Sunny Island 5048 and Battery Fan

[Tinbum]

MPP is maximum power point and not the max voltage of the panels and has no relevance on the max voltage at the batteries. They can go to a higher voltage than the MPP but the current will be less. In theory they could go to the VOC but in practice that won't happen with batteries connected.

[Eleanor]

Hi Bautsche, sorry to hear of your troubles. I haven't had time to read all of your posts properly but I think the Sunny Island SOC calculation error may be caused by the fact that battery capacity varies with the size of the load ie the bigger the load the fewer Ah that can be drawn from it (Peukert Effect). If the SI sees a big load it will recalculate the battery capacity and the %SOC will therefore decrease accordingly. Stephen posted the relevant page of the manual a while back and from memory the SI uses just two averaging factors for the capacity in the calculations even though the actual capacities at low currents will vary considerably. This leads to step changes in the %SOC readings which I don’t think reflect the real situation. It could also be worth checking that the correct battery capacity and Peukert coefficient are entered into the software.

I wonder if it would be best to AC couple the PV using a grid tie inverter? This way the SI will control all the charging. As Stephen has mentioned earlier presumably all the charging devices go through a common shunt so the SI is seeing all the current going in and out of the battery?

[bautsche]

Hi Eleanor.
Yes, the current sensor sees all the power going to the battery. Interesting re how the SOC is calculated. I'm going to upgrade the firmware this weekend anyway and there is some mention about better SOC calculation in the later versions, so I'll see what that does.
The reason I direct attached the solar is quite simply money, i.e. the money I don't have to buy a SunnyBoy. :-(
Looking over the graphs with a friend last night, it seems that this jump in SOC calculation is what caused the SI to start the generator when it wasn't required which in turn lead to the high temperatures....

Thanks you guys for all your help and pointers.
Eric

[stephendv]

Unfortunately, my battery bank is pretty shot, so I also start the generator when the battery voltage drops below 44V and then run it until the voltage reaches 59V.

44V!  That's far too low for a normal set of batts.  My guess is that the batteries are so shot that the SoC calculation in the SI is completely thrown off.  For example, the peukert constant would be quite different for new vs. damaged batts, and I think yours are tending towards the latter.

In all, a boost run doesn't happen more than once every day or two because before the SI decides to start the generator based on state-of-charge (SOC) the voltage has usually dropped below 44V. A boost run tends to happen early in the morning before the farm wakes up when the load on the battery bank is low but the SOC has dropped to the point where the SI starts the generator (the SI starts at 45% SOC). I experimented in the past with starting the generator earlier but that then keeps the generator running for such long periods of time that I can't afford the red diesel.

You can control gen runtime through the boost and full time settings.  As I mentioned earlier, I'd reduce the boost and full voltages to something like 2.5V and limit boost time to 30 mins - 1 hour and see how it behaves.

If you have a constant 1kW draw and 3kW of PV, then when the batteries are fully charged they're still being hit with a constant 2kW charge- that could easily overcharge them.  A 60A charge controller is not that pricey: http://www.navitron.org.uk/product_detail.php?proID=8&catID=119  compared to the price of a new set of bats.

[bautsche]

For historic reasons, I actually have a Trace 4048 (I think, same as Paul Camilli's anyway) as well as the SI (trace isn't doing anything other than sit there at the moment) and that has a spare relay that can be operated based on high battery voltage, so I plan on using that and a cable that already runs from the inverter shed to the house (to operate a relay to turn on the immersion heater). I just actually have to get around to taking the kitchen cupboards apart to actually get at the cable.... :-(
Eric