In December 2009 I installed a new Lithium battery pack in our EV. The pack has been working really well, not a single battery problem for 10 months. The previous lead acid pack was always causing problems, possibly because we pushed them a bit to get the range we needed. I didn’t mind so much, as I like tinkering with the EV. Other people I know have had better luck with lead acids, in particular when the depth of discharge was low. However a Lithium powered EV is a much more practical vehicle for the every day driver.
We have done up to 75km on a single charge. I have no idea if that was close to the limit as I don’t have an amp-hour meter. In fact I don’t even have a volt or ammeter at the moment. We just get in and drive, then plug in at night to charge.
Anyway after 10 months and around 7000km total on the Lithium pack it was time for some maintenance – I wanted to see if the pack was equalised. That is, make sure all of the batteries are in a similar state of charge.
All 36 Lithium cells are in one series string. Wired across segments of that string I have three chargers. The 96V charger handles 28 cells, and each 12V charger 4 cells each. The 12V chargers always finish first, and never trip the over-voltage cut-out. The 96V charger takes a longer, and one of the cells always trips the over-voltage cut out. This means at least one cell is a little “ahead” of the pack, hitting 4.1V and dropping out the charger.
My Battery Management System (BMS) uses CM090 BMS modules on each cell. These BMS modules have a series regulator feature. As the cell reaches full charge, the series regulator starts to bypass current around the cell. Each regulator can bypass about 800mA. Electric car chargers are designed with a certain charging “profile”. They start out with a bulk charge current (say 10-20A), then towards the end taper that current down to 1A or less. The low finish current allows the series regulator to bypass current (up to 800mA) around the fully charged cells allowing the slower cells to catch up.
However my 96V charger was designed for lead acid cells so it doesn’t have the right “profile”. It tends to charge at full current (8-10A) rather than “tapering” the current at the end of the charge. When the first cell hits 4.1V the BMS system shuts down the charger. So I am not getting the benefit of the series regulators and there is a chance that some cells may be out of balance.
A single EV charger designed for my pack would get them all to 100% every time I charge, by tapering the charge current at the end of the charge to 500mA.
The picture below shows the CM-90 BMS modules fitted to 4 Lithium Cells. The red LEDs indicate that each cell is over 3.6V. This means they are at about the same state of charge. At the end of a charge we want all red LEDs lit on all cells.
To balance my cells I first charge them normally, then connect a simple low current “manual” charger. This charger is a Variac followed by a full wave bridge rectifier and an Ammeter. It delivers an average DC current of 0-2A that can be controlled by the voltage on the Variac. However it has no automatic cutout so you need to be very careful when using it. I always connect the AC plug of the manual charger to the AC socket on my BMS controller, so it will be switched off automatically if a cell hits 4.1V.
To balance the cells I dial up a small current like 0.5 to 1A then wait for the LEDs on each cells series regulator to come on. The red LED indicates the cell is above 3.6V. The trick is to bring them up gently (500mA to 1A) so that no cells hit 4.1v – at that point the BMS controller drops out the charger.
However my cells are rated at 100A, so charging them at 1A can take a while. For example if one cell is 5AH down, it could be 10 hours at 0.5A. If I up the current to 5A the series regulators on the full cells won’t be able to bypass the current, those cells will hit 4.1V, and the BMS system will cut the power to the charger.
So to speed up the process I connect a 12V charger across blocks of 4 cells at a time. This way I quickly charge any low cells at 12A, which takes around 30 minutes.
In the picture below a 12V charger is connected to a block of 4 cells in the middle of the pack:
After a few hours I had all LEDs lit on nearly all cells. To bring the last few up I connected the manual charger at 500mA to get the last fraction of an AH. After about 30 minutes all cells had all LEDs on.
In the picture below just one red LED is unlit:
I estimate that the worst cells were no more than 10AH down (about 10%). I don’t know if they get more out of sync over time, or if they get 10% out and just stay there. The cells are likely to vary in capacity, so 10% doesn’t sound unreasonable over 9 months.