There are so many horror stories in the news about Lithium batteries that you may be wondering about their safety. The bottom line is cheap batteries and, in particular, cheap battery control systems can lead to less than stella performance and in extreme circumstances even fire.
We spent 3 years researching the right Lithium cells to make up our Ark battery and ended up selecting EVE Lithium Iron Phosphate cells which is the safest form of Lithium chemistry on the market today. We then looked at control systems and, after testing "off-the-shelf" systems, we were unimpressed with the lack of safety and battery health features in these. We decided the way forward was just to design our own from the ground up based on our real life experience with Lithium.
The result was the Arks built-in caretaker, Bob. This character is borderline paranoid about keeping the cells happy and even has back up sensors in the unlikely event the primary ones fail. The battery is the heart of the Ark and we want to make sure it stays healthy for a very long time.
Lithium batteries are great for supplying lots of energy in a compact package but they're picky about temperature. Too hot and you can't use them at all, too cold and you can't charge them.
Anything over 55°C and they can't be charged of discharged. The Ark has several measures to prevent batteries overheating, including a heat venting design, powerful (but quiet) fans, and a range of sensors which are all managed by Bob. This combined with the cells large thermal mass means they keep cool during short periods of increased ambient air temperature.
Anything below 0°C and charging has to be stopped to prevent damage to the battery capacity. This is clearly a waste of solar energy, especially in winter when sunlight is precious. To combat this the Ark has a heat mat under the battery that uses solar energy, not precious battery reserves, to warm the batteries.
Our fussy Lithium friend also doesn't like being constantly fully charged which can also end up impacting battery capacity.
Bob makes sure that when the battery is full, it's not kept that way. Instead it allows a small window of discharging to occur before charging starts again. You'll probably notice this on the screen, the battery is full but not charging even if you're using it and it's sunny. By managing the ‘fill level’ of the battery in this way Bob helps to maintain battery health and maximise battery lifespan.
Lithium also doesn't like being run down to completely empty.
When batteries are run down too low, the chemistry can be affected and life time capacity can be reduced. Bob keeps an eye on battery levels and gives you plenty of warning when the batteries are getting a little flat. The Ark never drains the batteries completely, making sure there's some left in reserve for protection purposes.
Finally the battery is made up of 8 individual cells. These have to be closely monitored to make sure they all stay synchronised with one another. We want to make sure the cells all reach full charge at the same time to avoid potential slower cells never quite getting there.
Bob manages cell levels ensuring that if one cell is a little bit more charged than another then the excess energy is routed to the lower cell. By doing this Bob compensates for the natural variations in individual cells which occur during manufacture and which can develop with age. This close supervision extends the cell life even further.