diy solar

diy solar

Newbie question - Do you guys have an on/off switch with an RV solar set-up?

pierre said:
My take on this is that ‘storage’ means not connected / inactive and do not top up periodically i.e. leave at about 70% SoC. ‘Floating‘ on the other hand is in an active system as Joey described above i.e. floating at about 13,5v or 26v.
Click to expand...

Yes I agree. Its just the closest related info I've seen, I don't know if a constant float current would change anything or not.
 
Dzl said:
Yes I agree. Its just the closest related info I've seen, I don't know if a constant float current would change anything or not.
Click to expand...

I'm not sure where I got the idea but its my understanding that leaving the battery exposed to voltage above full resting for extended periods is stressful/abuse.
But any voltage below "full resting" which I calculate to be 27.6 volts is use but not abuse.
See the state of charge table at this link https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/
 
Dzl said:
Yes I agree. Its just the closest related info I've seen, I don't know if a constant float current would change anything or not.
Click to expand...
Great graph , thanks Dzl. At that float level the charger is in constant voltage mode and either the BMS or the charger LFP profile will switch on/off with little float current.
 
pierre said:
Great graph , thanks Dzl. At that float level the charger is in constant voltage mode and either the BMS or the charger LFP profile will switch on/off with little float current.
Click to expand...

Sorry not picking up what you are laying down.
 
smoothJoey said:
I'm not sure where I got the idea but its my understanding that leaving the battery exposed to voltage above full resting for extended periods is stressful/abuse.
But any voltage below "full resting" which I calculate to be 27.6 volts is use but not abuse.
See the state of charge table at this link https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/
Click to expand...
Agreed. Once again this points to the importance of having a programmable BMS to set that point correctly.
 
pierre said:
Agreed. Once again this points to the importance of having a programmable BMS to set that point correctly.
Click to expand...

I'm just using a commodity BMS and a victron battery protect.
The BMS will disconnect at 3.7 volts per cell which the converter will never approach.
 
Here the topo for those who are interested
Code: 
usage model:
    isolated ac ups
design:
    24 volt system
    1500 watt hardwire capable inverter
    Discreet common off the shelf components
    commodity 100 amp atc fuse block
    commodity separate port bms
    battery_wire = 2 awg stranded thhn
    inverter_wire = 2 awg stranded thhn
    charger wire = 10 awg stranded thhn
diagram:
primitives {
    mrbf = marine rated battery fuse 100 amps
    lvd = low voltage disconnect(victron battery protect)
    |F| = fused screw terminals 30 amps
    |U| = un-fused screw terminals
    |S| = spare
    | = unfused m5 post 100 amps
    ... = repeat last object
}
battery {
    pos = mrbf<->cell_in_series<->...
    neg = shunt_sampler<->cell_in_series<->...
}
system {
   10 awg|F|->lvd_in->lvd_remote_pos->inverter_ext_3
   16 awg|F|->shunt_sampler
   10 awg|F|<-charger
   10 awg|F|<->inverter<->battery.pos(2 awg)

   10 awg|U|<->inverter<->battery.neg(2 awg)
   10 awg|U|->premises_ground
   10 awg|U|<-charger_equipment_ground
   16 awg|U|<-lvd_ground
   10 awg|U|<->bms_b<->bms_p->|->charger
                              |<-lvd_remote_neg<-inverter_ext_2(16 awg)
}
 
I guess I could get an overkill bms and set the HVD lower.
I could then omit the victron battery protect.
 
smoothJoey said:
I'm making a lifepo4 online ups and will therefore float my battery pretty much its entire life.
I can't find anything authoritative that says its bad practice.
If anyone has a link I'd like to see it.
For bonus points I would like to know why.
Click to expand...

Have to admit Joey that you got me there. I don't have any scholarly proof that is true. It is just something I have seen on several places on the internet. So it has to be true! :LOL:

Thanks for the chart, DZ. Where did you find that? Now that Joey has called me out on it I want to make sure it is true. If not, I wanna float my batteries where they should be.
 
boondox said:
Have to admit Joey that you got me there. I don't have any scholarly proof that is true. It is just something I have seen on several places on the internet. So it has to be true! :LOL:

Thanks for the chart, DZ. Where did you find that? Now that Joey has called me out on it I want to make sure it is true. If not, I wanna float my batteries where they should be.
Click to expand...
I float my packs at the first 'knee' voltage on the discharge curve. Seems to work out o.k.
 
smoothJoey said:
What does that equate to in voltage for your rig?
Click to expand...
I use the charge/discharge graphs as supplied by the manufacturer of the cells in each case. Presently with the pack I have it is 13,5v or 3,38v / cell.
So, for your 24v system it will be 27v and you are sitting at 27,6v or 3,45v/cell which I think is in the ballpark. Will be interesting to find out what that first knee voltage is for your make of cell. You may be spot-on. Temperature also plays a part and some manufacturers supply curves at various temps.
I try not to overthink this too much -- at the end of the day you lose a few battery cycles out of the total between 3000 to 5000. Pack will outlast me anyway ;)
Sorry for the hijack @Woodbender but all this ties in with your original post.
 
smoothJoey said:
Here the topo for those who are interested
Code: 
usage model:
    isolated ac ups
design:
    24 volt system
    1500 watt hardwire capable inverter
    Discreet common off the shelf components
    commodity 100 amp atc fuse block
    commodity separate port bms
    battery_wire = 2 awg stranded thhn
    inverter_wire = 2 awg stranded thhn
    charger wire = 10 awg stranded thhn
diagram:
primitives {
    mrbf = marine rated battery fuse 100 amps
    lvd = low voltage disconnect(victron battery protect)
    |F| = fused screw terminals 30 amps
    |U| = un-fused screw terminals
    |S| = spare
    | = unfused m5 post 100 amps
    ... = repeat last object
}
battery {
    pos = mrbf<->cell_in_series<->...
    neg = shunt_sampler<->cell_in_series<->...
}
system {
   10 awg|F|->lvd_in->lvd_remote_pos->inverter_ext_3
   16 awg|F|->shunt_sampler
   10 awg|F|<-charger
   10 awg|F|<->inverter<->battery.pos(2 awg)

   10 awg|U|<->inverter<->battery.neg(2 awg)
   10 awg|U|->premises_ground
   10 awg|U|<-charger_equipment_ground
   16 awg|U|<-lvd_ground
   10 awg|U|<->bms_b<->bms_p->|->charger
                              |<-lvd_remote_neg<-inverter_ext_2(16 awg)
}
Click to expand...

What is a "topo"? It looks like a computer language. I'm curious if it helps with a re-design I'm going through.
 
If you have an MPPT controller you should switch the panel side first. Having panels connected with no battery can let the smoke out of the controller. On any DC circuit you only have to switch one side. I recommend the positive. On my PWM controller I switch both battery side and panel side using a DPST battery switch from Blue Sea. There is no need for a fuse or breaker on the panel side. The typical 8awg or 10awg wires used can safely carry the short circuit panel amperage. There is some fusing consideration with more that 4 parallel panels as one bad panel can draw all the combined amperage from the other panels. With any type lead acid, the battery must be kept fully charged. Even if disconnected they will gradually lose charge to the gods.
 
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