End-of-charge Foibles…

Though I’m glad just to have an excellent charge control system for a great little scooter, I’ve been struggling to get the BMS end of charge (EOC) system to reliably switch off when the bank has finished charging. The ideal charge voltage for my system is theoretically 86.4V, which gives the exact 3.6V threshold per cell. In actual reality though, I’ve had to do quite a bit of fine-tuning of both the charger voltage and the EOC adjuster to get it to the point where it would reliably switch off once the cells were near enough ‘full’.

The charger voltage setting needs some fine-tuning

How it should work

In principle, it works like this. The charging voltage is delivered to the master terminals of the full pack, whereupon all the cells will start to charge. However some cells charge faster than others, and so a system is needed to make sure each individual cell stops charging once it reaches its 3.6V limit (in the case of LiFePO4) . This limit is the level to which it’s deemed safe to charge the cells without adversely effecting their lifespan, and is called the high-voltage cutoff (HVC) .

The HVC system comes into play towards the end of the charge cycle, usually in the last 10-20 minutes depending on how badly depleted or out of balance the cells are. When a cell reachs its HVC, a series of shunts divert excess voltage to some large resistors on the back of the board. which simply burns off the excess energy as heat. A series of orange LEDs – one for each cell – gradually come on one by one, indicating which cells are fully charged.

The shunt resistors, here mounted on the back of the board

As all the cells reach this limit, the pack draws draw less and less current from the charger, until the end-of-charge (EOC) cut-off point is reached. This cut-off point is based the setting on a 1K variable resistor (‘pot’) which is turned one way or the other to determine the lower current threshold at which it will cut off. Once the current drops below this point – around 1.0 amp in my case – the BMS shuts off and the orange main indicator light turns to green.

However sometimes I was finding that – whatever the EOC setting – it would not shut off at all. The shunts would just “run away”, continuing to draw a low current, but gradually heating up the whole unit until the case was uncomfortably hot. I found that this was particularly prone to happen when the BMS was in ‘pulse mode’.

“Pulse Mode”

In addition to this steady constant current (CC) charge mode, there is also another system that acts as backup mode to deal with any cells that are badly out of balance. This can happen if the bank has been deeply discharged and is left almost drained, or if there are one or more ‘weak cells’. However, this mode can also be triggered – in my experience – by having the charger voltage set too high.

In this mode, imbalances between cell voltage circuits shunts cause them to become ‘flooded’, and they cannot maintain their threshold HVC. This allows cells to potentially become over charged as the shunts collapse. One impressive feature of the Goodrum-Fechter “Zephyr” is its ability to cope with even very badly imbalanced cell combinations. If the shunts become flooded, then the unit goes into a cycle mode, where it switches itself on and off in pulses. These pulses allow the shunts to operate briefly and give lower voltage cells the chance to catch up, but then switch off just the point where the shunts can no longer maintain their HVC point. This cycling is visible as the orange LEDs brightening then dimming as the current is drawn in bursts. A video of the BMS in pulse mode can be seen here.

The cell circuit LEDs in ‘pulse mode’

Voltage threshold

Eventually I discovered that even though my charger voltage was set at the right level theoretically, it was causing problems for the EOC. In the calibration test the LEDs are best set so they are only just visibly lit. When my charger was running ‘live’, though, the LEDs seemed brighter and the case was getting hot when the shunts were on for any prolonged period of time.

So I decided to drop the voltage a little on the charger, dialing it down to 85.8V. This dealt with the problem of the case overheating, though with the voltage lower you couldn’t always see all the LEDs lit at the end of a charge. Nonetheless, it was clear from my range tests and monitoring off the cells that the bank was ending up fully charged and – once settled – nicely balanced.

However I wasn’t the only one having EOC problems: Richard Fechter had heard other complaints about pulse mode failing to trigger the EOC, and had done some more testing. He soon came up with a diagnosis as to what was the trouble was, and detailed a a fix for the EOC/pulse mode problem. Apparently smaller cells at higher charge rates were getting the pulse mode out of whack, and an extra diode needed to be added to make sure it could properly check the EOC between pulses.

Q3 times two

Since I had to take it apart to solder in the extra diode, this was a good opportunity to fit the second Q3 FET in the empty space under the other one. This extra FET is not included in the BOM (parts list), but is an optional extra for people who want to buy bigger chargers to take full advantage of the LiFePO4 cells’ phenomenal potential for fast charge rates. With an extra Q3 on board, the Zephyr should be able to take up to 20A, meaning that I could upgrade my 10A charger to a 15A or 20A one later on.

The charge control circuit (left), leaves space for an extra Q3 MOSFET

The position of the diode is marked in white on the picture above. Richard Fechter’s diagram deals shows it marked on a slightly different version of the board, but the locations is essentially the same.  The diode needs to go from the base (middle pin) of Q2 to the gate (top pin) of the main FET Q3. The cathode (the end with the black band) goes to Q2.

I elected to mount the diode on the underside of the board. Initially I goofed up and connected it the source of Q3 (the leg at the top in the photo below) and not the gate, but Richard Fechter pointed this out and I fixed it by moving it across to the rail serving the gate of Q3 (shown bottom in picture). Oops!  The other end of the diode just went to the bottom of the middle pin of Q2.

The LED between the gate of Q2 (top) and the rail for the gate of Q3

It’ll be some time before I know for sure that this works, as I’ll have to have run a few more charge cycles before I can see get an idea of whether it’s all better now. Between this fix, the slight voltage dial-down and slowly adjusting the EOC so it’ll trigger earlier, I hope to finally have this licked and have an arrangement that will charge fully, switch itself off when done, and won’t get too hot when the shunts are all on.

Below you can see the new Q3 FET, which should enable the board to handle charges of up to 20 amps.

The second Q3, opening the way for higher charge rates

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