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Discussion Starter #1 (Edited)
I've had several private emails about my DIY grid charger/discharger and decided to start a thread here so everyone can get the same info. I'm also curious how many people have built one of them from my article/schematic.

Here's the first question/answer to a recent post on the forum:

Coincidentally it's your charger that I'm planning on making once the parts finish slow-boating their way from the various ends of the earth.

I thought it might be mine when you said it had a discharge connector. :)

Any changes or updates to what you have on your site?
No changes after adding the discharge connector and wires.
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[edit 4/19/2019] Please read this, it is very important:

You MUST use current limited power supplies. Not constant current supplies for a grid charger. The difference is subtle but most constant current supplies wired in series will not track properly. Unfortunately eBay Chinese sellers of LED power supplies have almost all switched over to constant current supplies and the current limited supplies are very hard to find. If I find a current seller of a 45-90 volt supply I will post it here.

I have tested several different constant current pairs and typically the first supply that reaches it's maximum current (considering the load) will prevent the other supply from delivering 1/2 of the voltage to charge the battery due to a race problem when the supplies start up. This will cause one supply to operate above it's wattage rating. I will describe this further near the end of this section.

The following are the results of two typical Chinese 60-90 volt, constant current 300 ma supplies that I recently tested.
Test 1: Test both supplies individually for consistency.
Supply #1: (Tested by itself)
No load voltage: 126 volts
Loaded with two 40 watt light bulbs in series: 89.9 volts @280 ma (after 10 minute warm up)

Supply #2: (Tested by itself)
No load voltage: 131 volts
Loaded with two 40 watt light bulbs in series: 88.5 volts @275 ma (after 10 minute warm up)

Test 2: With both supply's outputs wired in series
No load voltage: 261 volts (NOT 180 volts as you would expect from two 60-90 volt supplies in series.)
This is actually normal for a constant current supplies. The no load output voltage has to be higher than the expected loaded voltage to deliver the maximum specified constant current to the load.

A limited current supply has a maximum voltage it can deliver (for instance 60-90 volts) and will if necessary, also limit the current so as to not exceed it's rated maximum output current. That is the subtle difference between the two types of supplies. The lower rating (60 in this case) is the lower limit for normal operation.

Test #3: Check individual load voltage of the two series connected supplies.
Load with two 40 watt light bulbs connected in series to simulate charging current: 120 volts @ 239 (that sounds reasonable)
Output voltage of each series connected supply: (Wait till you see the next test readings @ 239 ma!!)
Supply #1: 4 volts (Yes, FOUR volts!)
Supply #2: 116 volts

Obviously these two typical non-precision constant current supplies are just not going to work for our grid charger since each supply is rated as a 25 watt supply. One supply is overloaded and doing most of the work and the other one basically took the day off.

Here's what caused one supply to only output 4 volts. When the AC power switch is set to ON there is a voltage/current race problem between the two supplies wired in series until one of them reaches it's constant current condition.

Since the supplies are wired in series, power supply #2 with the lower constant current capability with that load will have also reached it's maximum capable voltage at that maximum current. Supply #1 then fills in the remaining voltage (only 4 volts in this test) because of the current supply #2 is already forcing through both supplies. The higher current capable supply #1 can't force more current through the series connected supplies to balance the voltages because supply #2 won't allow any more current to flow.

Current limited supplies don't have the race problem because they each put out the their maximum voltage listed (90 volts) and if necessary limit the current to their maximum specified current. Yes the lower max current supply rules but only after it reaches it's specified voltage. Since the lower current sets the current the other supply is able to also output 90 volts for a total of 180V.
[/edit 4/19/2019]
_____________________________________________________________________________________________________________

Here's some tips of what I did when building and using the grid charger that I didn't include in my website article.

1. I'd like to emphasize that if possible use a top mounted 120mm fan to make it much easier to mount the multi meter on the main part of the ATX case. And it gives you a little more leeway on mounting the LED driver supplies since the fan is not in the way.

2. Another thing that might help some builders: I used a normal 12v/5v Molex female power connector on the end of the charging harness. For the mating male connector on the grid charger I hacksawed off the male connector and a little of the metal work around it from a junk hard drive. I was then able to mount the male connector on the front of my grid charger and just plug the end of the charging harness into that connector.

3. I also place a normal flexible Molex power connector cover on the charging harness female connector when not charging the car. I find those black flexible covers on junk DELL desktop computers.

4. I haven't found any of the 3 digit multi-meters that show the voltage and current at the same time that display 1/10s of a volt above 100 volts. i.e. 165.3 will appear as 165 volts. I find it very useful to be able to see the tenths of a volt. Especially so near the end of the charge when you want to see the trend of voltage change when the change may only be 0.2 volt per hour!

5. I just checked and there are quite a few LED drivers that look the same as the ones I used but they have output voltage ranges that are either much too high or too low. So builders will have to be careful and only order the drivers that output 45 to 90 volts DC. I only found one vendor at the present time that has the voltage range needed and have updated the BOM to that eBay listing.
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[edit 6/7/2017]
Some builders using LED drivers have mentioned they hear a squealing noise from the charger after a discharge. It has been determined that some LED drivers do squeal when they are forced to output a voltage below the total minimum voltage rating of the series connected drivers.

If you let your battery sit after a discharge until the voltage has self risen to over 90 volts you won't get the squeal. I found that the batteries quickly rise to >100 volts on their own when the discharge load is removed.[/edit]

I would appreciate if you have ordered the parts, are building or have finished building one of these grid chargers that you go to my Google spreadsheet to give me an idea where you're at in the build. The grid charger/discharger has been the most read of everything on my website so I am just interested in how many people have built a charger from it.

Thank you.

Link to google spreadsheet.
 

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Discussion Starter #3
I looked through your site the other day, and loved it. Lots of great information there!
Thank you. Glad you like what I have presented.
 

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Great thread. :)

So quick question...

Originally I'd planned on just repurposing a wall wart I've got lying around the house, or buying one for $.50 at Goodwill in place of buying the fan PS listed in your BOM. But I forgot about it and the last of the parts showed up today, and since I've only got 1 car and 2 batts, I'm thinking I'm just going to use it as a bench charger and just setup my air mover to keep the pack cool while it's out of the car.

As such can I just simply cut out the fan and cell phone charger like attached, or will that mess something up?

Thanks!
 

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Discussion Starter #5
Great thread. :)

So quick question...

Originally I'd planned on just repurposing a wall wart I've got lying around the house, or buying one for $.50 at Goodwill in place of buying the fan PS listed in your BOM. But I forgot about it and the last of the parts showed up today, and since I've only got 1 car and 2 batts, I'm thinking I'm just going to use it as a bench charger and just setup my air mover to keep the pack cool while it's out of the car.

You ought to also use the grid charger to keep your battery in the car balanced. So I would recommend building it with a 12 volt DC supply to run the cooling fans (the charger and the car fan).

As such can I just simply cut out the fan and cell phone charger like attached, or will that mess something up?

Thanks!
I'm probably repeating what you already know but it's best to simplify the answers for other readers who might come along later.

What you have on the schematic will charge the battery and you will be able to read the voltage etc. But if the charger is built in a closed box it's components won't get cooled nor will the IMA battery because you don't have a 12 volt supply to run the fans.

It also needs to be isolated from the AC mains because if it isn't the IMA battery would very likely have the negative lead connected to the power mains or the ground terminal of a three prong AC plug. NOT good and potentially deadly.

You need to use switching type wall warts because they are all isolated supplies. You can tell a switching wall wart because it will be quite light for the current capability.

The power supply for the IMA fan needs to have a 3/4 to 1 amp rating at 12 volts. The supply needs the extra current capability for starting the fan. And you should NOT turn the ignition key on with a power supply rated at less than 1 amp because if the car's normal fan relay kicks in the 12 volts from your charger will be connected into the cars 12 volt system that might overload the charger 12 volt supply.

So I would just find a wall wart 6 to 15 DC voltage -isolated- supply to run the meter and use another 12-14 volt, 1 amp DC supply to run the cooling fans. The meter draws 25 ma or less so any isolated supply you find will work OK.
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Do you have a 914-6? Those were underrated but neat cars.
 

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Discussion Starter #8 (Edited)
Im sourcing parts and it seems these will work. They are rated 75-96v and you say it is critical to use the 45-90v drivers. Any reason this driver would not work?
Thanks
They will probably work ....... but ......... the unloaded voltage is a too high (192 volts) for the G1 Insight. Even 90 volt drivers are a bit too much.

In an ideal situation we would know what our "fully charged" pack voltage would be and at that point it would be good if the charger would automatically lower the charge current to be a very low trickle charge to hold that voltage until we turned the charger off. Unfortunately each pack and even each cell will have a slightly different fully charged voltage.

The chief cause is that all the cells of the pack are not running at the same temperature and nowhere near the manufacturers recommended charging temperature. And to further complicate things the temperature of the cells change while being charging which causes the voltage of the pack to change while being charged.

A typical manufacturers fully charged specification is 1.46 volts/cell @ 20°C.

I try to design so I always stay within the manufacturers ratings so the fully charged pack voltage at 20°C would be ~175 volts. By having a 198 volt capable charger the charge current will going full tilt even until you turn the charger off. If your pack has high internal resistance you might see a voltage much higher than a normal charged pack.

My 178 volt capable [edit] charger only tapers the charge by 0.010 Amps (10 ma). So I record the voltage every hour to get an idea of how the charge is going. When the voltage rise slows to ~0.2 volt per hour after reaching ~174 volts I usually stop the charge.

Another reason to try to keep the voltage close to ~175 is that once in awhile DIY simple (or dumb) chargers have been known to have a condition of surging current. The LED drivers are current limited but basically the voltage is allowed to swing quite a bit depending upon the load. I think charger surging may be caused by using two drivers that are either mislabeled and/or don't have the same output voltage rating at the actual charge current due to the battery load.

Also, it has been noted that even though the drivers are current limited they do not charge the battery at the rated limited current. Yet they do run limited when driving a resistive load. So having two of the drivers in series could cause surging if each unit had a slightly different actual voltage output at the charge current due to the battery load. So in affect they end up chasing each other trying to stabilize the output current (which may cause the load the battery presents to also change). A self eating watermelon .... :mad
 

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So I would just find a wall wart 6 to 15 DC voltage -isolated- supply to run the meter and use another 12-14 volt, 1 amp DC supply to run the cooling fans. The meter draws 25 ma or less so any isolated supply you find will work OK.
I just want to confirm the power supplies needed for the LED meter. Your diagram lists 6.2V and 350mA but the parts list has it as 12V and 800mA and the ebay link item is rated at 9V and 1A output.

AC 100 240V to DC 9V 1A Switching Power Supply Converter Adapter EU Plug M2 | eBay

So this will work to power the meter but if I don't want to power up the BCM I'll just need to source a different power supply that will output 12-14VDC at 1A?

Thanks for all your work, your website has been incredibly helpful.
 

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Discussion Starter #10 (Edited)
I just want to confirm the power supplies needed for the LED meter. Your diagram lists 6.2V and 350mA but the parts list has it as 12V and 800mA and the ebay link item is rated at 9V and 1A output.

Opps, (I had to look back at the BOM and the schematic to see what I had on them.)

The power supply that actually runs the multimeter needs to have an output between 6 to 15 volts DC @ less than 50 ma. Of course more current capability is OK but not really required.

I seem to remember that my multimeter drew 20 ma. Without thinking when I did the schematic I listed the values of the cell phone charger I had used then.

Any small switching power supply (cell phone charger) with 6 to 15 volts will be able to run the multimeter.


AC 100 240V to DC 9V 1A Switching Power Supply Converter Adapter EU Plug M2 | eBay

So this will work to power the meter but if I don't want to power up the BCM I'll just need to source a different power supply that will output 12-14VDC at 1A?

Yes, that will be fine to run the battery cooling fan and the charger fan. And at $1.84 with free s/h it's still the best deal around. :)

Thanks for all your work, your website has been incredibly helpful.
Thank you.

I've corrected the BOM on the website and will correct the schematic tonight or tomorrow. Thank you for the heads up on that.

[edit]
Schematic now up to date.
 

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I recieved the led drivers today. They are only rated 52-85v. I ordered the 60-90v drivers. Should I use these or try to return to China.
 

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Discussion Starter #12 (Edited)
I recieved the led drivers today. They are only rated 52-85v. I ordered the 60-90v drivers. Should I use these or try to return to China.
The Chinese sometimes seem to use specifications and orders as a "suggestion" of what they have and will deliver.

I would measure one driver at a time with only a voltmeter connected to the output and see what voltage each one really puts out. If it's really 85 volts for each one try connecting a 40 watt light bulb on the output of one driver at a time and see what the voltage is and what current they are delivering. You should see 300 ma +/-5% at some voltage. The main thing is the loaded voltage should be very close for each driver.

If they are not 90 volt max drivers (no load) I would email the seller and see what he can do other than you sending them back (which will cost many times more than the drivers cost).

It might be that you can use them but if they are really 85 volt max no load then that is just a little low on voltage for the two of them. 85 x2 = 170 volts is just a little lower than what we'd like to see. The IMA battery should have about 174 volts when it's fully charged IF the battery is fairly cool and in good condition. If the charger can only put out 170 volts you can't be sure the battery is fully charged.

Normally our batteries are much warmer than the spec calls for so the final voltage reads a little lower than 174 when fully charged. But we should still shoot for 174 volts when building the chargers for the times when it is cooler. My new battery normally reads no more than 172 volts when it's quite warm.

You could temporarily connect up a grid charger on a [real] bread-board as a test to see how it charges your battery. Note the end voltage (after 24 hours or so) since that will be the important measurements with slightly lower charger output voltage. Make sure you have the IMA fan running.
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eBay about a year ago got very hard on the sellers to deliver good and proper things we buy. A few bad ratings and eBay will get after the sellers. For that reason the sellers are really trying to get good feedback etc.

To give an example I bought two small computer fans for $3 last year and both of them didn't do what the seller claimed (the 3rd wire which is a speed feedback was not even connected internally). When I emailed the seller in China he wanted me to send the fans back and he would give me a refund. When I looked up the cost to send the two small fans back it would cost me $68!!

I emailed the sender again and he wouldn't budge. So I sent copies of the emails to eBay and they just took the money out of his eBay account and quickly refunded my money.
 

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The Chinese sometimes seem to use specifications and orders as a "suggestion" of what they have and will deliver.
In fact lying is OK in mainland China. It is a cultural thing. The problem is that most of the world does not know this nor other secrets of that strange place.

Assumptions cause much grief.

Oh yeah, the culture also ignores safety, thinks that contracts are meaningless , and certifications are easily faked. Be aware of this.
 

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Olrowdy:
Thanks for all the info on your webpage. I built your charger pretty much according to your instructions which I thought were great. I used these drivers-

As the wiring is a little cramped inside the case, I did not use either the 12V meter or the switches. They are not really needed, but I understand why they are part of the design.

Anyway, just wanted to give some thanks and feedback
 

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Discussion Starter #15
Olrowdy:
Thanks for all the info on your webpage. I built your charger pretty much according to your instructions which I thought were great. I used these drivers- Amazon.com: 2 Pcs 85-265V 30-90V 300mA Power Driver Adapter for 18-25x1W LED Light: Electronics

As the wiring is a little cramped inside the case, I did not use either the 12V meter or the switches. They are not really needed, but I understand why they are part of the design.

Anyway, just wanted to give some thanks and feedback
Thank you for the info how you built your charger. Glad to help out.

Do those supplies run at the 300 ma limited current while charging? The ones I used always run around 250 to 260 ma while charging.
 

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I've just received a couple of LED drivers similar to those described here. They are now described as 54-85 volts, but mine read 95.1 and 97.1 volts open circuit.

With the two output leads connected to the base of a (UK) 60 W bulb I get a pulsing output - the bulb flashes at about 3 cps. The bulb's cold resistance is 67Ω. Neither of my meters seems prepared to find a current, alas (scales of 0-10 A and 0-200 mA) The voltage is impossible to read with it cycling so fast. Looks as though I need to find a higher-resistance load? E.g 316Ω shoud draw 300 mA.

For discharging the 60W bulb looks to be a good option, a little bit on the safe side of what Eq1 suggests. Perhaps if it takes too long it would be worth substituting a 100W bulb for the early discharge, down to, say, 140 volts?

I found a meter which shows volts in tenths (reads up to 199.9 volts in red, and 9.999 amps in blue - ebay item number: 291398999846) - but I'm still waiting for it... I'm not really clear about why the meter power supply needs to be separate from e.g. the fan supply? I have a board which originally drove a computer hard drive, and has a 12 volt 2 A output and a 5 volt 2 A output. Can I use the latter for the meter (3.5 - 30 volts specified, with <20mA current)?

My housing has its fan in one end, so I could secure the bulb-holder to the top. Is there a reason not to do this? I plan to use a 'Rough Service' bulb, which is available in incandescent form still.

I'll see what I can find today.

Thanks for all your efforts on this!

Best regards, T.
 

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Discussion Starter #18
I've just received a couple of LED drivers similar to those described here. They are now described as 54-85 volts, but mine read 95.1 and 97.1 volts open circuit.

With the two output leads connected to the base of a (UK) 60 W bulb I get a pulsing output - the bulb flashes at about 3 cps. The bulb's cold resistance is 67O. Neither of my meters seems prepared to find a current, alas (scales of 0-10 A and 0-200 mA) The voltage is impossible to read with it cycling so fast.
Looks as though I need to find a higher-resistance load? E.g 316O shoud draw 300 mA.

Obviously there is either a defect or at least a design fault in those drivers that allows pulsating output under a variable load. It sounds like the current limiting is over compensating.

Also the no load voltage is not correct according to the label etc. This seems to be an ongoing Chinese problem; "Ship what we have, don't worry about the label." Or they don't have a meter to check the voltage.

If you have a junk computer power supply, here's something you might try to see if will stop the pulsating.

Computer supples have two rather large high voltage capacitors on them (usually black in color) that are rated something like 300 mfd at 200 Vdc or more. If you can, carefully remove those two capacitors, temporarily attach one cap across each the output leads of the LED drivers. Make sure you have the polarity of the cap and the drivers connected correctly.

First see what the voltage is and that the charger isn't pulsating with no load. Then try your light bulb load and see if the drivers have settled down. Let us know if that works or not.


For discharging the 60W bulb looks to be a good option, a little bit on the safe side of what Eq1 suggests. Perhaps if it takes too long it would be worth substituting a 100W bulb for the early discharge, down to, say, 140 volts?

I would guess if your battery is bad enough that it needs a discharge cycle you will get below 140 volts pretty quickly.

I tend to start with less than 350 ma at the start of a discharge cycle. That way I have less worry about a destructive cell reversal.


I'm not really clear about why the meter power supply needs to be separate from e.g. the fan supply? I have a board which originally drove a computer hard drive, and has a 12 volt 2 A output and a 5 volt 2 A output. Can I use the latter for the meter (3.5 - 30 volts specified, with <20mA current)?

Computer supplies have the negative lead of the 12v & 5v DC outputs grounded to the case. That would be an non-isolated supply and should NOT be used to power the high voltage meter.

I bought several different Chinese meters including one like you ordered and all of them have the power supply negative lead that runs the meter itself (typically 4 to 30 volts) connected internally to the negative lead of the high voltage you are measuring. So the meter power supply HAS to be isolated from ground to prevent grounding the cars IMA system to the metal of the car.

While grounding the IMA system shouldn't cause a problem for grid charging you really don't want to do that in case it causes some sort of expensive problem. And if you have the IPU case open it is a safety hazard while charging/discharging the battery.

The computer supply you have would be fine to run the 12 volt grid charger fan and the IMA battery cooling fan though since the car's 12 volt system is a grounded system. I would guess that the supply is rather large though. Lately I've seen 12 volt switching wall warts that are rated for 1 amp for $1 each on eBay.


My housing has its fan in one end, so I could secure the bulb-holder to the top. Is there a reason not to do this? I plan to use a 'Rough Service' bulb, which is available in incandescent form still.

I'll see what I can find today.

Thanks for all your efforts on this!

Best regards, T.
Glad to be able to help.

I would build the discharger separate from the charger since you normally wouldn't need to discharge the battery every time you grid charge the IMA battery. But there's no electrical reason why you can't include it on top of the charger. You will need to add a switch to turn the bulb off though.
 

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Hi,

First, sorry that my post shows that the symbol for 'Ohms' reads like a zero - makes some of what I wrote a bity confusing! I'll write 'Ohms' in future!

I wrote:
... LED drivers similar to those described here. They are now described as 54-85 volts, but mine read 95.1 and 97.1 volts open circuit.

With the two output leads connected to the base of a (UK) 60 W bulb I get a pulsing output - the bulb flashes at about 3 cps. The bulb's cold resistance is 67 Ohms.. The voltage is impossible to read with it cycling so fast.
Looks as though I need to find a higher-resistance load? E.g 316 Ohms should draw 300 mA.

Obviously there is either a defect or at least a design fault in those drivers that allows pulsating output under a variable load. It sounds like the current limiting is over compensating.
Yes, I guess so. I was consoling myself with the notion that this was because it was 'outside the envelope' - running into a load which was smaller than it could cope with.

Also the no load voltage is not correct according to the label etc. This seems to be an ongoing Chinese problem; "Ship what we have, don't worry about the label." Or they don't have a meter to check the voltage.
If you have a junk computer power supply, here's something you might try to see if will stop the pulsating.
I'll have a look. Don't have a dead computer PSU, but maybe something else.

First see what the voltage is and that the charger isn't pulsating with no load. Then try your light bulb load and see if the drivers have settled down. Let us know if that works or not.
I'll give that a go.

I wrote:
For discharging the 60W bulb looks to be a good option, a little bit on the safe side of what Eq1 suggests. Perhaps if it takes too long it would be worth substituting a 100W bulb for the early discharge, down to, say, 140 volts?

I would guess if your battery is bad enough that it needs a discharge cycle you will get below 140 volts pretty quickly.
Actually, the signs seem to be that my battery, although original, is not in bad shape. I left the car unused for almost three weeks and on my return it started up fine from the IMA system, then after a few miles it recalibrated down to maybe 1/4 charge, but in 40 miles or so it was fully charged again.

I tend to start with less than 350 ma at the start of a discharge cycle. That way I have less worry about a destructive cell reversal.
Ah, I was following Eq1's advice in May this year:

"...On deep discharge current, I'd choose a bulb or resistor based on the current at 140V (but within a range of about 0.5A to 2A) and how unbalanced your pack likely is. The more unbalanced, the lower within this range you'd want to be... 140V is about where you might see the first cell reversals... "

Do you think this is too much?

Computer supplies have the negative lead of the 12v & 5v DC outputs grounded to the case. That would be an non-isolated supply and should NOT be used to power the high voltage meter.
In my case it is simply a small board, which originally lived in a plastic box. No need (or provision) for it to be attached to the case. But perhaps the IMA fan is grounded, and would thus ground the supply?

I bought several different Chinese meters including one like you ordered and all of them have the power supply negative lead that runs the meter itself (typically 4 to 30 volts) connected internally to the negative lead of the high voltage you are measuring. So the meter power supply HAS to be isolated from ground to prevent grounding the cars IMA system to the metal of the car.
Ah, I understand!

The computer supply you have would be fine to run the 12 volt grid charger fan and the IMA battery cooling fan though since the car's 12 volt system is a grounded system. I would guess that the supply is rather large though. Lately I've seen 12 volt switching wall warts that are rated for 1 amp for $1 each on eBay.
While my supply is quite small I now see the importance of having a separate supply for the meter!

I wrote:
My housing has its fan in one end, so I could secure the bulb-holder to the top. Is there a reason not to do this? I plan to use a 'Rough Service' bulb, which is available in incandescent form still.

I would build the discharger separate from the charger since you normally wouldn't need to discharge the battery every time you grid charge the IMA battery. But there's no electrical reason why you can't include it on top of the charger. You will need to add a switch to turn the bulb off though.
Of course. My sense from the 'best light bulb for a deep discharge' thread here is that the deep discharge is more important than grid charging to restore a battery to full effectiveness. (Which is not at all to deny the importance of grid charging - just that deep discharging should ideally come first). Do you agree?

As to the discharger, I hadn't thought of making it separate! The advantage of having is integrated is that the meter can see what is going on - as in your schematic. For as much as it costs, perhaps a separate meter is a good idea. Probably easier to keep the dischager remote from the car, so one can watch it without having to shuttle in and out of the house all the time! Come to that, given the small currents involved, using the charger remotely as you suggest on your site is easy enough - one just needs a lead capable of carrying an amp or so.

Very much thanks for your input!

Best, T.
 

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The cheap led power supplies usually have hiccup current limiting, so they will pulse when connected to a low value resistance load which draws more than their rated output.

The Meanwell ones may be different but I haven't tested one.

The ideal is one that delivers the full 300 ma output into any resistance load (within range), varying it's output voltage between it's rated min/max to maintain 300ma output into the chosen load

I use proper 300R ceramic resistors for my discharge loads. Cheap as chips on e-bay and much more consistent than a lightbulb, allowing easy calculations of power/amps etc if you just log voltage.. as you know the value of the load ;) Ohms law does the rest..

The OBDIIC&C does logging of any parameters you want there is lots of info on that spread about.. ;)
 
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