Is this SoH OK?

[wattmatters]

The MG owners manual isn't written for people to easily understand this process

It's not written for technical people either. It's full of translated information that is sometimes inconsistent.

It says that in our MG manuals (for the ZS page 9)

The MG4 manual is pretty ambiguous about the need for a (near) full discharge prior to an equalisation charge.

It just says a "full slow charge" is required, which can simply mean charging the car to full.

Further in it gives expected charge duration for slow charging which includes an equalisation charge at the end, but again it does not explicitly state that prior deep discharge is required for equalisation to occur.

 

[Data]

Yes, the ZS manual is similar although does mention iirc taking the soc down to just below the low battery warning to do a full & proper equalisation. On my NMC 72.6KWH battery that usually happens at 20% soc. Anyway for clarification I went to my dealer & MG technical who told me to get the low battery warning then charge slowly non stop & let the BMS control the charge all the way to 100%. And don't interrupt it or it won't fully equalise.

 

[Everest]

My point being that very few people actually get issues with batteries. It is clear "it works ok" might be a better way of putting it. We know active balancing is better, but not really needed.

Agree - passive balancing works OK. And is probably safer in a (relatively) hostile environment like an EV. Active balancer can, and do, fail. When that failure happens due to poor connections and under-reading of a cell's voltage they can pump too much energy into a cell and cause over-voltage / bloating issues. The mode of failure is not possible with a passive balancer.

 

[MickeySw]

View attachment 44699

First of all thanks for sharing 'real life data', am I correct in assuming you used a granny EVSE with the aim to balance the cells? Did you by any chance check the result min/max cell voltages aferwards?

I would be grateful if some of the 'gurus' can help me understand what I'm seeing:

- for starters let's assume that there are no losses and all the energy goes into the HV battery (400DC @100%SoC, 200Ah cell capacity).

• as long as SoC is below 100% the current into the battery is about 3.75A
• however after being 'fully charged' 3.75A is still 'flowing' for another 35min (2.2Ah)
• the current then sharply drops to about 0.6A afterwards for approx 25min (0.25Ah)
• the remaining 2.5h (balancing?) are averaging around 125mA (0.3Ah)

What triggers the initial current reduction:
a) the highest reported voltage cell is reaching cut-off voltage
b) the highest reported voltage cell is getting close to reaching cut-off voltage
c)?

At what stage are the parallel bleeding resistors (post #127) activated?
a) after SoC 100%
b) after 1st current drop
c) during the 2.5h (balancing) section
d)?

Assuming that to increase the cell voltage by 1mV, approx 150mAh of energy is required, there doesn't seem to be any scope to significantly 'lift' low reading cells during the balancing phase?

 

[Data]

Agree - passive balancing works OK. And is probably safer in a (relatively) hostile environment like an EV. Active balancer can, and do, fail. When that failure happens due to poor connections and under-reading of a cell's voltage they can pump too much energy into a cell and cause over-voltage / bloating issues. The mode of failure is not possible with a passive balancer.

The safety issue is actually a valid one Everest. I understand from my guru friend that it can be an issue especially as the batteries age, often triggered by corrosion. Passive has few drawbacks apparently. Interesting, & probably another reason on top of cost, that it's not very popular.

 

[Everest]

The safety issue is actually a valid one Everest

Yes, I didn't make it up - has been an issue for at least a couple of home energy storage battery systems on a solar forum I'm active on.

 

[Data]

First of all thanks for sharing 'real life data', am I correct in assuming you used a granny EVSE with the aim to balance the cells? Did you by any chance check the result min/max cell voltages aferwards?

I would be grateful if some of the 'gurus' can help me understand what I'm seeing:

- for starters let's assume that there are no losses and all the energy goes into the HV battery (400DC @100%SoC, 200Ah cell capacity).

• as long as SoC is below 100% the current into the battery is about 3.75A
• however after being 'fully charged' 3.75A is still 'flowing' for another 35min (2.2Ah)
• the current then sharply drops to about 0.6A afterwards for approx 25min (0.25Ah)
• the remaining 2.5h (balancing?) are averaging around 125mA (0.3Ah)

What triggers the initial current reduction:
a) the highest reported voltage cell is reaching cut-off voltage
b) the highest reported voltage cell is getting close to reaching cut-off voltage
c)?

At what stage are the parallel bleeding resistors (post #127) activated?
a) after SoC 100%
b) after 1st current drop
c) during the 2.5h (balancing) section
d)?

Assuming that to increase the cell voltage by 1mV, approx 150mAh of energy is required, there doesn't seem to be any scope to significantly 'lift' low reading cells during the balancing phase?

Are you concerned about your own battery Mickey?

As I understand it there can be variations in the answers to your questions. It can depend how the BMS has been setup (how it's software has been written). Also, how the battery has been built. Don't ask me about the battery build variations. That's the answer my German friends gave me. They research & develop EV batteries & have knowledge far above my pay grade.

Q. What triggers the initial current reduction?
A. Usually it's the highest cell set reaching a predetermined voltage. This may or may not be the cut off voltage. Often it is a voltage just below the cut off voltage. This is done so as not to stress the cells.

Q. At what stage are the parallel bleeding resistors activated?
A. The resistors can be activated at virtually any state of charge. This could be 60, 70 80% during the final phase of the charging as long as smart charging is disabled. In other words an equalisation charge (a partial equalisation) can occur at various soc. They can even operate when the car is NOT charging to prevent a battery becoming very unbalanced. I'm not totally sure if that is the case for our MG's but I believe it is the case. Most car EV batteries can do this apparently. Given that MG batteries are high quality I'd be surprised if that wasn't the case. There is no predetermined length of time for the equalisation period. If the car battery is unbalanced it may take an hour, or 30mins. Equally it might take 2hrs or more. My car takes anywhere from 12 mins to 30mins. It's usually not very out of balance.

Dunno if any of this helps but these questions are not really very relevant to knowing how to look after your battery. There are often no precise answers. It doesn't actually help to know this sort of stuff unless you are a developer or battery tech. My answers have been checked with a battery guru.