Batteries: the most important part of an electric vehicle and the most expensive part, too! EV drivers always obsess about remaining capacity (…especially the ones that use their EV for more than just their daily commute) and which external factors can contribute to capacity loss…and what can be done to preserve capacity as much as possible.
When speaking to a non-ev-driver, there is much confusion about what exactly capacity is. Car manufacturers make things even more confusing with much electronic nannying and many abstraction layers.
Chemical energy storage is a vast field and I could start a full blog covering just this one topic. I’ve spent entire evenings discussing the merits of various lithium battery flavours and their ageing and cycle stability characteristics.
Although emissionless.ch is about electric motoring and batteries are a very important part of it, there are much better places to head to if you want to learn about batteries: one of the best places to go to is batteryuniversity.com.
Welcome back! Now that you know everything there is to know about TW560’s LiFePo4 battery pack, we can dive into the details 🙂
Bought back in 2010, TW560’s battery pack from dreifels has since been driven an average 23000+ km a year, clocking 15800+Ah discharged energy and an aggregate 2000+ cycles. (This equates to a whopping 6+ MWh delivered by the pack!)
TW560’s batteries lost most of their capacity due to regular usage, many full cycles and fast charging – not calendrical ageing.
The normalized line above shows a near-linear decrease of capacity over time. Capacity rises slightly in summer. As with the NiCd-pack TW560 came with, the capacity it emerges with after winter is the capacity I’ll be having during summer.
But why am I dedicating a full entry to this?
LiFePo4 batteries are more susceptible to drift than other battery chemistries.
High drift contributes to cell death. Why? With every charge, due to minute differences per cell, charges slowly drift apart and certain cells are either over-charged or – even worse – discharge beyond safe levels. Fortunately, if this happens, LiFePo4 batteries usually just go to 0 Ohm and short out. With a 120-cell stack like TW560’s the loss of 1-2 cells isn’t noticeable and can be easily repaired at a later point in time.
My TWIKE has had to replace a few cells during the last 5 years. The cells are much larger than the usual 18650’s used with many modern EV’s.
This format has distinct advantages: changing cells is very easy.
With 5 replaced cells there are newer cells with more capacity than all others. With such a set-up it is very important to balance the cells to be able to deliver maximum capacity. The graph below explains how and why:
So balancing of lithium batteries is a good idea – but how does it work?
There are two ways to keep a battery pack balanced: active/passive circuits as the one in the picture above or external balancing. With a 120 cell stack, in-pack balancing is quite expensive and heavy – TWIKE’s with LiFePo4 packs are non-balanced. Very soon I noticed that dreifels’ assumption that the pack wouldn’t drift very much didn’t apply to me and my usage pattern. My cells were drifting all over the place and cells were dying due to this.
I started requesting balancing cycles from Thomas at Möckli Elektrofahrzeuge at 200 CHF for each balancing cycle … he usually told me after such a cycle that my cells were looking very bad, but was unable to articulate on details.
This is why i decided to buy myself an external balancer and start balancing my batteries on my own every 2-3 months. (and learn how to interpret the results!)
Revolectrix offers a multi-chemistry battery workstation at a very attractive price. It can balance up to 10 cells at a time and has two operation modes – stand-alone and computer controlled. I decided to get a PowerLab8, able to cope with all battery chemistries I have at home. With the optional balancer and serial interface cable, I’m ready to get any battery back to its maximum capacity!
Before I start balancing TW560’s batteries I usually fully charge them using the on-board charger ath a very low maximum current.
When fully charged, I open the battery bay and check if every block has similar voltages – if not, then the blocks with a lower cell voltage need a cell replacement.
My TWIKE has 5 blocks with 24*2 cells each. Every block consists up of 3 strings of 2*8 cells @ 29.2V max voltage. Every string has a balancing port for 8 cells – a breeze with my PowerLab8.
After some light configuration, balancing 8 cells @ 2.5A max. is just a button press away. The software downloads the config to the battery workstation. Any real work is performed by the workstation – the PC is just a logging device. Up to 10 PL8 can be daisy-chained to speed-up the process.
Let’s start balancing!
Balancing can be just a ‘push-a-button’ thing with all intelligence left to the PowerLab8. Only when looking at the graphs, cell voltages and total Ah charged a trained eye can deduct how each string is performing – let me give you some examples:
All cells are stable and are uniformly charged. Discharging will look about the same – everything is normal.
Unfortunately, TW560’s blocks look mostly like the following two examples:
With one cell lagging, we’d better give it lots of time to slowly charge back to the level of the other 7 cells. Depending on the string, this might be up to 2Ah of 15Ah!
Balancing 15 strings can take up to 20 hours. Since I cannot stay with my computer all the time, I access the graphs and interface via my mobile phone, calling my wife when a string has completed a charge…and I ask her to re-plug the balancer:
At the end of each balancing session, there is a noticeable boost in total capacity – although I’m currently saving for a 50Ah battery pack, my interest is to keep my current pack at its current maximum performance and keep each cell as healthy as possible.
Have a LiFePo4 battery from dreifels and want to know more about how to keep your pack as healthy as possible? Get in touch!