RW,
Please consider me a complete idiot when it comes to electronics, please keep that in mind regarding my questions. I probably know enough to just confuse everyone.
Help me if you can.
You say you are using 48 volt in, 3.3 volt out DC to DC converters but you mention charging at 3.65 volt and also 3.5 volts. How? Are you putting 2 units in series and then using resistors? Some other method?
Many of the dc/dc converters are isolated, but not all. If you have a part number for the dc/dc I would be happy to check it out. Also most dc/dc converters have an adjustable output (or trim) of about +/- 10% and some have a wider range. 3.3V + 10% is 3.63V (3.65 is safe) I dialed it back to 3.5v to stay closer to the rated output current. The voltage is set by the resistor you can see in the picture, when I design the board it will be a 20 turn pot to dial the voltage in exactly.
When I look at your picture I can't quite figure what is going on. Can you sketch a diagram?
I believe my setup is exactly what you are trying to do, I have 3 parallel cells per group, with 4 groups in series (you'll have 50 but same idea) there is a dc/dc converter per group, now the wiring might look a bit odd though. The when you put all of the dc/dc converters in series to charge a series pack, almost all of the current flows through the large wires in the picture, so you are basically building a 175v power supply (50 x 3.5v dc/dc converters in series) the current will flow in one big circle, not 50 small circles. The only time current will flow through the small red wires (in my picture) is when the cells are out of balance. So when you build the charger it's better to use heavy connections between the + and - connections of the dc/dc converters and then run a wire to the cell (use heavy wire, not the small stuff I used) and fuse every wire at the cell end (VERY CLOSE TO THE CELL).
HERE IS MY PROBLEM
What I want to do is charge 50 individual 3 cell parallel packs with 3.5 volts to each pack. since the parallel packs are also connected in series and because I want to use a common power supply to power them, I'm told that the DC to DC converters MUST be isolated.
That is 100% true, if the dc/dc converters are not isolated it won't work and things will blow up.
The advantage for me for not applying more then 3.5 volts to an individual 3 cell pack is safe charging that does not require monitoring. The 3 cell parallel packs will never recieve more then the DC to DC output of 3.5 volts. The current output can be low because normally the tractor will have at least a few days between competitions.
That's the exact reason I'm designing my system, there is minimal chance of overcharging a cell. The chances of a dc/dc converter failing and putting out higher than 3.5v is very slim. There is the chance of a dc/dc converter failing and a group of cells not getting charged, if that happens you will probably destroy them if it isn't caught before you make a run.
I have a couple of options open for monitored bulk charging I won't need to go into here.
My main concerns are:
1.) Plugging in the charging system and walking away, then returning to a balanced pack.
2.) Reasonable chance that I will not return to a burnt to the ground tractor and storage unit.
3.) Not being required to spend more then the cost of the pack for a BMS system and LiPoFe charger and then still have to worry about burning to the ground with unattended charging.
1.) That is the same ideal I have, and providing a dc/dc converter doesn't fail it should be just that simple.
2.) There is minimal chance that a cell could be over charged (which is when they burn)
3.) Also the same reason I'm designing my system, a few grand for a charger, few grand for BMS, and few hundred for gauges isn't what I wanted to do. So my system will cover all of those for much less.
There are two negatives though, you are top balancing, so be careful not to go to 100% discharge or you could damage cells, and if a dc/dc converter fails you could easily destroy the cells that didn't get charged, so you need some way to monitor and confirm all cells got charged. (This could be as simple as an LED that turns on when the dc/dc converter turns on, with a blocking diode so that the cell can't turn it on) I will be doing individual cell voltage monitoring, but I'm thinking that some type of double check might not be a bad thing.
I will be monitoring each 3 cell parallel pack with Cell Log 8m units so I can set a high and low limit to shut down charging units but that requires monitoring. I want to be able to plug in the power supply and walk away knowing that no battery is going to get more then the output voltage of the DC to DC unit.
I'm not convinced that some other system turning off the charger is a good idea. (meaning reliable, and failsafe) Especially when monitoring voltage, if I do go with a system that will turn off the charger it will be based on current draw on the 48v line that powers the dc/dc converters. (too high or too low and it will turn it off) But probably a simple timer or something like that as the failsafe.
Do you know if the 3.3 volt and 3.5 volt DC to DC units he sells are isolated. I do not know where to go look for this information. Can you help?
The dc/dc converter I use is isolated and they make it pretty clear in the description, it's usually not something you have to go digging for. (depends where you are getting them of course) if you give me a part number I'd be happy to check it out for you.
On a side note from your later posts, I've used onlinemetals for years and the service is exceptional. I would recommend them to anyone, I believe speedymetals is a bit cheaper but they aren't so friendly shipping to Canada.
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