Hi Terry, Questions welcome! Sometimes answering them clears one's own thoughts.
As Barboots has shown, the thing is a dc dc converter. There are lots of them on ebay, some better built than others. The one in the pic there has input and output leads the same size, not good if input carries greater current......The one I got, although a bit cheaper, had about 10g input wires, 12 g output, fit for purpose. The heatsink would only be necessary if you pushed it to max 20A output (380 W at 19V). The specs I saw for similar gadgets suggested more cooling needed if ran at max output. I had the heatsink left over from another project so put it in just in case. Turns out was not necessary for the way I have it set up, but gee it is a real talking point when people have a squiz in the front boot.
I did not want the dc dc converter to have a meltdown while driving along! I really don't trust a lot of the ebay stuff! I have set the output of the victron mppt to 20A (at say 14V), which limits the input from dc dc gadget to only around 16A (at 19V) or so, allowing for only 90% efficiency in the mppt (it is probably better). Again, allowing 90% for the dc dc conversion the input from the car at around 13v is about 25A. I have a 30A circuit breaker in the car wiring and it does not trip. The BMV in the van shows 20A going into the lithiums. The car wiring (only 8b&s as suuplied by toyota) does get just warm after 30 mins. The dc dc converter did not even get warm. Can't give you exact amperages, the above are just my calculations, but so far it seems to do the job. I turn off the solar if we need to use the dc dc, which is rarely. If the solar is left on in the sun, then you still only get your 20A to the lithiums, and if in the shade there seems to be some back flow...no blocking diodes in the panels. Would I do it this way from scratch...no way. When we first set up the lithiums we towed with a Hyundai Terracan, consistent 14 to 14.1 V from the alternator, and with the losses in the wiring charged at about 10A into the lithiums when they were about 50% down, so just let it do its thing if needed. The BMS would protect from overcharge if we drove for 5 hours straight and forgot about things, but again, rarely used. The new Toyota Fortuner drops its voltage down to 13.9 or less and the wiring is so so, so only 2 or 3 A was reaching the lithiums....not worth bothering about. So if we wanted lots of good coffee in miserable weather the additiion of the dc dc gadget was a few dollars well spent. Besides, I wanted to see if it worked.
Barboots - I got the idea for this from some Morningstar tech notes. These suggested that this was a goer for their mppt controllers but not for their pmw ones. They did not explain why, however, or indeed if it would damage the devices....
Terry - Battery charger was in the van when we bought it. Turns out it is almost ideal for our use. Seems to still be made in Hong Kong but not available here. It has a power supply setting adjustable to 13.2, 13.5 or 13.8 volts. The same switch allows you to set the float voltage for any of the charge algorithms. If on a powered site and battery is under about 70% we use the 13.2 setting and very little goes in or out of the battery. If I do need to charge from 240 V (only for capacity testing or to check balanc e at top) I use the gel charge setting, 14.2 bulk and absorb (which never lasts more than 30 mins, sometimes only a min or so...don't know how or why and the basic manual is no help. It then drops to my 13.2 "float" whi h really just turns the charger off). Does bring the cells up to 3.55 V where the balance shunts start to work.
Current has already dropped below 2A at this time. As soon as a cell starts shunting, the voltage across that cell drops about 30mV. I have always manually monitored thhis, holding the meter on shunting cells and watching the voltage skip up and down between 3.52 and 3.55.
The max the shunts can move is 700 mA.
The shunts are part of the "Housepower" BMS made by Dimitri from Cleanpower Auto in the States. The company has since been bought out (by Lithionics I think) and the stuff is no longer available.
The only way to turn off the shunting is to remove the resistors from the board. As I understand it this will not affect their sensing functions.
The CB on the solar input is made for dc.
Hmmmm. Hopefully I have answered all the questions. Off to bed now, the grandkids wore me out this afternoon!
Cheers, Leslie
They would say you didn't know what you were talking about or didn't hold your mouth right or were just telling lies, been there done that and have the knife wounds for my trouble