DC TO DC efficiency

[thingstodo]

I would like to 'recycle' some forklift batteries in an ev conversion. Is it practical to use the dc boost stage (after voiding the warranty and taking off the cover) of a modified sine wave inverter to get 150 VDC from each of the 6 batteries?

The batteries each weigh about 150 lbs, so 6 fits in my GVRW. Then I need to series these 6 voltages to get the 900 VDC that my industrial VFD wants as input.

I searched the archives, but likely missed the answer there.

[IamIan]

Originally Posted by thingstodo

Is it practical to use the dc boost stage (after voiding the warranty and taking off the cover) of a modified sine wave inverter to get 150 VDC from each of the 6 batteries?

Most such inverters are not designed to operate in a series synchronized system like that ... care is needed else the magic smoke escapes.

How practical it may or may not be will depend greatly on the design of the DC-DC stage ( may not work like that at all ) , the person doing it, and how such this option compares with other alternatives for time and $.

[Ryland]

What is a VFD?
I would guess that a DC to DC converter like you are talking about is going to be about 85% efficient, just judging from other DC to DC converters that I've seen, but finding one that will deal with 6v is going to be hard and finding one that can deal with the high amps that you are talking about (500 amps or more) is going to be the really hard part.

[thingstodo]

Originally Posted by Ryland

What is a VFD?
I would guess that a DC to DC converter like you are talking about is going to be about 85% efficient, just judging from other DC to DC converters that I've seen, but finding one that will deal with 6v is going to be hard and finding one that can deal with the high amps that you are talking about (500 amps or more) is going to be the really hard part.

I would be happy with 85%. Re-using fork truck batteries is inexpensive and carrying a couple more for decreased efficiency seems reasonable. I need 35 HP out, so I'll need about 41 HP in.

A VFD, or Variable Frequency Drive, also referred to as an Adjustable Speed Drive (ASD) is an industrial motor controller. It takes three phase AC in, converts to DC - usually with 6 diodes in a bridge, has some DC filtering capacitors and inductors, then uses pulse width modulation to 'build' AC at the frequency that you want your motor to run and sends it out. This would replace the 'Curtis' controller, or the ReVolt Cougar.

The off-the-shelf DC to AC inverter I've tried takes 12V in and puts out 150- 155VDC. With 85% efficiency, that would be 12V in at 170A continuous, 340A peak. Output would be just over 11A continuous at 150VDC.

I should have said 'two fork truck batteries in series' for each inverter.

The DC output of the inverters appears to be isolated from the input terminals, and more importantly, isolated from the heat sinks. I'm testing so far - planning to put multiple inverters in series to add voltage, perhaps multiple strings of inverters to get the required current.

I need about 30A at 900VDC. That should give me 35 HP, about what I expect that 'SalvageS10' will need to make highway speed.

I've read about all of the AC conversions that I can find. No one seems to be using this method. I expect there are one or more reasons for that. I'm trying to figure out what the issues are and work around them.

[thingstodo]

Originally Posted by IamIan

Most such inverters are not designed to operate in a series synchronized system like that ... care is needed else the magic smoke escapes.

How practical it may or may not be will depend greatly on the design of the DC-DC stage ( may not work like that at all ) , the person doing it, and how such this option compares with other alternatives for time and $.

I would be surprised if the original designers had envisioned this as a possible end use. I'm trying to be careful.

There are two 'boards', the boost converter that takes 12V and makes 150V, and the output inverter that takes 150VDC and makes 120V AC modified sine wave.

I'm breaking into the DC between the stages, so I'm trying to be paranoid about isolation. The DC has some ripple. The DC 'bus' capacitors take up a lot of room - I have no schematic so it's difficult to determine if it's safe to series these inverters.

The input terminals appear to be isolated from the DC bus - and +, the case/heat sink, and the output AC. Similarly, the DC bus - and + appear isolated from the case/heat sink. The DC bus is not isolated from the output AC.

[dcb]

The efficiency is also a function of how much you need to change the voltage iirc, like going from 12 to 15 volts isn't bad but 12 to 900 volts you can expect an efficiency hit. Not sure though

[IamIan]

Originally Posted by thingstodo

The off-the-shelf DC to AC inverter I've tried takes 12V in and puts out 150- 155VDC. With 85% efficiency, that would be 12V in at 170A continuous, 340A peak. Output would be just over 11A continuous at 150VDC.

I need about 30A at 900VDC. That should give me 35 HP, about what I expect that 'SalvageS10' will need to make highway speed.

I've read about all of the AC conversions that I can find. No one seems to be using this method. I expect there are one or more reasons for that. I'm trying to figure out what the issues are and work around them.

I think that is the reason people don't use this method.

If you manage to get ~85% efficiency ... getting 30A @ 900V from 12V looks like about ~2,650 Amps from the battery side

That is some serious current ... need huge connectors and cabling... be careful with those huge cables or you will also be generating some serious size magnetic fields from that electromagnet.

For continuous operation of ~30A @ 900V ... from that type of inverter you're looking at about ~18 of those inverters.... 6 in series to Voltage 3 in parralel for current.

That is a lot of places to something to go wrong ... and a good size investment in time , money , space , and weight , in inverters ... plus the question of how the battery will take ~2,650 Amp discharge rates... so before you go out and buy ~20 inverters I would look into comparing to other options ... such as more batteries to make up the voltage instead of inverters ... or a different motor controller / motor that doesn't need 900V... the other options might not work for you ... but I would still recommend looking into them before seriously investing in this path.

[thingstodo]

Originally Posted by IamIan

I think that is the reason people don't use this method.

If you manage to get ~85% efficiency ... getting 30A @ 900V from 12V looks like about ~2,650 Amps from the battery side

... snip ...

I see a silly math error in my example, where 6 batteries would service 6 inverters. And I have not been clear that there is a separate pair of fork truck batteries feeding each DC to AC inverter. My apologies.

I agree that 2650 amps would be far more difficult to deal with than purchasing different equipment.

Let's try this example again with better numbers. 6 pairs of fork truck batteries. Each pair gives 12V at 500A to one AC to DC converter (DC Converter) which is sized better. Each 5000W (instead of 1750W) DC Converter puts out 30A at 150VDC, assuming 85% efficiency. String the 6 DC Converter outputs together in series to get 30A at 900 VDC.

This discussion of theory ignores the fact that I'm over the GVRW for the truck, and assumes that I successfully cool the heat sinks well enough that nothing melts.

The two biggest costs for an EV, from what I've read, are the battery pack and the controller. I have access to cheap big heavy batteries that are low voltage, and a cheap controller which is very high voltage. I've come up with this goofy idea to make the two work together. And I want to do it safely.

I'm nervous because I have not seen this method used thus far. I'm not arrogant enough to think that I'm the first to think of this. It's been thought of before and rejected for some very good reasons that I'm looking for. If I can find a solution to those good reasons - I'll try it.

One reason is inefficiency. Losing 15% of your battery power is a huge penalty. Not only is it lost, but I must find a way to get rid of the heat from all of those heat sinks - fans, liquid cooling, whatever. If I can carry 15% extra (cheap) battery to balance that off, and find a reasonable way to get rid of the heat, that one seems surmountable.

[thingstodo]

Originally Posted by IamIan

... I would look into comparing to other options ... such as more batteries to make up the voltage instead of inverters ... or a different motor controller / motor that doesn't need 900V... the other options might not work for you ... but I would still recommend looking into them before seriously investing in this path.

That is sound advice for any project.

Perhaps the effort of using the cheap parts won't work out, but a mental exercise is always worth it!

[IamIan]

Originally Posted by thingstodo

Let's try this example again with better numbers. 6 pairs of fork truck batteries. Each pair gives 12V at 500A to one AC to DC converter (DC Converter) which is sized better. Each 5000W (instead of 1750W) DC Converter puts out 30A at 150VDC, assuming 85% efficiency. String the 6 DC Converter outputs together in series to get 30A at 900 VDC.

6 inverters each rated 5,000 watts each , don't usually come very cheap... unless you have a very good source for them.

Also remember those peak rating numbers on inverters are only for surges ( a few seconds ) ... if you run the inverters for any significant amount of time at more than the continuous rating , you have a good chance of burning them out to an early death.

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

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.

And as you already noted ... even if you get ~85% efficiency ... those inverters alone will be pumping out over ~5kw of heat ... plus the additional heat from the batteries themselves ... plus the additional heat from the 900V VFD ... plus the additional heat from motor.

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