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I don't foresee much government resistance to such a plan, at least before factoring in utility lobbying. As far as I can tell,

To Block, or Not To Block

Electricity at our remote vacation cabin is supplied by six Arco model M55 solar panels and for charge control we use a Trace model C-30. Because of trees, we get only a little over 2 hours of shade-free solar each day for a gain of about 30 Amp-hours. Increased charge efficiency would be welcome.

Our panels have blocking diodes and we have read that diodes are no longer used since they reduce panel output. Instead, panels are matched with charge controllers that have automatic night-time shut-off to control reverse current flow.

Can we expect increased charging if we remove the blocking diodes and replace our charge controller with a unit that has the night-time shut-off feature? We are considering changing to the Trace C-30A, in any case, because it can be switched to accept an equalizing charge. I realize that two hours is too short a time for adequate equalizing, but since the batteries are full when we visit our cabin, the charge would come on top of full batteries. Ned Vilas, Davis, CA

Hello, Ned. Chances are that the diodes built into your ARCO modules are bypass diodes, not blocking diodes. ARCO (now owned by Siemens) and several other major PV manufacturers place a small bypass diode in ever module's junction box. This bypass diode is connected reverse bias between the module's positive and negative output terminals. A blocking diode is wired forward bias in the module's positive output lead. A quick look inside your J-boxes will determine if your diodes are by wired for bypass or blocking. If you see the diodes connected between the + and -- terminals of the module, then they are bypass diodes. FYI, almost all diodes have a band printed on their negative (cathode) side.

Removing bypass diodes will not give you any efficiency increase. Bypass diodes protect a shaded module from illuminated modules when the array is partially shaded. Blocking diodes are "one-way" electrical valves that keep the battery's stored power from flowing back into the PV modules at night. Removal of blocking diodes will potentially increase the voltage delivered to the battery by about 0.5 to 0.8 VDC. Since PVs are essentially constant current devices, this voltage increase will manifest only when system's voltage is high. This means when the battery is just about fully recharged.

Since you didn't mention the capacity or type of battery you are using, I can't determine if your present charge control is what you need. With a limited solar window such as yours, I'd go for the C-30A and set the disconnect voltage at around 2.5 VDC per cell. This setting assumes a lead acid battery (6 series cells times 2.5 VDC per cell equals 15 VDC battery voltage). How much water does your battery consume? Water consumption is a very reliable indicator of under and overcharging. If you are having to add water to a cell every two weeks or so, then you are overcharging the cell. If you only add water to the cell every six-eight months or so, then you are undercharging the cell. Richard Perez

Switching To and From the Grid for Charging

I have a solar system with eight 51 W panels, a 2500 W Trace Inverter and an SCI 30 A controller. Our grid is available for 8 hours during the day. I find that if I have grid power during the day and the batteries are floating at 9 am, there is no solar charging as the charging light on the SCI goes off. This is a waste of solar charging potential.

Is there an automatic control switch available (or can it be built?) to switch the inverter from the grid when solar power is available and the inverter is on float mode and then back again when usage is above the solar panels' output? Phillip Wilson, Pétion-Ville, Haïti

Hello, Philip. There are some problems lurking in your request. Consider what would happen if your system's load were just about equal to PV production. Here the electronic control could rapidly switch the inverter in and out of charger mode. This could damage the inverter and will certainly raise hell with any appliances on line at the time. Your problem is a little more difficult than just switching the inverter on and off of the grid because you are using the Trace's built-in battery charger. This charger transfers all the inverter's loads to the grid (or generator) when in charge mode.

I suggest that you use a separate charger and not use the one built into the Trace. The separate charger (I recommend the Todd or Statpower units) could be wired into the grid. The inverter would always be inverting and always be powering all the system's loads. Use a voltage sensing switch (like the Simple Switch from Photron) to connect the separate charger to the utility only when the battery voltage is low. This system would be user transparent. It requires no attention from the system's users. It will also make maximum use of your solar electricity. See the schematic on page 12 of this issue for a wiring diagram. Here Vladimir Nekola uses a Simple Switch to activate a Todd battery charger. Richard Perez

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