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Old 04-11-2021, 02:25 PM   #1
gambier
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Join Date: Aug 2017
Posts: 13
Default How to reprogram a new BMS for a 48V conversion

Hi folks!

I bought a custom-made Lithium battery pack for my Precedent back in 2017. A 48V, 100Ah system. All was fine with it except I forgot to put a 100A in-line fuse as part of the install, and last year something happened while friends were using the cart and the BMS blew. In the fall I took apart the battery, identified the blown BMS (not hard!), ordered a new BMS (but from a different manufacturer). I installed it last week, and presto, cart was like new again! Happiness.

Then I plugged in it to recharge (on the fancy charger that came w/ the battery), and after a few hours the golf cart read 0% full, and would not move. Sadness and dismay.

After a week of sitting outside in chilly weather (we're in BC), the battery now reads 100%, and it moves again! Happiness.

I think I know what is wrong, which is that the BMS I bought clearly needs to be configured for this battery pack, and I forgot to do that. I downloaded a random BMS from the app store (Xiaoxiang BMS) and it talks to my battery (amazing) and shows:

- total charge: 58.19V
- max V: 58.2V
- each cell is charged up to about 4.15V

I think the charger ended up overcharging the battery, until such time as the BMS said nope, and turned everything off. The cold week drained enough voltage that it's now in spec according to the BMS, so the golf cart works.

I haven't yet figured out how to use this app to change the parameters, but:

- I expect if I reduce the maxV to 48 (or whatever # is right for a 48V system), that it will again paralyze the cart. Am I right? If so, should I drive it until the voltage drops in the range that will be ok?
- what settings should I configure as part of the BMS config?

I am unfortunately not getting a lot of details from the battery manufacturer about the battery chemistry. It has 14 cells.

the spec sheet lists:
- nominal voltage: 50.4V
- discharge cut-off voltage: 42V
- charge voltage: 58.8V+/-0.2V

The BMS app has a lot of settings, including

total battery capacity
total cycle capacity
cell full voltage
cell self-discharge rate
cell 80% cap voltage
same for 60, 40, 20%

balancer config:
start voltage, delta to balance, etc.

and "protections"
cell over voltage 4250mV (trigger / 4150mV (release) / 2s delay
cell under voltage 2700 / 3000 / 2
batt over voltage 59500 / 58100 / 2
batt under voltage 37800 / 4200 / 2
charge over curr 130000 / 32 / 10
discharge over curr 130000 / 32 / 10
charge over temp etc.
charge under temp
discharge over temp
discharge under temp

I would love some expert guidance on what to change here.
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Old 04-11-2021, 11:22 PM   #2
Murby
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Join Date: Oct 2019
Posts: 226
Default Re: How to reprogram a new BMS for a 48V conversion

The battery chemistry for a 14 cell unit at 58 volts would be a standard Lithium Ion, of which there are several varieties that are all the same as far as a BMS or charger is concerned.

The variations in the different chemistries are irrelevant if you already own the battery. Some chemistries are just slightly safer than others, but not even coming close to what a phosphate battery is, some chemistries can deliver higher current but at the cost of lifecycle longevity. Other chemistries have higher operating temperatures.

But again, none of these lithium ion chemistries are important when it comes to a BMS or charger. When buying a battery, you really only have three choices, Lithium Ion, Lithium phosphate, or Lithium Titanate. Since you have a 58 volt battery with 14 cells, its a standard Lithium Ion which has a max charge of 4.2 volts. Actually, the max charge is 4.35 volts but the battery will only cycle a few times if you do that.. its really a theoretical thing and you should never allow the voltage to get that high on a cell.

All lithium ion have a "user" max charge of 4.2 volts and if you charge to that, you can expect 300 to 500 cycles from the battery depending on temperature, current draw, speed of charge, etc. If you hold the charge to 4.15 volts, that 300-500 becomes 500 to 750 cycles.. and if you lower it even more to 4.1, then you might get 1000 to 1500.

We charge our big EV off-grid batteries to 4.05 max and we expect about 2000 to 2500 cycles from them.

One immediate problem I see is that you're using a Chinese BMS.. all of them are garbage.. They have a high failure rate. That said, someone will chime in with anecdotal evidence that they work fine.. key word is anecdotal. With a failure rate of about 10% under normal use, I wouldn't trust them.

REC BMS is the way to go for a vehicle, after that, a ZEVA or Batrium. ZEVA has a really really small balance current but your pack is only 100ah.

Set your cell over-voltage to 4200 mV and release at 4150
I would set your cell undervolts to 3200. Draining a lithium battery to the bottom is really bad for it.
Your battery over voltage should be 4.2v x 14s = 58.8
Your charger should charge to 58.1 or your BMS should cut it off at that.

What should really be going on here is that your BMS should be talking to your charger and telling it what to do through a CAN BUS. These setups where the charge has a set voltage and the BMS has to deal with it, are bad.. cobbled together mismatched systems that can cause problems later.

We have an off-grid system on our home with a 25kWh (480aH) lithium battery. I wouldn't trust those Chinese BMS's with that investment but everyone is different and has different applications.
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Old 04-12-2021, 12:12 AM   #3
gambier
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Default Re: How to reprogram a new BMS for a 48V conversion

Thanks a lot @Murby! I should have asked here before buying the BMS -- if it fails (by itself), I'll know what to do next. I'll try your suggested settings.
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Old 04-12-2021, 12:29 AM   #4
gambier
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Default Re: How to reprogram a new BMS for a 48V conversion

I'm probably going to explore the lower voltage options, as I care a lot more about cycle count than power..

Just so I understand the math, suppose I was shooting for 4.1V / cell, I would have:


- cell over-voltage to 4100 mV and release at 4050
- cell undervoltage to 3200, release at 3250??
- battery over voltage should be 4.1v x 14s = 57.4, and release 56.7
- battery under voltage at 44.8v, release at 45.5 ?
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Old 04-12-2021, 12:23 PM   #5
Murby
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Default Re: How to reprogram a new BMS for a 48V conversion

Quote:
Originally Posted by gambier View Post
I'm probably going to explore the lower voltage options, as I care a lot more about cycle count than power..

Just so I understand the math, suppose I was shooting for 4.1V / cell, I would have:


- cell over-voltage to 4100 mV and release at 4050
- cell undervoltage to 3200, release at 3250??
- battery over voltage should be 4.1v x 14s = 57.4, and release 56.7
- battery under voltage at 44.8v, release at 45.5 ?
That will work fine. I would suggest you never let your cells go below 3.45 volts. If you inspect the charge/discharge profile curves, you will find there is a sort of "knee" at both ends of the spectrum. When a lithium battery gets below 3.5 volts, it starts to drop very quickly as only about 10% of the energy in the cell is below 3.45 volts.

If you're able to set your charger, set it for a max of 4.05 volts as it will make your battery last a long time.

Never store your lithium batteries fully charged. Unlike lead acid, lithium batteries do not like being at high voltage as it increases the rate of ongoing parasitic reactions within the cells.

Lithium is happiest when its at 3.6 volts and just above freezing temperatures. Even in storage, parasitic reactions continue to degrade the battery, the only question is: at what rate? The higher the voltage the more reactions.. the higher the temperature, the more reactions.

Worst thing you can do is to charge all the way up and then store it in a hot garage.
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Old 05-02-2021, 07:02 PM   #6
gambier
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Default Re: How to reprogram a new BMS for a 48V conversion

After a long delay due to not being on island, I did reprogram the BMS to change the cell over and under voltage, as well as the battery over/under voltage. I then plugged in the charger, and it still went into some weird mode where:
a) the battery LCD indicator shows 0%
b) the BMS app shows 0%
c) the BMS app has a hard time reading configuration from the BMS, saying to try again.

So it looks like something else needs to happen. I'll be measuring the voltage on the charger cable to see what voltage it's putting out. It has one unlabeled switch, we'll see if that changes the voltage it puts out. It still seems like there's something that I need to fix with the BMS so that it stops charging before it gets into this weird state.

Any ideas?
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Old 05-02-2021, 07:37 PM   #7
gambier
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Default Re: How to reprogram a new BMS for a 48V conversion



Hmm. After a while with the headlights on to try and drain the battery a bit, I’m now in this very confusing state.

The BMS reads flips between 0 and 100%, the onboard LCD reads 55%, and most worrisome to me the batteries seem wildly unbalanced between 0.5V and 4.18V.
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Old 05-02-2021, 07:38 PM   #8
gambier
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Default Re: How to reprogram a new BMS for a 48V conversion

https://imgur.com/gallery/G9KYZyI

This link may work to show what the BMS app is telling me.
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Old 05-02-2021, 08:28 PM   #9
PingEye3
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Default Re: How to reprogram a new BMS for a 48V conversion

Quote:
Originally Posted by Murby View Post
The battery chemistry for a 14 cell unit at 58 volts would be a standard Lithium Ion, of which there are several varieties that are all the same as far as a BMS or charger is concerned.

The variations in the different chemistries are irrelevant if you already own the battery. Some chemistries are just slightly safer than others, but not even coming close to what a phosphate battery is, some chemistries can deliver higher current but at the cost of lifecycle longevity. Other chemistries have higher operating temperatures.

But again, none of these lithium ion chemistries are important when it comes to a BMS or charger. When buying a battery, you really only have three choices, Lithium Ion, Lithium phosphate, or Lithium Titanate. Since you have a 58 volt battery with 14 cells, its a standard Lithium Ion which has a max charge of 4.2 volts. Actually, the max charge is 4.35 volts but the battery will only cycle a few times if you do that.. its really a theoretical thing and you should never allow the voltage to get that high on a cell.

All lithium ion have a "user" max charge of 4.2 volts and if you charge to that, you can expect 300 to 500 cycles from the battery depending on temperature, current draw, speed of charge, etc. If you hold the charge to 4.15 volts, that 300-500 becomes 500 to 750 cycles.. and if you lower it even more to 4.1, then you might get 1000 to 1500.

We charge our big EV off-grid batteries to 4.05 max and we expect about 2000 to 2500 cycles from them.

One immediate problem I see is that you're using a Chinese BMS.. all of them are garbage.. They have a high failure rate. That said, someone will chime in with anecdotal evidence that they work fine.. key word is anecdotal. With a failure rate of about 10% under normal use, I wouldn't trust them.

REC BMS is the way to go for a vehicle, after that, a ZEVA or Batrium. ZEVA has a really really small balance current but your pack is only 100ah.

Set your cell over-voltage to 4200 mV and release at 4150
I would set your cell undervolts to 3200. Draining a lithium battery to the bottom is really bad for it.
Your battery over voltage should be 4.2v x 14s = 58.8
Your charger should charge to 58.1 or your BMS should cut it off at that.

What should really be going on here is that your BMS should be talking to your charger and telling it what to do through a CAN BUS. These setups where the charge has a set voltage and the BMS has to deal with it, are bad.. cobbled together mismatched systems that can cause problems later.

We have an off-grid system on our home with a 25kWh (480aH) lithium battery. I wouldn't trust those Chinese BMS's with that investment but everyone is different and has different applications.
You are pretty good at slamming the Chinese BMS, I have built 34 packs for golf carts since 2015. I had one failure from an early one from 2015. The rest still work perfect for this application. I did build two packs with the ZEVA BMS. Those two Zeva's have now gone to the trash pile. Replaced both of them with the Chinese with BT. Working fine. Do not need CANbus controlled chargers. The Chinese work just fine and can be easily programmed to your max voltage.

All of that being said, if I was building an off grid system I would use the Orion BMS. I actually have the Orion Jr on my pack from 2015, the first I built, as the Chinese version was not available from Nick at EV Battery Center at that time.
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