Project Cost

 

Completed Electric Tractor

Right, time for a project wrap up, and... I bet you'll be keen to see what it cost!

Ok, let's start with the cost bit first. Total project cost me NZ$24,536 (US$15,600), including the cost of the old diesel tractor (NZ$2,600).

However, the total cost includes the cost of 6x 2nd-hand SLA batteries I used for testing, plus the cables to hook them up and 12V chargers to charge them all, and a Golden Motor VEC500 I bought that I didn't need (thought I needed it when I changed from 48V to 72V). So, excluding these costs, the build cost NZ$21,428 (US$13,660). Here's the breakdown:

Base Costs:
Item	Cost (NZ$)	NZ$ to US$	Cost (US$)
1967 David Brown 880 Donor Tractor	2600.00	0.67	1742.00
QS-Motor QS-180 & ND72850 Controller	2007.70	0.68	1360.00
Cycle Analyst CA3 for PTO	229.26	0.69	157.11
1-3/4" Pillow Block Bearing	82.00	0.69	56.58
7/8" Pillow Block Bearing	36.00	0.69	24.84
Golden Motor 10kW + VEC500	3160.35	0.66	2085.83
Cycle Analyst CA3 for Transmission	338.14	0.64	215.13
Various Switches & 12V Relays	161.17	0.64	103.15
300A Shunts	70.28	0.63	44.47
100Ohm 10W Precharge Resistors	20.91	0.62	12.87
10k Potentiometer (x10 turn)	11.22	0.61	18.39
600A Main Contactors (12V switched)	228.99	0.61	140.76
Battery Cables (battery to contactor then to controller)	291.08	0.65	189.20
T10 Timing Belt Pulleys (x4)	477.71	0.64	304.78
T10 Timing Belt Steel	36.89	0.62	22.89
T10 Timing Belt Steel	48.22	0.62	29.92
Machined steel Motor Brackets	1008.55	0.62	625.30
Hall Effect Foot Controller	57.90	0.62	35.90
Hall Effect Angle Sensor (Hand throttle)	51.03	0.61	30.89
T10 Idler Pulley	20.43	0.54	11.03
Timing Belt Pulley Machining	1101.70	0.57	627.97
200A Circuit Breaker	117.50	0.61	71.68
Shaft Collars (to hold pulleys in place)	155.60	0.62	96.47
160Ahr LiFePO4 Battery Pack & Charger	7980.24	0.62	4947.75
Kevlar replacement Timing Belts	90.30	0.63	56.89
Steel for battery mounting	251.90	0.62	156.18
Replacement front Tyre	100.00	0.62	62.00
Misc Nuts, Bolts, cables, cable ties, etc	693.34	0.62	429.87
NZ$	21428.41	US$	13659.84
Testing Costs:
Item	Cost (NZ$)	NZ$ to US$	Cost (US$)
4x Trojan T-1275 Batteries	750.00	0.62	465.00
Battery Balancer	93.38	0.61	57.30
13pin Plugs & Sockets	61.00	0.63	38.43
4x 12V Battery Chargers	128.50	0.63	80.96
2x 120A Batteries	400.00	0.57	228.00
Heavy cables (for test batteries)	156.16	0.61	95.26
Replacement 140A battery (replaced 1x Trojan)	405.13	0.62	251.18
Golden Motor VEC500	1047.38	0.65	680.80
2x 12V Battery Chargers	65.98	0.65	42.89
NZ$	3107.53	US$	1939.81

So, if this tractor got stolen (or someone offers me a great price for it 😄), would I do the same as a replacement? 

Yes, I think for our needs and the cost and effort involved that this conversion works out well. It is a bit noisier than I expected though, so perhaps I'd explore a more modern donor tractor (one without straight-cut gears and final drives). However, the David Brown 880's ability to lock the 3-point link arms in the up position is perfect for electric operation - means you don't need to keep the PTO motor running unless you're actually raising or lowering the arms. I guess if you converted a tractor that didn't have this then you could design and install some sort of arm locking mechanism.

Is there anything on this build I'd do differently next time?

Yes, definitely. It was a good experience trying two different motors and controllers, but for any other build I'd recommend just going with one model. In my case, I'd go for two Golden Motor 10kW motors and VEC500 controllers. The GM motor is quieter, runs cooler and the controller is simpler to configure.

Using two Golden Motors means I could've kept the voltage at 48V too. As a side-note, I've now found that most hybrid solar inverters only support batteries up to 48V. So, my plan to use the battery to bank some of our solar power on the days I'm not using the tractor has got quite a bit more complicated now that I'm using a 72V battery (if anyone has any ideas on how to use my 72V battery for solar storage please do let me know).

That's all folks! Wonder what my next project will be? 😉