Ryobi Zero-Turn Mower SLA to LiFePo4 Conversion - (Updated - Build Complete With Pics!)

[Nobodybusiness]

I was trying to upload a video and realized I can’t so made some screenshots.

I am using the original Ryobi Charger. It’s 7.5 amps.
JK 150 amp BMS.

The interlock still works.

Literally pulled out the AGMs.
Put in 16 280ah LIFEPO4 and it works great.

The original battery charge indicator Meter on the side panel works fine also.

I use the JK BMS app anyway for charge level.

It took me about 5 hours to do by myself mainly because of Trying to situate the batteries but they fit fine in the tray.

With the top bar screwed down in the middle of the pack it’s solid

Edit:
After Supervstech suggestion I’ll put in a pre charge resistor.

 

[Supervstech]

I was trying to upload a video and realized I can’t so made some screenshots.

I am using the original Ryobi Charger. It’s 7.5 amps.
JK 150 amp BMS.

The interlock still works and no need for a pre charge resistor.

Literally pulled out the AGMs.
Put in 16 280ah LIFEPO4 and it works great.

The original battery charge indicator Meter on the side panel works fine also.
No need to change.

It took me about 5 hours to do by myself mainly because of Trying to situate the batteries but they fit fine in the tray.

With the top bar screwed down in the middle of the pack it’s solid

You should be aware, lithium 48V (51.2/58.4) has much lower IR, so each time the contactor clicks on, MASSIVE surge can and will flow into the capacitors
It likely works fine the first few times you turn the key...
Repairing a blown control board doesnt sound fun to me.

 

[Nobodybusiness]

You should be aware, lithium 48V (51.2/58.4) has much lower IR, so each time the contactor clicks on, MASSIVE surge can and will flow into the capacitors
It likely works fine the first few times you turn the key...
Repairing a blown control board doesnt sound fun to me.

It’s using the same circuitry as before.

Been using it since September last year with no issues so far.
Just did the video today because I was getting it out to cut grass


How would you suggest adding a pre charge resistor or some circuit to keep from damaging anything?

 

[Nobodybusiness]

You should be aware, lithium 48V (51.2/58.4) has much lower IR, so each time the contactor clicks on, MASSIVE surge can and will flow into the capacitors
It likely works fine the first few times you turn the key...
Repairing a blown control board doesnt sound fun to me.

How do you suggest to put a pre charge resistor or circuit in place?

 

[Supervstech]

How do you suggest to put a pre charge resistor or circuit in place?

There are a LOT of pages here in this thread explaining the feature.
But, changing the contactor output to a resistor circuit, then adding a power on bypass ought to do it.

 

[Nobodybusiness]

There are a LOT of pages here in this thread explaining the feature.
But, changing the contactor output to a resistor circuit, then adding a power on bypass ought to do it.

Looked through the thread. Must have missed it.

 

[Supervstech]

Looked through the thread. Must have missed it.

With 44 pages, it isnt surprising.
Around page 30 is where the discussion turns to resistor need.
Here is a good response for it in the thread.

Post in thread 'Ryobi Zero-Turn Mower SLA to LiFePo4 Conversion - (Updated - Build Complete With Pics!)' https://diysolarforum.com/threads/r...ed-build-complete-with-pics.22922/post-639584

 

[Nobodybusiness]

With 44 pages, it isnt surprising.
Around page 30 is where the discussion turns to resistor need.
Here is a good response for it in the thread.

Post in thread 'Ryobi Zero-Turn Mower SLA to LiFePo4 Conversion - (Updated - Build Complete With Pics!)' https://diysolarforum.com/threads/r...ed-build-complete-with-pics.22922/post-639584

Thanks for that.
It makes sense.

I have never used one but for the price of a momentary switch and a resistor it can’t hurt.

 

[prophead]

You should be aware, lithium 48V (51.2/58.4) has much lower IR, so each time the contactor clicks on, MASSIVE surge can and will flow into the capacitors
It likely works fine the first few times you turn the key...
Repairing a blown control board doesnt sound fun to me.

I am a bit confused. I have been reading through this thread and noticed most people's issues with blown caps were after shutting down.

How does an pre charge circuit help with this? Or, are people experiencing two different issues, one where the older controller had issues that caused them to blow after being shut down and another where they are blowing when the key is being turned on due to too much in rush current?

I was under the impression the pre-charge pursuit was purely to keep from tripping the BMS upon turning on the key?

I just installed a CHINS pack in my RB540e and it is working fine without a pre-charge circuit but your comment "It likely works fine the first few times you turn the key..." has me worried. Should I be adding a pre-charge circuit?

 

[Supervstech]

I am a bit confused. I have been reading through this thread and noticed most people's issues with blown caps were after shutting down.

How does an pre charge circuit help with this? Or, are people experiencing two different issues, one where the older controller had issues that caused them to blow after being shut down and another where they are blowing when the key is being turned on due to too much in rush current?

I was under the impression the pre-charge pursuit was purely to keep from tripping the BMS upon turning on the key?

I just installed a CHINS pack in my RB540e and it is working fine without a pre-charge circuit but your comment "It likely works fine the first few times you turn the key..." has me worried. Should I be adding a pre-charge circuit?

I still have the original PB in my mower. I have only perused the thread. 800+ responses take a lot of time to go through.

Before I transfer in the lithium, I will be reading the entire thread and distilling the warnings, and solutions for myself.

The warning was due to seeing some of the issues others have had.

 

[MitchBradley]

Hello, I am new here and this is my first post. I am a retired electrical engineer with 50+ years of experience in electronics and computers. After reading this thread, I converted my 48ZTR100 to LiFePO4, using a Lossigy 48V@100Ah battery with Bluetooth. I bought it new-in-box for $1250 from a guy who had gotten more than he needed due to a shipping SNAFU - a common occurrence here in Hawaii. I used blocks cut from 2x4s to keep the battery from sliding around in the tray, zip-tying the blocks to the tray through holes drilled in the blocks. I used a metal strap for a hold down, bolting it down to a couple of holes drilled in the tray.

With the factory charge on the battery I mowed for about 30 minutes on my hilly property with no noticeable loss of performance. At the end, the Bluetooth app figured out how to connect so I could read the parameters. The charge was at 29%.

I put in on the Ryobi charger for 8 hours using all the original wiring. It was pushing 7A/400W which seems about right for that charger. At the end of 8 hours the SOC was reading 100% and the current had dropped to 1A. I took it off the charger and went to bed. This morning I reconnected the charger, observed that charging resumed, and came back after an hour. By then the current was 0 and the BMS was reading "protection mode". One of the cells was just over 3.5V which is the protection restart voltage. The rest were just under that. I disconnected the charger and drained the battery a little by bumping the motor gently, until that cell dropped below 3.5V and the protection turned off. I then reconnected the charger and watched the voltage on that cell run up to 3.65V (the protection trip voltage) over the course of a minute or so. Then the protect kicked in and the current went back to 0. All this seems entirely reasonable according to what I know about LiFePO4. The cells charge pretty much equally until they get to the knee of the curve, then they start going up fast until one "wins" and the BMS calls game over.

At that cutoff point, the max cell imbalance was 0.1V, but that started dropping as the batteries rested. I took it out for a mow and finished the rest of my property including the pasture, ending with 56% - and the imbalance equalized to 0.003V.

A 10A LiFePO4 charger is due to arrive tomorrow, but I can't think of a reason to use it. At high charge rates, a perfect charger might be better, but at 0.07C and an appropriate cutoff point on the BMS, I'm not seeing what the advantage would be. I can't find anything that explains a problem with low-C LiFePO4 charging. I have no need for high-C charging and I would keep that particular AC outlet lightly loaded.

The Lossigy 48/100 pack has no problems so far with inrush cutoff. According to the Bluetooth app, its max current is 317A.

 

[Supervstech]

A bms should NEVER be the charger limit.
You really need to be ising a charger with the correct voltage cutoff.

 

[MitchBradley]

> A bms should NEVER be the charger limit.
What bad thing will happen as a result? Just wanting to understand the details.

 

[Hedges]

Would you jump out of an airplane and rely on a parachute to take you safely to the ground?
For every flight?

 

[Supervstech]

> A bms should NEVER be the charger limit.
What bad thing will happen as a result? Just wanting to understand the details.

The BMS is a cell charge and discharge safety... kinda like a circuit breaker.
It is there to catch cells going out of safe range.
It is an indicator of a failing cell, or a damaged circuit...

It is NOT a charge controller.
Each time they trigger, there is a chance it wont trigger again.
The balance resistors should be able to bleed off runner cell before hvd triggers.
That means the charge voltage needs to be in range for the balance circuit to keep cell voltage in check.

Too high a voltage, and rapid overcharging becomes regular, and this weakens the cells.

Get a proper charger going.

 

[Nobodybusiness]

Would you jump out of an airplane and rely on a parachute to take you safely to the ground?
For every flight?

Statistically the odds are more against you every jump but what’s the alternative? Glider?

 

[Hedges]

Remain in your seat with seatbelt buckled.

Parachute (0.27 deaths per 100,000 jumps)

United States Parachute Association > First-Time & Student Skydivers > How Safe is Skydiving?

Explore meticulously researched data and industry trends to gain insights into the safety of skydiving.

www.uspa.org

Commuter flights (0.311 fatal accidents per 100,000 departures)

U.S. Civil Aviation Fatalities and Flight Activity Decreased in 2020

www.ntsb.gov

Oops, according to those, you're better off jumping than remaining in the plane to let the pilot land.
Although since each departure represents more than 1 person (at least one pilot and one passenger), plane is safer. Not comparing passenger to passenger.

Another, 1 fatal accident per 100,000 general aviation flight hours:

General Aviation Accident Dashboard: 2012-2021

www.ntsb.gov

Regarding batteries, I don't think we've heard from someone who's batteries got overcharged due to BMS failing to disconnect after previously disconnecting repeatedly. We have heard of quite a few overcharged without BMS, especially charging a single cell without CV set correctly on a power supply. Some series connected batteries that failed (one appeared to have no BMS and differing numbers of cells in parallel; that burned the house down.)

Possibly the FETs would be overstressed when they disconnect a charger. MPPT has current going through an inductor, and when BMS disconnects the inductive kick will deliver a high voltage. In most inductor circuits we think about turning off the source, which produces a negative spike. A clamping diode protects against that. In this case, disconnecting load after the inductor, so FET sees voltage kick high. That might cause FET to fail shorted and after that battery gets overcharged.

When MPPT operates, its design allows its FET to switch within voltage limits, and it ramps down current delivered as battery accepts less. Not overstressed like I think BMS FET would be.

Belt and suspenders, set your SCC to regulate voltage and rely on BMS only as second device for safety. And as Supervstech says, slower charge allows more time for rebalancing cells.

 

[MitchBradley]

I am going to test with the LiFePO4 charger when it arrives later today. I understand the concerns that have been raised, but I am not sure they apply in this case due to a confluence of factors. One is that the charger maxes out at 7A, so the entire charge cycle from beginning to end is at 0.07C. The fast-charge graphs tend to go CC at 0.4C up to say 3.5V/cell, then CV until the tail current drops to 0.05C. 0.07C is barely more than the tail cutoff, so in effect the just-before-cutoff slow rate is happening "forever". The other thing is that this battery is rated for a charging at 150A, 327A draw for 30 seconds, and peak current of 500A for a couple of seconds, so I suspect its BMS has some serious FETs. This is the kind of system-level problem where it is hard to know the answer without detailed knowledge of the circuits, components, and build quality at both ends. And even then it probably requires a lot of testing of obscure corner cases.

One other problem with the Ryobi charger is the fact that, after the BMS has cut off the charge, the cell in question will eventually drop below the restart threshold and there will be a brief restart. And that will keep happening. That may or may not cause actual problems, but it doesn't seem like a great idea.

 

[Hedges]

FETs designed for serious current will be lower voltage. If voltage spikes higher with FET off, could break down Drain to Gate, which would then turn gate on and resume charging.

It isn't the current you're charging at that causes damage, it is the voltage the battery reaches. Even if charged very slowly it will eventually reach excessive voltage, because unlike lead-acid, power isn't bled off by electrolysis. OK, maybe it is, but that is what causes swelling and eventual venting.

A charger should have a bulk (absorption) voltage, then set back to a float voltage. That would avoid repeated runs to full.

A spring-wound timer was common for dumb lead-acid forklift battery chargers. You could set one up for yours. I use one so air compressor doesn't kick back on middle of the night.

All we really want for your application is a circuit that cuts AC to the charger when battery reaches a certain voltage.

 

[Supervstech]

Yeah, 1A at 3.65+ voltage causes the cells to swell and degrade.
You want a charger that tapers off at 3.4V per cell… not one that gives CV of 52V and let’s the BMS trip out repeatedly…