first step in DS conversion to lithium seems easy

[BBBHC]

Keep in mind I am, at best a novice when it comes to electrical...

It seems plausible to me that this BMS can utilize the MOSFETs to cutoff for over charge and over discharge. I know that todays lithium powered hand tools (drills, impact guns, etc.) utilize MOSFETs to protect the batteries from these events. I am just not seeing in the data sheets if it specifies for sure if it does or not. Let alone, at what voltage(s) it would do so. Are you able to adjust any settings, through a monitor or computer?

Hopefully a guru will be along shortly so we can all learn...

[Volt_Ampere]

I have not seen a BMS that can handle LVC and HVC without external relays. The mosfets can't handle the current they would be seeing. The HVC has to interrupt the charge current and the LVC should interrupt the main drive current by opening the contactor. I do not use an active LVC on my cart - just a very loud alarm.

[Sergio]

One of the functions of a BMS is to monitor individual cell voltages (not pack voltage) and stop the charge or discharge of the batteries if the voltage is outside the specified range, either higher or lower.

There are many ways to implement the "disconnect" function, the type of BMS you have is really designed for E-Bikes that have much less discharge current.

The FETs themselves are not the issue, thermal management is the real problem since FETs have a small resistance when conducting and can get very hot during high current discharge.

You also have the issue of the circuit board copper trace, which appears to have been "solder augmented " in your picture, having to carry the controller current without melting.

I think the best approach to use that BMS with a Golf Cart is:

• Connect the BMS B- to Pack Negative.
• Connect the Charger Negative wire to the BMS P-
• Connect the Controller B- directly to the Pack Negative
• Use an inexpensive 48v relay like DaveTM has on his picture:
• • Connect one side of the the relay coil to the BMS P-
  • Connect the other side of the relay coil to "cold side" of the key switch terminal.
  • Cut the harness wire that is connected to the "cold side" of the key switch and run it thru the relay N.O. contacts.

Cold Side of key switch is the same as the switched side or the side that only has power when You turn the key switch ON.

That will allow the BMS to disconnect the charger in case of a charging issue and disable the cart in case of a high/low cell voltage condition by essentially turning the key switch OFF.

Depending on your situation You can also add another inexpensive 120v coil relay to disable the cart from operating when the onboard charger is connected to AC power.

[DaveTM]

FWIW....these are the relays I am using. Cheap and effective. Remember that these are 48-V relays.https://www.mouser.com/ProductDetail...rrencycode=USD

[dlwilson]

Originally Posted by Sergio

I think the best approach to use that BMS with a Golf Cart is:

• Connect the BMS B- to Pack Negative.
• Connect the Charger Negative wire to the BMS P-
• Connect the Controller B- directly to the Pack Negative
• Use an inexpensive 48v relay like DaveTM has on his picture:
• • Connect one side of the the relay coil to the BMS P-
  • Connect the other side of the relay coil to "cold side" of the key switch terminal.
  • Cut the harness wire that is connected to the "cold side" of the key switch and run it thru the relay N.O. contacts.

Thanks Sergio! I had to study your post for a while, but I think I've got it. I drew it out on the attached diagram. As you say, the cold side from the keyswitch runs through the 48v relay, and carries 48v as long as the key is on and the relay is energized by the BMS. When the BMS detects low voltage, it shuts off, which opens the relay and cuts voltage from the cold side, which shuts off the cart.

The BMS still manages the charging in its usual way, which I guess it's able to do because the amperage is so much lower than the discharge amperage?

If I use a 12vac relay as you suggest to protect the battery during charging, then I guess I could use the lowest capacity (least expensive) BMS available for this?

[Sergio]

You got it, as You have drawn the BMS is already providing charging protection since the charger current has to go trough the BMS P- before reaching the pack negative.

When the BMS senses a high cell voltage during charge it can stop the current flow from P- to pack negative so the charger can no longer charge the batteries.

The 120vAC relay is in case You want to prevent the cart from driving while you have an extension cord plugged in the cart.

I think a good Solution would be to use one of theses SPDT relays:

https://www.amazon.com/Zettler-Minia...SC8/ref=sr_1_4

Wire the coil terminals in parallel with your charger AC input so it gets powered when the cart is plugged to AC power, wire the contacts as follows:

• Connect the BMS P- to the Relay COM terminal.
• Connect the Charger Negative Lead to the Relay N.O. terminal.
• Connect the wire that goes to the new 48v relay coil negative to the Relay N.C. terminal.

That way when the AC cord is not plugged to the cart, the 48v relay is connected to the BMS P- and the charger is completely isolated from the battery pack.

When You plug the AC cord, the 120vAC Relay gets energized and the BMS P- gets connected to the charger output and the 48v relay cannot be energized to drive the cart.

If they make a version of that BMS with only 50 amps capacity it should work just for charging.

[dlwilson]

Now I understand about the relay to stop the cart from moving while it is charging. Thanks again.

The BMS is not programmable, I'll check the seller's eBay listing tonight to see if it specifies the high and low voltage cutoffs.

[Volt_Ampere]

I looked into using Solid State Relays (FET's) and I found that to handle the charge currents I am using required a fairly expensive one with a heatsink. I went with a conventional automotive relay that was very inexpensive and does the job to interrupt the charging current if my BMS senses any cell voltages over the limit.

[dlwilson]

I wired up the charging portion of the BMS tonight with a 50.4v 4A charger. All seems to be working well; it has gone from 47.3v to 48.4v when I shut it off for the night.

Below is the technical specs of the BMS from the eBay listing:

12S 150A Li-ion / LiPO Battery protection board.

Applicable for
43V (3.6V * 12S) lithium battery & packs
44V (3.7V * 12S) lithium battery & packs
50.4V (4.2V * 12S) lithium battery & packs
Lithium battery (Li-ion)
Prismatic Lithium Polymer battery (Li-Po)

Technical Parameters:
Balanced current: 60mA (VCELL = 3.90V when)
Balanced for: 4.20 ± 0.05 V
Over-charged Protection: 4.2 ± 0.05 V
Over-charged Release: 4.05 ± 0.05 V
Over-discharged Protection: 2.9 ± 0.05 V
Over-charged delay: 5mS
Over-current Protecton: 150 A
Supports Max. Continuing Discharge Current: 150A
Max Charge Current : 40A
Static power consumption: less than 200uA
Short-circuit protection function: disconnect the load from the recovery.

Dimension: 200mm * 110mm * 20mm (L*W*T) - with Radiating Board

The main functions: Over-charged, Over-discharged protection, short circuit protection, over-current protection, with Balancing function.

[BBBHC]

Looks like you are on your way! Glad Sergio chimed in to educate us all!

Originally Posted by Sergio

There are many ways to implement the "disconnect" function, the type of BMS you have is really designed for E-Bikes that have much less discharge current.

Originally Posted by dlwilson

Below is the technical specs of the BMS from the eBay listing:

Technical Parameters:

Over-current Protecton: 150 A
Supports Max. Continuing Discharge Current: 150A

I am wondering how this will work... At cruising speeds the continuing discharge current should be fine. However, on take-off, hills and a cart that has more weight on it, will go north of 150 amps. I'm wondering what this BMS will support for "brief" current loads. I know folks on here have seen north of 400 amps in scenarios. (Granted, that is with aftermarket motors and controllers)

Is your motor and controller stock? If you have an aftermarket controller you could set it to limit the available amps for these scenarios.