Enviromower Battery Upgrade

14 Oct 2015

Finished 'test' version. (Minus balance lead which I added next).

I've just finished building a replacement battery pack for my Ecomower.

The original battery pack consists of two 12V 10Ah Gel batteries which I got second hand via TradeMe.

The new pack consists of 84 recycled laptop cells (18650s Lithium-Ion) wired as 7 series groups of 12 parallel cells for a nominal voltage of 25V and 21Ah giving roughly 500Wh - or twice the capacity of the old Gel batteries when they were new. And it weighs 2 kg less than the Gel pack.

As you can see they are perfectly sized for their box. Lucky.

Next: add a BMS (battery management system) to help keep the groups balanced and to protect from over-discharging. In the meantime I'll use a balance charger to charge it and stick a Lithium cell monitor on it to warn of low voltages.

I tested it in the Economower and it worked great - but the cells are currently not charged so I'll charge them over night and mow the awn tomorrow and see how it performs in a real world test.

I'll give some more details on how I built it over on my Reusing Old Laptop Batteries page.

15 Nov 2015

I've added a BMS to the battery to keep the battery from being over-charged or over-discharged.

And I tested the system with a watt meter:

  • 1257 Watts nominal, 1600 peak load (from the motor)
  • 50 Amps nominal, +80 Amps peak current draw when the mower is stalling in thick grass
  • 28 Volt on the battery, dropping to 27 V at peak load.

21 Nov 2015

A young friend managed to kill the BMS by running it into thick wet grass till it stalled and holding it on. *Sigh*

I'm guessing it reached over 90 Amps. Which is a lot to ask from a cheap 30A/60A BMS.

So I've bypassed the BMS for the moment.

29 Nov 2015

Just mowed our lawn and got some more data.

Before mowing:

  • Battery was balance charged on my iCharger to 29.4 Volts a few days ago
  • Voltage had settled to 28.4 V

After mowing

  • 6.703 Ah
  • 154.3 Wh
  • 94.18 A peak
  • 26.77 V min (3.8 V per group)
  • 2025 W peak

30 minutes after mowing:

  • Resting voltage returned to 27.0 V

Based on the measured capacity of the cell pairs before assembling, the pack should have a total capacity of 21.8 Ah and 570 Wh (from 84 18650 cells).

Next I recharged it with my iCharger:

  • it took 3 hrs 21 min to complete the balance recharge
  • it reported 6.536 Ah capacity pumped in (Note: this is only 3% different from the 6.703 Ah reported by my Watt meter after the mowing session - so, pretty close.)
  • after settling for an hour the pack was at 28.62 V
  • Internal resistance of the seven groups was: 9 mΩ, 7 mΩ, 15 mΩ, 11 mΩ, 12 mΩ, 16 mΩ, 10 mΩ
  • Internal resistance of the whole pack was 80 mΩ

Note: 94.18 Amps peak current draw divided by 12 cells (in parrallel) equals 7.8 Amps per cell. That's quite a lot from a used cell. Assuming these cells are rated for 2C, then it's running almost 4 times it's rated load. Sub-optimal.

This is part one of a larger project to see how much can be done with recycled laptop batteries. Currently our local IT recycling center receives about 1000 laptop batteries a month - which they then ship overseas to be processed. I'd much rather see them re-used locally.

Cheers, Paul

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Page last modified on 19 January 2016 at 04:00 PM