Solarfun4jim
Solar seduced :-)
In the setup's below, which would function best for longevity or which would be a strict no-no for reasons given.
(Just experimenting with idea's at present)
(Just experimenting with idea's at present)
Question on SCC disconnect comparison
- Thread starter Solarfun4jim
- Start date
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
I would think either would work (assuming your SCC has an accessible on-off switch)
Your SCC is being effectively 'disconnected' every night when the sun goes down, so this is natural for it.
If your SCC has an on-off switch, then it is designed to be turned on and off... so it should be fine.
Of course the need for either is up for debate. A lot of people say disconnecting the battery while the panels are generating power will damage the SCC. Other people say it does not damage the batteries. I'll stick with Victron SCCs. They say they are designed for handling BMS disconnects.
Your SCC is being effectively 'disconnected' every night when the sun goes down, so this is natural for it.
If your SCC has an on-off switch, then it is designed to be turned on and off... so it should be fine.
Of course the need for either is up for debate. A lot of people say disconnecting the battery while the panels are generating power will damage the SCC. Other people say it does not damage the batteries. I'll stick with Victron SCCs. They say they are designed for handling BMS disconnects.
Solarfun4jim
Solar seduced :-)
@FilterGuy
So im assuming a SSR would have no issues disconnecting the variable PV input(so long as a DC output version was selected(preferably much higher rated, say 40A to avoid any issues)? This would mean a very low wattage draw for the coil.
I'm also viewing, turning off the power switch or disconnecting the PV input as decidedly different from disconnecting the output cable to the battery....ie termination of input as opposed to termination of output, both ways i think would be safer anyways???
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
The SSRs typically get in trouble with large inrush currents. Since the SCC is not a highly inductive or capacitive load, it should not have much of an inrush current. (Even if it did, the panels will limit the inrush). Consequently, this should not be a problem for a properly sized DC SCC. Size the SCC for something well over the Isc and worst case Voc of the array. Be sure to take into account low-temperature rise of the rated Voc.
Solarfun4jim
Solar seduced :-)
@FilterGuy
Thanks filterguy. While you are on, do you know if there is any issues with having two BMS's hooked up to one battery pack(parallel like)?
(note: only one BMS would be responsible for balancing, other deactivated)
The SCC i'm looking at handles 150V 70A..... my PV out put maxes out at 60Voc(*2) 11A ISC(*2), so well within specs for cold temp boost, but also, panels would be mounted flat, so less output that way.
Last edited:
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Theoretically, if they are identical BMS's it would work but in practice it could be problematic. No matter how good the tolerance on the BMS design and manufacture is, they won't be identical. The best way to explain is by an example. One of the BMS's might read a cell as 3.51V and the other at 3.54 volts. The two of them might then 'fight' with each other to balance the cell.
I assume you are thinking of this in order to handle a higher current than the BMS is designed for. If the battery is under load and the BMS decides to shut off (low temp or under-voltage), it is unlikely the other BMS will shut off at the same instant. This means the 2nd BMS will suddenly be faced with handling the full load. Hopefully the over-current built into the BMS will shut it down before any damage....but you never know.
Bottom line: I would not advise doing it.
Solarfun4jim
Solar seduced :-)
Different entirely. Only one BMS handling balance. Two totally different BMS's to be used. First one simply as a disconnect mechanism on the SCC only. The second BMS has the overall contol of the safety aspect/balancing. Simply using the first one as a diconnect on the SCC at a targeted voltage limit. If it failed, the second BMS provides the redundancy/insurance, being set just a little bit higher threshold.
EDit....good lady shouted me for dinner.
If you were interested, i could provide a sketch and outline of my idea later?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Sure
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
So the BMC that is controlling your SCC is just watching voltage and disconnecting the SCC before the other BMS does.
With this set-up, presumably your SCC is set to stop charging at a lower voltage than either of the BMSs. Consequently, you end up with a triple layer of protection.
With this set-up, presumably your SCC is set to stop charging at a lower voltage than either of the BMSs. Consequently, you end up with a triple layer of protection.
- Normally the SCC would stop charging and the BMSs would never disconnect.
- If, for some reason the SCC keeps charging, the 1st BMS would disconnect
- If, for some reason the SCC keeps charging and the 1st BMS does not disconnect, the 2nd BMS disconnects.
Solarfun4jim
Solar seduced :-)
@FilterGuy
OK, my theory is, that you need the highest possible PD(v) to drive the electrons for the quickest charge rate. With solar input being minimal, you want to capture it as quick as possible. You dont however want to terminate your charging at that high voltage end point. So, you set the charge controller to deliver 4v/cell @ whatever amperage the panels can give. You set the first BMS to disconnect once it sees a cell at 3.4v (only wishing to charge to less than 100% capacity) In this way, the SCC does not determine the end point of charging, the first BMS does....but in a 'gentle manner'...lol.
If any problems with termination at 3.4v, then the second(chargery)BMS will catch it at 3.5v/cell and disconnect the SCC through the charge relay to the on off switch(or to a SSR connected to PV input).
NOTE: the balancing on the first BMS would be deactivated, so no competition issues.
OK, my theory is, that you need the highest possible PD(v) to drive the electrons for the quickest charge rate. With solar input being minimal, you want to capture it as quick as possible. You dont however want to terminate your charging at that high voltage end point. So, you set the charge controller to deliver 4v/cell @ whatever amperage the panels can give. You set the first BMS to disconnect once it sees a cell at 3.4v (only wishing to charge to less than 100% capacity) In this way, the SCC does not determine the end point of charging, the first BMS does....but in a 'gentle manner'...lol.
If any problems with termination at 3.4v, then the second(chargery)BMS will catch it at 3.5v/cell and disconnect the SCC through the charge relay to the on off switch(or to a SSR connected to PV input).
NOTE: the balancing on the first BMS would be deactivated, so no competition issues.
Last edited:
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Wow. That strategy is the exact opposite of what most people do. The SCC is designed with multi-stage charge profiles and makes decisions on what to do based on where the battery voltage is. In this design, that is pretty much bypassed and all of the charging comes down to the voltage the BMS is set at.
The generally accepted strategy is to make the BMS disconnect the exception and let the charging be done by the SCC.
The generally accepted strategy is to make the BMS disconnect the exception and let the charging be done by the SCC.
Solarfun4jim
Solar seduced :-)
@FilterGuy
Yep....thinking outside the box....what issues do you forsee?
The SCC sets the applied voltage, but BMS sets the termination voltage....back up protection on overall BMS 2(chargery unit), some some redundancy?
I'm looking for a high input voltage but a much lower termination, as i only really want to charge to about 90% capacity....so this kills two birds with one stone as it were.
Also, deliberately over rated the first BMS, so it is only handling a third of its rated capacity.
Last edited:
Solarfun4jim
Solar seduced :-)
You also get two layers of cold temp protection as well....so another bonus 
Only issue i see,(thus far) is that if you plugged into grid power whilst the solar was charging at 'the high' voltage, the charger in the inverter charger, would not come on till the SCC output dropped off with cloud cover etc. (I would not set the inverter charger to these high voltage inputs, as i would only have the one BMS protection...so no go)
Only issue i see,(thus far) is that if you plugged into grid power whilst the solar was charging at 'the high' voltage, the charger in the inverter charger, would not come on till the SCC output dropped off with cloud cover etc. (I would not set the inverter charger to these high voltage inputs, as i would only have the one BMS protection...so no go)
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
I have to think about it some, but I suspect it might not fully charge the battery because it will disconnect as soon as it gets to the CV stage of the charge cycle.
When the battery is 'empty' the charging is limited by the amount of current the battery can safely take (or the charger can provide). As the battery charges the voltage will rise. Once it hits the charge voltage, you control the charge by limiting the voltage. At this point the current will eventually start going down till near zero. At that point the charge is complete and the voltage should be removed.
When the battery is 'empty' the charging is limited by the amount of current the battery can safely take (or the charger can provide). As the battery charges the voltage will rise. Once it hits the charge voltage, you control the charge by limiting the voltage. At this point the current will eventually start going down till near zero. At that point the charge is complete and the voltage should be removed.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
By letting the BMS control the charging, the charging stops as soon as it hits the charge voltage so you never get the 2nd stage of charging.
Solarfun4jim
Solar seduced :-)
Yes, but im terminating before the current reduces to zero, ie only wanting a 90% charge.
Solarfun4jim
Solar seduced :-)
yes exactly....what i was aiming for.
Im assuming when i hit 3.4v/cell it is about 99%-100% charged, but allowing for surface charge, might come to rest about 90% charge?
Attachments
Solarfun4jim
Solar seduced :-)
I really like the spec of the chargery unit, but dont like relying on contactors/relays, plus the wattage drawdown....so thought this might be a good workaround?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
I would use the charge profile of the SCC as the primary driver of the charge cycle. If you don;t like the relay of the Chargery, use the charge relay output to turn the SCC on or off as a fail-safe. Not as the primary.
Solarfun4jim
Solar seduced :-)
Ok thanks
Only trouble with that, if you only wish to charge to 90% capacity, is it would take much longer with solar at less than 0.1C charge rates. :-(
