DIY Lifepo4 Daly BMS cell voltage drop issue under load

[Volt_Ampere]

With my Yamaha carts, I can limit my current to 300 amps without a major difference in performance. We don't have any real hills here though. Going 25 on level ground draws about 75 amps at speed with my cart. Full acceleration on level ground will draw over 300 amps at startup. My Leaf cells handle that without any significant voltage drop. I don't know what the "C" rating of my cells is but it's way more than 3C.

[reiter284]

Originally Posted by DaveTM

It may very well be the cell(s) drop if the draw is big enough. I did the "hook up the laptop" to get the readout when I first installed my lithium pack.

When I would drive up the steepest hill I encounter (about 30-35 degree) my cart would draw 392amps. It did not drop any cell below the LV cutoff and I changed my controller so the max draw would be 300amps. I re-tested the cart going up the hill again and encountered no perceptible decrease in performance.

You may want to look at your controller and change the settings (dunno if you can do this with your stock controller or not). Or you may want to consider upgrading your stock controller to an Alltrax where such adjustments can be done.

FWIW

I have the stock controller (Danaher RxV) thats turned up to go 25mph with 23" tires. AFAIK you cant adjust it. I may have to go aftermarket which would also provide the ability to limit speed for the kids when they get a bit older and will drive.

[reiter284]

Originally Posted by WalterM6

What is the Max Amp rating of your BMS?

I'm using a 60amp BMS, but the BMS just cuts off a relay that cuts off the accelerator wire. The only thing running through the BMS is my 10amp charger and on the discharge side the 48v-12v reducer that powers a radio and lights. So may charge through BMS is 10 amps and max discharge is 3 amps.

[reiter284]

Originally Posted by Volt_Ampere

With my Yamaha carts, I can limit my current to 300 amps without a major difference in performance. We don't have any real hills here though. Going 25 on level ground draws about 75 amps at speed with my cart. Full acceleration on level ground will draw over 300 amps at startup. My Leaf cells handle that without any significant voltage drop. I don't know what the "C" rating of my cells is but it's way more than 3C.

That sounds about right, I thought the batteries may not deliver the full power, but I didn't think of the instantaneous voltage drop and the BMS cutting out.

I guess I had it in my head that the aftermarket solutions all head Lifepo4 and that it would all be ok and that a 100AH battery was reasonably large enough to handle it.

I'm going to rework all the BUS bars so they are uniform with the same exact washers, etc.. and all torqued the same and see if that helps but I'm not hopeful, will report back. Doubtful it helps.

There also seems to one cell thats a little (.1V) worse than the others under load, maybe after a few charge/discharge cycles, I can tune that cell in better or replace it.

If not, I may just go navitas and limit the amps, then I can upgrade cells down the road.

[Volt_Ampere]

Most of the available solutions are LiFePO4 now. Grade A new 200AH cells should be able to handle max currents in a cart without sagging much. One of my golfing buddies has a 105 AH box from RoyPow and he is loving it. The factory Lithium carts often come with only 60AH packs and they work fine also but my guess is that they are limiting the current somewhat. They are using very high quality cells in those.

[reiter284]

Here are two screenshots from BMS, one under load and one at rest. Cell 12 is always lowest under load. By .05-.10V

Batt1.jpg

Batt2.jpg

[Volt_Ampere]

That's a pretty big voltage drop. I would replace the lower ones with better cells. That might solve your immediate problem. You really need to know what current you were pulling under load to know if you can get away with reducing it. First thing I noticed when I switched to Lithium was how much less sag under load there was compared to my FLA batteries that I replaced.

[reiter284]

The more I have thought about it, the more I think it may be busbars. The cells where I doubled up busbars don't have significant voltage drops. Neither does cell 1.

I'm way out of my expertise but some quick googling of the existing busbars indicate they can handle 100amps. They are 14mmX1.2mm.

I ordered some 1" x 0.25" Aluminum flat bar and will be making my own. These will be 10X the cross sectional area and have full coverage over the battery terminals.

I will report back.....

[Volt_Ampere]

If you can, make them out of copper. It's a better conductor than aluminum by a bunch. If you move your BMS wires from one side of a bus bar to the other and remeasure, you can test your theory. My Leaf cell battery has thick copper bus bars. Not sure how thick but my battery meter shows very little sag under load.

[reiter284]

Originally Posted by Volt_Ampere

If you can, make them out of copper. It's a better conductor than aluminum by a bunch. If you move your BMS wires from one side of a bus bar to the other and remeasure, you can test your theory. My Leaf cell battery has thick copper bus bars. Not sure how thick but my battery meter shows very little sag under load.

Sorry, what do you mean move the BMS wire from one side to the other?

I can re-arrange the doubled up bus bars to other parts of the pack on different cells. Is that similar to what you mean to test it?