Low cell voltages when load applied - Golf Cart 36v

[chillybags]

I built my 36v 12s LFP battery and on first use had a short which melted the BMS balancing wires and burnt some foam i was using as a spacer. There only appeared to be minor cosmetic damaged to the top of the prismatic cells.

I have learnt form my mistake and I have rebuilt the battery and replaced all the melted BMS wires and secured them appropriately to prevent another short.

Everything seems to be working ok on that front. Photos below - I have replaced the two red cables running from pack + to the fuse with 50mm welding cable on either side of the fuse.

Got the battery installed in my golf cart and on startup when there is a decent current draw (up to 200a) the JK bms shuts down on UVPR on some of the cells (got a screenshot below), with some cells dropping very low. As soon as the load to the BMS is cut voltage comes back up to 3.3v almost instantly. The cells have been charged up to 3.65v, i havent done a top balance due to JK BMS active balancing. When fully charged when the first cell hits 3.65v most cells are at 3.6-3.65 but theres two cells sitting at about 3.58 - i know not ideal at this stage but didnt think it would make such a big problem right this second.

I have Grade A EVE 105a cells (purchased from amy wan from luyuan) that have a 1C rating up to 3C pulse, the JK BMS can handle 200a up to 350a pulse. I thought these specs should be ok for my needs but it seems maybe the cart is pulling way more current than the setup can handle?

If i am very careful and start the golf cart at a very slow pace and draw under 100a then once it is going it stays going.

 

[AntronX]

Bad interconnects or weak cells. Batteries are a bit cold at 16 C that could explain excessive voltage drop.

 

[chillybags]

Bad interconnects or weak cells. Batteries are a bit cold at 16 C that could explain excessive voltage drop.

Cleaned all fittings with isopropyl alcohol and tightened up all the fittings and that seemed to do the job. But unfortunately due to large lug sizes i have stripped the pack main + and - terminals. They are only tiny little m4 bolts. Planning to drill them out and helicoil them. Unsure as to whether go m6 or m8 though. The terminal is a total of ~14mm in diameter so if i went to m8 theres not a lot of area left for bus bar contact? Or will it be fine.


thought i would upgrade to grub screws instead of the bolts as less likely to strip. Found these grub screws with nylon tips which i thought would be good to protect the bottom of the terminals and lower change of puncturing through the bottom? Anyone used these before?

 

[AntronX]

Be careful not to drill through the posts or that will ruin the cell.

 

[chillybags]

Be careful not to drill through the posts or that will ruin the cell.

Yes very aware of this as an issue. I do not have a drill press and will be looking to buy or borrow one to complete this as it will be much easier and safer.

Did you have any thoughts on the hole size or the grub screws?

 

[AntronX]

No idea, never done this. Maybe some here knows.

 

[Steve_S]

Do NOT drill out the M4 to change them !
The screws are NEVER intended for power transfer, that is what the Pad Face is for. It's the contact patch that the power flows through properly.
Without first fully Top Charging and Fully Activativating the electrolytes, you will never get "everything" you can from the cells. All NEW cells and cells that have been in static storage for over 6 months need to be Top Charged.

Top Charging meaning bringing the cells up to 3.650V and allowing the Amsp Taken to drop to 1% or 1A for 105AH cells. Top Balancing is separate and follows Top Charging. You are now paying the price for a Shortcut taken !

A Note of LFP with heavy loading
The IR (Internal Resistance) os cells becomes more evident when heavy Amperage is moving in/out.
The lower the IR of the cell the faster it will charge/discharge, regardless of the rest of the cells in the pack.
If cells are in Parallel within a pack, the cell with lowest IR will drag down the "set". A BMS will only report on the "set" and this remains invisible until serious loading occurs, when you will see the Set decrease comparatively.
This is where & WHY Matched & Batched cells are more important, which have identical IR through the working voltage range. Cells need to have identical IR so their behaviours match throughout the cycle. Hence why EV Batteries are so expensive, they are all matched & batched "very tightly" so there is absolutely the minimum differential internally.

A Needed Tweak !
I noticed you have 2 Cables on each side of the battery. OOPS !
Either replace the two wires on both NEG & POS with a single wire, OR get a lug that can handle both wires & hydraulicly crimped very tight.

If you have or have access to a Cell Tester like a YR1035+ and charge all your cells to say 3.450V and test the IR of each cell, you'll quickly see the cells that drop voltage faster have a different IR then the neighbour that stays higher. The same cells will also show differently at 3.000V.

Hope it helps, Good Luck

 

[chillybags]

Do NOT drill out the M4 to change them !
The screws are NEVER intended for power transfer, that is what the Pad Face is for. It's the contact patch that the power flows through properly.
Without first fully Top Charging and Fully Activativating the electrolytes, you will never get "everything" you can from the cells. All NEW cells and cells that have been in static storage for over 6 months need to be Top Charged.

Top Charging meaning bringing the cells up to 3.650V and allowing the Amsp Taken to drop to 1% or 1A for 105AH cells. Top Balancing is separate and follows Top Charging. You are now paying the price for a Shortcut taken !

A Note of LFP with heavy loading
The IR (Internal Resistance) os cells becomes more evident when heavy Amperage is moving in/out.
The lower the IR of the cell the faster it will charge/discharge, regardless of the rest of the cells in the pack.
If cells are in Parallel within a pack, the cell with lowest IR will drag down the "set". A BMS will only report on the "set" and this remains invisible until serious loading occurs, when you will see the Set decrease comparatively.
This is where & WHY Matched & Batched cells are more important, which have identical IR through the working voltage range. Cells need to have identical IR so their behaviours match throughout the cycle. Hence why EV Batteries are so expensive, they are all matched & batched "very tightly" so there is absolutely the minimum differential internally.

A Needed Tweak !
I noticed you have 2 Cables on each side of the battery. OOPS !
Either replace the two wires on both NEG & POS with a single wire, OR get a lug that can handle both wires & hydraulicly crimped very tight.

If you have or have access to a Cell Tester like a YR1035+ and charge all your cells to say 3.450V and test the IR of each cell, you'll quickly see the cells that drop voltage faster have a different IR then the neighbour that stays higher. The same cells will also show differently at 3.000V.

Hope it helps, Good Luck

Hi Steve,

Thanks for the feedback. I will look at top balancing them, but i didnt have a dc power supply that i could use to do this. I will look into getting one.

As for drilling the cells, they are now stripped so what other options do i have?

I have already replaced the two cables on the positive with a single 50mm cable which is much neater. I will look at doing the same on the negative.

 

[Steve_S]

If you've stripped the threads (not hard to do sadly), the only practical option is helicoils. That means redrilling the existing holes very slightly (clean them up) and then recutting the threads for the insert. You can buy a complete kit for like $20 or so. Some do not have the T-Handle to use for the cutting bit. When drilling out the hole you have to go slow and make sure you don't go deep, your only "cleaning" out the current hole. Tapping, by hand, slow & easy back & forth, is safest. This is where a proper Tap handle is good, can be bought separately.
The video's are pretty good and explain it well, watch a couple and practice on something else before doing it to a battery cell. Just a piece of aluminum that you drill out and tap, so you have a feel for it.

M6 Kit I used on original EVE LF280's : https://www.amazon.com/Heli-Coil-Helicoil-5546-6-Metric-Coarse/dp/B0002KKPXK
there is everything from M3 on up.

Good Luck

 

[EastTexCowboy]

You can use a drill bit stop collar to limit how deep you're drilling. I would set the depth a little less than needed then sneak up on it if it's not enough. Make sure you get the collar set screw tight, but otherwise this low tech solution works pretty well.

 

[chillybags]

If you've stripped the threads (not hard to do sadly), the only practical option is helicoils. That means redrilling the existing holes very slightly (clean them up) and then recutting the threads for the insert. You can buy a complete kit for like $20 or so. Some do not have the T-Handle to use for the cutting bit. When drilling out the hole you have to go slow and make sure you don't go deep, your only "cleaning" out the current hole. Tapping, by hand, slow & easy back & forth, is safest. This is where a proper Tap handle is good, can be bought separately.
The video's are pretty good and explain it well, watch a couple and practice on something else before doing it to a battery cell. Just a piece of aluminum that you drill out and tap, so you have a feel for it.

M6 Kit I used on original EVE LF280's : https://www.amazon.com/Heli-Coil-Helicoil-5546-6-Metric-Coarse/dp/B0002KKPXK
there is everything from M3 on up.

Good Luck

Steve, this was the plan. Just first line of your original comment was “do no drill out the m4”

I have found a helicoil kit and planning to do this. My question was around what size helicoil to go for. Originally m4 bolts, do I just go up to m5, can I go to m6 or even m8 or will it reduce the contact area too much? The contact pads are about ~14-15mm in diameter on the 105ah cells. Are they same size on larger cells which already come standard with m6 bolts?

 

[Q-Dog]

Steve, this was the plan. Just first line of your original comment was “do no drill out the m4”

I have found a helicoil kit and planning to do this. My question was around what size helicoil to go for. Originally m4 bolts, do I just go up to m5, can I go to m6 or even m8 or will it reduce the contact area too much? The contact pads are about ~14-15mm in diameter on the 105ah cells. Are they same size on larger cells which already come standard with m6 bolts?

Use the helicoil size that matches the bolts/studs you already have. Unless you want to change all of them to a larger size.

 

[Steve_S]

Originally m4 bolts

Stick to M4 don't increase, it's a lot easier & stays the same for all intents & purposes. Only the hole gets enlarged for the thread insert, in which the M4 screw should thread inst.