BMS wiring mistake?

[Volt_Ampere]

No way you can predict accurate SOC by hooking up a new BMS to a battery pack. It might give a value but it's bogus until you cycle the battery - maybe more than once.

[MDLNB]

Allowed the golf cart to sit idle for about a month or so and when I went to use it, the battery was depleted. Granted, I did NOT turn off the BMS. Would the BMS constantly working to balance the cells, cause it to deplete the battery? I did have the golf cart in TOW mode while it sat. So, I put a new charge on it and then the day after, I turned OFF the BMS. I have no idea if that will make a difference. Note: when the battery pack was sitting on the bench for over a year, it hardly lost any charge at all.
Thanks again for all the information that you guys have given out on here. Hope others have appreciated it as much as I have.

[augiedoggy]

Originally Posted by MDLNB

Allowed the golf cart to sit idle for about a month or so and when I went to use it, the battery was depleted. Granted, I did NOT turn off the BMS. Would the BMS constantly working to balance the cells, cause it to deplete the battery? I did have the golf cart in TOW mode while it sat. So, I put a new charge on it and then the day after, I turned OFF the BMS. I have no idea if that will make a difference. Note: when the battery pack was sitting on the bench for over a year, it hardly lost any charge at all.
Thanks again for all the information that you guys have given out on here. Hope others have appreciated it as much as I have.

Do you have a dc to dc voltage reducer and is it wired to be on when the cart is off by chance? Many of them draw enough current even with no load on them, to deplete the battery in as little as 2.5 weeks
Also not sure what bms your using or the settings but many will shut off balancing once the cells reach a certain voltage like 3.4 or 3 volts.. If the settings are too aggressive or you have one cell that's not holding its charge the balancing can draw the whole pack down.

[Pat911]

Originally Posted by MDLNB

Note: when the battery pack was sitting on the bench for over a year, it hardly lost any charge at all.

With or without the BMS connected? A BMS shouldn’t drain a battery pack in a month. As Augiedoggy said, ensure balancing stops and check parasitic current drain. You have Leaf cells I believe, so set balancing to stop when under 4.0v and start at 4.02v.

I have the JK BMS on my 72v 280Ah battery and it’s been sitting without a charger connected for well over a month and SOC has hardly dropped.

[MDLNB]

Apparently, the BMS was leaching power from the battery by continuous attempts to balance the cells. I gave it a full charge and turned off the BMS was well as switched the cart to "TOW." It held the charge. In one month I used the cart twice for short runs of less than a mile. The analogue meter only slightly moved while in operation, staying in the top of the white "charged" area, even when I had it floored most of the trip. When I parked it the second time, the total charge had hardly moved. Unfortunately, I forgot to take a comparison reading from the BMS to see how much charge was actually used. My bad. But, apparently by turning off the BMS while the cart is sitting in the garage, the battery pack is maintaining a mostly full charge.

[Pat911]

Balancing should only occur above 4v per cell for NMC. If you can, change the parameter in the settings.

[Volt_Ampere]

If you are not going to use the cart for more than a few days, it's a good idea to disable the BMS so that it doesn't put any drain on the cells. My Leaf Cell system disables the BMS when the key is off.

[MDLNB]

Yes, thank you for the advice. I did turn off the BMS and it DID hold the charge. My BMS says it has 40ah with a 100% charge. When I used it a couple times for short distance rides to pick up the mail a few blocks away, and let it sit without charging (but turned off) the BMS said (when I turned it back on) that I had 31ah and 87%??? (trying to remember) remaining charge. I still haven't taken it on the ten mile round trip test run yet. Want to make sure that if I get out there and can't get back, I have someone to come fetch me. According to the BMS, the cells seem to be balancing pretty well, and I don't have any unusually low cell voltage on a particular cell. They all seem to be pretty closely balanced down to the hundredth of a volt. Temperature has been good too. Cheap Chinese charger is doing a bang up job. The charging amps start at 15 and as it brings up the voltage level, it lowers the charging amperage, which seems to be norm that we would want. It was a mistake trying to explain the system and how it works to the spouse. She is nervous, worrying about getting stranded or having it blow up on her. The more I get her to ride in it with me, the easier it will be for her to relax.