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

[AMDPower]

Hi. I'm new to the forum. I have a 48V Ryobi Zero Turn that runs off of (4) 100AH AGM batteries. It's been about three years and now I can't get through a full cut. I would like to upgrade to LiFePo4 batteries. I'm thinking of using these ones

https://www.amazon.com/gp/product/B08K7MKRF7/ref=ox_sc_saved_title_1?smid=A2EZ26GXN4H62H&psc=1

It is my understanding that the 50ah LiFePo4 should be about equal to a 75AH SLA battery. The Ryobi also has a 75AH AGM model. So, my question is... Could this be a drop in replacement? I store it in my shed so I was going to monitor inside temperature and shut off the AC Circuit automatically when below freezing (this battery doesn't seem to have low temp shutoff). Also, if I pre-balance and wire for 48V, can I use the existing charger? I think it only outputs 48V so probably not. If not, can I use a simple 48V 50A or lower charger and charge them in series as they sit? Since these have built in BMS units will they handle the shutoff automatically? Many thanks for any direction.

 

[Austan]

Good day. It appears those batteries you linked to will be able to replace the agm's in your mower. I would expect you can also use your charger without issue.
Thoughts; You do not have to match the amp hours, only the voltage. The size difference between the agm's and lifepo4 might make it so you can put 150ah or more in the same available space if you were so inclined. I'm sure I'm not alone in thinking that you could get a lot more battery for the 1200 you'd spend on those 4x 50ah batteries. I would encourage you to look into making your own battery with 16x higher capacity (amp hour) 3.2 lifepo4 cells wired in series, or 32 if you need more output for some reason. I have a feeling a 48v 120-150ah battery would come in under $1k as a diy pretty easily.
We can help you along with more specific recommendations if you are interested in pursuing it. It would help to know the space restrictions of the battery compartment in your mower.

 

[45North]

What are you mowing that grows, so you need to mow and charge below freezing?

 

[AMDPower]

Thanks guys. Regarding mowing the grass in the winter. Ha! ...uh, no. I store the mower in the shed which is unheated. I live in Ohio and winter season runs 4-5 months. I didn't want to leave it unplugged and sitting the entire time so I just planned to automate a disconnect on freezing.

More specifically, what would you recommend for cells/premade/BMS? The battery compartment holds 4 SLA batteries stacked 2X2 longways. It's pretty big. I figured I could probably get away with 50ah as a replacement for 100ah SLA. Happy to go bigger if I can find the cells. I briefly looked at some used Valence batteries on eBay that were $189 for 12V 40ah but that didn't seem efficient for the size and package. I also wasn't sure about going as low as 40ah. I have to mow 1.25 acres. When my SLA batteries were new, they would easily cut 2.5 acres. Now, it's more like .75 acres. I have a lot of electronics experience and tools so I have no problem building my own if I can find some good packaging and its safe. I'll be riding on top of the batteries so I'd prefer to keep my privates away from the fire.

 

[HRTKD]

When I went LiFePO4 cell shopping I did it based on physical size. I started with the compartment where they would be installed and worked my way backwards, finding the largest cell that would fit. The larger the cell in Ah, the less you'll stress the battery.

Your BMS choice will depend on the number of amps the mower can draw. The usual BMS option starts at around 120 amps for a 12v system and 100 amps for a 48v system. That's for a BMS that does not use a contactor.

 

[schmism]

Given the amount of space, I would look at building my own replacement out of 280ah LFP cells.

 

[AMDPower]

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?

 

[HRTKD]

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?

It depends on the dimensions. The Fortune 100 Ah cells are taller than the 280 Ah cells as I recall.

 

[AMDPower]

Hmm... I'm still not getting how this is less expensive for my operation though. I can see if you need maximum capacity for solar but I just need to cut my grass. Even if I look at the Fortune 60ah, it's still going to be more than the ready made 12v batteries I listed. I would have $1360 in batteries plus a BMS and have to get some sort of case.

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Wouldn't I be better off with the 50ah drop in I originally listed?

 

[HRTKD]

My preference is to build an actual 48v battery if you need 48v. Series connecting four 12v batteries will get you 48v, but it's more complicated.

Most of us are not buying our batteries domestically. Take this offering on Alibaba.com for example. 16 cells will cost $1093.

 

[Q-Dog]

One thing to consider is that batteries last longer if they are not used to their full capacity. I would at least build equal capacity to what you already have. BUT, if it were mine I would want to put as much battery as I could physically fit in there. ?️

 

[AMDPower]

I see... That looks really good. Have you ordered from that vendor before? I'm confused though. Why is 100/120/150 all the same price?

 

[Supervstech]

You certainly could build a smaller pack for even less...
Bigbattery has assembled packs of 100Ah 3.2V you could get 16 of those, wire it to a BMS and have a great setup, 1/2 the weight of the SLA...
Drop ins are pricy for what you get, and most will not work in series at 48V...

 

[noenegdod]

Personally I think Id go this route for that application: https://batteryhookup.com/products/new-battery-hookup-lifep04-32650-3-2v-5000mah-cells

 

[HRTKD]

I see... That looks really good. Have you ordered from that vendor before? I'm confused though. Why is 100/120/150 all the same price?

No, I haven't ordered from that vendor. I was throwing that out as an example. I did order my cells from Alibaba though. The trusted source that I would use can be found in this thread:

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[schmism]

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?

Depends on your time frame. The group buy here in the forums for 270-300ah LFP cells is place to start. The cells are drop shipped from china and Michael has said he can order any number of ah ranges (including 100-120). I ordered my 280's through him and he was great to deal with. Shipping over the holiday last year took right at 60 days.

Lishen 270ah-272ah cell group buy.

Ok, this will be a longish post but I want to lay some foundation before taking orders on these cells. Eve 280ah cells appear to only be available at this point from suppliers that want a premium price for them From everything I can find, Lishen is extremely popular in China but has yet to get...

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[Q-Dog]

This likely won't be less expensive than replacing with AGM initially, but if you build a proper sized pack and don't blow them up LiFePo4 cells should outlast 3 sets of AGM batteries.

I don't know if the 50ah batteries proposed in the OP can handle the amp draw, unless you double them to make a 100 ah pack. This mower has 2 traction motors and 2 motors for the blades. It isn't all about capacity. You also have to consider if the battery pack can actually provide the amps the mower will draw.

 

[AMDPower]

Thanks Q-Dog. That's exactly what I was trying to determine. I can't find any specs for current draw of the mower/motors yet though. May have to buy a DC amp meter. Mine is only AC.

 

[HRTKD]

Often, the amp draw limiting factor is the BMS. Motors often require a BMS that supports a very large amp draw. Until you know the amp draw of the motors (continuous and surge) you can't continue with your design.

A 100 amp @ 48v BMS may not be enough. (Don't know for sure, just spitballing here) If it isn't, then a second 16s battery can be created with its own BMS and that would give you double the amp draw. If you have to resort to that, you're getting into some serious money.

Is there a way to recondition an AGM battery? Maybe that would get your existing batteries back to running closer to where they were when new.

 

[AMDPower]

Just ordered a clamp.

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I want to find out what total draw is anyway. I think you can do some reconditioning. Will have to look into it. I still would like to do a conversion at some point though.