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Discussion Starter · #1 ·
Hi everyone, first post here.

I'm interested in converting my JCB robot skid steer from diesel to electric. I admit I have little experience with ev conversions (save for a simple 48v dc brushless conversion on a small david bradley walking tractor I use here on the farm). I'm also far from an expert when it comes to hydraulics. I'd like to brainstorm my thoughts and see if anything is off base. I welcome any and all feedback.

The target is this - my 2001 JCB Robot 165
123049


She has a reliable 44hp peugot diesel powerplant. I've had her in and out of the JCB dealer several times over the last year though. At this point I'm convinced she needs a head gasket (no coolant in the oil, but she blows coolant out of the overflow and starts getting cranky after 20 minutes of operating). The thermostat, water pump and radiator all checked out, so this is my next target. Diesel engines are a bit above my head, so I'm looking at a likely $1500-$2000 job to have a head gasket done. At that price, I'm really pushed toward swapping the whole power plant for an electric one which is what I really want. I don't want to underestimate the size of the project, but being a simple direct hydraulic drive for everything makes it seem simple (at least on paper).

As far as I can tell from the service manual, with standard flow hydraulics its rated for 14.5gpm with a relief pressure of 2650psi. Banging around some quick engineering calculations (Hydraulic Pump Calculations - Womack Machine Supply Company) 14.5 gpm flow requires 27 mechanical hp at 2700 psi.

I'd like to stick with a 48v nominal system (I have other projects and a solar off grid system that run on 48v). This limits my choice of motor, but it seems like the curtis AC 9 would fit the ticket. It should deliver more than the required power at 48v (with 600A controller). The other obvious win is that it's available with an SAE A hydraulic pump mount, meaning I could bolt it directly to a hydraulic pump without any custom fabrication.

AC-9 motor kit w/ SAE A pump mount - AC-09 AC Induction Motor Drive Kit

Operating time is going to be limited by battery pack size. I'll be sacrificing run time for sure, but for my use on the farm, if I could get an hour of hard run time (an hour of operating without hearing protection and diesel fumes) that would be fantastic. I'm a fan of the bmw i3 modules (48v 60ah). I can get a salvaged battery pack from a local yard for a decent price. I'd be looking at 20kwh realistic for an 8 module pack.

BMW i3 60ah pack with 48v modules -

Are there any glaring holes in my plan? Anything I haven't considered? I haven't seen anyone do a project like this. Does anyone have any links resources for other direct hydraulic ev builds or conversions?

Thanks!
 

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I'd like to stick with a 48v nominal system (I have other projects and a solar off grid system that run on 48v). This limits my choice of motor, but it seems like the curtis AC 9 would fit the ticket. It should deliver more than the required power at 48v (with 600A controller). The other obvious win is that it's available with an SAE A hydraulic pump mount, meaning I could bolt it directly to a hydraulic pump without any custom fabrication.

AC-9 motor kit w/ SAE A pump mount - AC-09 AC Induction Motor Drive Kit
Listings like that one from Electric Motorsport tend to lead people to believe that Curtis motors exist - they don't. That's an HPEVS AC 9-06.06 motor plus a 1236SE-5621 controller from Curtis Instruments.
 

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I'd use a second-hand forklift motor, considering it's almost the same requirements and working conditions and voltages. Standard routine is that you can get them for scrap price, $100-200.
 

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I'm a fan of the bmw i3 modules (48v 60ah). I can get a salvaged battery pack from a local yard for a decent price. I'd be looking at 20kwh realistic for an 8 module pack.
The nominal voltage of those 12S modules is about 45 volts, but that's compatible with the motor and the nominally 48 volt controller. Many people seem to assume that all batteries are multiples of 12 volts, and that's only true for lead-acid.

These modules are used in series in the BMW. Using all eight of them in parallel would mean that you need to manage eight strings, all with their own battery management systems... that's still 96 BMS channels, despite being only 12S overall.

The 60 Ah BMW i3 modules are the earliest and lowest-capacity - the latest ones have twice the capacity, and appear to be the same physical size and likely even weight, although they would be more expensive. Weight may not be an issue (the battery pack will be the counterweight for this machine), but space might be an issue and four 120 Ah module or six 94 Ah modules (to get the same energy capacity) would require less mounting structure, less wiring, and less BMS.
 

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As far as I can tell from the service manual, with standard flow hydraulics its rated for 14.5gpm with a relief pressure of 2650psi. Banging around some quick engineering calculations (Hydraulic Pump Calculations - Womack Machine Supply Company) 14.5 gpm flow requires 27 mechanical hp at 2700 psi.

I'd like to stick with a 48v nominal system (I have other projects and a solar off grid system that run on 48v). This limits my choice of motor, but it seems like the curtis AC 9 would fit the ticket. It should deliver more than the required power at 48v (with 600A controller).
The peak hydraulic power calculation makes sense.

600 amps at 48 volts is a bit more than 27 horsepower, allowing for 80% efficiency; however, the AC 9 motor cannot use anything close to that current or produce that much power given 48 volts without overheating. 600 amps is just the controller's rated limit; the actual current will depend on the motor. This motor with a suitable controller can't use more than about 184 amps from the battery at 48 volts while staying at a tolerable temperature for continuous operation, and that's only at the motor's optimal point of about 6,000 RPM, where it can produce about 10 horsepower (from the 184 A * 48 V = 8.8 kW, at 85% efficiency). Down at the speed where it should run to match the engine, you might get five horsepower.

Of course you don't need the power corresponding to maximum hydraulic flow continuously, but my guess is that you need substantially more than 5 hp continuously. How fast does the engine turn that pump?

The peak power curve from HPEVS does show 27.75 hp at about 2200 RPM, but power falls off sharply with higher speed (voltage inadequate to maintain power) or lower speed (current limited), and that peak can't be sustained without overheating.

You can have more continuous power from the same motor by providing more voltage, but only at higher speed (the higher voltage makes higher speed at the same current and torque possible).
 

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Are there any glaring holes in my plan? Anything I haven't considered? I haven't seen anyone do a project like this. Does anyone have any links resources for other direct hydraulic ev builds or conversions?
I think your calcs and assumptions are off. I could be incorrect, but the online formula must not figure in pump and motor efficiencies. For hydraulic pumps and motors, the efficiencies are not very good - at best, ~80% for each. So for a typical pump/motor set-up it's 0.8 X 0.8 = .64 or 64% overall efficiency. Applying this to the results from the calculations: 100% / 64% X 27HP =~ 42.2HP . This is close to the rated HP of your ICE: 44HP.

On the electric side, the ratings for the motor and controller are peak ratings. The continuous ratings are typically 1/3 to 1/2 of the peak ratings. Depending on the usage, the continuous ratings might be the better figures to use when calculating the HP available from a given motor/controller combination.

I always thought a skid steer's design lends itself to electric motors replacing the hydraulic drive motors. That is, if there is room. The hyd motors are much more compact. The electric motor set-up could be much more efficient. You would only need a small electric motor driven pump to run the hyd system. I suppose if you had large auxiliary hyd needs, a large drive pump would be good to have.
 

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Discussion Starter · #7 ·
I'd use a second-hand forklift motor, considering it's almost the same requirements and working conditions and voltages. Standard routine is that you can get them for scrap price, $100-200.
Anything you could recommend? I would think these would be much lower rated. An electric fork truck has a much smaller pump and lower demand. For reference, something like a front end loader with 2 lifting rams and single tipping ram uses a 5.5 gpm pump. The factory pump in this is 3x that capacity.
 

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Discussion Starter · #8 ·
I think your calcs and assumptions are off. I could be incorrect, but the online formula must not figure in pump and motor efficiencies. For hydraulic pumps and motors, the efficiencies are not very good - at best, ~80% for each. So for a typical pump/motor set-up it's 0.8 X 0.8 = .64 or 64% overall efficiency. Applying this to the results from the calculations: 100% / 64% X 27HP =~ 42.2HP . This is close to the rated HP of your ICE: 44HP.

On the electric side, the ratings for the motor and controller are peak ratings. The continuous ratings are typically 1/3 to 1/2 of the peak ratings. Depending on the usage, the continuous ratings might be the better figures to use when calculating the HP available from a given motor/controller combination.

I always thought a skid steer's design lends itself to electric motors replacing the hydraulic drive motors. That is, if there is room. The hyd motors are much more compact. The electric motor set-up could be much more efficient. You would only need a small electric motor driven pump to run the hyd system. I suppose if you had large auxiliary hyd needs, a large drive pump would be good to have.
Yes, I'm trying to keep in consideration the idea of peak vs. normal rated power. I think your efficiency number look pretty close. The electric motor claims an efficiency on the order of ~90%. That compares to your calc for the ICE (probably 60-70% efficiency at best).Of course, the motor is never operating anywhere near it's peak hp output. The torque is what matters. Stock engine claims 80+ft-lbs. I'd be ok with less than maximal speed on the cylinders. They can operate scary fast at full throttle and I never operate over 3/4 throttle, 1/2 throttle most of the time to be honest.
 

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Discussion Starter · #9 ·
The nominal voltage of those 12S modules is about 45 volts, but that's compatible with the motor and the nominally 48 volt controller. Many people seem to assume that all batteries are multiples of 12 volts, and that's only true for lead-acid.

These modules are used in series in the BMW. Using all eight of them in parallel would mean that you need to manage eight strings, all with their own battery management systems... that's still 96 BMS channels, despite being only 12S overall.

The 60 Ah BMW i3 modules are the earliest and lowest-capacity - the latest ones have twice the capacity, and appear to be the same physical size and likely even weight, although they would be more expensive. Weight may not be an issue (the battery pack will be the counterweight for this machine), but space might be an issue and four 120 Ah module or six 94 Ah modules (to get the same energy capacity) would require less mounting structure, less wiring, and less BMS.
Yes, sorry, these are 12s, 44.4v nominal. Not as ideal as 13s, but should have good compatibility. These are about the best price per kw/hr I can get, and they are available locally.

I'd run the modules in parallel. Each would need BMS (if using). High quality cells like these rarely ever go out of balance when treated well. I think one could get away with cell by cell monitoring (cheap arduino circuit would do it) with a low cell voltage alarm/cut-off and manually rebalancing as needed. I'm not convinced these cheap chinese BMS modules alone should be trusted to protect expensive cells. And other BMS can add half as much cost to a battery. Another thought I have is cheap BMS for charging only, and draw from the batteries directly, bypassing BMS.
 

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Discussion Starter · #10 ·
The peak hydraulic power calculation makes sense.

600 amps at 48 volts is a bit more than 27 horsepower, allowing for 80% efficiency; however, the AC 9 motor cannot use anything close to that current or produce that much power given 48 volts without overheating. 600 amps is just the controller's rated limit; the actual current will depend on the motor. This motor with a suitable controller can't use more than about 184 amps from the battery at 48 volts while staying at a tolerable temperature for continuous operation, and that's only at the motor's optimal point of about 6,000 RPM, where it can produce about 10 horsepower (from the 184 A * 48 V = 8.8 kW, at 85% efficiency). Down at the speed where it should run to match the engine, you might get five horsepower.

Of course you don't need the power corresponding to maximum hydraulic flow continuously, but my guess is that you need substantially more than 5 hp continuously. How fast does the engine turn that pump?

The peak power curve from HPEVS does show 27.75 hp at about 2200 RPM, but power falls off sharply with higher speed (voltage inadequate to maintain power) or lower speed (current limited), and that peak can't be sustained without overheating.

You can have more continuous power from the same motor by providing more voltage, but only at higher speed (the higher voltage makes higher speed at the same current and torque possible).
I'm looking at curves on HPEVs website at 350, 450 and 650 A. I understand these are peak and not continuous duty, but I suppose so is our HP requirement of 27 hp and 14.5 gpm of flow. The 650 A curve looks like I would be in the sweet spot for both torque and hp around 2200 rpm. 450A curve shows a peak closer to 3000 rpmCoincidentally, most hydraulic pumps are rated for duty around 1800-2500 rpm. Am I misreading this graph?



You mention 184 amps as a usable limit. Where is this coming from? I guess if I'm looking at 600A max, 1/3 or 200A should be a safe assumption for continuous rated duty?

I guess this begs an obvious question, is there a more powerful motor given the 48V limitation? I know the ME1004 dc motor is good for 200A continuous. If this AC-9 motor is limited to only 200A continuous, I guess it's no upgrade...
 

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I always thought a skid steer's design lends itself to electric motors replacing the hydraulic drive motors. That is, if there is room. The hyd motors are much more compact. The electric motor set-up could be much more efficient. You would only need a small electric motor driven pump to run the hyd system. I suppose if you had large auxiliary hyd needs, a large drive pump would be good to have.
I agree, but in a machine with working hydraulics I expect that the owner/builder would take the easier (but less efficient) route of keeping the existing system, replacing the engine directly. I see the same thing in projects to convert lawn mowers and small tractors, which usually keep the inefficient and indirect hydraulic system.

If anyone has a machine like this in which the hydraulic traction motors are shot, it would be a great target for a more thoroughly electric conversion.
 

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I would think these would be much lower rated. An electric fork truck has a much smaller pump and lower demand. For reference, something like a front end loader with 2 lifting rams and single tipping ram uses a 5.5 gpm pump. The factory pump in this is 3x that capacity.
Yes, a small forklift's pump motor would be way too small for the hydraulic demands of this skid-steer, even if the wheels were not driven hydraulically. The traction (wheel drive) motor of a forklift is larger.
 

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The electric motor claims an efficiency on the order of ~90%. That compares to your calc for the ICE (probably 60-70% efficiency at best).Of course, the motor is never operating anywhere near it's peak hp output. The torque is what matters. Stock engine claims 80+ft-lbs. I'd be ok with less than maximal speed on the cylinders. They can operate scary fast at full throttle and I never operate over 3/4 throttle, 1/2 throttle most of the time to be honest.
The HPEVS AC 9 probably never hits 90% even at the ideal speed and load, but 80% or better is plausible.

That compares to your calc for the ICE (probably 60-70% efficiency at best).
What are you thinking has that efficiency? The engine will be substantially less than 50% efficient, but since we're not looking at fuel flow the engine's efficiency is irrelevant. The engine's mechanical output is converted to hydraulic power by the pump, at some efficiency unrelated to what is driving it... that's where the "probably 60-70% efficiency at best" would come in, and it will apply to the use of power from the electric motor as well.

Of course, the motor is never operating anywhere near it's peak hp output. The torque is what matters. Stock engine claims 80+ft-lbs. I'd be ok with less than maximal speed on the cylinders. They can operate scary fast at full throttle and I never operate over 3/4 throttle, 1/2 throttle most of the time to be honest.
As always, torque by itself means nothing. The combination of torque and speed (which is power) is what matters. In driving a fixed-displacement pump, torque corresponds to pressure and speed corresponds to flow rate; you need both.

Do you mean the diesel engine is never operating anywhere near its peak power output? Yes, that's possible, although the engine is probably operating near its optimal speed, since it is designed for this sort of use (or at least this version is set up for this use) and chosen for this application. From other comments about JCB equipment of this era, it appears that the engine might be the XUD series, tuned for continuous lower-speed operation than the usual automotive application. 80 lb-ft (108 Nm) would be 44 horsepower (33 kW) at about 2900 RPM... is that about what it runs?
 

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Another thought I have is cheap BMS for charging only, and draw from the batteries directly, bypassing BMS.
I hope you're not passing this sort of power through a BMS at any time. The BMS should monitor, control the charger, tell the controller to limit power if required, and balance cells; it doesn't need to have the input or output current go through it, like the cheap BMS units used in 12 volt lead-acid replacement units do.
 

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I'm looking at curves on HPEVs website at 350, 450 and 650 A. I understand these are peak and not continuous duty, but I suppose so is our HP requirement of 27 hp and 14.5 gpm of flow. The 650 A curve looks like I would be in the sweet spot for both torque and hp around 2200 rpm. 450A curve shows a peak closer to 3000 rpmCoincidentally, most hydraulic pumps are rated for duty around 1800-2500 rpm. Am I misreading this graph?


No misreading, that's right... for short-term peak use which will overheat the motor if sustained.

The peak power comes at a higher speed with a lower current limit only because the lower current means lower torque. If you look at those two graphs, and recognize that their scales are different, they are the same data from 3000 RPM up (because they have the same voltage available), with a different torque limit being the difference below that speed. At any speed, the motor's power output is determined by torque and speed; at low speed the torque is limited by the current limit.

You mention 184 amps as a usable limit. Where is this coming from? I guess if I'm looking at 600A max, 1/3 or 200A should be a safe assumption for continuous rated duty?
The 184 amps comes from the continuous power chart. That's as much as they could push through the motor in testing without the temperature continuing to climb.

I don't think a percentage rule of thumb is useful, because this is highly dependent on how well the motor cools.
 

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Yes, I'm trying to keep in consideration the idea of peak vs. normal rated power. I think your efficiency number look pretty close. The electric motor claims an efficiency on the order of ~90%. That compares to your calc for the ICE (probably 60-70% efficiency at best).Of course, the motor is never operating anywhere near it's peak hp output. The torque is what matters. Stock engine claims 80+ft-lbs. I'd be ok with less than maximal speed on the cylinders. They can operate scary fast at full throttle and I never operate over 3/4 throttle, 1/2 throttle most of the time to be honest.
Backing up what brian says, torque is just a component of HP calculations. Also, with the skid steers I have and have operated, you run them at or near full throttle(and power output). It's how they are designed to operate. If you are moving the machine with a loaded bucket on level ground or up an incline, or digging into the ground, most of the engine's HP is used to drive the wheels(or tracks, as the case may be) through a large pump, control valves, and hyd motors. How do you know you are using all of the power from the engine? Because, even at full throttle you can usually stall the engine digging into the ground or a heavy pile of material, if you are not careful.

Simultaneously, usually through a much smaller pump, the cylinders of the boom and bucket are operated on a separate hyd circuit fed from an oil reservoir common with the drive system. This system has pressure relief valves built into the controls that usually activate long before the engine stalls.

As brian says, the efficiency of the ICE has no bearing in my calculations. Part of the confusion here is that diesel engines are usually designed for continuous use at their rated HP. Electric motors and controllers usually have a continuous rating and a peak rating.
 

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Discussion Starter · #17 ·
I agree, but in a machine with working hydraulics I expect that the owner/builder would take the easier (but less efficient) route of keeping the existing system, replacing the engine directly. I see the same thing in projects to convert lawn mowers and small tractors, which usually keep the inefficient and indirect hydraulic system.

If anyone has a machine like this in which the hydraulic traction motors are shot, it would be a great target for a more thoroughly electric conversion.
Yes, my intention would be to replace only the motor and hydraulic pump. I'd select the new pump based on the ideal operating range (in rpm) of the new motor. I'd select a pump with the right displacement to give me the flow rate I want at that rpm. There is only a single pump, delivering flow to both the drive motors, lifting and tipping rams, as well as the auxiliary hydraulic circuit.

Drive motors are a nightmare. I wouldn't dare try to swap them for direct electric motors. These use a pair of motors with an oil bath chain mechanism between the wheels. Not trying to re-engineer that :)

As always, torque by itself means nothing. The combination of torque and speed (which is power) is what matters. In driving a fixed-displacement pump, torque corresponds to pressure and speed corresponds to flow rate; you need both.

Do you mean the diesel engine is never operating anywhere near its peak power output? Yes, that's possible, although the engine is probably operating near its optimal speed, since it is designed for this sort of use (or at least this version is set up for this use) and chosen for this application. From other comments about JCB equipment of this era, it appears that the engine might be the XUD series, tuned for continuous lower-speed operation than the usual automotive application. 80 lb-ft (108 Nm) would be 44 horsepower (33 kW) at about 2900 RPM... is that about what it runs?
I think that's a good way to think of it. Torque = pressure, power = flow - and both are important!

Yeah, I think that lines up with where I find the sweet spot for running. I'm guessing where I am just above half throttle is probably ~3000 rpm. Higher throttle makes more noise, but doesn't really deliver any more power. Coincidently, even in a modern mini ex (Kubota KX040 is what I have the most time in), I've never operated at full throttle. I only notice more speed over the ground when driving around, but no more digging power.
 

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Discussion Starter · #18 ·
Backing up what brian says, torque is just a component of HP calculations. Also, with the skid steers I have and have operated, you run them at or near full throttle(and power output). It's how they are designed to operate. If you are moving the machine with a loaded bucket on level ground or up an incline, or digging into the ground, most of the engine's HP is used to drive the wheels(or tracks, as the case may be) through a large pump, control valves, and hyd motors. How do you know you are using all of the power from the engine? Because, even at full throttle you can usually stall the engine digging into the ground or a heavy pile of material, if you are not careful.

Simultaneously, usually through a much smaller pump, the cylinders of the boom and bucket are operated on a separate hyd circuit fed from an oil reservoir common with the drive system. This system has pressure relief valves built into the controls that usually activate long before the engine stalls.

As brian says, the efficiency of the ICE has no bearing in my calculations. Part of the confusion here is that diesel engines are usually designed for continuous use at their rated HP. Electric motors and controllers usually have a continuous rating and a peak rating.
I figured I could get away with some reduction in total pump flow to the boom and bucket circuits without issue. But the drive motors, especially climbing hills loaded, would really need that full flow from the pump. I even tried to do some calculations around duty cycle, but when I take the drive motors into consideration, that number moves up quite a bit.
 

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Discussion Starter · #19 ·
Are we coming to the conclusion that this particular motor (AC 9) is underpowered for my application? I have no experience with these motors but I had higher hopes. It seems a bit dishonest by the seller and manufacturer. Why sell a motor with a 600 A controller if it will set itself on fire with less than 1/3 that amount of current through it? The DC motors all seem capable of running at their rated current without issue (again, the ME1004 is rated at 200 amps, and will run 200 amps all day).

Can we squeeze more current with active cooling???
 
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