Change SLA to LiFePO4 in a Push Mower

Timberalan · Jun 4, 2024

I have a 36-volt Black & Decker mower (CM1936Z) that has a battery pack normally made up of three 12V 14Ah batteries. I replaced the SLA's about 4 years ago with 15Ah's. I understand 4 years is about average for SLA life expectancy. I'm ready to replace again as they are getting weak.

This 19" mower works fine with good batteries, so I wasn't really looking to replace the mower. I was considering using LiFePO4 because of the drop-in replacement capability. One battery distributor told me their 12V 15Ah LiFePO4 batteries could not be used as replacements due to the current that a lawn mower needs. Has anyone else done this successfully at a decent cost?

sunshine_eggo · Jun 4, 2024

Electric motors have very large surge currents, 4-5X their run current. The BMS in most LFP is the limiting factor.

Can you access the battery cables with a clamp DC ammeter and test what the MAX and run currents are?

Timberalan · Jun 5, 2024

I'll try to do that. If the surge current is the issue with the LiFePO4 BMS, would Lithium Ion be a better choice?

hwy17 · Jun 5, 2024

An appropriate contactor BMS would handle it, but that increases cost and complexity. Worth it for a riding mower but for this small lead acid is so affordable.

sunshine_eggo · Jun 5, 2024

Timberalan said:
I'll try to do that. If the surge current is the issue with the LiFePO4 BMS, would Lithium Ion be a better choice?

No. Lithium ion has the same limitations.

SparkyJJO · Jun 5, 2024

You might be able to get higher Ah rated LFP batteries in the same space as the old SLA, and those may have higher amp ratings. Find out peak and running amps first (how many does it pull when actually cutting some thick grass, not just free spinning the blade). Then go from there.

Timberalan · Jun 5, 2024

Very well! Thank you everyone. I'll most likely stick with replacing the SLA's for now to go another 4 years, hopefully. I'll try to get a peak current using these batteries and then again with the new SLA's so I have a worst-case peak current draw for future consideration.

Timberalan · Jun 5, 2024

My clamp ammeter is AC only. I've got an AC/DC arriving tomorrow to take the peak current reading.

Meanwhile, I see there are some 12V 15 Ah LiFePO4 batteries sold by THISS on Amazon that have a continuous/peak 30/60 amp discharge rate. They even say they are good for push lawn mowers in their description. Judging by the wire size connecting my batteries, I doubt my 36-volt mower even draws much over 20-amps peak. Thoughts?

sunshine_eggo · Jun 5, 2024

Timberalan said:
My clamp ammeter is AC only. I've got an AC/DC arriving tomorrow to take the peak current reading.

Meanwhile, I see there are some 12V 15 Ah LiFePO4 batteries sold by THISS on Amazon that have a continuous/peak 30/60 amp discharge rate. They even say they are good for push lawn mowers in their description. Judging by the wire size connecting my batteries, I doubt my 36-volt mower even draws much over 20-amps peak. Thoughts?

Wires, fuses and breakers are not sized for surge, they are sized for max continuous. Get data.

42OhmsPA · Jun 5, 2024

Don't buy 12V LiFePO4 to run in series for 36V, build a proper 12S 36V LiFePO4 battery.

Edit - welcome to the forum.

yodamota · Jun 5, 2024

our kobalt 40v push lawnmower operates with one 6amp battery. It works enough for our little needs.

How sharp is the blade on the mower? sharpen it regularly?

Timberalan · Jun 7, 2024

yodamota said:
our kobalt 40v push lawnmower operates with one 6amp battery. It works enough for our little needs.

How sharp is the blade on the mower? sharpen it regularly?

No problem with the mower or blade. I do try to keep it sharp. It's just that the lead acid batteries are getting old again and I'm looking into better battery technology.

Timberalan · Jun 7, 2024

42OhmsPA said:
Don't buy 12V LiFePO4 to run in series for 36V, build a proper 12S 36V LiFePO4 battery.

Edit - welcome to the forum.

Thanks for the welcome! I haven't explored building a LiFePO4. I didn't know it was a thing. I was looking at about $150 for three premade 12V 15Ah that fit in . Other than the charger, would building a pack be similar cost?

Timberalan · Jun 7, 2024

sunshine_eggo said:
Wires, fuses and breakers are not sized for surge, they are sized for max continuous. Get data.

I'd forgotten the basic fact that any brushed DC motor will draw heavy current for a moment when it is stopped and full voltage is applied. Then, current quickly drops as the motor comes up to speed.

In a number of ammeter tests last night, I got instantaneous 60 amps for a split second at power on, about 5 amps free spinning, and peaks of about 16 amps when running in heavier grass. I expect I could get slightly higher current if the batteries were in better shape.

So still considering the LiFePO4 12V 15Ah batteries with 30/60 continuous/peak. Question: Does a LiFePO4 BMS cause the battery to limit discharge or does it shut off the battery if/when that peak is reached?

SparkyJJO · Jun 7, 2024

Some may limit, but most simply cut off AFAIK.

42OhmsPA · Jun 7, 2024

Timberalan said:
Thanks for the welcome! I haven't explored building a LiFePO4. I didn't know it was a thing. I was looking at about $150 for three premade 12V 15Ah that fit in . Other than the charger, would building a pack be similar cost?

Building them is fun for some, overwhelming for others.
You will need a BMS when building your own.
If you can find smaller cells the cost will be similar.
These are the smallest I was able to find quickly. https://batteryhookup.com/products/new-3-2v-25ah-lifepo4-topband-navitas-prismatic (note that those don't include busbars)
12 may not fit in the space you have so you'll need to consider that as well.

This BMS would work and handle the current, I'm using 3 of them with my 48V packs.

7s-21s 300a Lithium Ion / Lifepo4 Smart BMS + Bluetooth

We have both lithium ion and lifepo4 versions (select from the dropdown menu). These come with the bluetooth already installed. These BMS units are brand new and work for all variations from 7s to 21s and everything in between. This BMS is rated for 300a continuous. Included is 4x 4AWG wires...

batteryhookup.com

The biggest issues with the 12V packs in series are limited discharge current and packs going out of balance.

Timberalan · Jun 7, 2024

42OhmsPA said:
Building them is fun for some, overwhelming for others.
You will need a BMS when building your own.
If you can find smaller cells the cost will be similar.
These are the smallest I was able to find quickly. https://batteryhookup.com/products/new-3-2v-25ah-lifepo4-topband-navitas-prismatic (note that those don't include busbars)
12 may not fit in the space you have so you'll need to consider that as well.

This BMS would work and handle the current, I'm using 3 of them with my 48V packs.

7s-21s 300a Lithium Ion / Lifepo4 Smart BMS + Bluetooth

We have both lithium ion and lifepo4 versions (select from the dropdown menu). These come with the bluetooth already installed. These BMS units are brand new and work for all variations from 7s to 21s and everything in between. This BMS is rated for 300a continuous. Included is 4x 4AWG wires...

batteryhookup.com

The biggest issues with the 12V packs in series are limited discharge current and packs going out of balance.

Thanks for the additional info! I'll read up more on this. Looks interesting

Change SLA to LiFePO4 in a Push Mower

Timberalan

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sunshine_eggo

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Timberalan

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hwy17

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sunshine_eggo

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SparkyJJO

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Timberalan

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Timberalan

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sunshine_eggo

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42OhmsPA

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yodamota

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Timberalan

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Timberalan

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Timberalan

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SparkyJJO

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42OhmsPA

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7s-21s 300a Lithium Ion / Lifepo4 Smart BMS + Bluetooth

Timberalan

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7s-21s 300a Lithium Ion / Lifepo4 Smart BMS + Bluetooth

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