DC TO DC efficiency

[thingstodo]

Originally Posted by IamIan

What kind of Lead Acid batteries are you considering? Do you have any model numbers or specs on them?

I'm not very picky. I expect that it will be a mix of whatever is traded in and has not been sent to the recycler. Some of them will have enough life left to help me. Some will not and I'll get them traded back. Sorry - no part numbers.

Originally Posted by IamIan

Lead is heavy to start with ... and Lead usually doesn't like high Amp current rates very much either ... plus the Peukert effects of the harder you push them the less you get out of them ... if you are pushing them too hard they might not have a very long service life under the stress either.

I agree. I don't know much about fork trucks or their batteries. I heard from the service guy I was talking to that they get traded in when they won't last a shift, or half a shift. He said that there is still some life in them when they are traded in for the core charge.

Originally Posted by IamIan

What kind of motor is it? Do you have any model numbers or specs? ...

It is a GE industrial motor, 575V, three phase, medium efficiency, rated 1766 rpm at 36.8 amps. Class B insulation (affects maximum temperature), service factor 1.15. Totally enclosed, fan cooled. This thing weighs a lot - maybe 500 lbs? I'm not really worried about this motor overheating. It will take a lot more abuse than I can dish out.

Originally Posted by IamIan

If the only reason to need this step up to 900V is to use the VFD as a motor controller ... than we can look at other motor controller prices ... which will put a cap on what the VFD + Inverters would have to be under to be a cost effective alternative.

The VFD is quite a bit over-sized. It is an Allen Bradley 1336 Plus II (circa 1998) rated at 302A, 150% for 30 seconds.

[Ryland]

What I would do is find someone who installs solar electric systems and offer to trade them a few forklift batteries for a few golf cart batteries, pound for pound you are going to be better off because for the same weight you can have a big pile of golf cart batteries giving you a higher end voltage... altho 900 volts is still way up there.
Either way, I think it would be worth trading up to equipment that will better suit a car,

[IamIan]

Originally Posted by thingstodo

I agree. I don't know much about fork trucks or their batteries. I heard from the service guy I was talking to that they get traded in when they won't last a shift, or half a shift. He said that there is still some life in them when they are traded in for the core charge.

Brand New Lead Acid batteries are usually less than about ~40wh/kg ... these older ones they are replacing will most likely be significantly less than that ... but you can test them easily enough when you get them.

Given what you know already about the batteries ... 12V and weighs about ~150 pounds ... that means it is safe to assume when they were new they had something around ~230 Ah... until you test them to better quantify where they stand now it is reasonable to expect them to be somewhere less than ~80% of the original capacity ... which means , until you can quantify differently I would expect these batteries to offer less than ~180 Ah @ 12V ... when discharged at a ~20 hour rate.

The rate is important because of Peukert effects on the amount of usable battery capacity at a given discharge rate.... Depending on the type of Lead Acid it can range from about 1.05 to 1.6 ... The high Amp rate needed for the battery side for this DC-DC step up concept forces a large Peukert hit to the useful battery capacity ... a far larger Peukert hit than one would see from a higher voltage lower current battery system for the same power output ... if we assume a conservative Peukert k value of ~1.2 ... @ ~500 Amps we might expect to see something around ~80Ah usable ... 12*80=960*6= ~5.7kwh expected ... of course you can improve the accuracy of that estimate with some better quantified specs from testing these batteries to get on real numbers and not just estimates.

You should not be at full power 100% of the time ... so if your vehicle can manage about ~4 miles per kwh average vehicle efficiency ... you might expect to see around ~23 miles of range ... +/- YMMV.... if you do 100% DoD ... which is a bad idea for Lead Acid battery service life... and you will also loose additional usable range when the batteries get cold... Maybe somewhere around ~12 Miles per charge would be reasonable to expect in the winter time.

Originally Posted by thingstodo

It is a GE industrial motor, 575V, three phase, medium efficiency, rated 1766 rpm at 36.8 amps. Class B insulation (affects maximum temperature), service factor 1.15. Totally enclosed, fan cooled. This thing weighs a lot - maybe 500 lbs? I'm not really worried about this motor overheating. It will take a lot more abuse than I can dish out.

Extra Large Motor + Extra Large Controller + additional inverters = a lot vehicle space and weight is getting used by non-battery / non-energy storing devices.

But it should be functional ... I still have some doubts about how practical it will be.

Originally Posted by thingstodo

The VFD is quite a bit over-sized. It is an Allen Bradley 1336 Plus II (circa 1998) rated at 302A, 150% for 30 seconds.

Well that has a lot of documentation for it.
Link

[jakobnev]

It "should" work as long as each converter is working, but what happens if one should shut down(for what ever reason) and suddenly sees -750v (yes negative) inside it? (in a place it shouldn't be possible no less since you hacked it)


(edit)
After posting i thought of two things:

1. if the outputs are galvanically insulated, you can make one 2s6p pack for all converters, instead of giving each converter it's own 2s pack, this eliminates the risk of one converter running out of input-juice.

2. You can give each converter a bypass diode to protect it if it shuts down.

[thingstodo]

Originally Posted by jakobnev

It "should" work as long as each converter is working, but what happens if one should shut down(for what ever reason) and suddenly sees -750v (yes negative) inside it? (in a place it shouldn't be possible no less since you hacked it)


(edit)
After posting i thought of two things:

1. if the outputs are galvanically insulated, you can make one 2s6p pack for all converters, instead of giving each converter it's own 2s pack, this eliminates the risk of one converter running out of input-juice.

2. You can give each converter a bypass diode to protect it if it shuts down.

Adding one diode between inverter stages and one as a bypass for each inverter sounds like a good fix.

What do you put between the old and unbalanced batteries to allow them to be run in parallel? Diodes?

With one 2s pack each, the supplies are isolated from each other. The loss of one reduces the output voltage by 150V but can be survived. Hopefuly I can figure out a way to 'enable' each inverter separately and can shut them down when the battery voltage drops below 11.0 V. Schottky diodes that can deal with 100 amps or so will drop about 3V. 5 diodes, 35A continuous, another 175W of precious battery power dissipated as heat ...

[thingstodo]

Originally Posted by IamIan

... ~5.7kwh expected ...
... ~23 miles of range ...

Thank you for the ball park figures.

My numbers for the drive to work are 30 HP (for 40 minutes), 90 km/h average - maybe 15 kw-h at 100% efficiency. Assume some efficiencies: battery to VFD = 85%; VFD to motor = 90%; I need just under 20 kw-h. Your numbers give me about 5.7 kwh, less than 25% of what I need. Maybe with 3 times the battery, the battery draw would be lower and the kwh would rise a bit, maybe not.

I guess it's a start.

[thingstodo]

Originally Posted by Ryland

What I would do is find someone who installs solar electric systems and offer to trade them a few forklift batteries for a few golf cart batteries, pound for pound you are going to be better off because for the same weight you can have a big pile of golf cart batteries giving you a higher end voltage... altho 900 volts is still way up there.
Either way, I think it would be worth trading up to equipment that will better suit a car,

I have a 1250 lb 'budget' for batteries. At 35 lbs each (lowest I can find for info online) plus battery frame, that would get me 25 - 30 batteries. The kwh add up pretty close, though.

[Ryland]

So if you traded your current free parts for golf cart batteries and a fork lift motor you could come in at about the same weight and have a 144v system that you know is going to work and work well while giving you full highway speeds.
I do like the idea of doing something different like this, but at the same time it's going to be complex, inefficient and heavier then using more common parts.

[thingstodo]

Originally Posted by Ryland

So if you traded your current free parts for golf cart batteries and a fork lift motor you could come in at about the same weight and have a 144v system that you know is going to work and work well while giving you full highway speeds.
I do like the idea of doing something different like this, but at the same time it's going to be complex, inefficient and heavier then using more common parts.

I don't know that I could trade the older fork truck batteries for others - I'd have to check that out.

If I can make these parts work out, I should have a good source of batteries for a few years at least.

I'd like to have something reliable (aka no experimenting) but I'd like to experiment ...

[IamIan]

Originally Posted by thingstodo

Maybe with 3 times the battery, the battery draw would be lower and the kwh would rise a bit, maybe not.

Peukert effect are exponential ... for example:
For instance the same batteries with a Peukert k value of 1.2 and 180Ah at a 20 hour discharge rate will give.

~80Ah when discharged at ~500Amps.
or
~111Ah @ ~100Amps
or
~176Ah @ ~10Amps

etc.... etc.

The thing to keep in mind about Peukert Effects is that the Wh of energy are not really all lost ... it is just that the chemical reaction of the battery can not keep pace with the faster rate of the electronics ... for example if you discharged these same batteries at ~500Amps ... you might expect to estimate running out of ~500 Amp load ability in about ~80 Ah ... but if you were to then let the batteries rest for an hour or so you will be able to pull more from them again ... as the chemical reactions inside the battery that produce the electrical potential take time to catch up with the faster electronics ... Some old time BEV people call it 'growing Amps'.

Not all batteries have the same Peukert effects ... some handle high discharge rates much better than others ... A123 is among the best ... Lead Acid is among the worst... but even in Lead acid there is a significant variation from battery to battery.

Originally Posted by thingstodo

I guess it's a start.

Sure it's a start ... and with that many inverters you could power a whole house from your battery pack in case of power outages... ~30kw of Inverter AC is well over 200 amps of house 120 level AC ... easily an entire houses 100% full electric load running just about everything in the house at once.

Originally Posted by thingstodo

I have a 1250 lb 'budget' for batteries.

Using that weight as your battery budget we can work backwards ...

Unless you can test and confirm better than the gross estimates.
~12V ~180Ah per ~150 Pound Battery = ~8 batteries max = ~17kwh
~17kwh from Batteries into 85% efficent Inverters = ~14.4kwh output
~14.4kwh from inverter to ~90% efficient VFD = ~12.9 kwh output
~12.9 kwh from VFD to ~90% efficient Motor = ~11.6kwh to wheels....

Unfortunately the batteries are not ideal batteries ... and you will have Peukert effects to deal with making this less than ... and you will have cold temperature effects to also deal with in the winter also making it less ... and it is not a good ideal to run 100% Do, also making it less.

Sorry ... I don't see this setup of batteries and components getting you ~15kwh to the wheels in ~40 minutes or less.

Maybe look at ways to reduce that ~15kwh ...

Or get some tested numbers on batteries that have more than ~180Ah @12V @ ~150 Pounds @the Discharge Rate you are aiming for ( not at some ~20 Hour Rate )