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

[AMDPower]

Sent them another email asking how the temperature sensor works with that profile.

Wondering if I should just pick this up and re-use the cable and connector from the Ryobi. This should be an appropriate charger right?

Amazon.com

 

[Hedges]

Any decent lead-acid charger will alter voltage according to temperature. Even car alternators have a temperature-compensating regulator, although it measures ambient not battery temperature.

While the charger is in CV mode (may need to be connected to lead-acid because BMS of lithium would disconnect), you could twiddle a potentiometer, observe voltage change. Or start with your fixed resistor, measure voltage. After testing the sensor at a couple other temperatures and seeing its resistance, you could determine the NTC thermistor characteristics (what it probably is). Plug in a different fixed resistor and observe voltage change. Compare to published temperature compensation for batteries.

Also page 19 of my manual, "0.004V/degree C per cell or 0.024V for 12V battery"

To drop yours from 4.2V to 3.645V/cell would take simulated 92.5 degree increase, from 25C to 117.5 degrees C.
The charge controller ought to say, "That's too hot, I quit!", but worth a try, it's just a resistor value.
Higher indicated temperature is lower resistance. You already know that open circuit (infinite resistance) disables charger. Don't know about reduced resistance.

Shouldn't charge to excessive voltage every time and rely on BMS to disconnect. That is supposed to be a last line of protection; for less chance of failure you want two mechanisms to limit over-voltage. The charge controller is likely a switching power supply, and sudden disconnect could cause it to experience a voltage spike. Even if it survives a few times, could be causing damage.

Batteries have caught fire and burned the structure they were in, so consider charging it outside.

If you can't fool the battery charger or get it reprogrammed, consider one that does allow lithium settings. Victron or Midnight SCC used with PV panels, for instance. Or an inverter/charger. That would let you run tools too. If it wants to know and manage SoC, a shunt could let it keep track of power drawn by the mower.

Your AGM bank together with battery charger could get a new life as an on-line UPS. Or the individual batteries could be carried in the trunk for jumpstarting yourself.

 

[rio]

AMDPower, thanks so much for undertaking and documenting this! I've been on the same boat with my Ryobi (exact same as yours) for some time and there is very little DYI content out there on this type of conversion. You are paving the way for many of us.

I'm following this thread with excitement and I'm going to do something similar with mine during the off season. I'll either do exactly the same thing you're doing (though I have a lot to learn), or I may pay a little extra for some convenience and go with something like this: https://shop.signaturesolar.us/products/48v-100ah-lifepower4-battery-by-eg4
I'm pretty comfortable with electronics and electrical systems and this even seems like a fun project. I can't understand why Ryobi has been so dismissive of LiFePO batteries, and they even just doubled down by releasing their 54 inch version with the same SLA batteries.

I was wondering if the machine would handle differently from losing so much of the weight after the conversion - did you notice any difference during your mow? I wonder if I may have to add weights to make up for the difference - I find the machine to be on the rattly side as is.

Thanks again and I hope you don't mind if I hit you up with some questions in the near future.

 

[AMDPower]

Hi Rio! It has been very fun and surprisingly easy thus far. Hardest part is waiting oon the cells (120 days). I do notice it’s lighter but still handles similar. Very sharp turns will spin wheels in the driveway. But I cut the grass at 8am first day with a bit of dew and it did great. Have a pretty big hilly yard too.

 

[AMDPower]

I got this back from Delta-Q. Wouldn’t this mean I actually need to INCREASE resistance?


Hi Jack,

Algo 259 is for AGM batteries. Leoch to be exact. The temperature works based on a thermistor. The charger use a 10K thermistor. Higher than 10K resistance will simulate > 25°C.

 

[Hedges]

My original batteries were 100ah. In my opinion it was too high. Probably why they died after 2.5 years!

How many cycles? Once a week for 2.5 years (even in winter) would be 112. That's half what the chart on last page of my manual says for 100% DoD. SunXtender are expensive premium batteries, while some others are lower price and shorter cycle life.

If you analyze the curve, you'll notice that lifetime Ah is lower with 100% cycles, but not much penalty doing 75% DoD vs. 25% DoD; 1/3 the cycles and 3x the power per cycle.

Lead-acid has one set of deterioration characteristics, lithium has a different set. We hear storage voltage/SoC and temperature matter, also charge rate and temperature. I don't use them (except a couple garden gadgets), but from what I read storing around 50% SoC helps, moderate temperature, and charging either while at moderate temperature or else low current if low or high temperature. In other words, avoid charging at 0.5C on a frigid day. Do you know if your BMS has low-temperature charge disconnect? Even if it does, it isn't going to prevent you from giving high charge current at 5 degrees C. You have to design the system to avoid that (which is something I'm preaching, but haven't seen recommended elsewhere.)

 

[Hedges]

I got this back from Delta-Q. Wouldn’t this mean I actually need to INCREASE resistance?


Hi Jack,

Algo 259 is for AGM batteries. Leoch to be exact. The temperature works based on a thermistor. The charger use a 10K thermistor. Higher than 10K resistance will simulate > 25°C.

According to that, yes. That would be a PTC thermistor. Test yours and find out.
You've already determined 12k not 10k at ambient.

For some reason, NTC thermistors are cheaper, and commonly used.
I've used NTC in an op-amp control loop with a resistor as heater to "oven stabilize" a circuit.
And PTC in the bias circuit of a bipolar amplifier, reducing gain as transistor gain increases with temperature.

 

[AMDPower]

The BMS I used in my build has high and low temp cutoff. It is one of the best out there from what I've read, is made in USA and is medical grade. The BMS in the Ryobi wall charger? Highly doubt it. Left it plugged in over winter in my shed. Maybe had something to do with it. Plan on leaving it disconnected with the Lithium and a 50-60% SOC. I'll also monitor through the bluetooth module.

 

[Hedges]

Good it has low-temp cutoff. Will tested some, found they didn't work as advertised.
Some guys here use heaters, so they can charge batteries in an RV while in cold locations.

It was this set of charts posted by Dzl that makes me say you should regulate charge current according to what battery can handle at low-temperature disconnect, or raise low temperature disconnect to where battery can handle your maximum charge current.

The charge controller ought to implement an amps/temperature curve so battery can be safely charged at any temperature above freezing, but haven't seen that in any products. Some batteries activate a heater until a minimum temperature is reached (but I think 0.5C would still be too much current). Probably only in EVs has it been completely engineered.

Why you cannot charge LiFePO4 below 0 degrees Celsius

I am not an expert either but my understanding is: The simple answer is just don't charge under 32*F The complicated answer is... its complicated, but still no.. ;) There are a number of variables to consider with low temperature charging (1) temperature (2) C-rate (3) SOC (4) degree of...

diysolarforum.com

 

[AMDPower]

Yeah, this is my BMS. It's pretty nice.

The Leoch battery that comes with the mower is rated at 600-700 cycles for 50% DOD. At half a charge cycle a week, even in winter, that would have been 24 years no? Definitely not 2.5 years. Anyway, the LiFePo4 is rated at 3000 full cycles. If I can manage it properly, I shouldn't need to worry about batteries again in my lifetime.

 

[Hedges]

Using it once per week to 50% DoD, 10 years would be 520 cycles.
High temperatures shorten life. Sitting at low SoC shortens life.
Being AGM, can't add water so over-charging (including excessive voltage for a given temperature) would kill it.
AGM should be OK fully charged at the start of winter and left with no drain or charging until spring.

Can't be sure the manufacturer designed the system to treat the battery properly. Pretty common for a designer to misunderstand or neglect something. Volume products may or may not ring that out over a few generations. If it gets past warranty (how long was yours?), they probably don't care. I've seen things like electric pumps operated outside vendor specs, as designed into other products. In the case of this battery/charger, voltage seemed high to me but it might be OK for fast charge with the tail current they selected.


Looks like some Ryobi products have a 3-year warranty on battery. Maybe you can get a free replacement?

Help Plus - RYOBI Tools

RYOBI Help Plus

help-plus.ryobitools.com

 

[AMDPower]

At 50% that would be 260 cycles no? A cycle is a full DOD. Oh wait... You mean 50% cycles. Yes.

 

[AMDPower]

It's out of warranty for the SLA. I have many Ryobi products and actually called them on this one. I think the SLA battery is only 1 year. They are really easy to work with though. Really like Ryobi/Rigid/Milwaukee/Techntronic!

 

[AMDPower]

OK. I top balanced all cells to 3.65 down to 1.0A, then let the BMS do the balance/bleed off thing for an hour then just drove the mower back to the shed and parked it. This is the final reading. I'd say that's close enough. I think I'm just going to get the proper charger for that I linked to earlier made for LiFePO for $90. For as much as I've got into this, I'd rather just get the right charger.

 

[eabyrd]

Sent them another email asking how the temperature sensor works with that profile.

Wondering if I should just pick this up and re-use the cable and connector from the Ryobi. This should be an appropriate charger right?

Amazon.com

How do you connect the battery? I don’t see any connectors, does ryobi have a matching plug?

 

[Supervstech]

I agree with your choice, getting a charger with lifepo4 settings built in makes sense.
Maybe get a charge controller and a pair of 100W panels too. Cool to be mowing the yard for free!

 

[AMDPower]

I agree with your choice, getting a charger with lifepo4 settings built in makes sense.
Maybe get a charge controller and a pair of 100W panels too. Cool to be mowing the yard for free!

That's the next project! Gonna try to make use of these 4 100ah half dead AGM's I have.

 

[AMDPower]

How do you connect the battery? I don’t see any connectors, does ryobi have a matching plug?

I already ordered it. The plug that comes with the Ryobi is serviceable/removable. It has like 6 screws. I'll just I'll just re-use it on the new charger.

 

[AMDPower]

Actually, since I'm talking to you solar dudes... I would like to put panels on my shed but it's a quite moderately (heavily) shaded area in Ohio where the shed is at. Do you think that would be enough over the course of a week to charge up the mower? What would you recommend to use to power the shed? I have 120V out there and would like a solar charger that could take grid in along with solar and the old batteries and just handle providing 120V to the shed. Is that possible? Anything "cheap"?

 

[Hedges]

0.037V delta much better than 0.217 delta
Of course that's after it settled. What to look for is how far apart at charge voltage.

Actually, since I'm talking to you solar dudes... I would like to put panels on my shed but it's a quite moderately (heavily) shaded area in Ohio where the shed is at. Do you think that would be enough over the course of a week to charge up the mower? What would you recommend to use to power the shed? I have 120V out there and would like a solar charger that could take grid in along with solar and the old batteries and just handle providing 120V to the shed. Is that possible? Anything "cheap"?

I think there are some $800 and up MPP hybrids that take AC in, PV in, battery, and make AC out.
PV is cheaper than utility rates, but only if a reasonable percentage is used. Net metering is one way. Zero export (using current transformers at utility feed to see which way power flow) could be a good way to do things without asking permission.
Maybe makes more sense closer to house, if you have 120VAC routed to shed. Then it can feed house loads too.

You'll have to evaluate the shading. Would think summer sun overhead would give a few hours. Check angles to obstructions, compare to sun's angle vs. season and time of day.

 

[AMDPower]

Noted. New charger will be here tomorrow. I'm not plugging that Ryobi charger back in again. I figure it will settle after a few good cycles. Thanks for the info on the solar.

 

[Hedges]

Figure out what you want to power. It's all about starting and running watts. Motors typically require 5x their nameplate rating to start, so larger tools need an inverter with second or longer surge rating that high.

Wide range of products, price quality and tolerance for overloads vary. Some fail trying to start a motor, some have run peoples cabins for decades. No-load current draw can be a big drain on a small system. If switched on only to run a tool, that is avoided the rest of the time.

 

[BJohnson]

Thanks for the link. I am planning to replace my batteries in the spring. You had several very good ideas such as your wood box and the foam to help with vibration. Now that you have done it I will have the confidence I won't be throwing my money away.

People who are interested in doing this, I think you may start to find used Ryobi mowers for sale because people are running their mower until it stops before you charge it. When they start replacing their batteries every year I think they will be looking for a sucker to buy their mower. You can put in some LiFEPo4 batteries and have a good mower you can use for another 10 years.

 

[cfent]

I have enjoyed this thread. I have an old Dixon ZTR gasser I might convert to electric.

On a different note about battery life. I built a 125ah LFP for my travel trailer, a couple years ago using Will’s design. It worked well until I had corrosion problems on the home made terminal strips. There was a serious drop in charge and discharge current.

I did I did the following.

1. Put in 6 mm stainless steel posts as described in other threads in this forum, epoxied etc.
2. Made lug and wire connectors between all the cells. I don’t have to worry about vibration and the wires and lugs are sufficient for the current requirements and expansion of the cells.
3. After the epoxy set I used 600 and 800 and 1000 grit emery paper to clean the contact surfaces. An electric contact cleaner was used liberally on the cell contact and emery paper to keep oxygen from the surface.
4. The plated lugs were cleaned with acetone and then a layer of contact cleaner.

Assembly took place while lugs and cell contacts were still damp with contact cleaner. I believe oxidation is now a non-issue. Performance this past summer was excellent.

The emery paper came from a automobile touch up blister pack I found in the paint department. It came with hook and loop fastener emery cloth to foam tool that made it easy to clean the cell tops. I punched a hole in the foam and emery paper to go through the 6 mm post and twisted it while applying pressure.

I should do a formal right up on this because I also put it in a new box and added twist posts, and Anderson plug, and an amp meter. The BMS is inside as well as a fuse to the Anderson connector and a circuit breaker to the posts.

 

[AMDPower]

Well, I got the new charger (getting that in one day from Amazon Prime feels like magic compared to 120 days for batteries from China!). I was originally going to reuse the charger cable and solder it to the new charger board because it has a nice long SO cable and the charger came with a crappy IEC connector and cable. However, I then realized it would just be easier to splice it in an electrical box rather than solder. That part went well. However, as soon as I tried to charge, I kicked my GFCI. Tried again and it won't charge at all. Then I started looking at the actual charger port. There's actually some other stuff going on with it. It has battery plus/minus and data for the sensor but also has a blue cable. I believe this is just some type of switch. It's a single wire. Now I'm looking trying to look it up. It's the same as an EZGO golf cart. Both made by Delta-Q.