Lawn Mower Starter Battery Part 2

rebelrider.mike

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May 25, 2017
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Continued fromhttps://secondlifestorage.com/showthread.php?tid=6251

Hey, Charlie, sorry for the late response, but yeah, I get what you mean. It looks like these chargers have a similar algorithm to the TP4056 chargers. That the end current cutoff is around 1/10 of whatever the maximum is set to. Knowing this has made a difference in how I charge and balance my batteries.

Here's a little update on the mower battery.
image_jyghsw.jpg

Sorry the image is a bit blurry, but you can see that after about 2 years of use, it's still got a fair amount of charge left. Although it is getting imbalanced despite my attempt to put similar capacity cells together. But it still works great, considering these are old laptop cells.

Just for fun, I did a little math, and found out that theoretically the battery could make 523 attempts to start the lawn mower between charges. Unfortunately, in real live, once the battery gets down to about 12V, it just can't produce the Amps needed for the starter.

On the other hand, just three 20A cells could replace this battery and make over 13,000 attempts between charges. For $12 I think maybe I'll try that and see what happens.

By the way, the SLA direct replacement battery for this mower is $70. A home-made LiIon battery would be cheaper, smaller, lighter, and way more fun.
 

Bigfoot48

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Feb 12, 2019
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I found your first thread and discovered this one. Good to hear that it is working out!

I have the same problem, my lawnmower also uses a battery but a replacement including shipping costs will be 75,- or a bit more.
I managed to collect the cells now, it's going to be the same size as yours 3s6p.
As for a BMS, I have one that can deliver 40A, do you think that will be enough?

I'm going to spotweld the batteries, I hope to get to it this weekend, will posts some photos of the progress :D
 

rebelrider.mike

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May 25, 2017
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My starter takes about 22A. I measured it by using an ammeter and shunt, along with a car battery.

If your's is similar, then a 40A BMS will hopefully work. I only had a 20A BMS and it did a great job of shutting off the circuit. So unfortunately, I have no safety features built into this one. But I also watch it very carefully!

I hope your project is successful. :)
 

Bigfoot48

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Feb 12, 2019
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Yes I understand :)

I am building it, school is over now, only work during the day so that leaves me some free time. :cool:

With the last cell I messed up. I used 0.3 nickelstrip and my spotwelder did put in too much heat.....
The protection melted and shorted the + side with the negative side......

This was the idea 4s8p.

image_pzanjb.jpg


And with the last cell.....

image_csqakv.jpg


image_egsira.jpg


image_eqtunf.jpg


The otherside:

image_pcwlwc.jpg


image_jwupwm.jpg


I ordered some new nickelstrips and some protectors.
This evening I will tear down the whole thing and start over.
Replacing the damaged cells also.
 

rebelrider.mike

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May 25, 2017
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382
Yeah, I've had some uh-oh moments like that too. Especially on past versions of this battery which has been mostly a learning project. I'm probably going to start using extra insulation on the positive end too for future batteries.

P7220007.JPG


Well, as much as I like to re-use old cells, I've decided that the convenience of new cells is how I want to go with this battery. Since the starter takes about 22A, I found some 25A cells at www.18650batterystore.com. This way I only need 3 cells to run the starter instead of 24.

This old battery has held up good for the last couple years or so. Starts the mower every time despite being old laptop cells which should probably only be used at 4A instead of the 22A I've been asking of it.

P2070001.JPG


Something to note, the first series is still at 4.1V from when I last charged it however long ago. The middle series is at 3.9V, and the last series, the Sanyos, are at 3.8V. I top balanced this battery every time I charged it. Interesting that the voltages have diverged right away. Although, 4.1-3.8V aren't very far apart on the discharge curve so maybe they're not as different as they look.

The new battery will have one of those really great Pegatron boxes to live in, which I got from BatteryHookup.com. The BMS I bought is way too big to fit in the box, so it will have to be attached outside.

P2070003.JPG


In addition to this BMS being good for 40A, I'm pretty sure it also has a passive balance function. I'm still learning how they work, but there are 12 transistors, 6 ICs, 3 big resistors, and 10 mosfets. I think the big mosfets are for current protection? Some of the ICs are for voltage measurement, and the 3 big resistors are for discharge balancing. All guesses though.

The 3 new cells were $22.78, and the BMS was $6.13, and everything else I'm using I have laying around (free). So $28.91 vs the $70+ SLA that I would have had to buy for the mower.
 

rebelrider.mike

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Finally got the new battery closed up and tested. Might be hard to see in the picture, but the cells all came very close in voltage before I even charged them. The tester, by the way, is only hooked up via the balance cable. The main cable I didn't have the main cables hooked up yet.
P3290007.JPG


So with the main cable added, and everything attached or secured with hot glue, I added a protective foam pad to the BMS board and wrapped the whole thing in duct tape. Here it is secured to the bracket that use to hold the SLA.
P3310008.JPG


I still have to figure out how I want to attach the bracket to the mower. The old SLA was part of what kept the whole thing together without fasteners. You can see here the size difference.
P3310009.JPG


Maybe I'll modify the bracket. Or tape some magnets to it. Haven't decided yet.

This started way back as a practice battery, because the mower is usually pretty easy to start with the cord. This year though, something has gone wrong, and the engine won't stay running. I've tried all the normal things: gas cap vent, spark plug, carburetor, air filter, coil, flywheel magnets. It's all good as far as I can tell. I have some things on order and hopefully I'll be back mowing soon. But the point is that this little battery has been an arm and back saver. I've used it more times this week than I normally would in a mowing season. And it's showing no signs of slowing down yet. :)
 

LithiumSolar

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Oh man, them shorted positive ends! Have to use the adhesive insulators lol.

Funny timing with this thread. My mower battery is completely dead and was debating building a LiFePO4 pack with headways as a starter battery. Ended up just grabbing a replacement LeadAcid though, $20 at Walmart and will hopefully work for another 5 years .
 
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