MateoMX
New Member
- Joined
- Jul 5, 2020
- Messages
- 6
Background
I run a dive center in the Mexican Caribbean and we use an 8 seat golf cart to move customers wearing gear from our dive shop to our boats. A bunch of lead acid batteries were near end-of-life and I had spent waaaayyyy to long watching Will's YouTube channel not to consider LiFePo4.
I opted for LiFePo4 because...
Based on patchy data I was able to find online, the golf carts draw a max of around 100amps when accelerating or going up-hill. Spoiler alert: It turns out with a full load the current is probably closer to 200amps at times. It really can't be too much for too long because most of the main cables are only 4AWG. But with most LiFePo4 cells having a peak discharge of 3C I figured this should be OK for a 100Ah cell.
What did I buy and why?
Will is clear with his advice - Buy high-quality, matched cells, from reputable suppliers
Unfortunately, I live in Mexico, where...
The build
December 2019
I charged the pack using a LiFePo4 charger through the BMS. The charger soon cut-out claiming a full charge, but the pack voltage was still low. So something about the separate-port BMS caused the LiFePo4 'smart' charger to think the pack was full before it really was.
To fix this, I connected the charger directly to the 16s pack and allowed it to charge, reaching a total voltage of around 58v before the charger stopped.
The BMS's mighty 0.25w resistors were bleeding off some of the cells by the end of the charge, but considering this was a 10amp charger, this 0.25w of discharge was negligible.
The cart worked really well! With significantly more acceleration and torque compared to the previous Lead Acid batteries (although granted they were near end-of-life).
After each short trip, I kept an eye on the pack voltage and everything looked pretty reasonable without much of a drop seen. Then, unexpectedly, during a trip the BMS hit its Low-Voltage Protection and cutout the relay and therefore removing power from the drive system of the cart. I knew this was a risk, so I attached a jumper that would allow me to force the relay to re-engage and I drove the short distance back to the shop. Once back at the shop I checked the pack voltage at it was still at around 52.8v (3.3v per cell), well above the low SOC voltage of 49.6, so I naively left the jumper in place and continued to use the cart for another couple of short trips.
I eventually saw the pack voltage dropped down to around 49.4, so I figured it was now time to re-charge. Before re-charging, I checked the voltage of the each cell in the pack and found that actually most of the cells were between 3.2-3.3v, however a few cells were 1.9-2.6v - so they certainly seem to be at very different levels of SOC.
Balancing
After going back to watch more of Will's videos I then started to understand the important of paralleling cells prior to use and particularly bottom balancing. I had incorrectly assumed that the cells being delivered at "about" 3.3v is actually not a good way of testing the true SOC of each cell. I followed Will's advice and left them in parallel for a day while I took a dremel to the busbars to allow me to reinstall the batteries in a more robust configuration.
I re-installed the pack and added better balance leads for a 2amp active cell balancer that I had ordered from AliExpress and I was awaiting delivery. I partly wanted the balancer for its balance functionality. Although, to be honest, I also wanted the ability to monitor the individual cell voltages via bluetooth so I could see voltage drops during use - kind of difficult to do manually with a cart full of people driving on the streets on Mexico! Believe me Will - you won't believe how jealous I became of your inverter + heat-gun loads. That would have been a much easier than trying to monitor batteries parameters while dodging pot-holes at 20mph!
Once re-installed, I charged the pack again and this time kept a closer eye on cell voltages between each use and topping up where required.
Problems continue...
It started to become clear that some cells would discharge faster than others and some cells would reach higher voltages during charging compared to others.
Once I received the balancer, this proved to be really handy. After running the cart low, I was able to do a bottom balance again, but this time it was much easier compared to removing the entire pack and putting all cells in parallel (albeit taking quite a long time). However, despite the efforts of occasionally bottom and top balancing the pack using the balancer, I realized there were serious differences in the characteristics of these cells.
Now, in July 2020 these issues to continue and I regularly see about several cells reach between 4-4.3v during charging, meanwhile others only reach 3.35-3.6.
Obviously, these 4v+ values are not good news at all and this is why I am reaching out here for advice.
What is the cause of the problem?
In your opinion what is the cause of the problem?
If you were me, what would you do?
I run a dive center in the Mexican Caribbean and we use an 8 seat golf cart to move customers wearing gear from our dive shop to our boats. A bunch of lead acid batteries were near end-of-life and I had spent waaaayyyy to long watching Will's YouTube channel not to consider LiFePo4.
New video idea for Will: Come to Mexico, help me out with this and I will teach you to dive and take you on some incredible dives, including underground caverns for free. You can also spend your time here in Mexico investigating how solar should be better used in poorer but sunny countries.
I'm not kidding - if you can make it here, I'll sort out the dives for you. We're one of the longest running and experienced dive shops in the area, you'll have a great time.
I opted for LiFePo4 because...
- I don't trust myself with 'spicy' lithium chemistries that have higher fire/explosion risks. I also wanted to see a big improvement of charge cycle life over lead-acid.
- I hadn't really seen much about Lithium Titanate and these cells seemed very expensive
- I wanted to move away from Lead Acid since I charge the cart indoors and didn't want hydrogen off-gassing. I also wanted to avoid battery maintenance.
- I wanted to reduce the weight of the cart to improve hill climb speed with a full load of divers
- I liked the idea that with liFePo4 most of the capacity would be between 52.8-54.4 and with the DC drive of the golf cart that should help keep motor performance as high as possible.
Based on patchy data I was able to find online, the golf carts draw a max of around 100amps when accelerating or going up-hill. Spoiler alert: It turns out with a full load the current is probably closer to 200amps at times. It really can't be too much for too long because most of the main cables are only 4AWG. But with most LiFePo4 cells having a peak discharge of 3C I figured this should be OK for a 100Ah cell.
What did I buy and why?
Will is clear with his advice - Buy high-quality, matched cells, from reputable suppliers
Unfortunately, I live in Mexico, where...
- there are no domestic suppliers (as far as I could find)
- import duties are incredibly high and unpredictable
- not all suppliers are willing to ship here
The build
December 2019
- The rear section of the golf cart has plenty of space for the batteries under the seat. I built a rudimentary fiberglass box using bits of fiberglass left-over from a recent boat re-modelling. This box would help keep the batteries away from the metal from of the cart, which seemed a reasonable precaution to me.
- All the cells arrived at about 3.3v, so I figured, great - they are all "the same". I proceeded to add them in a 16s configuration using the included busbars, which, didn't actually allow the cells to touch one-another which meant building a strong interlocking structure wasn't possible.
- I had only been able to get my hands on a crappy looking 50amp BMS, which I installed and used to control a 300amp relay, so the real load went straight through the relay, rather than the BMS.
- I added a 48v > 12v DC-DC buck converter to power the 12v systems of the cart (horn, lights, GPS tracker).
I charged the pack using a LiFePo4 charger through the BMS. The charger soon cut-out claiming a full charge, but the pack voltage was still low. So something about the separate-port BMS caused the LiFePo4 'smart' charger to think the pack was full before it really was.
To fix this, I connected the charger directly to the 16s pack and allowed it to charge, reaching a total voltage of around 58v before the charger stopped.
The BMS's mighty 0.25w resistors were bleeding off some of the cells by the end of the charge, but considering this was a 10amp charger, this 0.25w of discharge was negligible.
The cart worked really well! With significantly more acceleration and torque compared to the previous Lead Acid batteries (although granted they were near end-of-life).
After each short trip, I kept an eye on the pack voltage and everything looked pretty reasonable without much of a drop seen. Then, unexpectedly, during a trip the BMS hit its Low-Voltage Protection and cutout the relay and therefore removing power from the drive system of the cart. I knew this was a risk, so I attached a jumper that would allow me to force the relay to re-engage and I drove the short distance back to the shop. Once back at the shop I checked the pack voltage at it was still at around 52.8v (3.3v per cell), well above the low SOC voltage of 49.6, so I naively left the jumper in place and continued to use the cart for another couple of short trips.
I eventually saw the pack voltage dropped down to around 49.4, so I figured it was now time to re-charge. Before re-charging, I checked the voltage of the each cell in the pack and found that actually most of the cells were between 3.2-3.3v, however a few cells were 1.9-2.6v - so they certainly seem to be at very different levels of SOC.
Balancing
After going back to watch more of Will's videos I then started to understand the important of paralleling cells prior to use and particularly bottom balancing. I had incorrectly assumed that the cells being delivered at "about" 3.3v is actually not a good way of testing the true SOC of each cell. I followed Will's advice and left them in parallel for a day while I took a dremel to the busbars to allow me to reinstall the batteries in a more robust configuration.
I re-installed the pack and added better balance leads for a 2amp active cell balancer that I had ordered from AliExpress and I was awaiting delivery. I partly wanted the balancer for its balance functionality. Although, to be honest, I also wanted the ability to monitor the individual cell voltages via bluetooth so I could see voltage drops during use - kind of difficult to do manually with a cart full of people driving on the streets on Mexico! Believe me Will - you won't believe how jealous I became of your inverter + heat-gun loads. That would have been a much easier than trying to monitor batteries parameters while dodging pot-holes at 20mph!
Once re-installed, I charged the pack again and this time kept a closer eye on cell voltages between each use and topping up where required.
Problems continue...
It started to become clear that some cells would discharge faster than others and some cells would reach higher voltages during charging compared to others.
Once I received the balancer, this proved to be really handy. After running the cart low, I was able to do a bottom balance again, but this time it was much easier compared to removing the entire pack and putting all cells in parallel (albeit taking quite a long time). However, despite the efforts of occasionally bottom and top balancing the pack using the balancer, I realized there were serious differences in the characteristics of these cells.
Now, in July 2020 these issues to continue and I regularly see about several cells reach between 4-4.3v during charging, meanwhile others only reach 3.35-3.6.
Obviously, these 4v+ values are not good news at all and this is why I am reaching out here for advice.
What is the cause of the problem?
In your opinion what is the cause of the problem?
- Bad cells from the start
- Damage caused by initial use without bottom balancing
- Wrong cells for the job - 70amp constant, 150-250amp peaks
- Incorrect charging method
- Damage cause by running the cart after the BMS had hit LVP
- Something else
If you were me, what would you do?
- Charge the pack differently (would different charge profiles allow the cells to charge more evenly without a few of them raising in voltage disproportionately to others)
Request a refund(not a realistic option now I have had the cells this long and made mistakes myself)- Give up and buy some of Will's recommended cells (likely to be over $2,500 for 48v@100Ah after delivery and taxes in Mexico)
- Only some of the cells seem bad. Buy a few more and switch them out.
- Buy an entire extra set and go for a 200Ah config (16s2p), this will lower the C rate particularly for discharge and will mean the cells with behave less erratically
- Start a YouTube channel all about dive shops, golf carts and life in Mexico so I can afford to buy nice things rather than going for the cheapest options
- Get Will down to Mexico to help me out while I teach him to dive and he makes some great solar and battery videos