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Discussion Starter #1 (Edited)
I'm familiar with the idea that, with the IMA disabled/bypassed, the DCDC can't charge the 12V battery at idle engine speed because the engine is turning too slowly to produce a high enough voltage. But is that really the case? I know that it doesn't charge at idle speed - that's a fact. But the engine can charge the IMA battery at around 1100 RPM - I was watching that earlier, 6.5 amp charge, about 155V...

So, what allows IMA charging at low engine speed, but doesn't allow the DCDC to charge the 12V battery at low engine speed?

I'm guessing that the electronics connected to the IMA boost the voltage from the engine and allow charging, but when you bypass/go gas-only, you also bypass those electronics, and the DCDC doesn't have its own means to 'boost' the voltage (or invert, or convert, or whatever), though that seems a bit strange to me since we only need around 14V DC output to charge the 12V...

Can anyone better explain what's going on? Ideas?

Oh, also, the corrolary to this is, of course, if there is something that boosts the voltage at low engine speed that gets bypassed going gas-only, wouldn't there be some kind of simple electronic mechanism to add after the bypass that would boost low RPM voltage enough to get DCDC charging at low RPM?
 

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Far as I know the 12v runs off the big nimh battery at idle.

The funny part is a very discharged battery can be force charged at idle when demanded.

Makes you wonder if this is an induced issue that isn’t voltage related
 

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When switching over to a Meanwell PSU, charge is available at idle - a very small amount of charge, maybe 10a @ 14v - but still charge nevertheless. Obviously in this case the stock DC-DC is gone completely. My guess is that there is definitely something in the DC-DC that prevents idle charge.

But, the Meanwell cannot pull it's full 650W at idle - only about 150 or so watts from what Peter demonstrated in his video test of the unit IIRC, yet it is as you say, that a 6.5a @ 155V charge is available for the DC-DC at idle. At idle, the IMA motor on its own is only putting out 50-90V depending on RPM; how does it bring that up to 155V before converting to 12V/14V? Unless the voltage of the IMA motor is load based? I've always thought the voltage of the IMA motor was RPM based.

Perhaps if there was that kind of load on the engine at idle, since the car is producing so little power cruising around idle, that if the car was pulling a lot of power to charge the battery, that the car would easily get bogged down. Not charging the battery at idle makes it easy to drive at 800 RPM under light load and get >100mpg in 4th @ 19 or 20 mph. If the car was under the additional load to charge the battery at that RPM, the car would just lug or stall.

I don't pretend to be an expert on these things; it's more or less just a few random thoughts I had that may or may not help at all.
 

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Discussion Starter #4 (Edited)
Was just spending some minutes reviewing this idea and looking at a couple things to help me understand. I think my gross, macro conception is accurate enough - you bypass all the stuff that enables the DCDC to work at low RPM...

In the 'total IMA bypass' config, where you remove basically everything, you're taking a wildly ranging raw voltage/3-phase power from the motor, rectifying it with your bought unit, then passing along single phase to your choice of alternative DCDC converter, like the Mean Well units. I noticed that the input voltage for one of the suggested units is 120V-370V DC.

So the low value here - 120V - is still pretty high. It must be that conversions from high DC to low DC, or perhaps to have a decent range in both input and output, entail... tradeoffs, difficulties, limits? that make it hard to retain a low-ish input voltage yet decent power output. I guess the 'IMA electronics' can cope with a low-ish input voltage, enough so to charge the IMA pack at idle engine speed (about 1kW output). Whereas the DCDC electronics can't?... It'd be nice to have a little deeper understanding than this, so if anyone knows, please feel free to chime-in...

As far as I can tell, in the 'soft' IMA bypass, where you leave the left side stuff, including the DCDC, the 3-phase power from the motor goes to the Motor Power Inverter, and then to the Voltage Converter, and then on to the DCDC. So, 'this stuff' must still be active and creating the right power for the DCDC to operate. Maybe these items - the MPI and the VC - play a role in the DCDC not being able to charge at idle speeds, though I would guess that the DCDC just requires an input voltage that's too high for the low voltage output at idle...

Here's a link to the 'adding an alternator' thread, where Peter P. first presented the idea of the 'big' IMA bypass, using Mean Wells, etc., where there's quite a bit of info that can help brain-wrap this thread's topic: https://www.insightcentral.net/threads/adding-an-alternator.107497/page-2#post-1231585
 

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Yes it's difficult for electronics to operate over such a wide voltage range..

OK So we have three scenarios. This microchip pdf shows what is happening.

BLDC Regen

1) Hard bypass where we remove everything and use our own 3 phase rectifier feeding a meanwell. (page 4 in pdf)

2) Soft bypass were you leave everything on the left and again use a meanwell or the stock DC-DC. (page 6 in pdf)

In this scenario we are basically the same as 1, the body diodes of the inactive IGBT module are doing the rectification like our simple three phase rectifier. So our voltage output is very similar.

3) However with the ima system intact including an active MCM, the system is able to control the active IGBT switches to boost the regeneration output voltage at low levels. It changes the times the IGBT turns on and off to provide a voltage boost from low rpm. This doesn't happen once we remove the other IMA stuff from the system.
(page 13 in pdf.)

This is how the system is able to charge the IMA battery at low RPM, it actively manipulates the IGBT switch timing to increase output voltage and operates as a boost converter..

Google bldc regen etc for loads of papers on the subject.

You could test a few scenarios with minimal 12v load or dc-dc HV input unplugged.

1) Check the ima voltage output at tickover with the MCM connected battery on.
2) as above but battery off.
3) Check the ima voltage output at tickover with the MCM disconnected battery on.
4) as above but battery off.
 

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Discussion Starter #6
^ Thanks, this is very helpful...

So it sounds like the answer to my 'corollary' (copied below) would be something like this: the fully implemented/driven IGBTs boost voltage from the motor at low RPM and allow IMA charging. This is a pretty complex and robust system -- so any alternative would likely be expensive and/or hard to implement...

You can buy cheap low power boost converters, but I'm guessing the kind of 'boost converter' that might enable a higher voltage at low engine speed, and perhaps a better regulated input voltage to the DCDC (or Mean Well replacement), would be hard to find, expensive, etc...

Oh, also, the corrolary to this is, of course, if there is something that boosts the voltage at low engine speed that gets bypassed going gas-only, wouldn't there be some kind of simple electronic mechanism to add after the bypass that would boost low RPM voltage enough to get DCDC charging at low RPM?
 

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Switching the IGBT in the correct way, at the correct time, at the correct speed, to enable regen boost isn't simple.

However Honda did at least make it so that everything on the left is nicely isolated and can be driven with simple 5v hi/lo logic signals.. It's also probably protected to some degree from driving the IGBT incorrectly.

So maybe we have a couple of other options.

1) Do the tests I indicated, and if retaining the MCM means the boost still works, then just keep the MCM.
Swap out the OEM DC-DC for a meanwell and you are good to go.

2) However it may be possible with a pic or arduino to do the IGBT control but that is not a trivial or simple project.

E-bay will turn up cheap and simple sensorless 3 phase BLDC drivers like this..


Run from 5V that might be interesting, but of course you also run the risk of damaging the left hand side stuff in the car. Some scary testing would be involved. But used/redundant MDM/IGBT modules are cheap and plentiful in the US, so you can afford to blow one up.

In a tickover test scenario with the cheapo bldc driver attached to your MDM your IMA motor will be turning and you will activate the driver which will start switching the IGBT, depending on rpm/rotor position and zillions of other factors something might happen. It might be good or bad or variable.. Testing required.

A sensored driver would be best as we already have three nice commutation sensors fitted to our motor.
This would avoid several bad switching scenarios and make control simpler.

Good luck.

This one has inputs for hall sensors..
However some tweaking would be needed to get the output to 5v logic level needed by MDM.


The above is based on this chip..
 

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Discussion Starter #8
....So maybe we have a couple of other options.... 1) Do the tests I indicated, and if retaining the MCM means the boost still works, then just keep the MCM. Swap out the OEM DC-DC for a meanwell and you are good to go.... Good luck....
Hey, I didn't say I wanted to actually do this... I don't even have a bypassed IMA. It might be something interesting to look into down the road, though...

If I do the voltage tests (I can do that), do I measure voltage at the big cables between the MDM and pack, for all scenarios?
 

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Discussion Starter #10
....You could test a few scenarios with minimal 12v load or dc-dc HV input unplugged. 1) Check the ima voltage output at tickover with the MCM connected battery on. 2) as above but battery off. 3) Check the ima voltage output at tickover with the MCM disconnected battery on. 4) as above but battery off.
fyi, I measured 155.4 volts under scenario 1, and about 52.5 volts under all the others.
 
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