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Golf power caddy/trolley battery
#1
Photo 
Folks.  This post was written so it will get indexed by Google and hopefully easily found by anyone who is doing research on alternatives to buying a commercial off-the-shelf "12V" lithium battery for their golf power cart/trolley.  I've posted it in General as a power golf trolley isn't a vehicle, scooter or bicycle.  There's no big revelations here nor any questions, just some facts about my build and testing that I think will be helpful to anyone who is contemplating a DIY battery pack for use in a power caddy/trolley.  From my research, I found very little info on DIY battery builds for use in an electronic/power caddy/trolley.  I apologize for the verbose language but again, I want people like me to find this.  I did a lot of searching before and after embarking on this adventure and I found very little information on this specific application.

Starting from the outcome and working backwards, I used Lion 18650s and constructed a 4S8P 20Ah pack with a BMS which successfully replaced a 12V sealed lead acid.  The new battery has worked flawlessly on the MGI Hunter 200 cart, consuming less than half of its rated capacity driving the cart 5,000 yards which is the average number of yards the motor is running at my golf course.  Total test yardage is now around 30,000.  The lithium battery pack is half the size and weight of the SLA yet delivers better performance and should support at least 2X more cycles.

Here's a photo of the battery in the cart in one of my tests on my makeshift "dynamo".



As this was my first build, there were some planning and design issues, mistakes and questions that were not asked and/or answered.  Fortunately, I didn't make any fatal mistakes.  The one big potential issue that arose from my poor planning was the voltage differential between the "12V" SLA battery and the "14.8V" LiOn.  As people on this forum know, those voltages are the nominal/average voltages of the batteries.  A fully charged 12V SLA is ~13V and the 4S LiOn is 16.8V.  The first mistake I made (which I thought might have been fatal) was going with 18650 Lion batteries rather than LifePO4 batteries which have a nominal voltage of 3.2V or 12.8V in a 4S pack, making them direct replacements for 12V SLA batteries. The COTS batteries sold by the power golf caddy brands all seem to use LifePO4 cells.  I learned this after buying 100 18650s but decided to take the risk and experiment with a pack that was "over voltage".  My specific concern with the voltage differential was if/how the motor, motor controller and electronics in the cart would react to receiving up to 16.8V.  What I can say with certainty is that in the case of the MGI Hunter 200 golf power caddy, they react normally.  I suspect that a 4S Lion pack would work fine in any MGI 12V cart.  My cart has a 200W motor, a motor controller, an LCD display and a USB port (great feature, charge GPS or phone).

Suffice to say, before plugging the battery into the cart for the first time I was a tad worried about the bad things that could happen.  Before doing so, I had an electrical epiphany, recognizing first that motor speed is a function of voltage and secondly that the electronics in the cart, the LCD and USB port, don't run on 12V, rather 5V, so I knew there had to be some form of voltage regulation in the cart.  This gave me the gumption to plug the battery in and see what happens.  I knew the motor and electronics could run on less than 12V but the question was about the top end voltage threshold.  The electronics work fine and the only obvious consequence of the higher-than-spec voltage is that the motor runs faster than "normal" when the speed control (potentiometer) is turned to max.  I rarely run the cart at max speed so the motor isn't receiving 16.8V very often, if ever, since the voltage drops when the load is applied.

I used a cheap Chinese 4S BMS.  I have charged the pack with two different chargers, a Hitec X4 hobby charger and a cheap Chinese ($20) Lion brick charger designed for 14.8V batteries.  As for balancing, that is still "work in progress".  The BMS supposedly does balancing but my charge tests are not conclusive.  The Hitec charger has a balance port/leads which I connected to the pack and monitored charging of each group of cells.  The brick charger doesn't balance, rather, it (is suppose to) deliver constant current and voltage to the power ports on the BMS which in turn does the balancing. I'm suspect about how well, if at all, the BMS balances on its own when it is being charged via a two-wire charger even with the BMS "balance wires" connected to the B1, B2, B3 etc.  More testing required.

As the pack is working beautifully and has way more capacity than I need in a single round, I've been thinking about how to get the maximum possible life out of the battery.  While only a theory at this point, I could fully charge the pack and use it for two rounds or I could partially charge the pack.  The former would reduce the charge cycles and the latter would extend the life of the pack by not charging it to maximum voltage.  Everything I've read says that if you charge to 4.0 to 4.1V that you can increase the cycle life of the battery by a multiple of 3 or 4.  The trick is finding a way to do an unattended balance charge to less than 4.2V.  I know this is a topic that has been covered greatly on these forums but I've yet to find a solution.  I'm not sure that the science is settled on this topic either. For anyone interested in the topic, there's a really great lecture from one of the top Lithium battery experts on the planet, Jeff Dahn, available on YouTube. I'll admit bias here.  I don't know Jeff Dahn but he's a professor at Dalhousie University which is located in Halifax where I live.  They have a very elaborate battery testing lab at Dalhousie and the research they've done is impressive.
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#2
I think that BMS starts to balance only once a cell starts to exceeded 4.2V.  And then it can bypass only 80mA, so it's likely too little and too late for larger packs.

It's probably better to use a "smart BMS" such as the one linked below.  It can bypass only 50mA, but you can configure when it should start to balance.  I've set mine to start balancing when the cell has reached 3.8V and is more than 0.015V higher than the lowest cell.

https://www.aliexpress.com/item/32959601568.html
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#3
(07-04-2019, 10:35 AM)ajw22 Wrote: I think that BMS starts to balance only once a cell starts to exceeded 4.2V.  And then it can bypass only 80mAh, so it's likely too little and too late for larger packs.

It's probably better to use a "smart BMS" such as the one linked below.  It can bypass only 50mAh, but you can configure when it should start to balance.  I've set mine to start balancing when the cell has reached 3.8V and is more than 0.015V higher than the lowest cell.

https://www.aliexpress.com/item/32959601568.html

Interesting.  A configurable BMS is exactly what is required.  As I said, I was suspect that a $5 BMS would balance at all.  Have you actually used the BMS which you referenced?
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#4
Yes, I use 3 in my powerwall. Look for "AJW22's modular..." thread. I have the 14s60A version, each managing a 14s100p 18650 pack, and it has been working fine for 6 months now. But one time I had a leaking cell, and the balancer got noticably behind after a few weeks.
The bluetooth module + app has other neat functions you might like: it uses the phone/tablet gps data to calculate speed and theoretical remaining range. see link
https://www.lithiumbatterypcb.com/produc...2-2-2-2-3/
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#5
(07-04-2019, 11:44 AM)ajw22 Wrote: Yes, I use 3 in my powerwall.  Look for "AJW22's modular..." thread.  I have the 14s60A version, each managing a 14s100p 18650 pack, and it has been working fine for 6 months now.  But one time I had a leaking cell, and the balancer got noticably behind after a few weeks.
The bluetooth module + app has other neat functions you might like: it uses the phone/tablet gps data to calculate speed and theoretical remaining range. see link
https://www.lithiumbatterypcb.com/produc...2-2-2-2-3/

I will review the thread.  I found this one Ali https://www.aliexpress.com/item/32959601568.html 

A question that comes to mind is this: If the balance voltage is configurable, say to 4.1V, what happens when you use a CVCC charger?  Also, does the BMS retain the settings?
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#6
Yes, I think it's the same series. The APP screenshot looks the same. You need the optional bluetooth adapter to modify the basic settings like balancing. Advanced settings, like calibration, can only be done with the USB adapter.

I use a 2 leads CVCC charger and it works just fine, provided the cells are reasonably balanced (capacity and SoC) to begin with. In your example, all cells exceeding 4.10V will have their charging current reduced by ca 50mA, provided there is at least one cell with less than 4.07V. That 0.03V threshhold can be adjusted, too.

You can also adjuat the hard cutoff setting like overvoltage (mine: 4.2V), undervoltage (3.0V) overcurrent (30A), low/high temp, etc.

Yes, the BMS retains all settings... But if you disconnect it, the "remaining capacity" value it keeps track of by measuring the current flowing in/out is lost, and it restarts tracking from 0.
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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