevitable (unless you are buying cells that are suitable for NASA). This  BMS not only governs the charge of each cell during charging it it also ensures that if any cell falls below the minimum safe voltage it shuts down the access to power from the pack.

Normally with just 6 cells there is less chance of an imbalance, which is why most small scale solar systems use 12V ... but it of course has inherent limitations in terms of the voltage and therefor the amount of Amps that are required to power much. As you should remember my system actually has 12 cells (6 in each "car" battery) because its 24V and this has no doubt created the situation where somewhere (and we know that its in B1) at least 1 cell is down. This is then going to place more stress on B2 in terms of over voltage charging because the charger only knows the entire voltage of the battery.

Is there something like this for Lead Acid batteries?

Myself I think there is, but its a bit of a fudge (and beyond the scope of this article) so the answer is basically no, and perhaps the "why we don't see things like this in Lead Acid is:

• Lead Acid is an old technology most commonly used in 12V configurations
• its perceived as less needed because there is no safety hazzard, and (probably) 
• because lead acid batteries are very recyclable and rebuildable unlike Li-ION.

Further most companies don't warrant car batteries to be used in Solar Power Systems, so if you kill your battery early then its all on you.

Right now what I see is that its on me to basically prevent this by keeping an eye on the system (and turning off the panel and disconnecting the battery from the Solar Charge Controller) when I'm not in the shed and just let it sit there essentially in storage. Fortunately that's something that Lead Acid is pretty good at.

I'll put it into another blog post for what my solution is on another day (see here for that). Mean time its been a fun ride with my system. I hope you've found this interesting and helpful so far.

Until Next time