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

[AMDPower]

Thanks. I'll do that. I need to take it apart to get the true measurement of the compartment anyway. They always sit on the charger as recommended. I measured voltage and they were all even at full and at "dead" using the regular charger but I assume that it will balance on draw anyway. How do you recommend I do a proper balancing? Should I run them down and then fully charge using an external charger?

 

[Austan]

Regarding a case for your build, if you were to get the 100ah cells linked in post #10 by Jim, 16 would fit in this milk crate. The crate measures 12x18 internally while the pack would measure 11.4x15.4. You would have to sawzall the top off as it is a bit high at 11" and said you have 9". The batteries in the link are just under 5.25" tall so plenty of room on top to add BMS. It would be good to ask them for size verification while checking for door to door pricing.

Just some thoughts.

https://www.amazon.com/Jezero-Mutli-Purpose-Crate-Mc-24-Rectangle/dp/B074V9J8SD

 

[Substrate]

Thanks. I'll do that. I need to take it apart to get the true measurement of the compartment anyway. They always sit on the charger as recommended.

Fine. Your onboard mower charger might be becoming flaky, or in many cases inadequate to start.

I measured voltage and they were all even at full and at "dead" using the regular charger but I assume that it will balance on draw anyway. How do you recommend I do a proper balancing? Should I run them down and then fully charge using an external charger?

NO, do NOT run them all the way down. Simply charge each one up *individually* with an external charger once. Use a decent one, not a wall-wart. Set for agm (I'm assuming they are agm and not gel) That will bring them and their internal cells into balance. Now operate as usual, and use the mower charger for regular storage and charging, because individually charging every time you want to mow is not practical!

Now you'll reduce some variables. Either your bank got out of balance, or it may simply point to the fact that you wore them out from normal use. But inadequate charging and premature death from abuse is more common.

I just don't want your grass growing too tall while you are shopping for LFP!

 

[AMDPower]

I just don't want your grass growing too tall while you are shopping for LFP!

Thanks for looking out! I will give it a try.

 

[AMDPower]

Thanks for everyone's help on this! I ended up getting in on a group buy discount with Michael and went with Ganfeng 100AH cells and an Overkill 48V 100A BMS. I'm currently searching for an ABS flight case that I might be able to use to house everything. Excited to get everything!

 

[ascottdude]

Thanks for everyone's help on this! I ended up getting in on a group buy discount with Michael and went with Ganfeng 100AH cells and an Overkill 48V 100A BMS. I'm currently searching for an ABS flight case that I might be able to use to house everything. Excited to get everything!

Any updates on how everything worked out? Is the BMS sized big enough to get up the hill with blades spinning? I need to get new batteries and want to upgrade. I cut 12 or so lawns a week so I'd be interested in the 280ah if they fit.

 

[A.Justice]

.... I live in Ohio....

Are you by chance the guy who hit me up on craigslist for the blue 90ah cells I have a few months back? Someone was doing the EXACT same project you are, and lives close by.

 

[AMDPower]

No, that wasn't me. That's pretty funny though. I'm still waiting on the cells unfortunately. I ended up going with an overkill solar 16S 100A BMS. It will be plenty. It's 100A continuous and peaks much higher than that for up to 30s. I have the BMS and a Pelican case that I'm using to house everything. All I have is a shipping label thus far.

 

[Short_Shot]

Nice picture. Yep. That's exactly it. But 280ah! Damn... I'd be able to cut the whole neighborhood with that. That would seem to be expensive no? Any suggestions/links on a low cost 100ah+ cell and a a good housing that I can strap these together in?

Believe it or not... usually it's way cheaper.

 

[Short_Shot]

No, that wasn't me. That's pretty funny though. I'm still waiting on the cells unfortunately. I ended up going with an overkill solar 16S 100A BMS. It will be plenty. It's 100A continuous and peaks much higher than that for up to 30s. I have the BMS and a Pelican case that I'm using to house everything. All I have is a shipping label thus far.

When I was looking into an electric mower I kept balking at the lead acid batteries in the ryobi.


Then I looked at the lithium cub cadet and laughed at the price.

Then I bought a gasser cub cadet and am pretty happy about it. In hindsight, I could have put a few solar panels out back and would have been happier, but here we are.

Knowing what I know now, I'd have done exactly what you did and just upgrade to lithium after a few years.

 

[AMDPower]

I got my cells! Just built my battery pack and will show pictures later but I have a question. All cells identify properly and are currently balancing with the overkill BMS. I see 16 cells. When I measure the OUTPUT of the BMS, it only measures 13V. The batteries themselves prior to the BMS measure 47.9V. All cells when I measured for assembly were between 2.96 and 2.98. They sat on a dock in Cali for a while I'm sure. My question is this...

Why am I only reading 13V on the BMS output? The BMS app says that says undervoltage error and is in the process of balancing. However, I can't charge because it appears that the BMS is shutoff. Do I just let it do its thing? Or do I manually "turn on" battery or what? What is the proper procedure here?

Thanks.

 

[AMDPower]

Hmm.. I just realized that the BMS software says Alert Cell Undervoltage Protection. I think this is a hardware cutoff. Reading through the BMS manual I might be able to lower the undervoltage release temporarily to "unlock" it which should then allow my charger to work as well. Maybe???

 

[AMDPower]

Yep! That was it! Cell must have dipped before during shipping. RTFM for the win! Charging up now!

 

[Short_Shot]

Yep! That was it! Cell must have dipped before during shipping. RTFM for the win! Charging up now!

Don't forget you need to charge *and* top balance the cells.

If you just slap them together you will have capacity issues with the bms cutting off inappropriately.

You may need to buy a cheap power supply to do this. There are top balancing videos around the net and tutorials here on this forum.

 

[AMDPower]

I just am glad I got them back in action. Doesn't the BMS handle the top balancing? What exactly is the process? They are all already within 1/100v.

 

[Short_Shot]

I just am glad I got them back in action. Doesn't the BMS handle the top balancing? What exactly is the process? They are all already within 1/100v.

It can do some of the top balancing but not a lot.

As for the voltage being close, that's just how lithium works. The voltage curve is pretty flat until you get to the extreme top and bottom of capacity, then it goes nuts.

You may find that when charged it's a LOT more than that.

 

[AMDPower]

It works!!!! Just got done with the first cut. Had 58% battery left after 1.25 acres! No bogging down or anything. I can do burnouts in my drive too! The weight of this rig is about the same as just one of my 4 SLA batteries. So pumped. Here are build pictures along with descriptions. It's my first build so if you guys see anything obvious that I should have done differently just let me know.

The biggest thing I would change is the enclosure obviously. I had planned on using a Pelican case and even purchased it but I just couldn't get it to fit right within the frame of the mower. I'm in the process of remodeling my kitchen and had a bunch of scrap maple cabinet grade plywood that was already sealed and finished on both sides. I used pocket screws and glue and then applied some adhesive roofing flashing I had around the edges. The hole is for a long bolt which goes through the existing tray to secure the battery to the chassis.



I got the cells from Michael Caro's group buy and used an Overkill 16S BMS. They are 100AH cells. The two larger groups are taped with Kapton. The cable that joins those blocks was used in my original SLA configuration. The reason for the layout is to allow the long bolt to pass through and secure it to the frame. I packed everything tight with foam as it's likely to get rocky in the mower. The bottom is lined with foam as well.



I used the existing quick connect cable and bolted the negative leads together and used shrink.


The box sits in a metal tray that can can be slid out on the existing rails. Here it is in the stripped down mower. Complete removal of the sides isn't necessary. I had some cleanup to do on the mower though.


Two bolts secure the tray in the rear and the one long bolt through the center. Drill twice, measure once..



Fully reassembled and charging.


BMS reading after full charge.


BMS reading immediately after full mow (1.25 acres).


Notes:
Because of the voltage and curve differences, the on-board battery level always reads 100%.

The charger had an error on it when charging was completed. It's designed for SLA though and is an intelligent charger. Might just be something I have to live with. Possibly because of the voltage level and impedence? Not sure.

I'm charging to full again and then plan to open up and inspect the pack and see if any errors or anything are thrown on the BMS during the process.

Anything else I should do? Thoughts? Also, thanks everybody for your help!

 

[Supervstech]

Very nice! I need to find me one of these.
Did you seal all the plywood edges?

 

[AMDPower]

Very nice! I need to find me one of these.
Did you seal all the plywood edges?

Thanks! I used that flex tape on all edges except for the top (as well as wood glue acting as seal). It's actual intent is for ledger flashing. It's from when I built my deck last year. I probably should have sealed the edges of the tops as well though.

 

[AMDPower]

One other question I have... Since I don't need anywhere near the capacity. Do you think I should change the max charge to something less than 100% to maximize battery life? I know this is the case with my car which uses Lithium Ion. Is LiFePo4 similar with max charge and longevity?