While reading any of my post, feel free to let me know about spelling or grammatical errors, I am an Engineer, not an English expert. : )
My First Post!
After much encouragement from others, I have decided to start a blog where I can post about projects I have been working on and things that I am interested in.
As an Electrical Engineer I Love electronics, but I also get almost as excited about the source of power for those electronics. In many cases it is batteries! I like all batteries, they don’t have to be rechargeable. The great thing about rechargeable batteries is they require electronics to charge them (wow, like the perfect mix). I really appreciate the latest Lithium-Ion or Lithium-Polymer batteries, but because they are also pretty straight forward to charge (it’s more a problem of discharging with them) then I kind of find NiMh more exciting. NiMh actually has about the same energy density as lithium but because of the higher self-discharge rate of NiMh they seem like less. I know someone is going to want to throw out "Paul, what about Low Self-Discharge NiMh batteries!?” Well it would seem that by being Low Self-Discharge (LSD) that would then make them the same as lithium, which we all know they are not, so that would mean I have no clue what I am talking about. As it turns out, the LSD part also takes away from the overall capacity.
Anyways, NiMh are still very popular rechargeable batteries for devices designed to use standard AA or AAA sizes. However there is a problem with them and it isn't the batteries, but rather the chargers.
My very first Digital camera used rechargeable NiMh batteries, AA size. It was a great little point and shoot. Eventually the day came when the batteries stopped holding a charge. It surprised me because I had only had the camera a year, however I figured the ones they include with the camera are pretty cheap and therefore I would just buy some awesome third party ones. I was pulled in by attractive marketing and went for those with the highest mAh rating. Guess what, they died even faster! I wanted to know why. I didn't know what I was getting into, but this was the start of my battery obsession.
Going back to my statement; the problem is in the chargers, it’s mostly true. In the situation above, I had been sucked into a bad trick. Several years ago (pre-LSD rechargeable batteries) when my experience took place, a few companies started really pushing the capacity limit too far. There is only so much energy you can fit in a given physical dimension. That limit had arguably been reached for NiMh. Then someone said “Hey, we could make the plates (not necessarily a plate, it can be more like a sponge or many other forms) thinner in the battery in order to make more room for the chemicals." Sure enough, it did increase the capacity over competitors', but at the cost of durability. The batteries were wimpy, and couldn't take much abuse (like charging a camera flash or getting slightly reverse charged) before giving out.
The Good News, most companies have moved on to the Low Self-Discharge path and I would say NiMh’s now are generally of better quality then they were (I said generally).
So Now, the chargers. Have you seen batteries advertising 1000 or even 1500 (even more) cycles? Not many people actually get even close to that many cycles out of their batteries. The reason is the Chargers! There Dumb! No really, they are. Many of the chargers that come included for free with a new set of batteries are actually battery killers. They over charge them, they charge them in pairs (which is bad, I will explain later) and they are inconsistent in their charging. Some of the worst are those that have instructions that say “You” must unplug the charger (and therefore stop the charging) after so many hours. If someone doesn't know that part, then they will end up with VERY cooked batteries.
Now there are some good options out there at reasonable prices (not the Free ones). The easiest way to spot them is if they have some sort of indicator (LED or presence on an LCD) for each individual battery. For example, if the charger can charge 4 batteries but only has one LED indicator for each pair… Stay away! That being said there are still some chargers that handle only two batteries that still have just one indicator. These kind of chargers require reading the packaging to find the minimum number of batteries they can charge. If the packaging says the minimum is two, that’s a giveaway that it is charging in pairs. If it says one, consider buying it.
Here are some examples:
Now although there may be some great chargers at affordable prices, Charging is all they do. You may be thinking “What else is there?” Well for one thing, when the batteries are all charged you have no idea of their actual capacity, health... anything! Just that they are supposedly all charged. If you want that information you are going to have to pay a little more money. In this category great examples are Maha or LaCrosse.
Another not so well known fact is that a brand new set of rechargeables will not have peak performance after the first charge. You actually have to “break them in” a bit before you will see the advertised capacity and performance. Many of these more expensive chargers will do that for you. Similarly, if a set of batteries have been through a bit of abuse, or are coming out of a long storage, you can cycle them a bit to kind of bring them back to life. These chargers can do that too.
Great, sign me up! I want one!... right? Well yes, they are great, but could they be better and maybe even do more, while still being reasonably priced? Finally the real purpose of my post!
Believe it or not, there are those who buy one of those nice, expensive, full-featured chargers and then spend even more money on some sort of data logger or data acquisition box to connect across the terminals of the batteries and track the voltages while they charge. The other side of this would be monitoring the batteries while they drain to see their capacity and performance into different loads.
I decided to start with draining batteries first (it’s easier).
Using a microcontroller, an N-Channel MOSFET and an LCD to display everything, I started draining all sorts of batteries. One thing led to another and two years later (and a lot of late nights) I am on my fourth version NiMh Battery Charger/Analyzer that does a lot more than just charge batteries!
It handles each battery independently.
Each battery has its own RGB LED at the base to indicate all sorts of stuff; Charging, Discharging, Done, Bad Battery, Checking Temperature, etc.
Everything is stored on an SD card so you can analyze all aspects of your batteries (or if you just want to see how the charging/discharging went).
A dedicated temperature sensor (in the middle of the image above) is thermally coupled to the metal battery clips.
Charge Plot. For the reason that charging is so critical, I measure the voltage on the battery both while charging, and during a short (few second) drain every three minutes. Older batteries have a higher impedance while charging and so this difference between voltages is much greater. This allows me to detect a failing battery during charging that may be right on the edge of its life and passed the initial "Bad Battery" check before charging began. The end of charge is detected by looking at Voltage (drop or flat above certain voltage) Temperature, Impedance and a few mixtures of all three.
The charge Current is measured and recorded. You can see it held very tight to the 400mA charge current I had it set for.
Discharged to 0.9 volts. The little lip at the beginning of the plot is where the battery was resting after charging but before discharging began.
Temperature plot. Everything before the red line is while charging, after is while resting and discharging. The first thing you may notice is the jaggedness of the plot, this is due to the use of the on-board 10bit ADC in the microcontroller. The resolution will actually almost double in the next version (by using a 1.8 precision voltage reference instead of the 3.3V power. Same number of bits but almost half the range). However, that being said, you are still looking at a resolution of 0.3 degrees C which is not bad. You can also see that the battery only got up to about 31C at the end of charging.
I will post more data after I get back the boards (and populate them) for the latest version. All of this is working towards a 4 battery version with an LCD ( the 4 batteries will be arranged around the LCD, keeping them spaced apart for cooling ) and a machined aluminum case.
Oh, and if you read this whole post and are still wondering why it is bad to charge in pairs, it's because unless the batteries are perfectly matched, either one is going to end up over charged or one is going to end up under charged. If you then use them in a device that uses the two batteries in series, you run a greater risk of driving one of the batteries in to reversal at the end of discharging them. This can permanently damage the battery.