Cordless electric enthusiast, new to forum.

[rscamp]

I really should have started a new thread for this. :smile:

Anywho, the proposed replacement batteries are eight of these. HobbyKing R/C Hobby Store : Turnigy 5000mAh 5S 20C Lipo Pack .

These would be used in a 10s4p (ten in series, 4 in parallel) arrangement to provide a maximum of 20Ah of capacity at nominally 38V. There is a derating with these also. To maximize cycle life, they will only be charged to 90% of capacity (90% SOC) and will have at least 10% left (90% DOD) giving this a nominal capacity of 16Ah.

This capacity is further derated by discharge rate but in this case, the discharge rate is so low relative to the current these batteries can comfortably deliver (easiliy > 100A!) that this is irrelevant. But let's say the capacity is only 14Ah to be conservative. This is much better than twice the 4-6Ah of the OEM battery - and with a savings of more than 10 pounds (5kg)!

 

[rscamp]

Time for the obligatory warning regarding use of LiPo (Lithium Polymer) batteries. Relative to other battery chemistries they have excellent energy density and power density. Lithium Iron Phosphate and Lithium Manganese are also good choices and are more forgiving but don't match the performance of LiPos. However, LiPos present a risk of fire due to their volatility and need to be used within strict voltage limits to avoid damage.

High current LiPos like these must never be discharged to the point that they have a rest voltage below about 3.75V. The result will be reduced capacity or immediate failure. They must also never be charged above about 4.2V. The result will be reduced capacity or much worse. Fire is very unlikely but the consequences are dire - charging is best done in a place where no serious damage will occur if one of the batteries decides to become a flare.

LiPos can deliver so much current that shorting the wires can be catastrophic.

LiPos are not inherently self-balancing. Some means of maintaining voltage balance between parallel cell groups must be employed. On board real-time battery management is not necessary - a suitable regimen can be just checking balance on charge every few cycles.

So LiPos are great but you must know how to handle them. If you aren't comfortable with the above warnings and the maintenance and use requirements, don't use them.

 

[rscamp]

Okay, so things have progressed a little faster than I anticipated. Home Depot had one of these environmental rebate programs on for replacing old gas mowers with push and electric mowers. The program was ending soon so we took in an old mower to get $100 off the 19" self-propelled model.

The battery was fully charged so I tried it out shortly after getting home. No surprises after reading the various reviews. I managed to get the front and back lawn done with it. The grass wasn't long, but it was fairly wet. Cut height will also have a huge impact on run time. I'll know better when I put a power meter on it.

I've got it on charge now with a computer charger so soon I'll know how much capacity was used (within reason). Charge rate is set at C/10 (1A for these 10Ah batteries) with a standard Pb CC-CV charge cycle. Charging is through the main power connector - I used the connector from the 'spare' battery pack to make the charge connector. I didn't use the small green and white Anderson charge connectors as they have what appears to be reverse-polarity protection diodes in the harness which may affect the voltage seen by the smart charger.

 

[rscamp]

Capacity in on recharge was 3.56Ah. Based on the previous analysis, I don't think it would have run much longer.

 

[floating ground]

Wow Rob, it sounds like you know what you're doing! Glad to see someone knowledgeable experimenting with this. I'm like the guy who knows enough to be dangerous :smile: Please do keep us informed of your progress.

Phil

 

[rscamp]

Wow Rob, it sounds like you know what you're doing! Glad to see someone knowledgeable experimenting with this. I'm like the guy who knows enough to be dangerous :smile: Please do keep us informed of your progress.

Phil

Well Phil, I don't know much about electric lawn mowers!

I tip my hat to you for the improvements you have made to your mower. Electric power in general is excellent and can outperform IC engines in every meaningful way except for one thing - the batteries suck. What we can do here is make the batteries suck a lot less.

In fact, we can improve the batteries so much with these mods that the motor won't be up to the task. By that I mean with the right battery replacement we can have 3 or more times the energy and 3 or more times the electrical power capability compared to the OEM batteries but the motor is still a relatively low power brushed commutator motor designed for the original crappy OEM battery. In heavy grass that the batteries can now handle easily, the motor may repeatedly go into thermal shutdown.

Even though batteries have a long way to go there is NO REASON an electric mower TODAY can't be as powerful or more powerful than the IC engine equivalent. NONE! I think manufacturers need to make a quality product with good batteries and a good brushless outrunner motor/controller. Sure it will cost more but it will have a wider usable power range and open a few eyes...

 

[rscamp]

Batteries arrived! I should be able to try them out this weekend...

 

[rscamp]

Here are the 8 X 5s 5000mAh LiPo batteries fitted in the battery case with all wiring harnesses connected for power and balancing. A block of foam separates the batteries at the front and keeps them in place. Some double sided tape was used to make the batteries more secure. The battery wiring at the top will be compressed a bit by the top cover to further secure the batteries in place.

 

[rscamp]

Here's a close up of the parallel balancer adapter that joins the balance leads from the 4 parallel batteries in each series group. This was fashioned from some project board cut to size with mating PC mount connectors purchased at DigiKey. The common set of leads run through 20AWG wire to AMP Microfit connectors. I had already standardized on these for balance connections to the batteries I use for ebikes.

 

[rscamp]

Here's the finished battery. The power leads running to the "Whattmeter" exit through slots cut in the top cover. This matches how slots are already cut inside the top cover for routing the wires. Balance leads exit through grommetted holes in a similar manner. It isn't necessary to open the enclosure for normal charging/balancing activity with all these connections brought to the outside.

The charging connection is made by disconnecting the input to the Whattmeter and connecting the charger to the plug going to the batteries. Disconnecting the Whattmeter also resets it for the next use. Any meter can be used or the leads can be connected without a meter in place.

It will be interesting to see the actual voltage/current/capacity/power used under different conditions. I was going to put the meter on top but angling it like this makes it much easier to read in use.

I'll mow the lawn this afternoon and see how it works!