Charging of 3x 12v Lithium Batteries using 3 x 12 chargers

[RichSA]

Hi All,

So this has got me stumped :( I have tried everything else that could possibly cause this issue but think it may be my setup with the charging. So here’s a summary:

I have a 36v Ezgo golf card which previously ran on 6 x 6v Trojan Lead Acid batteries. As a company we have recently got into the solar field so wanted to "experiment" with converting the cart to Lithium batteries as a side project. After quite a bit research I settled on 3 x 12v 108Ah Blue Nova Mobile batteries in series.

To cut a long story short we did the conversion and it works brilliantly. Then came the issue of charging. One option would have been to use the 36v charger I already had for the cart. However, my concern there was that the "charging algorithm" on the charger was for lead acid. I did some research on Lithium chargers and found one expensive 36v option. I then got chatting to my Local Victron dealer and he suggested I use 3 x Blue Smart 12v (25A) chargers and install them in the cart permanently wired up to the batteries. What I liked about this idea was that I would be certain that each battery would be charged equally and would never get a battery imbalance (and it worked out cheaper) 😊 So I went this route….

Fast forward 2 weeks – cart is going great, no issue and then suddenly battery 2 & 3 turn off (being battery 1 connected to main positive and battery 3 connected to main negative) ☹ i.e. the internal battery BMS shuts off… I have gone through COUNTLESS different theories on what could be the cause (from parasitic drain, to wiring etc) but at the moment I’m wondering if the following could be happening (but need someone VERY knowledgeable people to prove or disprove).

So could the following be happening?:

1. Battery 2 and 3 discharge just ever so slightly more than battery 1 during use (as battery one is connected to main positive and batter 3 to main negative).
2. Then during the charging “somehow” the three chargers pick up on when the first battery is full (battery 1 in this case) and then all three chargers move to the next charging phase (say from bulk to absorption).
3. This leads to battery 2 & 3 losing out on just a little of a full charge each charge cycle.
4. In my case the cart is used daily and uses about 30 % in a day. After about 2 weeks maybe that “ever so slight” loss on every charge cycle drops the battery to a low level and the BMS turn off the battery to protect itself?
5. It is a “theory in progress” as the batteries where turning off every few days until I separated the 3 batteries, charged them, put them back in series and then for the past 2 weeks it has been perfect (until yesterday when they shut off again). So last night I separated the batteries again, charged them up and ill see how long they last.
6. So in summary, I suppose what I’m asking:
a. Can you use three of the Blue Smart 12v (25A) charges on 3 x 12v batteries connected in series (i.e. one charger connected to each battery)?
b. In such a setup would they in any way affect the operation of each other?

As a last note, I did install a BMV-712 Smart Meter BUT because of the way the chargers are connected directly to the battery the “input” from the chargers cannot go through the shunt. So its set to reset to a full state of charge at a certain voltage (approx. 42v). This has helps me see what the parastitic drain is (approx. 0.01A) which I think is acceptable. This was verified with a clamp meter.

Any other suggestions would be appreciated 😊

Thanks,

Richard

[Sergio]

A few basics first, when batteries are connected in Series they discharge the exactly same amount regardless where they are on a string.

There is no issue on having three separate chargers connected one to each battery, they do not interfere or have any interaction with each other.

Questions about these chargers:

1) Are they designed for lithium?
2) Are they configured for on-board use meaning they reset when AC is disconnected?

A lot of the batteries with a built-in BMS have low thresholds for current discharge and over voltage charging.

If the batteries are going into safety disconnect during charge it means the charger is not setup correctly for the battery chemistry or voltage.

Do those batteries provide any external disconnect wire for you to wire a relay to stop the charger when they get full?

[RichSA]

thanks so much for taking the time to reply Sergio.

To be honest I agree with you 🙈 hence the reason I did it that way. But I have tried nearly everything and thus seems to be the only thing that is left.

To answer your questions:

The chargers are Victron Blue Smart 12v (25a) chargers and they do have a Lithium mode and stay on the Lithium setting even when disconnected from the AC. So, everytime the chargers come on, they come on in a Lithium charging mode with a bulk, absorption and storage mode.

I am worried that somehow the chargers are interacting 😬😬🧐🧐

[RichSA]

And sorry - the battery BMSs almost always turn off when the chargers are not connected to AC. And it always battery 2 and 3 in the bank.

[Sergio]

Do You get an error code from the batteries about why they went offline (over current or under voltage)?

Is that a Series or Sepex with Regen cart?

[RichSA]

No error (it’s an internal BMS). All I can do is reset the battery by disconnecting and reconnecting the batteries. Per the manual the BMS will turn off on low or high voltage. Or current. They are Blue Nova mobile batterie (1.4kw)(you can google them as I can’t paste links yet per form I’m rules 😂).

The golf cart is a 36v ezgo.

[Sergio]

The spec for that battery is 110 amps max continuous current with a short burst capacity of 160 amps.

While 110 amps will work for cruising current, the cart will use more than 160 amps during acceleration.

I understand the cart is an ezgo, but if it is a Sepex system You also need to worry about the Regenerative brake current which cannot exceed 50 amps for those batteries.

If You had an Alltrax Controller You could limit the maximum battery current to prevent the internal BMS from triggering.

The internal BMS on those self contained batteries going into a safety disconnect is typically the issue when paired with a Controller that cannot limit the battery current.